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Secondary Mirror Blank Source

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#1 Rainney

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Posted 02 June 2020 - 04:09 PM

I have made quite a few mirrors over the years. I have never made a secondary flat mirror. I have a 16” mirror, and want to make the secondary for it. I thought I would start off with a small 1.3” for one of my 8” telescopes. My mistakes will be less painful and expensive with the smaller mirror. My problem so far is finding blanks. I have tried Newport Glass, but they just don’t seem to want to get back to me. I can’t find blanks anywhere else. I have never had good luck drilling glass, so I don’t know about making my own blank. Is there anyone out there that actually sells secondary blanks?

#2 Steve Dodds

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Posted 02 June 2020 - 04:25 PM

United lens has them


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#3 JohnH

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Posted 11 June 2020 - 09:42 PM

You could always regrind one that has a scratch
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#4 stargazer193857

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Posted 11 June 2020 - 10:22 PM

I have made quite a few mirrors over the years. I have never made a secondary flat mirror. I have a 16” mirror, and want to make the secondary for it. I thought I would start off with a small 1.3” for one of my 8” telescopes. My mistakes will be less painful and expensive with the smaller mirror. My problem so far is finding blanks. I have tried Newport Glass, but they just don’t seem to want to get back to me. I can’t find blanks anywhere else. I have never had good luck drilling glass, so I don’t know about making my own blank. Is there anyone out there that actually sells secondary blanks?


Please tell us about your glass drilling experience.


I'm pretty sure your 8" needs more than a 1.3" secondary. In the most optimistic scenario, a 0.5" gap from aperture to tube, 4" for low profile focuser focal plane to tube, and 4" half aperture, is 8.5". Suppose f9 for 0.95". Plus 0.5" for reasonable 100% illuminated area. That is 1.45". Ok, 1.3" might work there. But for 8" f6 and standard focuser, 2" is needed. A 1" gap is better.



What size blank do you want for the 16"? What material?

#5 stargazer193857

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Posted 11 June 2020 - 10:27 PM

https://www.unitedle...est-plates/19mm

Looks like you might have to contact them. I don't see them listed. You could polish a small blank and core, but it would be thick.

Please update us on their availability and pricing.

#6 Garyth64

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Posted 12 June 2020 - 12:35 PM

An 8" f/9 with the secondary 8.5" inside the focus, the secondary should be about 1.5".

 

For an 8" f/6, the secondary should be about 1.77.

 

(according to Texereau)


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#7 arcainemachinest

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Posted 12 June 2020 - 01:38 PM

Texereau covers making a secondary in his book. swift glass in new york can water jet cut an oval blank for you, i would have them do a few as the price is based on setup time. they are used to dealing with telescope makers.



#8 Rainney

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Posted 13 June 2020 - 11:38 AM

Thanks for all the information. My experience drilling glass has been with aquariums. It has gone bad. lol.gif

I could not find any blanks, so I switched gears. I’m hoping to turn one of these into an optical flat. I started grinding the backs, which were the worst sides, first. I’m using 120 grit, center over center with about 1/3 overlap for five minutes at a time. At 2:30 I add a pinch more grit. My B had a swimming pool size depression in the middle, that I thought would never go away. Should I finish the backs to polish, and then start on the fronts, or should I work the front and back through each grit at the same time? Any advice or recommendations? Is my strategy completely wrong, and I’m just making hockey pucks? 

 

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#9 Garyth64

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Posted 13 June 2020 - 01:45 PM

You really could just leave the backs alone on those blanks. 



#10 Rainney

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Posted 13 June 2020 - 02:30 PM

You really could just leave the backs alone on those blanks. 

What I consider the fronts have clear sections, but the backs were pretty far from clear. The picture doesn’t do justice to the horrible finish.  Wouldn’t both sides need to be clear? 



#11 davidc135

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Posted 13 June 2020 - 06:10 PM

Rainey  How will you test the diagonal/flat? You'll need to be able to look through either the back of the diagonal or the back of the test flat to see the fringes.  David


Edited by davidc135, 13 June 2020 - 06:14 PM.


#12 Rainney

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Posted 13 June 2020 - 06:11 PM

 The backside of a mirror or blank does not have to be clear.  Why do you think it does?

 

It would only serve for aesthetics.  The backside doesn't affect the front side.

 

EG:  Here a picture of my 6" mirror.  You can see that the backside is ground.  And do you see the big chip on the edge?

