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!
Thanks for reading!