Jump to content


Cleaning C8 SCT Secondary Mirror- Lesser Evil Way?

  • Please log in to reply
4 replies to this topic

#1 Murrfk


    Explorer 1

  • -----
  • Posts: 58
  • Joined: 06 Feb 2012
  • Loc: Western MB, Canada

Posted 18 March 2012 - 01:15 PM

In an effort to try to determine why my images appeared fuzzy, I took a good look at my mirrors with a flashlight. I was able to see the the primary mirror of my C8 orange tube was in pretty good shape. It looked bright and clean and mostly dust free. However, when I saw the reflection of the secondary (looking at the secondary reflection in my primary) it was clearly dirty. In fact a quarter of it was quite unclear.

I have read about the care that must be taken in cleaning mirrors but I am left trying to determine which is the lesser of the two choices that I see:

1. Remove the collector plate to access the secondary for cleaning. However, this requires putting it back with careful alignment (which I believe I can do). However, I am worried that there might be delicate shims that I might not be able to replace.

2. Clean from the eyepiece end of the telescope up the tube with long q-tips or something similar. While this doesn't require dismantling the scope, I worry about the ability to clean efficiently and without scratching the secondary. Also, I will be hard to remove the water from inside the scope if any should pool at all.

What are your thoughts? Any and all opinions welcome.

#2 Murrfk


    Explorer 1

  • -----
  • Posts: 58
  • Joined: 06 Feb 2012
  • Loc: Western MB, Canada

Posted 19 March 2012 - 08:28 AM

I decided to remove the collector plate. It wasn't easy. My Orange tube c8 has a metal centre for the secondary. When I removed the eight screws I could not get the ring off. It seemed held in place by a gasket. However, I could by gentle pressure with a wood block remove the entire end ring, including the collector plate. There were no screws holding it in place. Only what appeared to be some thin sealant. It came off fairly well, and I found that the secondary was actually quite clean, so I put it back together being careful to realign.

I am curious as to whether that inner ring should have came off the collector plate more easily. It really seemed that it would take a lot of force to remove it, and I was worried about breaking the collecter plate.

#3 woodsman



  • *****
  • Posts: 2409
  • Joined: 12 Mar 2008
  • Loc: Colorado Springs, CO

Posted 19 March 2012 - 08:49 AM

I had to clean my c90 secondary as well as my vintage c5. I didn't have a way to clean either, by removing the corrector, so I did just as you are thinking. I took straws and taped them together. I then taped a large cotton ball on the end being careful to keep the tape on the back side of the cotton ball. I made two, one for saturation with windex and another dry one for wiping afterwards.

It worked well. I had big black chunks all over the c90 sec. spot and they were gone afterward. Same with the C5. I was very careful in my rubbing. The c8 will take a long straw, but I bet you can do it. Its the easiest way if you are reticent to remove the corrector. Good luck.

#4 rmollise



  • *****
  • Posts: 16037
  • Joined: 06 Jul 2007

Posted 19 March 2012 - 12:02 PM

Well, the thing is _the secondary will not make your images "fuzzy." I've seen secondaries with most of their coatings gone that produced sharp images. Secondly, looking at one with a flashlight, if that is what you are doing, is no indication of its condition. Any mirror will look dirty under those circumstances. Best bet. Leave it alone. If you cannot, clean it with USP cotton and distilled water with 1 drop of dishwashing liquid to a liter. Rince with more distilled water. And be VERY careful to reinstall it in the same rotational orientation. Best bet? LEAVE IT ALONE and look to collimation, cooldown and seeing as the cause of that derned fuzziness. ;)

#5 ultraviolet001


    Vostok 1

  • -----
  • Posts: 101
  • Joined: 14 Jan 2009
  • Loc: UK : Norfolk

Posted 19 March 2012 - 12:16 PM

I would check that your collimation is spot on before attempting any cleaning whatsoever - a little bit of dust ain't going to hurt your views , but if you have an SCT that is not in collimation , that will do it - lots of articles on CN on how to do this simply.....

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics