Introduction

Internet search engines are powerful things. The tiniest remark one lets slip in a forum, a newsgroup discussion, or a blog is indexed and associated with many key words discussed. As a former product and software developer, now retired, I have only good wishes for the many fine entrepreneurs who serve the hobbyist community, and only the hopes that their products get better over time. It's not fair to lambaste a product, in the heat of annoyance, and then have one's hasty remarks turn up 'til the end of time whenever a deep search for information is instituted.

For almost six months I have pondered the wisdom of posting my own particularly personal experiences with the integration of some products which, separately, are good or even excellent ones: but which, in my experience, did not work properly together to give me the success I had desired in using a GOTO scope. Finally I have decided that if I limit myself to some generalities and remain absolutely fair to the suppliers, with no exaggeration but only a clinical report of my experiences, I will do no harm to the community of developers, as I help end-users. Thus, I may sometimes seem below to be a bit evasive: it's to give manufacturers plenty of time to catch up with the experiences of the large group of users and their diverse needs.

Apologies if this article is very long and detailed; there is so much information that has to be presented in a structured fashion that it took many statements to convey the steps of an investigative process that took me more than a year to accomplish.

The Anticipated Result

I was still a telescope salesman when the first GOTO instruments were brought out: primitive compared to the ones sold today. I found them hopelessly complicated and unreliable, and continued to use traditional methods of finding objects to observe: star charts (augmented by printouts made from my home computer running a planetarium program using the Hubble Guidestar catalogue); a non-magnifying reflex site (I always prefer the "Telrad", by the way); an upright/correct image low power finderscope; and occasionally, on some of my appropriate telescopes, setting circles.

But, in 2003 my wife Regina Roper and I attended a star party to observe the favorable close opposition of Mars, which she reports about here. We were both incredibly impressed with the performance of one gentleman's fine new Meade GOTO scope, which reliably slewed to objects and positioned them in his eyepiece field of view with a minimum of fuss. He said that if he slewed all the way across the sky there might be a bit of hunting; but in a local region, object location was very reliable. Call up a Messier or NGC object, and =PRESTO!= there it was. We mentioned in the article about the star party that this was a scope with which you could really do some observing, not merely struggling. And as I have said elsewhere in these essays, I consider myself reasonably normal -- compared to most people -- in being able to read a star chart and find an object... but just not a gifted object hunter like so many advanced observers whose reports of challenging objects are found all over the net. If only I could improve my efficiency; after decades of star-hopping experience my skill level seemed to have "topped out" and wasn't getting any better.

By the end of the next year I had done my homework and decided to invest in a GOTO scope that would fulfill the following goals: (1) give me at least a magnitude better depth in discerning faint extended objects than the 8" Dob that I had used for the previous decade; and (2) assist in finding objects that were small, obscure, faint, and hard to miss in a complex star field. Recalling an experience I'd had with astrophotographer Chuck Vaughn in the 1990s -- struggling for nearly 2 hours to try to confirm the tiny nebula Minkowski 1-92 in the staggeringly pure and star festooned sky at 5,000 feet altitude on the California coast -- I wanted to be sure not to have to suffer that kind of struggle ever again. Furthermore, age is a factor. I wonder how many good years I have in me? Time's a-wasting! Why spend a third of one's observing session HUNTING?

My decision of telescope brand is not of major concern here, as I had good experiences with a variety of makes and relatively few real disappointments. And as a former salesman for several companies that sold amateur astronomy products I had visited the factories of both Meade and Celestron and was used to dealing with both companies. Suffice it to say that my final personal choice was to buy a Celestron C-11: it yields light gathering that was, theoretically, 1.89 times my old 8-incher. Both scopes (old and new) would have advanced low-loss reflective coatings so something close to the theoretical calculation could be expected: a significant improvement, breaking through my old "ceiling" of about 12.5-13th magnitude, the brightness of galaxies that were just about at the limit of the 8. But more importantly, I would have the ability to use a computer program to point my scope, almost precisely, where I wanted to look.

The Celestron telescope ads, reviews, and local dealer's salesperson promised the ability to locate an object from the internal database (all the IC, NGC, and many other objects) and position the scope within approximately the field of the lowest power eyepiece supplied: in this particular case, a 40mm Plössl which yields 70x and a FOV of about 0.6 degrees. Finding an object right overhead would be supremely easy, compared to my old 8" and 17.5" Dobs, or my even older pipe-mounted 10" Newtonian. Only my former antiquated C-8 had been able to offer almost effortless views of objects right at the zenith. Now, the new 11" scope would far surpass it in every possible way (and...not to keep you in suspense: it certainly does.)

The Initial Performance

The telescope salesman was just about right -- as far as using the scope with its internal database and hand controller. I could immediately, by doing even a fairly quick "GPS Align" procedure that takes a couple of minutes, get the C-11 to point at Messier objects and much fainter NGC galaxies and planetaries that were in the internal database, which numbers some tens of thousands of objects and stars but, sadly, not some of the obscure catalogues that I craved to explore. One found an object by using the 40mm ocular but often the very smallest diameter, faint galaxies would be almost impossible to discern and identify at 70x. Sure: a Messier like M27 jumps out at one; even if it's over to the edge -- as it would be from time to time -- centering was not difficult. Gradually I found that I was able to use a 30mm ocular, or even a 25mm one, for initial object location. But it was likely that an eyepiece with a narrower field would miss many that landed outside the zone. I estimated that the object location ability was at best a circle with about a 20 arcminute diameter.

However, everything went immediately to pieces when I used my new laptop and expensive "state of the art" star chart program (which shall, I'm sorry to say, remain nameless.) However, I discovered though that some "freeware" control programs were better and had location errors that were not much worse than Celestron's internal database. And my scope operated very reliably if I laboriously typed in accurate coordinates.

But the expensive program was frankly useless. It had a consistent systematic error that placed the telescope position about 20 arcminutes further away from the "normal" window of error. The screen star chart display would show the telescope slewing across the object, and way off to one side: even when the scope was pointed dead on the object. No assistance was offered by the program's user forum: the best advice I was given to repeated queries was "have you read all of our documentation and checked your installation?" (I deal with some of this frustration in a companion article about astro software.)

