The importance of SCT Collimation
"The revival of a 8"SCT"

Dear All, This article was origionally printed in our Astronomy Club Newsletter (The NJAA Astronotes). But may be of interest to all SCT owners. The story relates the importance of perfect collimation to the SCT design.

  As the leaves started to fall off and the nights started to become colder my attention turned to the two bright stars located near the Hyades star cluster in Taurus. Actually these were not stars but the planets Jupiter and Saturn, both bright and nearing opposition. I could remember that cold night almost one year ago just before the start of the Leonids Meteor shower, when I viewed beautiful detail on Jupiter. This year I was waiting in anticipation for some great views of Jupiter's activity.

  One night in October proved a real eye opener when viewing Jupiter through Wayne Petcko's 6"f8 Newtonian. Jupiter was amazing with festoons clearly visible and detail to be seen with in the two major belts. Then I was in for some disappointment, after looking at Jupiter with my 8"SCT (Schmidt-Cassegrain Telescope). Some of the features were there but not nearly as obvious as in Wayne's smaller Newtonian. Only lower power (150x) seemed to help, but even the two major belts were not that distinct while detail within the belts was nowhere to be found. After comparing my 8"SCT with Wayne's 6" Newtonian on several occasions the results were always to be same. The smaller Newtonian showed a wealth of detail that just was not easily detectable in my 8"SCT. After getting a chance to use both scopes on a few different nights it appeared the gap between them was greater on nights that were colder.

  This soon led me to believe that the true reason for my wows were the closed tube of my SCT coupled with its big obstruction. I investigated into the possibility of cooling the air within my SCT to help it come to ambient temperature quicker. After communicating with other amateurs it appeared that it was best to just put the scope outside a while before using it, especially if expecting to view the planets. This did help improve the image, but it was still not as good as the 6" Newtonian. Even an experiment in stopping down my scope did not really improve the image. As for the central obstruction, nothing could be done about that.

  After reading a few articles about Jupiter on the web and one in Sky and Telescope Magazine, many SCT owners were claiming big rewards for paying close attention to their SCT's collimation. Upon close inspection it appeared that that my scope's collimation was off just slightly when viewing the airy disk of a bright star at 225X. An attempt to fix this would have to be made but it probably wouldn't make much of a difference. To me it seamed best to attempt this when the moon was out, because I would not be as busy at that time observing variables. My scope had never seamed to need collimating in the past five years of ownership. It was a daunting task that might even take hours for all I knew!

  In the mean time Wayne's 6"f8 had impressed me so much that it seamed a viable option to construct my own considering the low cost. A deal was struck with my sister in laws boyfriend to grind a 6"f8 mirror for me that would be ready by this summer. With this project I could learn the skills to make a telescope (dobsonian). In addition the 6" scope would be a nice portable instrument which I could add to my arsenal of astronomical equipment. But that would be a way off and Jupiter and Saturn where visible now. What to do?

  After reading some messages posted on the message forum I finally came across someone who was claiming great things from a 8"SCT by perfecting its collimation. His posted message sounded to good to be true. He stated that his collimation had been just a tad off and only noticeable at 300X, then after he fixed the collimation error, bang!, he could see great detail on Jupiter. That he said taught him the importance of he watching his SCT's collimation very closely. Well it sounded to go to be true, but the moon was out and I was stalling (for fear of messing with my collimation).

  That night around 6:30pm I place my scope on the balcony to cool. Tonight was the night to tackle this collimation beast and see what happens. A few hours later my scope was setup outside and it was time to a to tinker. I decided to view Jupiter first to see if it would be possible to notice a difference before and after collimating. There she was, big and slightly blurred with just a glimpse of detail visible. Pointing the scope at a bright star near the zenith I popped in a 2X Barlow and my 9mm Nagler. The poor 2X Barlow never got much use because it was almost impossible to even focus well on object above 300X. After centering the star at 450X, I slightly defocused it to see the airy pattern, and there is was a slight miscollimation to the left. Well this was the moment of truth, I turned my top left screw turn and eagerly looked in the eyepiece. Then disaster struck, the star was nowhere to be seen. But wait, there it was just a little out of the field and it looked much different. After refocusing the star appeared different, it was a bright point with a faint circle around it. What a stroke of luck, perfect collimation on my first try. It was now time to pointed my scope at Jupiter and lowered the power.

  The sight was incredible! Jupiter at 200X was showing detail with in the two major belts (streaks and dark spots). Unbelievably other belts were now obvious, coupled with festoons that were now readily visible. A good look at Jupiter showed a white line (Streak) visible with in the Southern Equatorial Belt. After popping in my 9mm Nagler for a whopping 450X with my 2X Barlow, I could not believe that a good focus could be achieved, and gone was the usual blurry image. But visibility of the detail was much better at 200X. Then it was on to Saturn and the Moon.

