Ninety Minutes With Leo 1 - An Article by Steve Waldee INTRODUCTION In the list of "DEEP SKY CHALLENGE OBJECTS" printed in each edition of the OBSERVER'S HANDBOOK, published by the Royal Astronomical Society of Canada, noted authors and observers Alan Dyer and Alister Ling recommend a minimum aperture of 30 cm (11.8 inches) for observing Leo 1, a dwarf galaxy and satellite of our Milky Way, although they encourage users of smaller instruments to tackle their selected sky objects: a collection including some of the most obscure and low-surface-brightness galaxies, nebulae, and clusters that one could ever hope to investigate with a visual telescope. On p. 1061 of the 2nd volume of his CELESTIAL HANDBOOK (published back in 1966 before the advent of today's popular fashion of observing with large-aperture altazimuth scopes), Robert Burnham, Jr. writes a perceptive section on the galaxy Leo I, and its 'little brother' Leo II, never discovered visually in the heyday of the great eagle-eyed observers of Barnard's era. I really can't fault Burnham's warning that Leo I is "almost impossible to observe visually in any telescope owing to the extremely low surface brightness" (especially after spending a large part of two moonless observing sessions with my eye glued to eyepiece fields of the region of Leo I for about an hour and a half!) The object requires at least several satisfactory conditions, and will not be perceptible to any and every observer who merely trains a telescope on the galaxy's coordinates: especially if the viewer intends merely to "sweep around" the field of brilliant Regulus in order to find the galaxy. Yet, since in the amateur astronomers' newsgroup we have had recently at least "2-1/2" reports of viewing Leo I with scopes varying from 15 cm to 25 cm in aperture, Leo I is not a challenge that any observer should overlook, in order to stretch the boundaries of viewing skill. OBJECT DIMENSIONS AND CHARACTERISTICS This observer first tried in early February, 1997, and reported an equivocal and ambiguous sighting. After reading the comments of Jay Reynolds Freeman and Akkana Peck, studying the original Palomar discovery image, thinking about the effects and performance of telescope optics, and preparing a foolproof chart, the results on a second new Moon night in April have been distinctly better. As this writer reported earlier here, Leo 1 (UGC 5470 or MCG+02-26-027) is located near Regulus in the constellation Leo at: 10h 08m 28.12s RA; +12d 18m 23.4s Dec. It is a dwarf-class galaxy of large apparent angular diameter: The Deep-Sky Field Guide to the Uranometria 2000.0 (p. 189) specifies 12x9 arcminutes, while the Saguaro Astronomy Club Database (used for the computer program "The Arizona Database") rates it at 10.7 x 8.3', while it may be measured along its longer axis for some 28 arcminutes in the deepest exposures with huge telescopes, according to Burnham. There is a corresponding discrepancy in visual magnitude ratings according to these sources: either 10.2 or 11.3 respectively. Were the galaxy a narrower edge-on type of smaller angular diameter, but the same visual magnitude, even a Messier in smoky Paris might have spotted it in a small refractor: yet the vague light of Leo I's stars is so spread out that it is barely above the background luminosity and sky glow. 
On the original Palomar Sky Survey discovery plate [above, cropped and enhanced by the author; available on the web: see reference section below] with which by A. G. Wilson first observed the galaxy in 1950, most of the 20 arcminute angular diameter between the brilliant star Regulus and Leo I is suffused with stray light, which bounced around the surfaces of the glass plate used for the exposure in the fast 48-inch Schmidt Camera at the Hale Observatory at Palomar mountain. That photograph partially resolves Leo I into a mottled clump of stellarings of 20th magnitude and fainter, with numerous RR Lyrae-type variables (old, bright giants that are good "distance candles".) If the surface brightness as reported in the Uranometria is indeed 15.1 mag/square arcminute, then Leo I is at least one- fifth as bright as typical galaxies often viewed by amateur astronomers (which are often in the range of 12-13th mag/sq arcminute.) Leo 1 is NOT included in the large group of objects in the constellation discusssed in Skiff & Luginbuhl's OBSERVING HANDBOOK, featuring items detectable in various apertures from 6 cm to 30 cm. David Eicher makes no particular recommendations and gives no description in his book THE UNIVERSE FROM YOUR BACKYARD, other than to describe the object's faintness and obscurity. It is to be found neither in