The Celestron company has very responsive tech support and I had a number of interesting conversations by phone. But when I reported to them that the scope did deliver reasonable object location accuracy by means of its own hand controller and database, that was about as far as they could go. The problems of interfacing to OTHER company's products would be mine to work out with other suppliers: which is an understandable attitude. Celestron developed the command set for their control signals and it was up to software designers to use it to communicate with the scope; they could not be responsible for how a different company's star chart program algorithms succeeded or failed.

So: the scope performed worse at locating objects when using the expensive deluxe star chart program than by keying in the numbers of coordinates or using the Celestron object database. So poor, in fact, that the star chart was useless as a scope controller.

First Steps Toward Improvement

I investigated this by querying search engines with appropriate complaint statements, and found alternative software, including the freeware programs Cartes du Ciel by Patrick Chevalley, and RTGUI by Robert Shaeffer. Despite the many advocates of the former program, I found it unsatisfactory in almost every respect, but did admire and appreciate the author's dedication. The latter program, however, was much more useful. It could locate objects by means of my C-11, better than the deluxe software that had cost me over $150; but it had some peculiarities. One function confounded me several times: the software could be used to set the laptop's clock by means of the time downloaded by the telescope's GPS receiver. When things went badly wrong on several occasions and NOTHING came out right, I discovered that my laptop had been set to "2065", not 2005! After this happened several times, I emailed the author, expecting to have him dismiss me as a hopeless crank. But, no: it seems that the GPS date and time string was at fault, causing the Celestron software to set an erroneous year. Robert added a new subroutine fixing the trouble: "Celestron GPS readings containing obviously invalid years will be rejected." If the computer's clock differs in year when the GPS time is downloaded, the user is told to check and see WHICH is the valid setting. Problem solved!

I found that the telescope could point closer using the freeware than with the expensive software, but often still seemed too far off for my taste. And control via the interlocked relationship with "Cartes du Ciel" was extremely unreliable, that program occasionally freezing.

I once again used the Net to find other resources, and located the NexStar Resource Site - Celestron NexStar Telescopes by Mike Swanson, an authoritative Celestron expert. I read every page. It turned out that I had already discovered and followed almost every bit of advice about optimizing my scope's performance: I had installed a counterweight below the tube assembly (as the scope needs to be "front heavy" to avoid backlash); I had no problem at all with my firmware, controller motors, or alignment procedure. My telescope-to-computer interface was accomplished by means of one of the recommended serial to USB adaptors. My edition of Windows XP had all updates.

Mr. Swanson also supplies some freeware for controlling Celestron scopes: and it's a good effort called NexStar Observer List. Unfortunately it did not have available the many faint, small, and obscure objects that I desired to view, and also interfaced to Cartes du Ciel, which I had found clumsy and unsatisfactory in its present state. Furthermore, I prefer not to use "lists" but rather to use a star chart program as I would a printed sky atlas, and to click on a region of the sky. List generator programs are hard to use at night without wrecking my dark adaptation. I seriously suspect that MOST observers who use such software are not going after 15-16th magnitude objects, as I am!

A Glimmer of Hope

By now, nearly a year had gone by and I was still frustrated. I had resigned myself to using the telescope's hand controller, which meant punching in the coordinates for every PK planetary I wanted to observe, many asterisms, and other objects not found in Celestron's database: hard to do at night, especially by a near-sighted user. Hold the controller up to your weak eyes, and the reddish-orange glow will WRECK your dark adaptation for at least a while: then the 14th magnitude and fainter objects will not be seen!

What Is Required to Point the Scope Via Software Commands

With an engineering background, I felt impelled to investigate as much as possible what goes on under the hood. Having created both analogue and digital control systems for industrial applications, I knew very well how complex logic structures could be developed. I knew something about communications and electronic interfacing. I was familiar with celestial coordinate systems. I had worked on three-axis digital control systems for microscopes. Why not intuit what the telescope and star chart software designers would do?

I would start with a successful professional computerized telescope installation: none other than the120 inch Shane Telescope on Mount Hamilton, at Lick Observatory! I had been a volunteer for the observatory, years ago narrating and producing the audio track for the visitor's center at the Shane; and was sure I'd receive some helpful information.

I phoned my friend Tony Misch, whose title is "Faculty Specialist" at Lick Observatory: he maintains a website (not exactly for the larger public, and which I won't give as a link as it is really for staff and students), and who is one of the people directly in charge of creating "order" at Mt. Hamilton, and of setting up the system used for observing. When I explained to him of the chaotic world of amateur astronomical software -- riddled with catalogue errors, and lacking standards -- and my telescope interface problems, he explained how these issues are controlled by professional astronomers. In fact, he told me more-or-less exactly what I expected to hear.

For one thing: the numerical accuracy of the equatorial degrees-to-binary conversion of coordinates is done by the observatory's system with a higher number base than the hexadecimal conversion of Windows software and supporting languages. Their UNIX system, which also runs on Macs, has much higher resolution: so the rounding off problems of cheap home-PC programs don't occur. Conversions can be made through multiple steps, and the results after many passes are still accurate.

Next, he said that every object is resolved, before it's fed to their telescope software, by a complex program that uses the most academically- scrutinized and edited catalogues and data imaginable. All those catalogues are actually on the Net. But, I've observed that most amateur astronomy programs do not use them directly; most use "digests" from one source or another, such as via the lists compiled by members of the Saguaro Astronomy Club. These are used by developers of software to skirt various copyright restrictions contained in the original, accurate catalogue, or possibly just because it's so easy to do so -- and each one of those re-hashes, put together by volunteers years ago (before amateur computers with digital setting circles, or GOTO scopes) adds a layer of human error: a mistaken decimal value in an object coordinate was not very important, say, back in 1980 when the catalogue was used merely to locate an object by eye on a star chart.

But this level of ambiguity is not satisfactory for an observatory, where there time is too precious to waste. Lick's program bypasses potential mistakes of this kind: so even if a grad student hands them a slip of paper with some numbers for (say) an obscure UGC object, it is corrected by their program even before the numbers are sent to the telescope -- human or machine data mistakes are weeded out, and precession is automatically readjusted as needed.Finally, the telescope control modeling programs, which of course have been carefully tweaked by their advanced technicians, correct for ALL systematic errors, mechanical errors, nutation, refraction, etc.: so that the telescope gets within a few arcseconds of the precise target, even with the Cassegrain focus of the 120" Shane telescope, a narrow field indeed!

