Gordon Waite
Vendor-Waite Research
Reged: 06/11/07
Loc: New Jersey
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Making twin 20" quartz mirrors
#3624128 - 02/14/10 01:31 PM
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A couple of months ago you might have seen a thread started by JadeSmith about his two 20" quartz blanks. His posting intrigued me, so I offered to make the two blanks into finished mirrors, keeping one for myself and sending one back for Jade. I promised to keep a diary of the process here for all to enjoy, as the blanks are ground, polished and figured into Newtonian primary mirrors. Jade liked the idea, and I soon found two huge boxes waiting on my front step!
I'll do my best to entertain you with this big adventure. Maybe you can all help keep my spirits up as the "challenges" unfold and we see if this turns out to be a comedy, a tragedy or a triumph.
Gordon Waite
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kfrederick
Carpal Tunnel
Reged: 02/01/08
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Re: Making twin 20" quartz mirrors
[Re: Gordon Waite]
#3624308 - 02/14/10 03:25 PM
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Great are they pregenerated ? A 20 inch is a nice size .
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Gordon Waite
Vendor-Waite Research
Reged: 06/11/07
Loc: New Jersey
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Re: Making twin 20" quartz mirrors
[Re: Gordon Waite]
#3624310 - 02/14/10 03:26 PM
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Session #0: Here's a photo of one of the 20" mirror blanks, sitting on one of my turntables. The blank is sitting on three black hockey pucks.
The blanks have issues that make the fabrication a little interesting. There is a central hole, with different diameters on front and back. There is a little recess milled in the face edge. There are two holes drilled into the back of the mirror that extend toward the front face. And there are four finger grips milled into the edges of the blanks. This first session is just a look at metrics on the two blanks. They turn out to be very nearly identical:
Diameter: 20"
Weight: 41 pounds each
Thickness: 1.75"
Central hole: About an inch in diameter
Central hole: Over an inch and a half on the back
The two blind holes that are drilled through from the back can be seen through the face of the mirror. It would be nice to limit the sagitta on the mirror so that I don't risk grinding through and open up the two blind holes. If I go to f/4.5, that would be a sagitta in the center of .278". I'll see as the grinding progresses if it is possible to get all of the 180" radius without risk.
The faces of both blanks have had some milling work. There is a rim around the edge which is higher than the rest of the face, which appears to have been mildly milled or ground down. The face definitely isn't flat, and it seems to be a little uneven and not a surface of revolution. The back of the blank is actually nicer, but I don't want to deal with the larger central hole and the blind holes.
Gordon
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Gordon Waite
Vendor-Waite Research
Reged: 06/11/07
Loc: New Jersey
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Re: Making twin 20" quartz mirrors
[Re: kfrederick]
#3624319 - 02/14/10 03:30 PM
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are they pregenerated ?
No, the blanks are both close to flat. Roughly 44 cubic inches of quartz to hog out from each of the blanks.
Gordon
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Gordon Waite
Vendor-Waite Research
Reged: 06/11/07
Loc: New Jersey
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Re: Making twin 20" quartz mirrors
[Re: Gordon Waite]
#3624542 - 02/14/10 05:14 PM
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Here are links to some images that show the problematic characteristics of the 20" blanks...
Hole in center of face
Center hole in back side
Blind hole from back side
Blind hole as seen from the face of the blank
Finger grip on edge of blank
Notch milled in edge of face
Raised edge of face of blank
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Mirzam
Post Laureate
Reged: 04/01/08
Loc: Lovettsville, VA
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Re: Making twin 20" quartz mirrors
[Re: Gordon Waite]
#3624558 - 02/14/10 05:22 PM
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Hmmm. Could you fill all the holes with some epoxy mixed with glass particles to approximate the thermal and textural behavior of the blanks? Or maybe you already have another plan?
Jim
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Gordon Waite
Vendor-Waite Research
Reged: 06/11/07
Loc: New Jersey
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Re: Making twin 20" quartz mirrors
[Re: Mirzam]
#3624622 - 02/14/10 05:56 PM
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Hi, Jim,
Quote:
Could you fill all the holes with some epoxy mixed with glass particles to approximate the thermal and textural behavior of the blanks? Or maybe you already have another plan?
I filled the center hole partway with dental plaster. I filled it from the front, going back to about 1/2" from the back of the blank. I just taped over the front of the hole, and put the blank face down on the turntable. Then poured in a bit of dental plaster. A half hour later it was hard and cold and ready for grinding. Here's a photo...
I'm going to leave everything else as is. The center hole is so small relative to any reasonable diagonal mirror size that any figuring defects due to the hole will be inconsequential. The plaster plug will let me grind with the mirror face up without letting all the juice drain down through the center.
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rboe
Reged: 03/16/02
Loc: Phx, AZ
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Re: Making twin 20" quartz mirrors
[Re: Gordon Waite]
#3624693 - 02/14/10 07:01 PM
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That center hole should be able to play a nice role in a mirror mount. Not exactly a conical mirror but I bet you could do away with the strap.
