Astronomy Tips and Tricks, Page 2

Measuring Periodic Error Using an ST-4 and CCDTRACK Software

Aquiring the Data

One of the most important things to have in astrophotography is low periodic error on your mount. Low periodic error isn't everything though. All mounts, even at large professional observatories have at least some periodic error. No machining, no matter how precise, can create perfectly round or smooth gears. But it's not just a matter of how much error but also what kind. A person with high but smooth periodic error can get better results than a person with low but very uneven or jerky periodic error. A person or an autoguider can more easily correct smooth periodic error. Jerky periodic error is harder, especially for autoguiders, to correct. Santa Barbara Instruments Group has software for use with their ST-4 autoguider, found here, that automatically appends tracking data to a text file which can then be loaded into a spreadsheet program like Excel for analysis. I will describe the process here using an ST-4, though it can be done with any autoguider compatible with CCDTRACK.

First thing's first, you will need a computer loaded with SBIG's CCDTRACK software and a sreadsheet program, an ST-4, and your telescope set up as it would be used while you are in the field and polar aligned. Polar alignment makes things easier and allows you to test for longer periods though it doesn't have to be perfect. Align the CCD head so that the X and Y axes of the CCD chip are aligned parallel to the axes of the mount. The ST-4 needs to be connected to the computer through the RS232 port using the proper 9-pin cable. Connect the mount to the ST-4 via the relay cable. Power up the ST-4 and start CCDTRACK. The software will automatically connect to the autoguider. If it doesn't you may need to switch serial ports or change the port settings in the PC menu in CCDTRACK.

The software must be configured so that it will track but not send corrections to the mount's drive corrector. To do this go to the Relay Setup dialog under the Tracking menu. Set both the X and Y relays to "inactive." It is possible to set the declination relay to active to eliminate noise caused by declination drift. To do this, set the relay that is parallel to the declination axis to "active" or "reverse" if the CCD head is upside down (this may require testing to verify position).

The autoguider must now be setup. First block the tube of your guidescope, go to the Grab menu and change the "Take dark frame" option to Yes and press Enter/Return. This will take a dark frame and store it for later use. NOTE: Anytime you change an exposure or gain setting you must take a new dark frame. The ST-4 must now be focused. If you have a parfocal eyepiece you can skip this part. Find a star close to the celestial equator and place it on the chip as you normally would. The star should be bright enough so that you can use shorter exposures (1-2 seconds). To start the focus mode in CCDTRACK go into the Focus menu and press Enter/Return. You can use the image downloaded onto the screen or the pixel values displayed to focus the star. Once completed press Esc.

To begin testing go into the Tracking menu, highlight "Go" and press Enter/Return. It will now take an exposure and download the image. A 32X32 pixel box will appear superimposed over the image. Move this box using the arrow keys so that it is over the star you wish to use for the test (it does not have to be centered). Pressing Enter/Return will initiate "guiding." To the left of the screen is a latest image of the star with cross hairs superimposed over it. Pressing "G" will cycle through several different views on the right hand side of the screen. I like to use the one that graphically plots the errors of both axes (see image at right). At the bottom of the screen are instantaneous and RMS error values as well as inital and previous image pixel values. Allow the software to guide for 2-3 turn of the worm gear. This will allow you to sort out guiding errors from bad seeing.

Data Analysis

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