SkyDude Astronomy


AT8RC COLLIMATION

It's easy to tell when a
Ritchy-Chretien scope is out of collimation. The tell-tale sign is oblong stars at one side of the image while the other side looks better. You may also notice a slight coma-flare on stars, most noticeable at the center of the image.

But collimating an Astro-Tech Ritchy-Chretien scope can be a daunting experience for the following reasons:
After much fustration, I finally embarked on a step-by-step experiment and discovered the following:

Secondary  Mirror:
Primary  Mirror:
Armed with this info, I arrived at the following procedure:

Preparation:
Warning1: If you de-focus inward, the donut gap will be on the bottom (opposite of where you placed your hand).
Warning2: Failure to confirm the rotation may result in adjusting the wrong screw!

Primary Mirror Collimation:
  1. Using the focus mode of your software, de-focus OUTWARD slightly so that the stars are very tiny donuts.
  2. Select a star near the center of the image, and determine which side has flare or is fatter. The other side will be brighter which is sometimes easier to see.
  3. Loosen the primary black-screw that corresponds to the flare by no more than 1/16 of a turn, and tighten its mating silver-screw.
  4. Repeat steps 1-3 until the star-donut is balanced.
Secondary Mirror Collimation:
  1. Focus the scope, switch on guiding, and take a 1 minute image.
  2. Examine the corner stars and choose the corner with the most oblong stars (stars stretched toward and away from the center of the image). If you are using a field-flattener, this may require some careful examination to determine the worst corner.
  3. Tighten the corresponding secondary screw by no more than 1/16 of a turn, and loosen the other screws by the same amount.
  4. Repeat steps 1-3 until each corner is balanced with its opposite corner. Don't be surprised if one set of opposite corners has more elongation than the other set. This is normal for a Ritchy-Chretien.
Note1: If at any time the stars look like triangles, the secondary mirror is pinched. Loosening all 3 screws will fix it.
Note2: If you are not using a field-flattener, you may notice the secondary adjustment causes the center stars to become slightly oblong. Back off the adjustment a bit to reach a happy compromise.

After collimation is complete, and assuming you have one of the above mentioned field-flatteners, you should have well shaped stars across the entire image.

Example Images:

Pics/Donut.gif
Primary Collimation: The right side of the donut is flared and the left is brighter. Loosen the right-hand black-screw and tighten its mating silver-screw.

Pics/Edge1.jpg
Secondary Collimation: The stars at the top are the most elongated. Tighten the top screw and loosen the other 2 screws.

The good folks at Deep Sky Instruments offer a similar collimation procedure and some great info on Ritchy-Chretiens. The main difference between our methods is that I found it is easier to see oblong edge-stars on an image that is in-focus, rather than out-of-focus:
http://www.deepskyinstruments.com/docs/DSI_Collimation_Procedure_Ver_1.0.pdf

Another Astro-Tech RC user with a similar method, and great images:
http://astroimages.weebly.com/collimating-an-at8rc.html


Comments welcome: mark.eby@twc.com