I have sat in the control room of the Shane -- years ago -- and watched (without too much comprehension or concern) the operators routinely and very efficiently slewing the scopes to take spectra, images, and so forth. I remember that even back 20 years ago, when at least one of the scopes (the Nickel 40") was being run on what I seem to recall was a PDP-8 or PDP-11 computer programmed by paper tape (!), that the results of pointing accuracy were remarkably accurate. There was very little, or no, "hunting around". They had this worked out, at least intellectually, a long time ago, in a primitive era of computer development.

Furthermore, I needn't worry too much, said Tony, about the "warnings" I had been given by people who claimed that at least part of my trouble was that I was using a telescope with an alt-azimuth drive. The Shane too uses alt-az calculations! The only problem with alt-az is of course field rotation, unimportant to a visual observer.

After a great deal of thought, and tests with my scope and laptop, I came up with my conclusions about where the bottlenecks might be in my setup. They can be summarized as follows:

• 1. Precessional disparities and even gross position errors exist in object catalogues;

• 2. Mechanical tolerances, overshoot, and axis sensor resolution limitations may be factors;

• 3. Possible latency in the data control interface could affect control system regularity;

• 4. Is the control system a feedback loop, or a feedforward predictive system? If the former, is there overshoot; if the latter, is there a systematic error built in?

• 5. Accuracy of initial celestial coordinate alignment will affect all subsequent positions.

• 6. Possible errors of rounding due to the limit of data resolution of the 16-bit numbers used by Windows programs could affect accuracy.

Along the way toward my odyssey of exploration I joined several Yahoo users groups related to Celestron and other GOTO scopes, and made inquiries. At long last, I achieved some significant and positive RESULTS!

With respect to No. 1, above (object catalogue errors) I confirmed that there were many such mistakes in my expensive star chart program. I am still sorting them out now and also investigating replacement software. Often there are positional errors of up to 10-15 arcminutes worst-case even for objects in the IC and NGC catalogues, and in versions of the Saguaro Database used by the program. These have to be dealt with on an individual basis, as I have reported in this article about trying to observe faint galaxies.

No. 2: I had determined that my telescope was properly balanced and had no mechanical overshoot problem. And I learned in direct communication with Mike Swanson that the C-11 axis optical sensors had a resolution of 1.2 arcminutes, which I had more or less intuited, based on my own tests that nothing closer than 2 minutes could be achieved by sending careful pointing commands. This error, for a visual observer, is almost inconsequential; I was worried about errors that were an order of magnitude greater, or even worse, when the star chart program communicated with the scope.

No. 3: Latency. AH, YES! A big problem, and possibly the major crux of my difficulty!

It required two big changes:

(1) replacing the "native" Celestron GPS 11 telescope software driver used by the star chart program with a new one, written by the public-spirited contributors to the ASCOM Initiative. I had already installed the latest ASCOM platform for some of the freeware scope control programs I had tried; so all I needed to do was to download the new Celestron GPS Scope Driver and follow the directions for installation in my star chart program. (The one that I am using now is not the very latest one that has been written for the new Celestron firmware, so I cannot comment on any differences that may be noticed compared to the driver I downloaded in the spring of 2006.)

(2) The second big and essential change in my setup was recommended by a helpful astrophotographer who operated a large amateur observatory, using his scope via an Internet connection. There was a crucial setting in my star chart program that was, for all practical purposes, undocumented. As I said, earlier, I'm not mentioning the name of the software, primarily because I was so let down by the tech support of the company. I frankly would not recommend it in its current state. But, with the new setting, my huge bugaboo of the systematic cursor misalignment was finally fixed! Here it is (those of you who have the program may recognize the commands and menus):

Changing the screen update timing parameter of the program:

   Click DATA...

      Time...

          Time Menu...

             Time Options (the little display box that is the 
               chart icon in the lower right hand corner)...

                 Set SCREEN UPDATE to 30 seconds

                 Set STELLAR UPDATE to 30 seconds

[Note: Since this article has sat on my website for two years without drawing any outraged protests from either telescope or software makers, perhaps it is now safe to admit that this process, described above, is actually found in the program "TheSky VI", which I often use for controlling my scope. Now that I have added the ASCOM driver, I am reasonably satisfied with this application, and do prefer it to certain other commercial and freeware programs of its type. -- srw, 6.13.08]

This last setting seems to compensate for latency in the system, as far as I can tell. Now, the "systematic error" of the cursor constantly overshooting the desired object and moving to a spot somewhere around 10 to 20 arcminutes away from it has been ELIMINATED. It does not affect the scope motion but the way the star chart shows the position of the scope; so the trouble would seem to be in the way the program worked since the object position coordinates being fed back TO the computer were correct, by my actual repeated tests. Other star chart programs may have a function in the telescope control settings that can affect the latency of the system: check with their documentation or user groups for advice.

No. 4: Topology of control loop: feedback or feedforward? I don't know the answer to this, and neither Mike Swanson nor Celestron tech support could explain it. My suspicion is that it is entirely feedforward, and dependent on many factors to determine the success of scope pointing, many of them covered in recommendations on the NexStar Resource Site. Particularly important is scope balancing, and maintaining a slight front-end heaviness to reduce overshoot and gear backlash.

No. 5: With respect to the intial alignment of the telescope: the instructions make this a simple matter. But I have now evolved a more complex one, advancing further than the suggestions I've read on the Net.

A.  Set up a "zero power" reflex site on top of the scope, aligned with the main optics. I use a rifle red dot site with the brightness all the way down; it cost me less than $15. I attached it to the finder. When I first do the "GPS Align" the scope does NOT reliably turn to either guide star and get them within the 7 degree FOV of the Celestron optical finder; the scope is often 10 degrees away, even though I've set the magnetic compass alignment according to instructions. Slewing to the guide star is painfully slow, so much that one often can't tell which direction the scope is going: so I watch the red dot and can see if I am getting close enough for the guide stars to show up in the finder; then I switch to the finder to do my first level of alignment.

