Distorting primary

[earth titan]

Is it possible to distort the primary by screwing the mirror retaining clips in the wrong order / too tight?

Just thought I would ask as a point of interest after hving cleaned and reinstalled my 12" mirror. I'm thinking the glass is too thick but just wanted to know.

[Moonshane]

I have never done it but reading other posts, yes it is possible to create astigmatism in the mirror by having clips too tight. they should be loose enough to get a piece of paper under them without sticking is the general rule I think, so very close but not touching and allowing for a little expansion. I think this also applies to collimation locking screws if your scope has them. as far as I know though, you'd need to leave them too tight for a long time to create permanent damage? someone more knowledgeable will need to answer the last point though.

[FraserClarke]

Yes, very possible. The glass is thick, but you only need to distort it by 200 nanometers (0.000008" in old money) to make a visible difference.

As Moonshane says, the retaining clips should *not* touch the mirror. They are there to stop the mirror falling out if something bad happens (you turn it upside down by mistake). They're not in contact in 'normal operation'.

You won't cause permanent damage though; when you back the clips off, the mirror will come back to it's original shape.

[NGC 1502]

As others have said, if the mirror clips are too tight, it can cause problems.

On the other hand, if the mirror can flop around all over the place, then it's

likely that collimation will be more of a problem, especially when transporting

the scope.

Best regards, Ed.

[earth titan]

Thanks for the advice. Have slacked the clips off until they only just touch.

Will see if it makes a difference.

[barkis]

Being too thick is not a problem, it's the thin ones that can give trouble if they are not supported properly.

A 12" mirror will not lose it's figure under it's own weight, but as a matter of interest, how is it supported?

As the other guys said, the retaining clips can distort if too tight, and also, if there are any lateral bolts that may be used to centre the mirror in it's cell, are too tight, that also can strain the figure.

A thick mirror will also require a longer period of time to reach thermal equilibrium, which is another important element allied to it's performance.

Ron.

[Moonshane]

hi Ron

can you answer a query for me which is related to the thread. I am thinking about using a blob of silicone sealant on the inside of each of my clips on my 6" dob and cutting them down a lot as they are rather large (see http://stargazerslounge.com/equipment-discussion/115753-do-my-clips-look-big.html). My theory is that if I cut them back and grind the shape of the clips to a shallow D they will be kinder to the image than the current ones.

Any ideas if silicone will be OK to use (the idea being it prevents any flop in the mirror and retains the collimation a little better) and also what the effect of the current clips would be visually if any?

Thanks and I hope that the OP does not mind these related questions as they may also assist them.

[NGC 1502]

Hi Moonshane, hope you don't mind me butting in here, to answer the question

you asked Ron.

The mirrors on my O.O. 6" F4 were recoated earlier this year. When I came to

remount the mirrors, I simply copied the original method that O.O. had used.

Both primary and secondary had been siliconed to their cells, no clips were used.

I used silicone intended for building aquariums, as it's obviously meant for fixing

glass, and is guaranteed to stay flexible. It's more expensive than regular silicone,

but worth it. The primary is fixed with 3 large blobs. If you also used slimmed

down clips, then you could relax about any chance of the mirror detaching itself

from the cell.

When I came to collimate and star test, out of focus stars show no sign of mirror

stress, and although a 6" F4 is obviously not a planetary scope, I have been

pleasantly surprised by how the planets look thru this scope.

I do think that over large mirror clips would hurt the image, the same as thick

secondary spider vanes will.

HTH, Ed.

[Moonshane]

hi Ed

that's excellent cheers!

I will need to get silicone anyway so I agree the aquarium based sounds a good idea especially as it will remain flexible.

I will definitely be doing this mod soon, always nice to squeeze every last bit of performance you can from a scope!

I love this 6" f11. I just wish they'd pay attention to detail on the other bits that they do on the superb mirrors. that said, when I contacted them with comments they seemed to listen.

[NGC 1502]

Hi again Moonshane, yes, I agree that O.O. can be a bit lacking in some areas.

I suppose that they are trying to be competitive in a price driven world.

But I'm convinced that they make good mirrors, and most Dobs are "a work in progress" at least mine are ! It means I can do "astronomy" when it's cloudy !

If the planets look ok in a 6" F4, then at F11 they will blow your socks off !!

Cheers, Ed.

[Moonshane]

Absolutely mate agree with all you say. On the other point, I have been sorting out battery power for my dob fans today for when I eventually - get out in the field. I'll cool them off the car at 12v and then run the fans at 6v for observing. I love twiddling with things so am quite happy that there's always a job needs doing. the f11 is lovely. it's 1/6pv and HILUXED too which I did not know when I bought it!