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41.75" (1060.45mm) reflector

Started by brucesdad13, 01/23/2016 02:32PM
Posted 01/23/2016 02:32PM Opening Post
I recently acquired a mirror blank that is 41.75" in diameter and 10" thick. Needless to say it was VERY hard to move. My primary interest is astrophotography and I'm contemplating various designs. I figured I would post and see what comes to mind when you all think of constructing a telescope around such a large primary mirror. I haven't decided on focal length although I believe f/3 would be pretty cool based on "Le1m" telescope (Google Chrome will translate it for you) Perhaps something that could be craned onto a mobile base for outreach and back onto an EQ mount for photography. I have no time frame or budget for the project completion.

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brucesdad13's attachment for post 60012

~ Charlie Stevenson

8" f/5.7 String Telescope - 1st Scope Build; 2nd Place Stellafane 2016 Optical Award for Newtonians 12.5" and Smaller
10" f/4.5 Newtonian (June 2015) mirror refigured by Optic Wave Labs P-V WaveFront 1/14.24, Strehl Ratio 0.993 (Aug '15)
Criterion RV-6 seems to be circa 1973 (June 2015) [For Sale]
Celestron C8-A XLT (January 2015) [For Sale]
Celestron PowerSeeker 70AZ (Christmas 2014)
Aldrich Astronomical Society member since 2015
Posted 01/23/2016 03:20PM #1
Hi Charles,
Yep, thats a big blank. Considered briefly taking a similar project on, then helped my friend transfer ownership for another 40" mirror a few months back. It took 6 of us to move it into a truck by hand and it was in a welded mount with wheels.

As you appreciate first hand, its a lot easier to work massive optics when you are setup for it, we did them at the Perkin Elmer Garden Grove CA shop in the 80's-90's, we owned and worked with Boller and Chivens in Pasadena. Large machines for generating, grinding, an over head crane, large isolate test tunnels, and a roll off roof for star testing.

The Le1m is a good looking instrument!

Looks like a great project, your on the wrong coast for direct help so here is a wish for the best of luck!

Posted 01/24/2016 04:18AM #2
Wow! Hi Charles and Dave. Diam Big but it's the Thickness of your Solid that gets me! Like Dave, I've dealt with big stuff at work, but only the testing for me and watching other people man handle them. At home I stared at meter+ a couple of times and finally settled on a finished and coated fastish light-weight 36, which still awaits the mount... That one is pretty close to feasible (effort and $) because "all I need" is the Whiffle-Tree and plywood to build the Dob. I already have all the other parts. Even with creative mounts, the big stuff is still way non-linear function of size by any metric you choose: cost, effort, time, weight, logistics, materials, available shops, testing, coating, ladders, comfort of use, tracking, storage, transport, maintenance, thermal stabilization, danger...? BUT it all becomes worth it when you finally go operational and approach the eyepiece or camera --- holding your breath and hoping for the best! The Tracking for Imaging not to be short-changed for something SO massive: Three approaches to consider that could make that aspect less stressful: Mallincam, CCD TDI, Night Vision streaming video. Keep us posted as things progress! Tom Dey

29-inch Dob in a dome
36-inch upgrade soon
LUNT 80/80 solar scope
FLI 6803 cam
APM 100mm APO Binos
JMI RB-16 Night Vision Binos
Zeiss 20x60 IS binos
Posted 01/24/2016 10:02PM #3
Wasn't it just over a year ago that you entered or re-entered the astronomy hobby? And in quick leaps and bounds you're now planning you own "Palomar"?? Go "Buzz Lightyear" More power too you. 8) Thomas M.
Posted 01/24/2016 10:40PM #4
Charles, thought of something else: Do you know what the material is? From your picture it looks quite clear. In order of increasing value and utility would be "white" crown, Bk7, Pyrex (aka Borosilicate), fused silica, ULE. Older Pyrex and fused silica have bubbles; older ULE is blueish. New generation are quite clear and free of inclusions. If you lucked into fused silica or ULE, thermal behavior could be quite good, even exceptional. ULE CTE is nearly zero at room temp. i.e. it is a function of temp tailored to be identically zero AT operating temp. There are easy ways to ID the exact material if even only one surface is flat and polished. Equipment to do that you probably already have. Tom

29-inch Dob in a dome
36-inch upgrade soon
LUNT 80/80 solar scope
FLI 6803 cam
APM 100mm APO Binos
JMI RB-16 Night Vision Binos
Zeiss 20x60 IS binos
Posted 02/19/2016 02:22PM #5
If you are curious still about this material, there is a way to gauge its refractive index.

Get some liquids that are close to you suspected materials list, and make some pools of them on the surface.

If the reflection from the liquid/glass interface is very dim or non-existant, you can reasonably infer its index to be close to that liquid.

Zerodur and quartz can be almost clear or in the case of Zerodur, nearly white. I own the old 24" primary from the Innisfil Observatory. Its primary, a 24" Zerodur from a f/11 DK is almost clear.