B.  Obtain a cross hair eyepiece with moderate magnification that is a bit higher than the lowest powered ocular supplied with the scope. Astro-guiding eyepieces will be useless: their fields being too narrow and magnifications too high. As it happens, with the four Orion telescopes I purchased, I received a quantity of standard Orion "Sirius" 25 mm focal length Plössls. In my GOTO 11, the ocular provides a FOV of about 28 arcminutes (just under half a degree) at 112x. I had therefore several spares, and took one of them and glued in some No. 30 enameled wire in a home-made crosshair, positioned in focus between the end of the barrel and the field lens. It took only a few minutes to find the correct spot; tape the wire; and then apply small dabs of glue, taking care NOT to allow it to get on the lens. This saved me a considerable amount of money, as precision cross-hair oculars are expensive. You do not need PRECISION, for this application, but need approximate repeatability: knowing when your guide star is about to cross the central axis of the FOV.

C.  When aligning, you must press the buttons on the hand controller to anticipate a bit of latency in the Celestron software. I explained it this way in a letter I wrote to a private correspondent who asked how I set up my scope:

When you use GPS Align and do your second (final) alignment star, after you have "positioned it in the finder" and press ENTER, you are supposed to center it in the eyepiece and press ALIGN.

Well, it's this second step that requires close attention. After you press ENTER, the scope *seems* to start drifing. Using a star diagonal and looking into my reference crosshair eyepiece, the second alignment star is moving slowly from RIGHT to LEFT, drifting along the crosshair that I have adjusted to be horizontal.

If I *wait* until this star is EXACTLY on the center, and press ALIGN, there is a delay. By the time the alignment position is registered by the scope, the star is actually now OFF the center, left of it. And THAT spot is the "reference" and your objects will be placed THERE, not at the place where the star was the exact moment you hit ALIGN.

So what I've learned to do is to anticipate this a little bit: to press ALIGN just before the star crosses the hairs at the center. My reference eyepiece is a utility Plössl 25mm with some wires that I glued inside the thing, between the field stop and the field lens. It's actually not that precise a gadget; but it does show the approximate center, within a few arcseconds. I get the second alignment star on the horizontal crosshair and hit ENTER: it starts to slowly drift toward the left, toward the center. When it is an arcminute or two TO THE RIGHT of exact center, I press the ALIGN button. If I get the timing just right, the scope accepts the alignment JUST as the star hits the center, and the next object I slew to -- within say 10 degrees -- will come up EXACTLY at this spot.

As you move away from the region of that second alignment star, the error builds up. If I slew the scope 180d along the horizon, and ask for an object there, it will often be nearly outside the FOV of my low power ocular. To deal with this I slew to an ALIGNMENT OBJECT: a known star or bright unmistakable object like a Messier that I cannot possibly misidentify. It comes up off center; but I use the hand controller to position the object sort of close to the center -- with ANY eyepiece, not necessarily the one with crosshairs! -- hit Sync in the Object Display panel under the Telescope tab, and the program is now corrected. Or, if I am not using the computer at all, I merely tell the hand controller to "replace the star" used as the second GPS Align reference, with that new object. I get about the same positioning accuracy this way, with or without the computer: amazingly good, and MUCH better than I got when I first started to use the scope.

No. 6: Arithmetical rounding errors? Can't do a thing about them. And I suspect that with only 1.2 arcminute encoder accuracy, they are not particularly consequential.

Final Step of Improvement: Implementing and Using "Sync"

From discussions I have had with members of the ASCOM group on Yahoo, I have been told that Celestron's native command set for the NexStar scopes lacks the "built in" sync command (as one engineer who has studied the algorithms claimed to me; he however asserted that "sync" was a specific command in the Meade control command set.) I have no way of confirming this without examing all the parameters supplied by both Meade and Celestron to the software developers; and I haven't taken the effort to do so. I can say that when I used my star chart program initially, it showed a "Sync" button in the telescope control box, but pressing it resulted in an error message that sync was 'not supported by the hardware.' This erroneous and misleading message made me think that the scope was to blame, and I initially gave up on the hope of being able to use "local sync" to fine tune the calibration of the scope's position.

Sync is useful for digital imagers in particular: you can position the scope on a very close guide star, hit "sync", and then PRECISELY calibrate the telescope's position. You may even slew to an absolutely invisible faint object that does not show up in the eyepiece or finder, within a degree or two, but get it onto the narrow CCD detector chip. Similarly, I had planned to "sync" to stars near faint, small diameter galaxies and planetaries, so that my scope positioning could be refined down to errors of only a couple of arcminutes. But "sync" was disabled!

However, I found out in short order that an ASCOM software writer had come to the rescue, writing a NEW software driver for the NexStar GPS scope that implemented sync in an original manner. I installed this driver in my star chart program, replacing its native Celestron GPS scope driver. VOILA! Sync now works! And no "hardware" was changed, proving that the software warning message, described above, was basically A LIE: it was not a "hardware" problem. By using my now-enabled sync, I have been able to get 2 arcminute pointing accuracy near a guide star, within a few degrees! This is more than an order of magnitude BETTER than the scope does from its internal database. It's very easy to use the hand controller to position the last object you've looked at in the exact center of the FOV of your ocular, and press the "sync" button in the star chart; the NEXT object will then be more closely found, if it isn't too far away. Errors grow as distance accumulates. When you start missing the objects, find a fairly bright recognizable star in the region (I use my "bad" eye that I don't employ for observing, to preserve my dark adaptation); center it in the scope; hit it with the cursor of your star chart program; press "sync", and you are on the nose again.

Unfortunately, not all star chart programs will allow you to use the ASCOM driver. Celestron has said that it has implemented sync if one replaces the firmware by getting a new hand controller. I decided not to do that, in order to keep the original alignment process, which is no longer being provided; nor have I had to do it, now that sync works via my laptop program. Personally, my decision has been to upgrade my star chart software only with programs that will allow both the use of the ASCOM driver AND the sync command, and I will preserve all the functionality that I now have. (Unfortunately, my queries to certain software suppliers have been met by vagueness in some cases, and in others with a definite "no: ASCOM is not supported".)

