Cleaning mirror on telescope (2 Viewers)

[Mono]

I have a 130mm Dobsonian. I don't use it much. I got it out the other night to look at Saturn and noticed a build up of what looks like dust on the primary mirror.

What is the best way to clean it? Access is my biggest concern down at the bottom of the tube. It looks like you can unscrew it from the base but after replacing it it would surely need alignment.

Any thoughts?

It is this scope...

Sky-Watcher Heritage-130P Flextube Tabletop Dobsonian

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[Tringa45]

No direct experience, I'm afraid, but it looks as if you will have to remove the mirror.
The aluminium coating, enhanced or not, is probably more vulnerable than the usual anti-reflective coatings on most refractors so extreme care would be necessary.
Perhaps it could be washed in luke warm water with a few drops of dish-washing fluid usuing a cosmetic tissue under water.
The excess water could then be flushed off with acetone to obviate mechanical drying. Acetone, however might attack any plastic components or paint.

John

 

[albie...]

Years ago I cleaned and center spotted the primary mirror on a Skywatcher 10" dobsonian . I shared the entire process over on Cloudy Nights , most of it was about using Catseye collimation tools .
I started by removing the mirror cell at the bottom of the scope . Then I removed the mirror from the cell (carefully) . I then carefully placed the mirror in a sink with luke warm water and a few drops of Dawn dish soap in it . I gently rolled cotton balls over the reflective surface of the submerged mirror until I felt that the mirror was clean enough . Then I put everything back together carefully and collimated the scope . When you re-attach the mirror to the cell don't over tighten the clips , you want them to be a little loose but not so loose as to cause the mirror to move around .
I like to recommend that people shouldn't shine lights at the mirrors in their dobs as it looks worse than it actually is . The first pic below was the dirty mirror before cleaning although it looks as clean as the other two pics . I cleaned the mirror because I shined a flashlight on it

 

[glennd]

total agreement with albie...'s post.

i'll add a couple of thoughts. when i expect that the mirror will take some real work, I'll put a towel in the bottom of the sink as a cushion. running tap water over the surface can remove a lot of stuff but later work should be with distilled water to avoid mineral deposits. try to avoid working a spot too hard. the perfect can be an enemy of the good. at some point you run more risk of damaging coatings than improving them. a blower bulb can be helpful in the later stages. be super gentle and never apply those cotton balls to a dry surface.

try not to worry much about coating defects, sleeks, pinholes and the like. not that you want to add to them, but they don't actually detract as much as one might think. think of it in terms of affected area. i run a high school astronomy program. one summer mice nested on one of the scope mirrors. turns out this is not good. i cleaned the mirror, found intact coatings on about 80% and went with it because i would not be able to get a recoat in time to start the term. even at 80%, it still gathered more light than a 5" mirror and the scope performed beautifully, although diminished.

when replacing the mirror in the cell, the clips should be adjusted to about a business card thickness clear of the mirror surface. nothing should pinch the optic.

yes, replacing the mirror will require collimation. plenty of gizmos sold for this process, but your eye is really good at this. once you have your eyeball centered in the focus tube again (centered in the reflection of the secondary) take the scope out on a good night, place a star in the center field of view, defocus and check for concentricity of the diffraction rings. tweek the collimation adjusters until they are concentric. if your mirror cell has locking screws, gently and evenly apply them. recollimate if the locking process throws them off. collimation will be improved by doing this at higher magnification.

and +1 on albie...'s caution not to shine a light down there!