  Saturn was well defined at 200X but what was really surprising was viewing Saturn at 450X. Saturn was now large and beautiful showing even detail in the rings, which I had never seen through my scope before! The beautiful colors of Saturn were obvious even at this high power; I wanted to scream from happiness! Then it was on to the Moon. AT 200X craters on the moon were crisp and well defined. But again the view at 450X was like sitting inside a crater. Was I imagining this and was this my scope?

  The best way to describe the image difference was that before perfecting my SCT's collimation, it always appeared as if the atmosphere was turbulent. But after perfecting my scopes collimation and allowing it to cool to ambient temperature, the image through the scope showed very little turbulence at all, unless it was with out a doubt a bad night. The soft image that I had viewed previously was not due to the secondary obstruction, but rather the small misalignment of the secondary mirror.

  Two weeks later during the night of the Leonid Meteor Shower and NJAA Research meeting, I had a chance to show Ernie Rossi and Wayne Petcko views through my 8"SCT that now had perfect collimation. That night Saturn at 200X showed the elusive Encke division, plus the more obvious Cassini division and Crepe ring. At 350X with a TV Plossil and 2X Barlow, Saturn was nice and sharp. After showing this to Ernie he stated that my scope had excellent optics and he was impressed. (Coming from Ernie Rossi, this means a lot.) A view of the Trapezium in Orion even revealed all six stars. Later that night I got a chance to show Wayne the moon at 450X using my 2X barlow and 9mm Nagler, Wayne's words were "That's incredible". These were beautiful words to my ears. To add to my happiness the Leonids that night put up a fantastic meteor shower. The display showed at least 100 plus meteors at its peak even with the moon out! In fact one fantastic meteor to me looked like a plane crashing. But that's another story.

  From this experience it was possible to come to a few conclusions. First it became apparent to me that I had greatly overestimated the limitations of the SCT design (mainly a larger secondary) and greatly underestimated my scopes ability to produce a good planetary image. One wonders how many others have done the same with their telescope whatever the design. I should have learned from watching Newtonian owners who meticuasly aligned their optics that my SCT needed just as much attention. My unhappiness over the scopes planetary image led me to false believe that this was due to the optical design of my scope. Unfortunately the scope had been shortchanged from a lack of attention being paid to its proper collimation. After talking to a few owners of SCT's it became apparent to me that many of them don't touch their collimation. I think Michael Covington in his book "Astrophotography for the Amateur" said it best when he wrote, "most Schmidt Cassagrains in actual use are seriously out of collimation. A simple adjustment can greatly improve performance"

  Yet how could such a small miscollimation of my SCT's secondary produce such a difference in the performance of the scope. Before perfecting the scope's collimation the error was only visible by looking at a defocused star at high power, and it was not a significant error. Basically I noticed when looking at my defocused air pattern that the two innermost circles were not concentric. This led me to the conclusion that there was a slight error. (Note* The scope had a good general alignment, that needed to be tweaked) After further researching the SCT collimation issue, it became apparent how a slight miscollimation can have a major effect.

  This is due to the optical design of the Schmidt Cassegrain telescope where the secondary of the scope acts similar to a barlow lens, which magnifies the image coming from primary mirror. Hence it is easy to see how a small miscollimation of the SCT's secondary can have a major effect. But this is unlike the secondary of a Newtonian, which does not magnify the image it receives. This is not to say that collimation of a Newtonian's secondary is not important, but just to stress the fact that a SCT's secondary needs small tweaks to reach perfection

  No matter what type of scope you have something can be learned from my experience. Owning a scope for 5yrs and finally learning how to collimate it is not a good thing. Collimation that will give a better planetary image will no doubt produce better images of all objects. This is reason enough to invest time in learning how to master your scope what ever its design. Your scope wants to show you its best, but it can only do this if you learn how to properly collimate it. A scope may or may not be excellent optically, but paying attention to its collimation will help it perform to its best.

  Below is the address of an excellent website that can help SCT owners learn how to collimate their scopes. This site contains a good explanation of the procedure and pictures to help demonstrate what you should see through your scope. It appears to be the best one on the web at present. Also if any owners of SCT's have had similar experiences I would also be interested in your story. Please email me at During the December research meeting Rod McCrea gave a good presentation on collimating a Newtonian. Interestingly Rod told those present about a similar experience with his Celestron 8" SCT. He had stated that after paying attention to his scope's collimation it performed beautifully. This further convinced me of the importance of perfecting a SCT's collimation. Wayne Petcko probably pegged it right when he told me that "Most scopes that perform badly are not collimated well". Boy was he right.

The following link contains a very good article on how to collimate a Schmidt Cassegrain---> Collimation
<---Click here to go back to Bill's Astronomy Website
This site is maintained by WilliamAnthony.