At last first light for the Astrotech 12″ RC Truss! Finally after collimating, establishing a new pointing model with the new payload and working out some glitches with the new image scale in the various control programs a successful 2 hour test run was accomplished. For our test I chose a globular cluster. This is an excellent way to assess the general alignment and collimation of your optics. Stars are unforgiving especially at the full resolution of your set up. How many times have you seen images posted of nebulae etc which look great at fractions of the full image scale only to show the egg-shaped stars when the true full resolution version is revealed! Don’t be like those guys!
Stars are either round or they are not. A star cluster especially a globular has lots of stars so they make an excellent test for your system. Now there is a quantitative way to determine “roundness”. It’s called aspect ratio. This is the ratio of the width to the height of the star image in the case of stars. The program CCD Inspector (CCDWare.com) is able to determine that for us. The values in the program are read as percentages. For example if there is a 5% difference between width and height the aspect ratio is 5. In this calculation therefore lower numbers are better. My own experience tells me that when you go much above 15 you start to see the stars becoming egg-shaped. So as a matter of convention I tend to throw out any subs with aspect ratios over 15 or so. CCD Inspector is very handy in that you can simply place your cursor over the star and it automatically reads the aspect ratio and full width at half maximum. Last night the seeing was excellent so most of the subs registered in the low 2’s for FWHM
Now about our target! M15 is well positioned right now in Pegasus almost directly above us so this is great for our test object. It is one of the most densely packed globulars in the Milky Way with an enormous number of stars in the center. It has a number of variable stars and pulsars in it as well, AND the first planetary nebula discovered inside a globular!
I think overall I am satisfied with the test results and we can now move forward with our imaging projects!