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Re-machining a 16" Mirror

Started by mdvolle, 03/19/2015 08:48PM
Posted 03/19/2015 08:48PM Opening Post
This is a multi-part question about compromises.
I have a 16" parabolic mirror - 3" thick pyrex - roughly 132" FL - it needs re-coated but seems OK.

Calculating the mount for such a mirror, it is clear that it will deform significantly even with a very complicated arrangement of support.

Has anyone tried "re-machining" a mirror like this into more of a truncated conical back? I calculate that I could remove more than 12 lbs off of the 45 lb mirror but am not sure what that would do to the shape of the front.

In theory, it would be easier to mount and would cool faster?

Mark
Posted 03/19/2015 09:24PM #1
Mark Volle said:

This is a multi-part question about compromises.
I have a 16" parabolic mirror - 3" thick pyrex - roughly 132" FL - it needs re-coated but seems OK.

Calculating the mount for such a mirror, it is clear that it will deform significantly even with a very complicated arrangement of support.

Has anyone tried "re-machining" a mirror like this into more of a truncated conical back? I calculate that I could remove more than 12 lbs off of the 45 lb mirror but am not sure what that would do to the shape of the front.

In theory, it would be easier to mount and would cool faster?

Mark

I don't have the knowledge to say if it could be done without having to redo the front also but both in theory and practice it would definitely be easier to mount and also cool a LOT faster at 2" or less thick as compared to 3"....of that I am certain.

20" MidnighTelescopes f/5
8" f/16 Muffoletto Achro
Meade 6" AR6 f/8.0 Achro
Celestron C11
Parallax PI250 10" F/5 Newt
Vixen FL102S,NA140SS,R200SS
80mm Stellarvue Nighthawk with WO APOupgrade lens, 90mm f/16.67 Parallax Achro
LXD75/LX200, Mini-tower, GP-DX, CG5 ASGT, LXD650, GM-8, G11, GM-100, GM-150EX, GM-200 and a Gemini G40
4" portable AP convertable 44" or 70" pier
Stellarvue M7 Alt-Az, TSL7 Pier/Tripod
12.5" f/5 MidnighTelescopes DOB Swayze optics
Statham GA
Posted 03/20/2015 06:58AM #2
Here's what I think I know: removing material from the back side of the mirror creates the risk of changing the figure on the front because working on the back may release otherwise unreleased stresses within the blank. There's also the risk of scratching or gouging the front surface accidentally when working on the back side. If it was my project, I'd talk to one of the mirror makers (Carl Zambuto, Steve Swayze, Mike Lockwood, Normand Fullum, Galaxy, Pegasus, OMI, etc.) for his opinion of the risk and the cost to make the mirror right again if it needs to be refigured.
Posted 03/20/2015 10:54AM | Edited 03/20/2015 11:01AM #3
Hi Mark,

A thick mirror deforms less than a thin mirror so its mount can be less complicated. Thinner mirrors have the advantage of reaching thermal equilibrium faster and weighing less but they require a more complicated mount to maintain their optical figure. A 16" F/8 scope with 3" thick mirror sounds more like an observatory-based instrument where portability (less weight and size) is not a concern.

Tom
Posted 03/22/2015 09:34PM #4
Thanks to all for the input - I suspect that I need to build a scope to at least try the mirror before considering further work - the coating is good enough to do a star test.

I think that I would only reshape the back if the front also needs help - It will make for a heavy scope but my mount will already handle it fine, so I have little to lose by trying it ASIS first!

Thanks