Proof of Results

I am happy to say that now I am quite satisfied with the entire system of integration (though the star chart I'm still using does have database issues, which I talk about here.) There is not 100% total reliability at all times. Rarely, an initial "GPS Align" won't 'take'. On a couple of occasions, I have had the scope start mistracking and finally have had to stop and reboot it and re-align; the problem disappears and I then observe for many hours without a hitch. Every once in a while, my star chart program starts misbehaving and I have to 'disconnect' its communication from the scope, close the program, and re-start it. But, often I will go through a period of many weeks of observing with no difficulties or glitches: just trouble-free accurate object positioning without much bother.

My initial scope accuracy was a window of about a quarter to a half degree positioning ambiguity at best, using the hand controller's database. With my first tries using the star chart software, the error was at least twice as bad. Now, optimally I can position an object within a window of 4 arcminutes, and sometimes within 2 arcminutes if things go perfectly! On occasion I have even left in a 9 mm eyepiece (311x, FOV of 12 arcminutes) while slewing from object to object in the same general region of a constellation. On one very "good" night, for a long period of time I left in my 6 mm eyepiece while moving from one tiny planetary nebula to another! I was astounded at the improvement in my ability to find and confirm difficult objects. To read of my success, please consult the observing articles in my Faint Fuzzies list. My telescope is showing me things that are far fainter than expected: because I can get them into a narrow field of view, allowing me to observe them at high magnification. It's a joy to be able, at last, to detect 15-16th magnitude galaxies -- and with a scope of only 11" aperture!

Steve Waldee

 

NOTE: The information provided above, first posted in 2006, is NOT guaranteed to be absolutely correct in all of its assumptions or explanations; it's my best attempt but may contain some mistakes or improper descriptions. Telescope and software makers are very fussy, and sensitive to criticism. Sometimes the best advice is to wait for upgrades when other users have complained about the same problems: at a certain point, the developer will eventually respond with improvements. This is one reason I'm not naming certain specific products. I do not guarantee that following my suggestions will help your system if you are not using Celestron hand controller firmware 2.2, or the same star chart software, the same ASCOM driver, or the same type and speed of computer that I use: and I'm not being specific about those details. Many things can cause your results to vary. I have provided a narrative of my personal experiences not anticipating that readers will COPY them, but will use them as guidelines in their own investigations.

 

 

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ADDENDUM (June 2008):

If you have come directly to this "Addendum" without reading the article above, I would suggest that you stop and do so before proceeding.

Some time after I had first posted the above article on my own website, I read a question in a certain astronomy forum from a user of a similar GOTO scope, asking for advice in correcting poor pointing accuracy. I replied with some of the suggestions in my article, and a link to it. Immediately I was (in my opinion) "negated" by one of the forum regulars, who disputed what I'd said (he did NOT have the same telescope size or model); furthermore he assured the original questioner and all readers of the discussion that I was being harsh in criticizing the pointing errors, which he found normal and acceptable (therefore, being -- essentially -- inconsequential ones.)

But now I have discovered that the same individual has later done his own series of tests with a smaller Celestron GOTO scope, just using a list of stars (not deep-sky objects in the scope's or a computer program's database) and came up with maximum errors that were similar to mine. He reported that he sometimes found objects 20 arcminutes away from where the scope would point to (which he says he considered "OK"); and when he did the star test, determined a maximum pointing deviation of nearly 13 arcminutes (though many stars were not nearly that far away from the target position.)

Any kind of useful report of this type increases the knowledge of the amateur community, and I for one very much appreciate his efforts.

However, the analysis seemed -- to me -- to beg many questions, which went unasked and unanswered. It is very important to remember that one should not over-generalize from specifics; and I have tried (in the article above) to remind readers that my personal experiences are contextual and depend largely on the exact setup I use: what model scope, firmware, and control software; what star chart program and object catalogue; the scope driver file; which firmware method of scope/laptop interface; how scope alignment is performed; and the way error offsets are measured all effect the final results; changing any one will have a cascade of effects (see my "NOTE" above for caveats regarding the interpretations one might form about the findings of my article.) In order to determine the universality of his conclusions, and their applicability to other situations, much more should be understood of the context of his tests (in my opinion.)

Since he's providing figures within a presumed accuracy window of 6 arcseconds (one tenth of an arcminute), I don't feel that it is unreasonable to ask some questions about the methodology employed. Here are the issues that came to my mind when I first looked at his analysis:

