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GSO CC 6 inch Collimation Method (New?)

catadioptric collimation
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#1 isogroup


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Posted 02 July 2020 - 10:23 AM



I recently purchased the GSO 6 inch Classic Cassegrain (CC) to use for planetary viewing and some brighter DSO's. When I got the scope for AgenaAstro, they had done a nice job of alignment, but it was slightly out. Just a little bit out of alignment and the image was mush, bad, not good. Don't worry, once collimated this scope does amazing things. Read on.


I had read that these things can be very difficult to collimate, however, I tried a different method and had great success. Lacking a good collimation eyepiece I found a good substitute, my Olympus PEN digital, mirrorless camera. I attached the 2 inch camera adapter and put it in place of my diagonal. Then I turned on the cross-hair setting for the viewfinder. Bingo, cross-hairs with graduation marks. So easy to put the final dot of the mirror image in dead center (mine was already there) and measure how far out place was any part of the reflection. I could look at the image while pointing the scope at a blank wall and see the problem. Image 1 shows the starting condition of alignment in my scope. Taking a photo allowed me to analyze the problem and also send the same photo to AgenaAstro's tech department for their opinion. We both agreed that the main mirror was slightly out.


With the same camera set up, I put my hand over the end of the main tube to see where the out of alignment circle's closest point was in relation to the front of the tube. Where the white circle is pinched to one side, as seen by where your hand lines up with the pinch, you select the screw closest to that location on the back of the scope. Tiny, tiny turns of the screw (after you make a tiny loosening of the locking screws) brought the circle back to equality all the way around. See image 2. Now you gently tighten the locking screws while watching the image in the camera screen.  99% done in less than 10 minutes.


After getting the second image's results, out to the deck to put the scope on a bright star and use a 6mm eyepiece for 300x. Again, tiny, tiny turns of the alignment screws to bring an out-of-focus image of the star into perfectly round and multiple diffraction rings. I got seven perfect rings when I hit perfect alignment. Then I test my final setting by focusing on the bright star and turning the focus just a 1/64th of a turn to watch the point of the star expand into a tiny ring of light. If this ring opens at the same time and equally around the ring, then we are home. Took about five minutes.


These were very minor tweaks (maybe 1/16th a screw turn at most) but the change in final image was dramatic. Moon and planets suddenly clear and sharp (4 inch+ refractor sharp). Stars were round points of light with equal diffraction spikes, both in brightness and length, thus telling me I had hit perfect focus. Jupiter's moons were round and different sizes and colors. M22 showed some star by direct vision (formally a grey blob before this procedure). So, $500 scope around 10 pounds that can be deployed quickly means the CC6 is near the top of list for bang for the buck.


However, be aware that the CC is not a scope for beginners. If you have little or no experience aligning optics, then beware. This one takes patience and a delicate touch. I have owned a lot of scopes with mirrors, so I knew to get a second opinion on what to align before I touched anything. On the plus side, the hurdle was worth it. I have owned or regularly used Cassegrains from 5 to 16 inches. This includes both the Vixen VCM200 and VC200L. The CC6 if the best of the small Cassegrains I have ever used. No soft focus image from a corrector plate, not much cool down time with the open tube, no dew on nights of 80 degrees with 90% humidity.


In summary, I found using a camera with cross-hairs a fast and easy way to get rough alignment, and as always, a star test at high-power the best way to finish the job. Feel free to ask questions, hurl insults, make snide remarks. Just no stories about your recent operation, how your scope is the best of all, or anything using the word "rare."  Really, can we get that word banned from the classifieds?


My Two Cents


Lake Boardman Observatory

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#2 jgraham



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Posted 02 July 2020 - 11:16 AM

Excellent! I've had a similar experience over and over again; a Cat with marginal performance that blossoms into a beautiful instrument by carefully zeroing in the colimation. I had this same experience just last night with my new to me 8" RC, it is always a thrill to see the field come into sharp focus as you do that last bit of tweaking, and a camera definitely helps.

Thanks for sharing!

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