Telescope Maintenance

By Rock Mallin

Have you ever wondered if your Schmidt-Cassegrain telescope needs to be serviced? Is your telescope more than 20 years old? Do you use it and store it in a dusty environment? Well what ever the case may be, if there is enough dust in there, it will diminish the performance of your scope. How much dust will reduce it's performance? That depends on how much detail your observing requirement is. In 1997, I purchase a used 8" Criterian Schmidt-Cassegrain telescope, complete with deluxe tripod,and I must admit, I liked the color...White and light blue!! But I was warned by the store owner that the telescope was a "junker" and was reluctant to sell the scope as it sat in the back, away from every one to see it. My intention was to buy a used Schmidt-Cassegrain for use as a solar scope, with the proper solar filter of course. When I purchased the scope, I brought it home and find out many defects with it. The motor drive didn't work, and the day time views were awful!!! I said to myself, I have lot of work ahead of me for this project. After fixing the dual motor drive, and solidifying the rest of the mount, it was time for a star test...........Ho boy!!!!! What a mess!!!!! My bargain was no longer a bargain. But after 2 hours of unjaming the collimating screws, I realized that some one has tempered with those screws. I pointed the scope to Polaris, that way it would be a fixed reference star to start with. After another 2 hours, I had a not bad view, but far from perfect. I decided to put away the scope for the night. Next night i started all over again to perfectionized the collimation. The result paid off. The same night, I went to our usual spot on Armstrong road behind the Ottawa Airport, to meet with observing partner Pierre Martin, and Rob Young, and others.

After unloading my other scopes, I pulled out the "junker" out, and set it up near Pierre Martins' Celestron 8"Schmidt-Cassegrain. And later during the evening, I decided to peek in the scope under darker skies, where we were. To my amazement, I could not believe the view out of this 21 years old scope! So, Pierre Martin came over to check the view out. As it turned out, the optical quality were equal if not better than the Celestron 8"! Pierre did a series of star test and concluded the optics where in fact first rated! And I myself also compared the scope to other 8" Schmidt-Cassegrain, with stunning result. The work I performed on the scope paid off. One more thing that bugged me was the amount of dust accumulated over the years inside the optical tube. So I decided to take the scope apart for a major clean up. And the result are given here, in the form of photos I've taken during the tear down. Make sure there is enough dust in there to justify the disassembly!

Make sure you mark everything, such as the ring around the front plate holding the assembly, and remove the holding ring, and be very careful not to damage the front corrector plate.

Be very careful not to break any seals, and pay attention on how it is installed, as you will haveto reverse the process for reassembly.

You must look for a marking done at the factory, for alignment of the corrector plate with the rest of the optical system. If none found, MAKE ONE! use a light marking device or a thin line with some water proof paint. It can be seen in this picture as a engraving. You must have this, other wise you will never be able to realigned the optics.

Next, be careful not to touch the secondary mirror! Do not remove it unless absolutely necessary! Use a can of air to blow any dust particles on the secondary mirror. Be careful not to touch the corrector plate with your fingers. Hold it by it's sides.

  

Make sure to point the optical tube downward, to avoid any extra dust from entering. This is the position to clean the inside of the optical tube and it's mirror with compressed air. Do not touch the primary mirror, or attempt to remove it, the only way to clean the mirror is with a can of compressed air.

In this case we had to remove the secondary, to fix some of the threads, and the reposition of the ball bearing support. Again, if necessary, prior to removal markings must be done to replace the secondary in it's original position.

Make sure to remove dust with a can of compressed air prior to clean the corrector plate. Use only medical grade cotton, and do not apply hard pressure on the surface. In this case, because of the age of the scope, a thin film of haze was evident. Extra cleaning was required.

Make sure to change your medical cotton often, to avoid collecting small dust particles, and scratching the surface of the corrector plate.

  

Make sure to clean both sides of the corrector plate, then reassemble

  

Make sure to alingne the markings of the corrector plate and the optical tube ring mount, and reassemble the front ring, making sure it is installed at the same position as before, and pay attention to the amount of tork used to tight the screws. they must be equally torked. Next is collimation, and expect not to have it right the first night. Take your time to understand the collimating procedure.

  


Cleaning solution:
1 litre of distilled water
1 500 ml. isopropyl alcohol 99%
1 drop of dish detergent soap.

*Note: Please do not attempt this if you are in any way unsure of what you are doing, the slightest mishap can render your scope unusable.