• We don't have any idea how he measured the centering of the stars. What magnification was employed? Was a cross-hair eyepiece used? How did he determine exactly the deviations to tenths of an arcminute? Were the error measurements derived optically, through the eyepiece, or by repositioning and comparing positions using the control program, MegaStar?
• I have measured my own C-11's image shift during focusing to have an error of at least +/- 6-7 arcseconds. How does the tester's C-9.25 compare in this regard? If it has significant mirror-induced image shift while focusing, how did the tester compensate for it; were focusing-induced position errors factored in when star position errors to a tenth of an arcminute were derived? Was re-focusing EVER done during this test? Was mirror-shift measured at all points of the scope's excursion across the sky?
• What happened after the error offset was derived, when the tester slewed to the next star? Was the scope ever re-positioned; was the computer program "synched" to the scope pointing spot, with the correct coordinates substituted? My own personal email exchanges with Emil Bonanno, author of MegaStar, yielded the fact that the program cannot interface with the ASCOM platform, and uses only the Celestron native driver and command set. This fails to implement "sync" in the Celestron control software version used by my telescope. How was the complete system of scope/laptop recalibrated on each star, once the error had been measured? WAS it recalibrated?
• Did MegaStar, at all times, show an absolutely exact correlation between the scope's position in the sky, and the star chart display coordinates? My experience is that every program tried is slightly different, and that there is always an error of some sort -- but the versions of MegaStar that I've used do NOT have the telescope control capability, so I lack experience with it and wish to compare its performance with (say) TheSky, Starry Night, or Cartes du Ciel. Sometimes it is necessary to adjust screen display refresh rates and position update intervals in the software: was this ever an issue with MegaStar?
• The tester is a very experienced observer, and can be expected to be able to identify each of the stars from his solid knowledge of astronomical constellations. Yet, one wonders what care was taken to insure that, indeed, the correct target star was in the field, and how it was verified? Is there any chance that even one star was mis-identified (which would significantly affect the statistics derived for the entire series)? Were stars recognized in a finder view, then compared to a star chart; or were they identified on the laptop display? No disrespect is intended to a fine and experienced astronomer, but even one misidentified star will skew the final average error data.
• Did the tester try repeated runs to the same objects? This is not clear, though a later post infers that for at least one star, Algieba, a repeat slew with a greater error was indeed done, but had been accidentally left out of the calculations of the first posting of the results. If so, which other stars -- any? all? -- were repeated; and how many times?
• Why did he reset the alignment to Denebola in the middle of the series? This tantalizing fact is mentioned but not explained. Was it perceived that errors were building up?
• What was the rule or scheme employed for selecting the sequence of stars and their separation? What were the actual slewing distances between each star? (This might be derived from the list by a reader, but if the data were already known by the tester, it would save much effort .)
• Did the experimenter isolate and subtract any position deviations observed in the MegaStar data for each one of the target stars (preferably to the arcsecond, or at the very least within his given data resolution of a tenth of an arcminute) with respect to professional catalogue measurements derived via the SIMBAD or VizieR websites? Did he investigate various star catalogues to compare appropriately precessed positions to the arcsecond (or within his resolution window), with the "current" positions generated by MegaStar from the J2000 coordinates it has in its database? If not, does he know ANY single example of a star position discrepancy or disagreement between MegaStar's positional calculation, and the derivations obtained from an authoritative catalogue? Can he assert definitely that none exist? (As good as MegaStar or any other star chart program may be, discrepancies could accumulate that might possibly be non-trivial with respect to an attempt to determine scope position within a tenth of an arcminute. I could check, laboriously; but it's HIS test procedure, not mine; so I am not obliged to falsify all the data and check for any possible systematic errors before publishing it.) Without isolating and eliminating the possibility that occasionally the star chart program may not exactly precess the positions precisely, we cannot know if any resulting scope positioning error is due to the scope's performance, or the star chart's calculation; or due to a combination of the errors of both (which, in some cases, could be subtractive, IMPROVING pointing accuracy; at other times would be additive, INCREASING the position errors.) This may seem to be a very fine point of objection: but, remember, we are concerned with a resolution of accuracy of 0.1 arcminute, as asserted by the tester in presenting his results.
• Why did the experimenter use only MegaStar to control the scope, and not report the results obtained by using the star coordinates in the Celestron database, using the hand controller? Why were not any of the free telescope control programs also tested (such as RTGUI or Cartes du Ciel), which would cost the experimenter only a few minutes of time to download and install them?
• What initial scope alignment process, of the various ones available in the hand controller, was employed? What were the firmware numbers of the HC and motor drivers? What was the version number of the Celestron software?
• How was the scope interfaced to the laptop? Did it use a serial connection, or a serial-USB adaptor, and -- if so -- which one? Celestron suggests certain preferential models. There are potential issues of latency variations to be considered in the control loop.
• The C-9.25 tube assembly is very much smaller and less massive than the C-11 unit. Likely the same encoders and motors are used in the hardware. Scope balancing with the C-11 is very critical to the success of the initial alignment, and to the accuracy of slewing; I have found that my C-11 would not work reliably unless I added the aftermarket adjustable counterweight system. One wonders how the C-9.25 was balanced, and if it had the added weight system? Did it use the stock Celestron finder or another lighter or heavier unit?
• Was a dewshield employed (which would slightly alter the center of mass and the weight)? Whether or not a dewshield was attached, what were the local wind conditions during the slewing tests? Could wind gusts have affected at any time the scope positioning accuracy?
• How much difference was noted in the seeing conditions at high and low elevations? What was the maximum or mean displacement of the center of any of the star positions caused by seeing? My own experience during decades of observing at a high altitude Pacific coast site near Lick Observatory has demonstrated that seeing variations can displace star images up to several arcseconds. Was any test and calculation done to determine a deviation of error of the optical discernment of star position, as affected by seeing?

The tester concludes that "The scope routinely puts all the objects well within the one degree field of my 2" 40mm Mk-70 Konig during my entire observing sessions, so its performance is reasonably satisfying to me at least." However, this is NOT the ocular supplied by Celestron; it's made by a different company; requires a 2" diagnoal (not supplied); and from my research apparently it was not supplied as an accessory by Celestron with either the C9.25 or the C-11 telescopes.

In fact, included with my NexStar 11 GPS scope was a Celestron E-Lux 40 mm 1.25 inch eyepiece with apparent field of 43°, and a 1.25" star diagonal. The Mk-70 König is no longer listed as being available on the official University Optics website. However, a review of it by Otto Piechowski on Cloudy Nights states that the 2" eyepiece has an apparent field of 70°; I have indeed confirmed this with several alternative sources, catalogues, and lists. I used my own "Eyepiece" program and derived the estimated visual fields of both eyepieces, which are only approximate calculations based on the published apparent fields (but might be close): in the C9.25 scope the supplied Celestron 40 mm eyepiece could yield an ideal estimated widest field of about 44 arcminutes; the Mk-70 König might yield as much as 71.5 arcminutes: a huge difference. So, the tester's own low power eyepiece (which of course also requires a 2" star diagonal, not supplied as a stock accessory with the telescope by the manufacturer) is not at all directly comparable in performance with the instrument's original equipment. The total cost of upgrading to this combination would be about $315.90 ($195.95 for the ocular, and at least $119.95 for the Celestron-brand 2" SCT diagonal.) Therefore, it is very obvious that the tester is NOT evaluating the performance of the telescope as supplied by the maker, but has added a costly upgrade in order to achieve a much wider field of view in his low-power ocular.

If we go by the calculated field estimates, this König eyepiece -- having a visual field in this telescope of a calculated 35 degrees either side of the exact center -- could in fact position an object within its field, while the "stock" Celestron ocular -- about 22 arcminutes either side of center -- would not show it at all. Assuming that the tester, who states that the visual field of the eyepiece is "one degree", has actually done a careful star drift measurement (very likely) and determined that the field is actually narrower, the ocular still has a significantly wider field than the Celestron E-Lux supplied. What is "satisfactory" to THIS tester -- with a modified scope employing an expensive aftermarket ocular that is no longer sold by its maker, and thus might be hard to acquire when existing stocks run out -- is a test situation that does not represent at all the context of the scope as sold and supplied by the maker: the situation that I am reporting about in my article.