 

attachicon.gifmirror.jpg

 

In grinding with 120 grit, I chipped the edge.  No problem, I flipped the mirror and started grinding over again.

 

I guess, if the backs were really rough, you could grind them a bit to smooth it out.  But no sense to go past 120, and to me, insane to polish it.

This is for an optical flat, not a mirror. Unless I’m completely wrong, the flat side goes towards the object being tested, and you view the fringes through the glass. If the back side is not clear, how could you see the fringes?  I have never worried about the back side of mirrors, but this is my first time trying an optical flat. I would not be surprised if I am completely wrong, but this is kind of exciting.



#13 Garyth64

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Posted 13 June 2020 - 06:13 PM

Aha, my bad.  I believe I over looked that you were making a diagonal, and needed to see to the front to see the fringes.

 

So forget everything I said, sorry.


Edited by Garyth64, 13 June 2020 - 06:14 PM.


#14 Rainney

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Posted 13 June 2020 - 06:16 PM

Rainey  How will you test the diagonal? You'll need to be able to look through either the back of the diagonal or the back of the test flat to see the fringes.  David

This is all new to me. I have read so many techniques about testing an optical flat. I think my best bet may be to buy a Russian optical flat off EBay to test my optical flat to test my attempts at secondary mirrors. Quarantine may be affecting my thinking process. 



#15 Peter Nance

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Posted 14 June 2020 - 02:30 AM

Please clarify you are making round disks, not 45 degree elliptical diagonals?

 

How are you going to shape them?

 

I can send you a blank 1.3" minor axis for free.

 

Pete



#16 arcainemachinest

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Posted 15 June 2020 - 12:49 AM

Texereau did say that american made float plate is almost perfectly flat is this still true? If so , could one could block a few 1/2 inch thick bits up on a 45 and trepan 3 out. And then test them with the ball bearing test, and figure the best of three with pitch laps?



#17 Rainney

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Posted 15 June 2020 - 09:57 AM

Please clarify you are making round disks, not 45 degree elliptical diagonals?

How are you going to shape them?

I can send you a blank 1.3" minor axis for free.

Pete


I am making round discs. I switched gears when I couldn’t find any other blanks. Thanks for the generous offer. I would be happy to pay you for a blank.

I have them ground down to 5 micron, and ready to polish. They look so nice when they are wet.

#18 stargazer193857

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Posted 15 June 2020 - 10:39 AM

Texereau did say that american made float plate is almost perfectly flat is this still true? If so , could one could block a few 1/2 inch thick bits up on a 45 and trepan 3 out. And then test them with the ball bearing test, and figure the best of three with pitch laps?


I read the waves have amplitude 5 thousandths of an inch. But I've also read people hunting over the glass for a flatter patch to cut out.

Either way, you need a way to test it. Also, the sheet is not supported well and so will be forced, foiling your test till after you cut somewhere.

It would be nice to know what grit size can be started on with float glass. I'm guessing 9 micron. I thought I read 25 micron, though Gordon Wait sometimes starts with that on generated curves and spin grinding his parabolic mirrors.

#19 ed_turco

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Posted 15 June 2020 - 11:46 AM

You might check out Surplus Shed.  The last time I looked, they had a few diagonal blanks.



#20 DAVIDG

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Posted 15 June 2020 - 12:03 PM

 Having made flats, you can save yours sometime by grinding  the ones with large depression first against  a metal plate to get them relatively flat.  If you make yourself a small spherometer you can zero it against one of  the surfaces your grinding flat and then test the other surface you used to grind against.  So instead of three surface to grind and doing the ABC method your just grind two surfaces. When you zero the spherometer on one surface and test the other your measure 2X the difference from being mechanically flat. The sign that you measure tells you if that surface is concave or convex . If is concave you put that disk on the bottom and other on the top which has to be  convex and grind a few wets to flatten it out. The goal is to keep reducing the reading you get with the spherometer until it is zero. That means that both disks are mechanically flat and within a handful of waves from optically flat. Now you can polish the surfaces and testing by interference against a known  Master surface  to get them optically flat.

 

                    - Dave 


Edited by DAVIDG, 15 June 2020 - 03:39 PM.

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#21 Rainney

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Posted 17 June 2020 - 07:16 PM

I polished the back of one piece. There is a kind of clear window on the front, which I have not touched yet. I thought, why not throw it on something and see what happens. I was super excited to actually see some fringes under a normal fluorescent light. I know it doesn’t mean much, but it sure is encouraging. 

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