Loose screws ?

[Lorstin]

I have just removed the primary mirror cell ( 12" Sky Watcher collapsible dob) for cleaning and noticed that the screws that secure the three brackets/clips that hold the mirror to the cell are slightly loose. Are these screws supposed to be loose? maybe to allow for expasion/contraction of the mirror? or was it just poorly assembled? If I could get an answer before I put the cell back in that would be great.

 P.S I followed the instructions elsewhere on this forum to remove the cell, I did it in the verticle position and the tube lifted off easily, it couldn't have been easier.Thanks.

[John]

There should be a very slight gap between the mirror holders and the surface of the mirror. A gap of around "business card" thickness is often recommended.

If you tighten them you could well distort the mirror slightly which will result in odd star images.

[Lorstin]

Thanks John, I can now go & put it back together. The mirror certainly needed cleaning, unfortunately the second tissue I used left alot of lint behind, ah well these things happen.

[cjdawson]

The best way to clean a mirror is to first use an air duster to get rid of any loose dust and debris.    Then use lots of distilled water to wash the mirror, this will take a while but should result in a spotless mirror.  Distilled water will also not leave a residue, or streaks etc.

As for the mirror needing cleaning, I'll bet that it didn't.  Since I bought my scope in 2001, the mirror has never been cleaned, and is not likely to ever get that treatment.   Ok, my scope is an SCT, so the mirror is enclosed, but the same is true for most netonians as well.

You'd be suprised about how much muck you can have on a mirror before it needs a clean.

However, now that you have cleaned the mirror and left lots of lint on it.  The lint will attract dirt, so it would be worth using a lint free microfibre cloth to remove the lint.  Just be very careful as mirrors are extremely delicate and will be damaged easily.

[Lorstin]

Thanks for the advice CJD, if I'd have thought quicker I would've taken a pic of the mirror, it's been living in a not too friendly enviroment for several years. Regarding the lint - I used two brands of tissues when cleaning the mirror, for the initial clean I used an el cheapo (which was fine) and then finished off with a more expensive one - this was the culprit that left all the lint behind. I used another el cheapo and removed the lint.

I think "the boss" has some of those micro fibre cloths you mentioned, I'll pinch one when she's not looking.

[cjdawson]

Take a photo of the mirror now and post it so we can see before you do anything and risk damage.

[Charic]

Most reflector mirrors are exposed to the elements so they can get dirty, contaminated, dusty but it takes a lot to really affect the performance of the scope,  dried on Cola may cause you a problem? however your mind won't settle  and you will clean the mirror regardless.

Sky-watcher coat their mirrors with silicon dioxide to offer more durability and protection to the mirror. If you take your time and err on caution, and use the appropriate  method,  you should be able to clean as often as you want, although this is unnecessary in real terms.