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: Making twin 20" quartz mirrors
[Re: Gordon Waite]
#3624701 - 02/14/10 07:07 PM
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If the hole is exactly in the center, you could just bevel it, and put a piece of duct tape on the back with a thin layer of contact cement. I know it sounds crazy, but I've fiddled with perforated mirrors before. If your polishing regimen is up to snuff, it won't pose any issues at all. It has the advantages that there's absolutely nothing to effect strain while polishing. FWIW.
Mark
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Moggi1964
Vendor - Astro Laugh
Reged: 11/07/05
Loc: Madison. NJ. USA
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Re: Making twin 20" quartz mirrors
[Re: Mark Harry]
#3624713 - 02/14/10 07:13 PM
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20" quartz.......and TWO of them 
I can't wait (or perhaps Waite) to see these two when they are complete.
Lots of pictures Gordon, it's not every day we get to see something like this.
What Focal length are you aiming for?
M
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jdownie
scholastic sledgehammer
Reged: 02/24/06
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Re: Making twin 20" quartz mirrors
[Re: Moggi1964]
#3624781 - 02/14/10 07:51 PM
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100" or maybe 90".
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jgraham
Postmaster
Reged: 12/02/04
Loc: Dayton, Ohio
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Re: Making twin 20" quartz mirrors
[Re: jdownie]
#3624814 - 02/14/10 08:06 PM
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Neat! My 16.5" blank was right at 2" thick and being old-school I didn't want to make it any thinner so I generated the curve with a 9" tool while taking almost no glass off of the edge. Also, having had enough of figuring short focal length mirrors (some as short as f/3) I went with a fairly long f/6.7 (109" focal length). It has proven to be a wonderful scope!
Have fun!
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kfrederick
Carpal Tunnel
Reged: 02/01/08
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Re: Making twin 20" quartz mirrors
[Re: jgraham]
#3624925 - 02/14/10 08:56 PM
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I would grind the back flat.
Edited by kfrederick (02/14/10 08:57 PM)
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Gordon Waite
Vendor-Waite Research
Reged: 06/11/07
Loc: New Jersey
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Re: Making twin 20" quartz mirrors
[Re: kfrederick]
#3625194 - 02/14/10 11:24 PM
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I would grind the back flat.
You are right, but I don't have to with these blanks. They are already flat. I measured both at 13 random spots with my spherometer using a 5" head. Average deviation from the reference flat was .000052". All the deviations were concave. I don't know how these two blanks were machined, but whoever did it knew what they were doing! It's a beautiful piece of work.
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Gordon Waite
Vendor-Waite Research
Reged: 06/11/07
Loc: New Jersey
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Re: Making twin 20" quartz mirrors
[Re: rboe]
#3625212 - 02/14/10 11:32 PM
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Quote:
That center hole should be able to play a nice role in a mirror mount. Not exactly a conical mirror but I bet you could do away with the strap.
An excellent point. I normally use a cable sling on my test stand, and that won't work given the slots milled in the edges of the blanks. I had experimented a lot with test stand supports before settling on the cable sling. But I'm going to take your suggestion to heart and knock together a lug to support the mirrors on the test stand using the center hole. I'll still put the cable sling under the bottom as a safety, though, but I won't let it carry any weight. Should be interesting to see if mounting by the center hole does as well against test-stand 'stig as the cable sling.
Thanks!
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kfrederick
Carpal Tunnel
Reged: 02/01/08
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Re: Making twin 20" quartz mirrors
[Re: Gordon Waite]
#3626259 - 02/15/10 01:37 PM
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Maybe put the cable sling in the finger slots
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kfrederick
Carpal Tunnel
Reged: 02/01/08
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Re: Making twin 20" quartz mirrors
[Re: kfrederick]
#3626339 - 02/15/10 02:18 PM
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MY first 20inch I made is a f5.25 when i did the math for my eyepieces the numbers are odd !! The numbers are 666 mag for the 4 mm 555 mag 4.8 nangler 444 6mm 333 8mm 222 for the 12mm 133 for the 20 and 66power with my 40 mm kevin frederick
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Gordon Waite
Vendor-Waite Research
Reged: 06/11/07
Loc: New Jersey
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Re: Making twin 20" quartz mirrors
[Re: kfrederick]
#3626542 - 02/15/10 03:58 PM
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Quote:
Maybe put the cable sling in the finger slots
Easy enough to try your idea. I can compare that versus the support via the central hole. Thanks!
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Gary Fuchs
Pooh-Bah
Reged: 05/22/06
Loc: Easton, PA, USA
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Re: Making twin 20" quartz mirrors
[Re: Gordon Waite]
#3626946 - 02/15/10 07:05 PM
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Hi Gordon,
Nice (big) project, and nice documentation.
Gary
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BradleyB
professor emeritus
Reged: 09/01/06
Loc: Olympia, WA
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Re: Making twin 20" quartz mirrors
[Re: kfrederick]
#3626957 - 02/15/10 07:11 PM
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That may be because your f/l is 2 2/3 meters.
Brad
Quote:
MY first 20inch I made is a f5.25 when i did the math for my eyepieces the numbers are odd !! The numbers are 666 mag for the 4 mm 555 mag 4.8 nangler 444 6mm 333 8mm 222 for the 12mm 133 for the 20 and 66power with my 40 mm kevin frederick
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