How to clean a small Mak (C90)...or not

[Saturninus]

I loaned a friend my grab-and-go C90 scope for a night. When I got it back and tried to use it a few nights later, I noticed a thin film of dust or grime on the inside of the corrector lens. What I think happened is that he went from a humid, warm sea level location, and drove up to a summit area that was significantly colder. I'm guessing that he didn't open the tube to let the warm moist air out, and as a result, condensation occurred on the inside of the corrector lens. Once there was condensation on the lens, the dust from the air inside the tube must have deposited on the meniscus, and now there is this thin film coating the surface.

So, how does one clean the internal surface of the meniscus on a small Mak? Or is this the kind of thing that one has to send it to the factory for? I used the scope mainly for terrestial viewing, so it's not that big of an issue. But when I do use it at night, I can see a haze around the moon, which I think is caused by the residue on the meniscus. 

If it is reasonably easy to do, I'll do it. If this something that you don't do at home, then I'll live with it...

[Peter Drew]

On my Meade 90mm Mak the complete corrector cell unscrews allowing access to the internal surface for cleaning, if the Celestron one is similar then it is a straightforward job. Screwing the cell back on until it is firm should cause no problem with the collimation. 

[Saturninus]

Thanks - i'll take a look at my scope when i get home. I seem to remember only seeing screws on the rear cell. I hope that will give me access to the meniscus

[Tonys]

On my Meade 90mm Mak the complete corrector cell unscrews allowing access to the internal surface for cleaning, if the Celestron one is similar then it is a straightforward job. Screwing the cell back on until it is firm should cause no problem with the collimation. 

It's the same with the Skywatcher 90mm Mak. You need to be careful how you clean it though, to avoid scratching it. There's plenty of advice on this on the forum if you do a search.

[Tonys]

Thanks - i'll take a look at my scope when i get home. I seem to remember only seeing screws on the rear cell. I hope that will give me access to the meniscus

Just to clarify. If it's like the Skywatcher the meniscus cell itself is threaded and screws directly onto the scope tube so there aren't any screws like on the rear. You shouldn't have to touch the rear cell to gain access to the meniscus.

[NGC 1502]

On my Meade 90mm Mak the complete corrector cell unscrews allowing access to the internal surface for cleaning, if the Celestron one is similar then it is a straightforward job. Screwing the cell back on until it is firm should cause no problem with the collimation. 

Same on my 90mm Mak, it's the tube assembly from a Meade ETX 90.

Go carefully, you should be ok.  Cleaning the main mirror is a different job altogether, best left alone.

Hope you sort it, Ed.

[Macavity]

Yes, a lot of "smaller" MAK corrector cells do unscrew. "Grip the OTA firmly between thighs" etc!  

But, as noted above, the "mirror end" (rubber seals etc.) might best be not disturbed overmuch...