Disassembling a Meade SCT OTA

starfinder · Post by **starfinder** » Sun Nov 13, 2005 3:09 pm

Cleaning the inside of an SCT

I bought my Meade LX-90 8" SCT thru Mr. Lee soon after it was launched about 4 years ago.

I've never kept it in a dry cabinet bec I don't have one large enough. I've brought it to Mersing about a half dozen times, and have found it to be a superb instrument overall.

Over the years, some light amounts of "grime" has built up on the inside of the Corrector Plate. It may have to do with exposure to dew and humidity. The primary and secondary mirrors had also accumulated some small specks of dust.

I've never opened an SCT before, because I did not want to risk damaging anything.

Well, I was in Sydney last week, and visited one of the main telescope shops/dealers in Australia, called the Binocular and Telescope Shop, or Bintel, at No. 55 York Street, near the main shopping district in town:
http://www.bintel.com.au

There, I spoke with one of the technicians, a very friendly person, Mr. Don Whiteman. He told me that he worked at Celestron in the US for 20 years, and now goes to work in Meade in the US for a fortnight every year. Apparently, Bintel is the main Meade dealer in Australia.

Don described to me how to open up the LX-90 and clean it.

So, yesterday morning, I finally got the courage to remove the Corrector Plate to do some cleaning. I think that in my case, the amount of accumulated dust and grime justified opening the SCT.

What follows is a brief step-by-step description of what I did. I've attached several photos below.

First preface: Examine your conscience. (haha). Do you *really* need to undertake major surgery? Or do you just have itchy fingers? The first rule in cleaning optics is, "Do no harm", and sometimes, too much cleaning can do more harm than good. Assuming you decide to proceed....

Second preface. Remove your watch, rings, etc. Cut your fingernails and smoothen them. Wash your hands to remove oil, etc.

Step 1: Removing the Corrector Plate is surprisingly easy. First, take the scope off the tripod and set it down on a carpet, etc. If anything falls down, there is a least a chance that it will not crack.

Step 2: There are, on my LX-90, six hex screws on the inside perimeter (see photo). Tilt the scope up a little, and tighten the altitude (up/down) knob. Remove the 6 screws with a small hex key wrench.

Step 3. The Retaining Ring (that's what I shall call it) can now be removed. Simply pry it off with your finger and pull it away.

Step 4. You will now see the whole extent of the Corrector Plate. Take note that bec of the optical design of an SCT, the Corrector Plate, after removal, must be placed back in exactly the same rotational (i.e. clockwise/anti-clockwise) orientation. For this, at one spot on the bottom of the Corrector Plate and the OTA area next to it, Meade had placed a spot of white paint, and marked off a black line across. This is to denote that the two are to be lined up there when the Corrector Place is replaced. If there is no such black line, make your own markings. You could use some masking tape with a line drawn on it.

Step 5. To remove the Corrector Plate, simply hold on firmly to the central Secondary Mirror Assembly and pull it away. Note that the Corrector Plate and Secondary Mirror Assembly set weigh about one pound or more. When this is removed, the scope will tend to tilt up automatically, due to the changed weight balance. Be prepared for this. Set the Corrector Plate onto the carpet/bed, etc. Be careful not to drop or knock it, bec it is supposed to be quite fragile as it is relatively thin.

Step 6. You can now do the cleaning. Mirrors:
I did not clean the primary mirror bec it only had specks of dust on it. I used a large blower to puff away all the dust. I then used a wet tissue to wipe away the accumulated dust on the inside of the OTA. Beware that the mirrors (primary and secondary) are quite easily scratched. In comparison, the Corrector Plate surface is a lot tougher. If you need to clean the mirrors, i *think* you should just use cotton wool with water (preferably distilled water). I invite views on this point.

Step 7. To clean the Corrector Plate, it’s best to use an isopropyl alcohol-distilled water solution, though for moderate cleaning, I think plain distilled water will do. Don at Bintel in Sydney told me to use soft tissue paper. Lightly wet the tissue with the cleaning fluid. Before cleaning, use a blower to blow off all the dust, which may cause scratches during wiping. Then gently wipe the Corrector Plate. To dry it, use a dry crushed tissue paper. To be safe, wipe in straight lines and not in circles. I did this process a few times.

Step 8. After cleaning, replace back the Corrector Plate, remembering to align the black line so as to ensure the correct rotational orientation. Then, place back the retaining ring, and screw back the screws. Do not overtighten them.

That's it! The whole process can be done in 1-2 hours. Just remember to use excessive caution during the process.

After that, you may need to do collimation again, though it was not necessary in my case.

Remember, "A clean optic is a happy optic, but a dirty one is better than a damaged one."

starfinder · Post by **starfinder** » Sun Nov 13, 2005 3:11 pm

Photo 2...

starfinder · Post by **starfinder** » Sun Nov 13, 2005 3:12 pm

Photo 3...

starfinder · Post by **starfinder** » Sun Nov 13, 2005 3:13 pm

Photo 4...

starfinder · Post by **starfinder** » Sun Nov 13, 2005 3:14 pm

Photo 5...

starfinder · Post by **starfinder** » Sun Nov 13, 2005 3:16 pm

Photo 6...

starfinder · Post by **starfinder** » Sun Nov 13, 2005 3:18 pm

Photo 7 (final).

The Corrector Plate interior does look a lot cleaner, after wiping away 4 years' accumulation of grime.

Tachyon · Post by **Tachyon** » Sun Nov 13, 2005 4:32 pm

Thanks for your post! Wished you had done it earlier when I had difficulty with my C8i:
here

kayheem · Post by **kayheem** » Sun Nov 13, 2005 6:01 pm

Hi Gavin,

I noticed that yours is the one with UHC coatings. Did you use any special cleaning solutions for it?

In your 6th pic (the one with the blower), you will notice how shiny the internal of the tube is. I feel you should have taken the chance to flock it to increase contrast.

Just my 2 cts

Kay Heem

starfinder · Post by **starfinder** » Sun Nov 13, 2005 7:09 pm

Thanks Kay Heem for the suggestion. Well, the interior of the OTA is painted flat black by Meade. The shininess I think was because the OTA was near a window and it's concentrated light cast by the primary mirror onto the OTA interior. I think that at night, there would be very little stray light that would reflect off the OTA interior.

However, perhaps you're right and I may want to flock it one day. Do you have any suggestions of what material to use and where to get it?

My LX-90 was purchased a few months after Meade launched it. That was before they introduced UHTC to the market. So, this is the non-UHTC version.

When I was at Bintel in Sydney and asking Don Whiteman how to clean the LX-90, he promptly went to the shelf, took a small bottle and placed it in my hand: "It's a present, gift!". (I bought a few calendars there, so maybe this was complimentary. Anyway, he is really nice).

It is called "Bintel UHTC Lens Cleaning Solution". The instruction sheet says: "It is a mixture of Reversed Osmosis Water, Technical Grade Isopropyl Alcohol, Blue Windex and a wetting agent". I used this to clean the Corrector Plate yesterday.

When I told him about the general problem of fungus in Singapore, Don related to me that several years ago, he was tasked to go to New Caledonia, in the Pacific, to clean one large observatory scope there which had a lot of fungus. He said that to tackle fungus, he had to use "Shell X55", which is somekind of white spirit, to remove and kill the fungus growth. He ended up cleaning lots of amateurs' scopes there.