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Mirror recoating


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I've got an old mid 90's Orion optics 6" f5 europa n the loft, my first reflector. Is this a mirror worth recoating? It'd need rings and probably a new focuser if I want to use it for imaging but wondering it it might be better to get it back in good order or just buy a new 150 pds if i ever want a reflector again.

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28 minutes ago, Merlin said:

Personally, I don’t like waste, I’d renovate the ‘scope. The optics might turn out to be better than a newly-acquired one.

 

That's what I'm thinking. I always heard good things about Orion optics UK quality. Just hoping someone has some concrete knowledge as to whether it's worth it. 

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23 hours ago, Anthonyexmouth said:

That's what I'm thinking. I always heard good things about Orion optics UK quality. Just hoping someone has some concrete knowledge as to whether it's worth it. 

I’m not sure why the quality was like back in the 90s. If it was before they started Zygo testing then I imagine it was quite variable. You could do a star test with it as it is to see how it performs  and then decide?

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4 minutes ago, Anthonyexmouth said:

What do I need to do that? 

For the basic star test, aim the collimated scope with a 5mm or similar eyepiece at Polaris and allow time to cool.

Firstly, the optics can be checked for astigmatism by racking through focus, the out of focus blurs being distorted into ellipses the major axis of each at 90 deg to each other as you pass from intra to extra focus.

To then check for correction the size of the 2ndry shadow should be compared at exactly equal distances either side of focus. Its usual to check at +/-10 waves defocus which, for an F/5 optic is 1.1mm. It's not too crucial so long as it's the same either side. You might even try 1p pieces which are 1.5mm. You'll need two spacers.

Pull the ep out of the drawtube by the thickness of one spacer.

Find focus with the focuser.

Now pull the ep further out from the drawtube by another spacer to give the extra focus image. Note the size of 2ndry shadow.

Now push the ep back into drawtube to give the intra focus image and compare the two shadow sizes. 

If the inside f shadow is larger the scope is under-corrected and if it's the outer that's larger then over-corrected. I think a maximum reasonable error of 1/4 wave corresponds to a 3:2 proportion. Hopefully it'll be better, even 1/10 wave can be spotted.

A still cooling mirror adds over-correction.

A perfect mirror would look exactly the same either side. Zone and edge imperfections will affect the strength of the rings that surround the shadow.

David

 

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28 minutes ago, davidc135 said:

For the basic star test, aim the collimated scope with a 5mm or similar eyepiece at Polaris and allow time to cool.

Firstly, the optics can be checked for astigmatism by racking through focus, the out of focus blurs being distorted into ellipses the major axis of each at 90 deg to each other as you pass from intra to extra focus.

To then check for correction the size of the 2ndry shadow should be compared at exactly equal distances either side of focus. Its usual to check at +/-10 waves defocus which, for an F/5 optic is 1.1mm. It's not too crucial so long as it's the same either side. You might even try 1p pieces which are 1.5mm. You'll need two spacers.

Pull the ep out of the drawtube by the thickness of one spacer.

Find focus with the focuser.

Now pull the ep further out from the drawtube by another spacer to give the extra focus image. Note the size of 2ndry shadow.

Now push the ep back into drawtube to give the intra focus image and compare the two shadow sizes. 

If the inside f shadow is larger the scope is under-corrected and if it's the outer that's larger then over-corrected. I think a maximum reasonable error of 1/4 wave corresponds to a 3:2 proportion. Hopefully it'll be better, even 1/10 wave can be spotted.

A still cooling mirror adds over-correction.

A perfect mirror would look exactly the same either side. Zone and edge imperfections will affect the strength of the rings that surround the shadow.

David

 

Ah, ok, no eye pieces or mounting rings and the original focuser is junk. 

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Tricky! But I'd want to check out the mirror + diagonal before spending much on the parts you mention and I'd certainly not assume that they were good quality, although they may be.

Orion Optics offer a mirror evaluation service either basic or advanced prior to re-coating which is worth thinking about but the costs mount up.

Or you can investigate the Foucault knife edge test using stuff around the house.

David

PS A search shows quite a number of good reviews of the Europa scopes.

Edited by davidc135
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55 minutes ago, davidc135 said:

Tricky! But I'd want to check out the mirror + diagonal before spending much on the parts you mention and I'd certainly not assume that they were good quality, although they may be.

Orion Optics offer a mirror evaluation service either basic or advanced prior to re-coating which is worth thinking about but the costs mount up.

Or you can investigate the Foucault knife edge test using stuff around the house.

David

PS A search shows quite a number of good reviews of the Europa scopes.

One of the reasons I bought it back in the day was the good name of their scopes. It's just the OTA I have now, and maybe the old eq1 style manual drive head but that can stay in the loft. I know i'll need a new focuser as this one is knackered and not suitable for imaging but the tube is straight and spider is good. Might have to drop orion and email and see if I can get an honest opinion on it. 

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