Secondary Mirror Size

[Hans Tucker (Hans)]

Looking at purchasing an Antares 1/30 Wave Secondary Mirror for my 10" f4.8 Newtonian in a Pimp your Scope Project. Orion Optics UK installed a 63mm Secondary in their VX Newtonians which equates to 2.48" imperial. Antares offers in their standard off the shelf sizes 2.14" or 2.6".

Question. Would it be best to go for the 2.14" or the 2.6"?

[AG Hybrid (Adrian)]

I got a 1/18 wave Antares secondary for my 12". Haven't installed it yet but 2.6" was the smallest I could go. 2.14" was too small with probable light loss.



Not sure why you want a 1/30 wave secondary though. I think its a waste of money if your primary isn't 1/30 wave. Hell the 1/18(which according to test documentation is 1/19) for mine is over kill and I only got a new secondary because the coatings on my old secondary was getting pin holes.


I'm sure there some formulaic calculation that can tell you the bare minimum the secondary needs to be depending on aperture to focal ratio.

[RyanJones]

Hi Hans,

It very much depends on what you are doing with the scope. If you are using it for visual then you can lean towards a smaller major axis size and take advantage of better contrast from a reduced central obstruction . If you are imaging with it then to me 2.6” sounds considerably undersized. Depending on your back focus distance, I would expect a desirable size to be much more like over 3”.

Hope this helps

Ryan

[glend (Glen)]

The smaller the better if the scope is visual only, for the reason outlined previously. There is a tendency among Newtonian manufacturers to install larger secondaries, due to the increasing size of camera sensors available now (ie, Full Frame).. However don't be seduced into thinking that means better performance. Most cameras with APS-C (Canon) sized sensors work just fine with reasonably small secondaries. As an example, Skywatcher (upgraded) it's wonderful Mak-Newt the MN190, by increasing the size of the secondary from 52mm to 63mm, to reduce the chance of vignetting in larger sensor cameras used on the scope. In doing this they ruined a fine visual and APS-C photographic scope. It is impossible to buy the small secondary MN190s now, and this has made the original 52mm equipped scopes highly desirable.
So do your research and know why your considering a change, as bigger is not necessarily better.

[anj026]

This page from Mike Lockwood is a great reference.

http://www.loptics.com/ATM/diagonals.html

[astro744]

I would not glue a 1/30 secondary. Get a proper holder with a lip for it. It will sit in place without stress if properly packed and the lip will mask any surface defects at the very edge of the secondary.

As for size it depends on the fully illuminated field you want as well as how much magnitude drop (if any) you want at edge of field for your lowest power eyepiece. You need to provide a few more measurements to determine secondary size correctly. Search for Mel Bartels secondary sizing.