Lorenzin's 1000+ nor Sanford's OBSERVING THE CONSTELLATIONS. Surely this object could be considered a likely candidate for compulsive-obsessive observers with a certain masochistic streak! PREPARATIONS FOR OBSERVING Experience has long taught this particular user of a small amateur telescope (20 cm or 8") to suspect that the simplest representations of faint objects like Leo I -- as seen on chart 13 of the Sky Atlas 2000.0 or p. 189 of the Uranometria -- will be unsatisfactory. True, if perhaps the bright M-31 were situated a mere 20 arcminutes from brilliant 1.4 magnitude Alpha Leo (Regulus) it would be easy to spot. But if one is an ordinary mortal, observing with typical equipment, merely "trolling" for Leo I near Regulus will NOT likely succeed in securing it in the eyepiece field. The writer's first attempt was made more or less this way (although a narrow-field printed chart using faint stars from the Hubble guidestar catalog was available for corroborating reference.) Leo I was suspected at the outset, but the detailed chart soon corrected the misperception. When the precise star field was located, the glow of the galaxy was so diffuse that the author was unwilling to make an unambiguous claim that it had been "definitely seen." For the second observation, more elaborate preparations were undertaken. First, the optics of the scope were considered. As assumed -- and quickly confirmed in practice -- it is simply NOT possible using any mere portable amateur scope of typical design, to position the precise starfield of Leo I in the visual field of view of any usable ocular, avoiding the direct image of Regulus, AND to simultaneously and totally eliminate that star from the actual imaged prime focus field of the telescope. With any of the author's eyepieces (2.5 to 35 mm) in the instrument and the scope's position carefully adjusted so that the precise star- field of Leo I was at the edge of the field, placing Regulus' glow as far out of view as possible, the brilliant star was immediately visible in the prime-focus image seen through the focuser barrel as soon as the ocular was withdrawn. Thus the bright star's glare will bounce around inside the telescope optics. Less than the best baffling, the diffraction artifacts from the vanes of the diagonal mirror's spider, reflections off the plating on a focuser drawtube, or even light scatter from dust particles on the optical surfaces will tend to degrade the absolute contrast range available. So will poor coatings on telescope optics, or inferior eyepieces, degrade the images. It is recommended that the observer take extra care NOT to view Regulus in either the telescope optics or even in the finderscope employed! This author noted that when his eyes were fully dark-adapted (and 2 hours were allowed for Leo I) that if the scope were accidentally swept across the position of Regulus, he cringed in discomfort! The brilliant image left a visible bright "trail" in his vision that took 2 or 3 seconds to disappear. Too many of these experiences will diminish one's ultimate ability to detect the faint glow of the galaxy. In fact, judging from hindsight, the author should not have spent his day outdoors doing the spring window-cleaning! Even using dark sunglasses, his eyes' sensitivity would have been somewhat diminished, compared to staying indoors in low light as William Herschel often did when planning difficult observations. The finder chart employed by this writer for the second viewing of Leo I was far better than the printed chart made from a planetarium program (even a sophisticated one that showed Hubble guide stars down to nearly 15th magitude.) Instead, an actual picture was utilized, prepared to agree with the expected eyepiece field of view and image orientation. On Saturday, 6 April [1997], after a discussion with the present writer, Mel Bartels uploaded to his website (see below) a CCD image of Leo I that was prepared with a 6-inch f5 Newtonian. After downloading the file, this author used a graphics editing program to invert the image for black stars on white background (to minimize the ink needed for printing), with 'stretched' contrast that increased the density of the faint smudge of Leo I. An image with field size of about 1 degree was then output on a desk jet printer, and served as the finder chart, to be viewed with only the faintest red light illumination. The picture, though prepared with equipment within the reach of amateurs, including everything visible in the Hubble guidestar plot, as well as some fainter field stars. There was also -- in similarity to the Palomar discovery plate -- a diffraction spike from Regulus that intersected Leo I's image, plus a diffuse glow that extended over halfway to Leo I after the contrast was enhanced. That turned out to be not dissimilar to what was actually observed by eye. CONDITIONS FOR OBSERVING Successful observations of the faint Horsehead dark nebula require special attention to sky transparency. Now that the efficient hydrogen-beta eyepiece filters are available, it is no longer necessary to seek the absolutely darkest skies or to use only the biggest scopes. But the object should be seen when closest to the meridian, and when sky transparency is best. Leo I is a barely-visible faint patch against a field that may be infused with some scattered light from Regulus. The Horsehead, however, is a dark object seen in relief against a noticeably brighter nebular glow. In both cases, the ultimate viewing contrast is the deciding factor of success, not merely the amount of light gathered. And achieving that contrast demands excellent sky transparency. Observing in smoky or hazy skies will most likely nullify the chances to view these two faint objects. Attempting to observe the objects when they are placed at a low elevation above the horizon will certainly diminish one's likelihood of success. Luckily, recent high winds across Northern California had swept most of the air pollution particulates out of the atmosphere where this observer views on private property at an altitude of 3,400 ft. above sea level, not far from the Santa Clara valley. There is still some noticeable light pollution (which is greatly reduced in some directions as well as overhead), but on the outset of the observations reported here, the naked eye stellar magnitude limit was little better than 5.5 magnitude (at around 0600 UT, the stars 52, 46 and 37 Leo -- at 5.5 and 5.4 -- could be held with averted vision, and 42 Leo -- 6.1 -- was suspected.) The sky darkness was no doubt somewhat better: the nearsighted author's old, poor uncoated spectacles reduce the limit about a half-magnitude compared to the experience of other far-sighted observers. 2-1/2 hours later, with Leo I elevated only about 45 degrees above the horizon, the sky darkness had dramatically increased: now, SAO 99266 (5.8 mag, near 5.5 mag Leo 51) and other faint stars stretching between Regulus and Zosma (68 Delta Leo) became directly detectable. Yet, this venue offers by no means "pristine desert skies", so observers near urban areas may take heart! During the two sets of observations of Leo I reported here, seeing varied from an initially somewhat-rough "5" to a much steadier "7" or better after midnight (on the Pickering scale of 1 to 10, from terrible to excellent.) At first, there were several diffraction rings visible around Algeiba's members, but they were broken and turbulent. However, Mars showed excellent detail at magnifications of 425 to 500x: yet careful scrutiny revealed sudden bursts of turbulence -- lasting for fractions of a second -- though the long-term image sharpness was good. Later, as usual for this mountain site, calmness of seeing improved dramatically in time for a second, better sighting of Leo I. The author has often noted when observing the faint central stars of many planetary nebulae, that somewhat turbulent seeing will make the star wink out completely if it is near the limit. Under gross conditions of air turbulence, this has been noticed with the faintest and smallest of narrow-diameter extended objects, too. Since it is probably necessary to verify faint field stars near Leo I, the author would recommend waiting for good seeing to aid in achieving absolute identification of the stellar field of this faint galaxy. The foreground stars located in the boundary of the galaxy -- which range from 14.3 mag to 15.5 mag -- were not clearly seen with the relatively low powers used to view the galaxy: to achieve the absolute magnitude limit of the author's 8-inch scope it is often necessary to boost magnification to 200-400x. At such high powers, the background glow is reduced to near pitch black, and best contrast on point sources is achieved. But the surface brightness of a diffuse object like Leo I would fade to utter invisibility at such magnifications, at the exit pupils obtained in a small- aperture scope. Yet the author was surprised that at the lower powers used to detect Leo I, there was no difficulty in seeing the prominent field stars of the CCD image, down to about 13.7 magnitude. In fact, the visual field glow from Regulus was almost as bad as the Palomar picture, or the enhanced-contrast Bartels CCD image. In addition, the author's Newtonian had a nasty spider diffraction spike that threatened to interfere with the