Folding Refractor Astro Flat Mirrors

[glend (Glen)]

I am considering building a 127mm f12 folded refractor from parts i already have, but i will need two high quality well coated flat mirrors for bouncing the light path within the 'box'. Given that the objective is 127mm, the flat mirrors do not have to be very big. Anyone know of a source of high quality flat mirrors? I am tempted to just use newt secondary ellipical mirrors and blacken the unrequired ends with flocking. These are readily available, i just need the minor axis, and can use secondary stalks to mount them and adjust. Anyone see a problem with that approach?

[dannat (Daniel)]

nope a guy on CN who built a 5" bino uing 2x127mm obj. just used 2nd'ary mirrors -he used holders so he can collimate it easily also. he said it was hard to source good quality flat mirrors, so ordinary secondaries did the trick, he didn't bother blackening any section -but he didn't have the light path crossing over either

[Merlin66 (Ken)]

Glen,
I use many front surface mirrors in my DIY spectroscopes...
Obtaining the shape (Square/rectangular/elliptical) the required surface accuracy, and preferred coatings is an expensive challenge.
For my smaller mirrors I used Knight Optics in the UK, but I think you're on the right track with your thoughts of using secondary mirrors.
Bang per buck they give a good quality alternative and are readily available.
No optical issues other than the sizing of the mirrors (for required FOV) and the mounting plates (to give X-Y-Z alignment).
Sounds like an interesting project.

[ZeroID (Brent)]

OK, so you've conquered the cooled DSLR problem, now you're going to fold scopes in half ......
This should be fun to watch ...

[glend (Glen)]

Originally Posted by ZeroID

OK, so you've conquered the cooled DSLR problem, now you're going to fold scopes in half ......
This should be fun to watch ...

Actually in thirds Brent, and only considering at this point. I need to work out the diameter of the flat mirrors required and see if there is a standard newt secondary mirror that would suit. Drawings to do, calculations to make, then a decision.

[billdan (Bill)]

Glen you are probably aware that Edmund Scientific sell Optical Flats at selectable wavefront surfaces.

They are expensive though, a 50mm diam one tenth wave flat is $200.

Cheers
Bill

[glend (Glen)]

Originally Posted by billdan

Glen you are probably aware that Edmund Scientific sell Optical Flats at selectable wavefront surfaces.

They are expensive though, a 50mm diam one tenth wave flat is $200.

Cheers
Bill

Yes Bill i have looked what Edmund's offers and while its great stuff it is far too expensive for my project. A GSO secondary will do the job and are pretty cheap.

[Kunama]

4" and 3.1" secondaries might do the trick Glen.

[ZeroID (Brent)]

Originally Posted by Kunama

4" and 3.1" secondaries might do the trick Glen.

Sounds about right ... with a wee bit of room to wriggle depending on placement

[OzEclipse (Joe Cali)]

don't forget that wavefront errors propagate. A threefold two mirror system is effectively a three mirror system when you add a mirror diagonal. Are GSO flats 1/10wave p-p or rms? If they are p-p rms then 3 refelctions will give you a 1/3 wave system, if they are rms, you'll have more like a 2/3 to 1 wave system. Sorry, long day at work & too stuffed to do the exact calcs. If you use GSO's you'll end up with a very very low power telescope.

http://www.nova-optical.com/Flat%20Mirrors.htm
expensive but good.

Joe

[glend (Glen)]

Never mind, change of plans, will not be deconstructing my nice iStar f12.

Thread closed. Thanks to all.