If the reader would like to try to duplicate this tester's setup, they may indeed have difficulty locating the exact discontinued UO eyepiece he employed. I checked and found that the closest current 2" barrel models with 40 mm FL and 70° apparent field are three: Pentax XW Series 40 mm (as of 6/13/08, $579 "sale price" from Scope City); Meade SWA Series 5000 40 mm (68° apparent field, "sale price" $399, ditto); and William Optics SWAN Series 2" 40 mm ($117.95, from Adorama). So, it could conceivably cost as little as $238 -- to as much as $779 -- to get approximately the same field width performance as the setup of the tester, if his exact eyepiece was not available. And, I can predict with fair certainty that these oculars, and the tester's König, will significantly differ in their "edge clarity" in any single given scope. While an f/10 SCT is more forgiving than a faster Newtonian, 40 mm eyepieces with wide fields often demonstrate considerable problems of vignetting, coma, or blurring the further away from the center of the field one positions an object. A very dim galaxy or small planetary nebula might be severely "smeared" at the edge of the field, up to the point that it disappears from ready detection. This aspect of performance will vary predictably between these four eyepieces, introducing yet another factor in the apparent success of a GOTO object acquisition, and showing how judgment of "GOTO satisfaction" can be very context-dependent, possibly related to the exact choice of eyepiece employed.

Note: The current closest production model of the alt-azimuth GOTO Celestron C9.25 is the CPC 925 XLT, which is supplied with a 40 mm E-Lux Plössl identical to the one sold with my NexStar 11 (and indeed the same ocular supplied by Celestron when the NexStar GPS 9.25 scope was in production.) However -- oddly -- the Celestron Advanced Series C9 1/4-SGT (XLT) (same basic tube assembly, on a German equatorial mount) is sold with an accompanying 25 mm Plössl (52° apparent field) which would yield a calculated visual field of 34.5 arcminutes: much narrower than the ~44 minute field of the "stock" equipped alt-az C-9.25. Hmmm! This does suggest in fact that the Newtonian version might have "tighter" GOTO performance since, of course, it is widely reported that the scope will get objects 'in the lowest power eyepiece field', and presumably in the FOV of the ocular supplied with the scope.

Bright star positions have been very well defined for the last 150 years, but the visual-center coordinates of many deep-sky objects are at variance with some catalogue values, achieved by photometric measurements that are not at all in accordance with how objects "look" to the eye in various telescopic views. So, for instance: certain large open clusters may indeed not necessarily seem to be properly centered in an eyepiece, though the scope is set near or on the 'textbook' coordinate position; the same problem applies to many large diffuse nebulae. Furthermore, some of the catalogues used by certain star chart and observing planner programs contain significant position errors. The perception of GOTO accuracy may, to different observers, seem to vary, depending on their individual familiarities with an object and and understanding of the totality of its isophote plot, versus the "visual" phenomena that may be perceived. I can accept that stars will be less ambiguous; but testing only stars does not replicate the typical experience of deep sky observers nor that of the seekers of planets. The issue of GOTO scope errors may in fact be perceived differently by observers with different levels of experience and taste, as well as in regard to the objects being sought: for instance, faint nebulae or diffuse looking galaxies can be invisible at the EDGE of an eyepiece field; the author of this test acknowledges that objects did sometimes show up at the edge of his lowest power eyepiece. What objects? What kinds of objects? Were they detectable without changing to higher magnification?

Even a considerably advanced observer may have great frustrations with certain astronomical catalogues. While I'm not competent to qualify myself as being "advanced", or not -- I leave that determination to my friends and readers! -- I do from time to time look at very obscure objects. I found, while testing the program "Starry Nights" to control my GOTO C-11 scope, that objects from the Dolidze-Dzimselejsvili cluster catalogue were sometimes almost impossible to see in my lowest power eyepiece: they were off the field. Starry Night slewed my telescope to them -- allegedly -- but the clusters were not all even remotely in the center of the field with a 32 mm Orion Q-70 eyepiece (which yields 87x at approximately 48 arcminutes field diameter); and even once a cluster was more than partially OUTSIDE that field: an error of about 1/2 degree, or perhaps larger. (Some of these clusters, and others that are not rich and are barely detached from the star field, are hard to perceive visually, so are not immediately or easily identifiable in the eyepiece view: especially if situated partly beyond the field-stop.) But, this is what I'm used to achieving in many cases: that's why I generally need to use a local synched reference to a known object when hunting for something really obscure...at least Starry Night lets me do that; some star charts will not (TheSky does, since it can use the ASCOM driver.)

I must say that my first reaction to what I personally felt was a curt and very dismissive "put down" of me was very discouraging: in fact, I did not bother to post again on that forum for at least a year. It annoyed and pained me to be negated, when I had recorded my own experiences very thoroughly and accurately; furthermore, it was troubling to be told that my opinions about this level of performance, and my goals, were faulty. I was in effect informed that it was inappropriate to say that these errors of pointing were unacceptable; my opinion about that was wrong.

That is, in my view, an absurdity: as it is not possible for another individual to get "into my head" (especially if he does not have the same telescope, and did not share all the same goals and attitudes of a different amateur astronomer.) If I believe that it is not acceptable to have typical pointing errors of 20 minutes or more, I am entitled to SAY that: really, there is no way it can be criticized except on another individual's PERSONAL subjective grounds (and therefore there is almost no limit to what we might waste our time debating.) A reasonable person might feel that we can in fact BOTH say opposite things; we're allowed to hold opposite views regarding a subjective, personal judgment. Because of that very real fact, I did not dispute him, and let his strong dissenting opinion stand, with no comment from me.

That he FINALLY acknowledged that the performance of the GOTO scope does indeed -- under some circumstances that he documents -- fall short of precision is a step forward. Is it too much to hope that he will be more tolerant of other amateur astronomers' differing opinions in the future?