exact region of Leo I. Much care and patience had to be exercised in the exact positioning of the scope and establishing just the right true field of view from the eyepiece, as well as in obtaining the best possible image contrast by choosing the correct exit pupil. VISUAL VS. PHOTOGRAPHIC SIZE The Bartels CCD image -- even if cranked up to highest possible contrast -- depicts Leo I's outer extent as being no more than about half the diameter as shown in the computer program GUIDE 3.0. And, correspondingly, this author's visual impression reduced the apparent diameter even more. The best guess from many averted glance "vision peeps" (to use Lowell's phrase) was a detectable diameter of no more than 3 to 5 arcminutes. The best analogy the writer could offer would be to a defective view of NGC-2491, the faint and distant globular cluster nicknamed the "Intergalactic Wanderer", which measures 4.1 arcminutes diameter. If the sky suddenly went haywire due to clouds or fog and the globular faded out to unresolved near-invisibility...that would be the impression Leo I gave in the best averted-vision images obtained during this session. EXIT PUPIL AND FIELD OF VIEW When the author used low powers and a large exit pupil, it was almost impossible to ignore the upper-atmospheric airglow as well as field glare from Regulus. At 50x, with an exit pupil of nearly 4 mm, the region near the two 11.5 magnitude "bright" stars nearest the galaxy was not utterly dark. Despite the fact that the visual diameter of Leo I would be found to be at least twice the distance between these stars, the galaxy's faint glow did not immediately "leap" into one's ken. A variety of magnifications yielded no satisfactory confirmation. Finally, the author employed his standard "small galaxy" combination of eyepiece and Barlow which yields about 125x at an exit pupil of about 1.6 mm. After trying all kinds of scope maneuvers and rotations, a best-placement for the field was determined, so that the galaxy would be ideally located for averted glimpsing, but would not be stuck up against the eyepiece field stop. At the appropriate magnification and positioning, the background glow was diminished as much as possible, yielding best contrast. Thus, in the first half of the Leo I observing session (when the object was closest to the meridian, but not at the best sky limiting magnitude) the galaxy was finally detected at the absolute outposts of dimness that could be perceived! No outer boundaries could be judged, and no image could be held for an indefinite period. It was necessary either to sweep the eyes continuously across the field (keeping the region generally averted) or gently to rock the telescope. The result was not much more satisfying than the previous attempt to view the galaxy in February. Had this been the author's only experience with Leo I this session, he would have suspected that the object was "assuming facts not in evidence." The eye and brain can perform remarkable feats of self-induced tomfoolery. Even a modest observer who intends no self-deception can be taken in. So many other oculars were tried, with fields narrow and wide, and powers high and low, but with no better confirmation. The observer of this report did not have his own Wratten #12 "minus blue" filter available for this session; he had merely an inappropriate Wratten #15 Deep Yellow, with slightly less transmission and a different spectral balance. No enhancement was expected, and none was provided. Theoretically, to judge from the report by Jay Freeman (Sky & Telescope, March 1997, p. 117) the Wratten #12 may help increase visibility of dwarf galaxies with populations of old red stars. Initially, a general purpose light-pollution rejection filter provided no enhancement when the galaxy was near the zenith, and the naked eye limit was about 5.5 to 5.8. Later that night, however, around 0830 UT on 4/7/97, as stated earlier, the background darkened, though the galaxy had slipped in elevation to the western horizon. Yet this time, now that seeing was steadier, and about a half-magnitude darkness improvement had been gained, the galaxy was less ambiguous. Also, the author had already spent an hour experimenting with the charts, telescope, and oculars, and was supremely confident of the field stars and positioning. Leo I was spotted without much difficulty, and in less elapsed time, after 0830 UT. Standard averted vision was essential. Furthermore, some prolonged gazes of a second or two were attempted before shifting the eyes. Perhaps this helped slightly to accumulate faint light; in addition, now with the broadband LPR filter, the galaxy showed up better against