Bit of a problem with my mirror..

[bish]

I put off cleaning my mirror for quite a while because I am a bit clumsy and thought a dirty mirror is better than a broken one. However,  it looked very straight forward, and it was. My mirror is (almost) spotless.

Now for the problem. The clamp bolts were very tight and 2 of them broke off. There are 10 more, so no worries. I assumed that being so stiff that the bolts must have been screwed down really tight.  I expect you know what"s coming. I screwed the clamps down hard, collimated the scope. 

The first light after cleaning, the stars were all streaky and Mars all elongated. I loosened the bolts quite a bit last night and it did seem improved but not cured. I intend to loosen the bolts as much as possible without the mirror moving around. If it is not better I will assume that I have warped the mirror and just buy a new scope (although there is quite a wait at the moment!).

The main thing is that I have learned from my mistake!

 

[banjaxed]

You don’t say what scope you have but it it is a decent make I can’t see how you could warp the mirror as they are normally a big chunk of glass.

[johninderby]

Overtightening the clamps on the mirror can distort the mirror. I tighten them until they just barely touch the mirror then back off a tiny, tiny bit.

[bish]

12 minutes ago, banjaxed said:

You don’t say what scope you have but it it is a decent make I can’t see how you could warp the mirror as they are normally a big chunk of glass.

It is a Skywatcher 250px. It does seem like a fair chunk to warp.

[bish]

1 minute ago, johninderby said:

Overtightening the clamps on the mirror can distort the mirror. I tighten them until they just barely touch the mirror then back off a tiny, tiny bit.

Thanks. I will try that and hope I haven't done anything permanent.

[johninderby]

No permanent damage. The distorsion caused by overtightening is very, very small. You would really have to overdo it to crack the mirror. Very unlikely.

Edited October 23, 2020 by johninderby

[rl]

I'm not familiar with this particular scope but it's probably reasonable to assume that the cell fixings will contract faster than the mirror as the scope cools down. You need to allow enough slack so that the glass wont get pinched even in the coldest nights. I always leave enough clearance to get a couple of pieces of paper in between the metal and the glass. If you're that worried about movement a couple of blobs of silicone goo will hold the mirror without putting excessive force on it. 

[niallk]

You only need the clamps to stop the mirror from falling out ... there's no clamping action at all   I had a 250px and man those clips were tightened down - difficult to remove.

On my 15", the mirror hangs in a sling, and there are 3 prongs which are about a centimeter clear of the mirror to catch as a fail safe.

I think the only risk is if you transport the tube on its side or turn it upside down.  there should be no contact at all.

I doubt that the glass is permanently deformed - fingers crossed.  Can you loosen them to a point where the mirror is free to move just a little?

Edited October 24, 2020 by niallk

[bish]

4 hours ago, niallk said:

You only need the clamps to stop the mirror from falling out ... there's no clamping action at all   I had a 250px and man those clips were tightened down - difficult to remove.

On my 15", the mirror hangs in a sling, and there are 3 prongs which are about a centimeter clear of the mirror to catch as a fail safe.

I think the only risk is if you transport the tube on its side or turn it upside down.  there should be no contact at all.

I doubt that the glass is permanently deformed - fingers crossed.  Can you loosen them to a point where the mirror is free to move just a little?

Thanks. I have loosened the bolts today. They don"t press down at all but hold it in place when it"s collimated. Just need a star to look at now!💭

[niallk]

16 minutes ago, bish said:

Thanks. I have loosened the bolts today. They don"t press down at all but hold it in place when it"s collimated. Just need a star to look at now!💭

Fingers crossed

[knightware2]

I hope the star test goes well. Once you have backed the clamps off so that they don't quite touch the mirror surface, you should shake the tube a little when pointed at the zenith so that the mirror settles in its cell. Then collimate. I usually do this, and after an hour or so of observing, check collimation again so that temperature changes can be addressed. My telescope if f/4.5 so collimation is important for good crisp images.

Good luck!

- Phyllis

[bish]

Managed to grab a quick look through a lot of passing cloud. I think the scope is OK. Had  a great view of M103 and a crisp view of Mars and The Moon. Seemed to be a lot of coma at the edge, so had to re-focus the edge to get it sharp. I think it was always like that but it has never bothered me and I am looking out for flaws now. The main thing is I enjoyed my grabbed quick session and the view was massively improved.

[Pixies]

54 minutes ago, bish said:

Managed to grab a quick look through a lot of passing cloud. I think the scope is OK. Had  a great view of M103 and a crisp view of Mars and The Moon. Seemed to be a lot of coma at the edge, so had to re-focus the edge to get it sharp. I think it was always like that but it has never bothered me and I am looking out for flaws now. The main thing is I enjoyed my grabbed quick session and the view was massively improved.

If you had to refocus to make the edge sharp, then it's going to be field-curvature, rather than coma, I think.

[niallk]

Sounds positive! 👍

[bish]

8 minutes ago, niallk said:

Sounds positive! 👍

Yes it was a big relief when I focused on M103 and the stars were sharp.