Furthermore, I do still have a difficulty in accepting that 20 arcminute errors are "OK" and "normal". As this user's test report indicated, he by no means obtained exactly repeatable results. Pointing accuracy would get better for two or three successive stars, and then deteriorate greatly; it could be worse at lower elevations (for a while) and then, surprisingly, the errors in subsequent slews at that elevation would not be as great. He also had the opposite experience of your present writer: his laptop control pointed the scope more accurately than the Celestron hand-controller could do; I had the reverse experience (with a different software program and a later version of the operating system.)

Therefore, I can only conclude that part of his comments do indeed verify many of the initial complaints I had with my early C-11 GOTO performance; but that his test (arguably not very thoroughly explained) does not by any means agree enough with my own procedure to provide any conclusions from it that I find useful: as he employed a different sized, smaller Celestron scope (how was it balanced?); another software package; a different type of laptop (was he using a serial connection or a USB-serial adaptor?); a different OS; and stars but not deep-sky objects; and a different, larger star diagonal and wider-field 2" barrel lowest power eyepiece. I also conclude that without attention to the elimination of systematic error, the test result cannot likely be as accurate as he posits.

The contexts of my tests, and his test, are quite different. I have taken great pains to insist that my experiences are not universal and that other situations can provide different results; but in his original post of test data, the writer gives no such specific caveats (for instance, he only includes the phrase "satisfying to me at least" in one of his answers in the fifth post in the thread.) In this post he explains: "If the pointing accuracy is consistently better than 20 arc minutes, things are very probably working about as they should" and "You might consider it 'poor', but it is what most people will see as a 'maximum deviation' from the Go-To target using the Nexstar 8, 9.25 and 11 inch SCTs when used in the altazimuth mode."

I find no actual existing evidence backing up these broadsides, nor reliable analyzed social data to show 'how GOTO users expect their scopes to perform'; they're arguably just his speculations, likely drawn from absorbing various anecdotes expressed by diverse users with a wide range of sophistication. Even so, if my results and conluding judgments differ from any old GOTO scope user, this does not logically suggest I'm wrong. I have just as much right to MY opinion, as the other user has to HIS opinion, except that in this specific case, mine is based on the facts I've derived, while his claim of "what most people see" seems -- since no evidence is given -- not based on anything definite or concrete that has been systematically derived. He cites (at least in the original post dated 9/12/06) no surveys, no industry findings, no magazine reviews, studies, or metastudies: NO independent sources to back up his opinion of what is expected by purchasers of such telescopes.

This point is not as argumentative as it may seem: for in his original critique given to my explanation of problems I had wrestled with, he painted (I believe) a picture of reliable and accurate GOTO performance, alleging that I was not even slightly reasonable. Then, his subsequent test report shows that he too gets position errors (which he variously categorizes as "20" arcminutes, or within a "5 to 15 arc minute area", or "10.7 arc minutes", or "12.7" arcminutes, or "just a bit less than 13 arc minutes", or "at maximum 13 arc minutes" depending on the exact statement in a given post in the discussion thread), and concludes that this is "good performance" -- for him. Does it have to be 'OK' for me? Or, am I permitted to have a different opinion, without being subjected to criticism that relates not to facts but largely to differences of subjective opinion?

Forum and Newsgroup "Negation"

As I asserted above, I recall very clearly being quite perturbed when the facts I presented were glossed as being "OK" or normal operating performance, and I became so discouraged by it that I decided not to post again in any of the CN technical or observing forums. About a year or so later, I did make a post in the astronomical drawing forum, and posted one to their "pets" forum. However, I am not nearly as bothered by his disagreement two years later -- especially after reading his own test reports that confirmed many of the essential points in the critique contained in the above article I've written: water over the dam, as it were.

But, the issue remains: forums and newsgroups often contain what I'd call "knee-jerk reactive criticism" and objections that in the long run do not seem to be reasoned or fact-based, but rather influenced by feelings and general opinions (not to mention personal animus.) I came to that forum only specifically to answer a question posted by a GOTO scope user who had a very similar problem that I believed I had solved; and I gave my explanations and some general recommendations. I wasn't thanked by the "expert" respondent, nor apparently appreciated or respected for the work I was reporting. Furthermore, when he APPARENTLY came up with very similar errors in his later test, he made no reference to mine, not even referring to it as one that corroborated some of his results.

I believe that often there is a tendency of certain types of technically minded individuals to argue narrowly in what I'd define as being "extreme fundamentalist reductionism": one of its aspects is arguably to fail to recognize the individuality of other people (in this instance, other amateur astronomers.)

While there are certainly some very basic and definable physical principles involved in analyzing issues of astronomical objects and coordinates, telescope functionality, computer algorithms, and kinetic functions, over-riding the sum of these parts in a complex system involving the machine-man interface are the human elements of perception and judgment in performing tests, deriving judgments, determining solutions, and forming opinions. These are very complex processes.

Furthermore, in my opinion many of these alleged disagreements are like Freud's "distinctions without differences": they are subjectivities that are inflated into a larger importance than they really have. If my debater tests, and finds that he gets almost the SAME errors, as well as many similar inconsistencies and erratic results, as I do: why not leave it at that? Allow other people to decide if I have any justification for wanting better performance than this, without declaring the matter one way or another. Hasn't a combination of my report, his reaction, and his subsequent tests -- followed by my analysis and response -- created a synergy that has increased general knowledge?

If my data are actually verified and correct, then categorizing the nature of my judgment is the only remaining issue; and it's unimportant. I don't agree with my critic. I simply cannot agree. I have a different perception of what "acceptable and reliable GOTO performance" should be. And by following my process, I solved the problems to a great extent. To argue beyond this and cast aspersions on personal subjective value systems, goals, and judgments is to take the discussion away from objectivities, into a very nebulous realm where almost any opinion can have any justification, or be found faulty, on any subjective grounds one might propose. This serves no purpose; helps no one; solves no pointing errors; encourages no further collegiality and cooperation; fails to promote respect; and forges no friendly bonds.

Furthermore, "negating" what the original poster believes is a reasonable criticism and judgment does not assist the manufacturer to know what type of performance is expected by its customers, nor offer any inducement to improve future versions of such products. -- srw, 11-13 & 17 June 2008

 

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