an absolutely black background at 125x and an exit pupil of 1.6 mm (this is one of the very few instances the author has experienced of such a filter having ANY appreciable help with a galaxy!) That the observing conditions were now quite fine was confirmed by sightings soon made of some other objects: NGC 5903 in Libra: 11.5 mag, 2' x 1.7'; NGC 5898 (same) : 11.5 mag, 1.7 x 1.7'; both galaxies easily seen at about 85x; in fact, a great pleasure and tonic to the eyes, after the struggle for Leo I! NGC 5663: galaxy in Libra, 1.0' by 0.8', 14.5 visual mag, surface brightness 14.1: just barely detected at over 200x. Planetary PK342+27.1, known as Merrill 2-1; visual mag. 11.6 (Marling/Uranometria); 7 arcsecond disk (16th mag. central star of course not seen with 8" scope!): nebular disk easy to detect with OIII filter. NGC 5897: 8.6 Mag globular, 12.6' in diameter; 13th magnitude brightest stars. Despite Eicher's caveat that "small backyard scopes" cannot reveal the stars of this distant globular, the object was not only partially resolved at 85x, but also numbers of stars were detected across its face: from 6 to a dozen of the brightest were held with averted vision. Were they foreground field stars or actual cluster members? Sharpless 2-1: emission and reflection nebulosity field; traces of irregular illumination were detected with a narrowband nebular filter. CONCLUSIONS Since respondents on the Internet newsgroups have been naturally- selected for their disputatiousness, there will be much room for disagreement, although no one else has the present author's particular set of eyes! So, while the specifics of the reported observation cannot readily be challenged, the conclusions definitely may be. The present writer suggests that Leo I is a remarkable "test case" object for small to large amateur scopes. The smaller instruments lack the light gathering to force enough photons through the eyepiece to make much of a shattering impression. However, the larger scopes may suffer from intense glow around Regulus, and may even pick up atmospheric light scattering from this bright star that could tend to wash out the contrast. The author suggests that the condition most appropriate for viewing Leo I is to achieve best contrast, not merely with scope aperture, but by the most careful attention to the observing technique employed, the sky conditions, and the faithful documentation of the position of this elusive object. Not a casual "star-party" object, Leo I bears careful preparation and long scrutiny when observed under less than "world-class" conditions, or with small telescopes. Now, on to Leo II! However, this incredibly faint dwarf of 17.5 mag/ sq arcminute surface brightness will certainly elude an 8-inch aperture visual telescope, so its photons must be gathered with a larger instrument and in a darker sky than those afforded the author. Sincerely, STEVE WALDEE, Waldee-Wood Astronomical Software REFERENCES: Finder Chart: CCD Image with 6" f5 Newtonian, by Mel Bartels. Available at his website, at page of CCD images: www.cdcc.usp.br/cda/telescopios/hp-mbartels/ccd/ccd.html/ (The author gratefully acknowledges the cooperation of Mel Bartels for providing this remarkable image on his website!) Palomar Discovery Plate: STSci Digitized Sky Survey at: http://stdata.stsci.edu/cgi-bin/dss_form (NOTE: first, request coordinates for object, as given above in article; then request download of field width desired up to 60' in GIF or FITS format.) Acknowledgments are required for use of the images, and may be found here. Sky Charts: SKY ATLAS 2000.0 by Tirion; Sky Publishing; Uranometria 2000.0, vols. 1, 2; Tirion, Rappaport, Lovi; Willman-Bell; THE DEEP-SKY FIELD GUIDE TO URANOMETRIA 2000.0, Cragin, Lucyk, Rappaport; Willman-Bell BURNHAM'S CELESTIAL HANDBOOK by Robert Burnham, Jr.; Dover; Vol. 2, p. 1061. PROJECT PLUTO: Guide CD-Rom Planetarium Program, incorporating Hubble Guide Star Catalog; Project Pluto, 168 Ridge Road, Bowdoinham, ME 04008. The_Sky IV: Guide CD-Rom Planetarium Program, incorporating Hubble Guide Star Catalog Charts prepared courtesy of R. Wood. OBSERVER'S NOTEBOOK, Sky & Telescope, March 1997, p. 117 THE UNIVERSE IN YOUR BACKYARD by David Eicher, Kalmbach Publishing, pp. 109-110, 113. OBSERVER'S HANDBOOK, 199_, Ed. Bishop; Royal Astronomical Society of Canada. SAGUARO DATABASE, Catalogue: http://www.saguaroastro.org/content/downloads.htm Charts for Observing:
Photographic finder chart: upright, correct - print out and use at the telescope (reverse the image before printing, if needed, to correspond to your telescope's image orientation); based on Mel Bartels image, used by permission. |
