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Help with astigmatIc 14 inch mirror

mdstuart

By mdstuart
in DIY Astronomer

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Moonshane

Moonshane

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it could well have been and sounds like your summary was right - this is likely to be the cause of the apparent astigmatism. taking it off and regluing to the current secondary holder with three blobs of silicone equally spaced if the best option without sourcing a new holder. if he wanted to create a flat surface then your suggestion would be good although with a single blob or three blobs as above. personally I'd stay with the current holder as adding a flat surface would introduce another possible failure point. although in truth it's relatively rare for a secondary to come unglued.

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mdstuart

mdstuart

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Thanks all. Lots of hints and I like the safety rope!

Thinking of cutting an oval piece of wood, coating it with epoxy and then using the spaces and silicon blob approach.

I will work out how to attach the oval to another piece of wood and then attach that to the hollow plastic holder...

Then its fingers crossed!

Mark

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mdstuart

mdstuart

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My wife suggested PVA bond rather than epoxy to coat the plyboard. I am sure it will be fine and help the silicon to stick!

Now I have to cut an oval in plyboard with a hacksaw...hmm should be fun.

Mark

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mdstuart

mdstuart

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Shane. I am...I have put some wood supports in and here is my oval mirror base..

I will now pva the wood oval...

Then wait and then I can use silicon blobs to attach the mirror to the oval...

Mark

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Moonshane

Moonshane

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that will certainly work OK. personally though I like as few possible failure points as possible. make sure (as I always recommend with larger secondaries like this) you use a blob of silicone with a safety rope around a spider vane too. I am sure it will be fine and improve the views a lot. top modding!

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ollypenrice

ollypenrice

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Came late to this and was thinking 'secondary' as I read through because my 20 inch was astigmatic due to this. Ralf Ottow made me a new flat and fitted it. He was insistent that the 3 blobs method is not the best. He makes a small ring of sillicone of matchstick method thickness. The scope is transformed.

Anyway, secondary is good news! Best of luck.

Olly

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Moonshane

Moonshane

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Interesting Olly. I'd be keen to understand why the single thin ring is considered best vs. the single/tri-blob approach. I am always up for new methods if I understand the logic :0)

the good thing is that whatever route is used, it seems that if it does not work well, change the method of fixing should hopefully correct it one way or another at little cost.

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Astrobits

Astrobits

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+1 for Shane there.

I too cannot see how a ring of silicone would be better than 1 or three blobs. It might be that the ring being "matchstick" thick ( and not very thin ) that prevents movement of the holder from transferring to the flat.

I have not tried PVA as a primer for wood/silicone bonding. I tested quick setting epoxy and as that worked well stuck with it :grin: ( sorry about the pun ). As the quick setting epoxy can be used within minutes and PVA would need to be left for hours to fully dry I cannot see the advantage of PVA. Also, as a polymer chemist, I would expect that PVA would not make as strong a bond as epoxy and I would be very nervous of trusting my flat to it without testing the concept first with something less important.

Nigel

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mdstuart

mdstuart

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When you say epoxy do you mean the glue with two compounds you mix together? Do you have a link to some online?

Base now ready...yes I know there are screws...this is non practical Mark here!...but I will make the spacers thicker than the screws..

Mark

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Moonshane

Moonshane

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hi Mark. anything like this http://www.ebay.co.uk/itm/like/160825524485?lpid=83&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=83&ff19=0 would be OK. you could take the screws out one at a time and countersink slightly to get them just below the surface? how is the flat held onto the plastic holder? or are the support pieces glued in somehow? maybe some screws in from the sides might be a good idea for belt and braces? this is why I'd have just used the holder but I am sure it will be fine.

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mdstuart

mdstuart

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Thanks..

The wood supports inside the holder are screwed in to the base so its very secure...

I will try the counter sinking idea..

Mark

I really appreciate everyones input! Accounting is my skill not this stuff..

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mdstuart

mdstuart

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Update....

No good still the same after all that..got so desperate I tried my secondary from the ten inch and the stars were still astigmatic..:(

So it is the main mirror or its cell..:(

Plan A visit astrobits in Somerset to test the mirror and perhaps put something strategically behind the mirror to correct the mirror...long shot!

Plan B put on astro buy and sell as spares or repairs for say £100..after all it has a great secondary, good focuser and a reasonable base..and put it all down to experience..

By the way I looked at the nova and some double stars in my 10 inch with my new 12mm x cel lx to console myself and boy those stars looked good as small dots! :)

Mark

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mdstuart

mdstuart

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Yes absolutely you can use sealant around a large secondary directly on to a plastic hollow tube..

Overnight I have decided to silicon the main mirror to try that. I will post a pic later of the mirror cell without the mirror.

Again I suspect the mirror but I have the silicon so I might as well try.

There is a paturn on the wooden cell which I will use.

Mark

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