While we prepare the Lhires spectrograph for the telescope indoors, next step outdoors is to collimate the C14. There are a thousand ways to do this and much of this can’t be covered in a single blog entry. Everyone has their “take” on it. For myself I have used many different scopes and while lasers and other collimating instrumentation can work, they can be expensive and difficult to reproduce consistent results. Laser collimators generally work well on Newtonian-style telescopes with a flat secondary mirror. On Cassegrain-style telescopes, which have a curved secondary mirror, a laser collimator will work properly only if the secondary mirror is perfectly centered above the primary mirror. Unfortunately, telescope manufacturing tolerances are such that slight secondary mirror offset is inevitable. Under these conditions the laser collimator may give a false indication of collimation. Using a star is much more accurate. The bottom line is you are doing this to get the best possible STAR image you can so it makes sense to me to do the collimation while observing stars. My preferred method is to use a software program called CCD Inspector (CCDware.com) . You can use the program free for 30 days. Afterward the cost is $150. This is still cheaper than many laser collimators, and the program has many other applications for imaging. Definitely worth a look
The classic mass produced Schmidt Cassegrain telescope has 2 spherical mirrors, one primary and one secondary. The secondary is attached to a “corrector plate” with usually 3 Allen screws (see diagram below) These screws can be replaced with easy to turn thumb screws called “Bob’s Knobs”
The collimation procedure is pretty straightforward. Several 2 second exposures of a defocused 2nd magnitude star are taken. While you take an exposure or 2 place your hand over the front of the telescope and observe where the shadow appears in your exposure. The software will tell you what the current collimation is in arc seconds (you have to enter your image scale in the program settings) and most importantly a line (arrow) will appear showing you what direction you need to move the star image to properly collimate the scope. For an SCT you have to move the corrector in the direction OPPOSITE the arrow or line to get the star movement correct. Position your hand over the scope as above until you see the shadow of your hand directly opposite the line on your 2 sec image. This is the direction you need to move the corrector. See where your hand is in relation to the Bob’s knobs and use the knobs to move the corrector exactly in this direction. I use the excellent diagram produced by Astro-Tech (see below) to figure out which knobs to tighten or loosen. Only move the knobs slightly at first to get a feel for it. Then take another exposure. You should see the star image move in the direction of the line. Repeat this process until collimation is reached, which for most seeing conditions, should be less than 5 arc sec. I started at around 26 or so! It looks like we’re good! Note that at first the line will stay in the same direction until you are close to being collimated, then will start shifting around to different locations. Then you know you are as good as you can be for those seeing conditions!
Classic SCT optical arrangement. On the left is the secondary which is mounted on a “corrector plate”, not a spider . 3 Allen screws secure the secondary and can be adjusted to tip tilt the mirror for collimation. Reproduced from the text book “Telescopes, Astrographs, Eyepieces” by Smith, Berry et al
Bob’s knobs. This is the front of the OTA. These replace the Allen screws so in the field you can collimate the telescope very easily without fussing with an Allen wrench
This diagram is excellent and shows you which knobs to loosen and tighten in order to move the star in the direction shown. For an SCT you have to move the secondary in the direction opposite the arrow in the CCD Inspector program using your hand’s shadow to figure out where on your scope that is. (see text)
And this is the result. 2.5 arc sec collimation! Took about 15 minutes to do!
Thanks for reading!
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!
This is a raw 6 minute luminance image of M15. Yes there are significant star blooms (vertical streaks) however this is the trade off with a highly sensitive CCD and these can be processed out
At full resolution the central stars are most definitely round! At the periphery (not shown) there is a subtle decrease in roundness due to the field curvature from the optics but this is expected
This is the CCD Inspector Viewer pane which shows the very nice feature where you can display the object parameters by just moving your cursor over it. Shown is a core star with aspect ratio and Full Width at Half Maximum values. I think this is a keeper!