Gray
professor emeritus
Reged: 01/31/11
Loc: Hixson, TN
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20" mirror blank question
#5600841 - 01/02/13 08:29 AM
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I've been looking into ATM. I've found a 20" blank made of plate glass that is 1" thick. They say to use thin mirror techniques for attempting this project. I would like to grind this out myself to around f/5. I'm wondering if this blank is too thin and can not be done or is it worth the time and cash. Thanks in advance. 
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dpwoos
Pooh-Bah
Reged: 10/18/06
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Re: 20" mirror blank question
[Re: Gray]
#5600878 - 01/02/13 09:08 AM
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There is no way that I would tackle this job as your first attempt. No way.
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Norm Meyer
member
Reged: 02/08/09
Loc: Warren, ME 04864
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Re: 20" mirror blank question
[Re: dpwoos]
#5600889 - 01/02/13 09:25 AM
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Yes a 20" x 1" glass can be made into a mirror. No
you shouldn't try it as a first mirror project.I've made
dozens of smaller mirrors and I would think about it
very carefully. You should start out with a 6" or 8" mirror
for a first project...maybe even several of them.
Regards Norm
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73pitch
newbie
Reged: 02/25/12
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Re: 20" mirror blank question
[Re: dpwoos]
#5600896 - 01/02/13 09:38 AM
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Please don't do a 20" mirror as a first project. I would suggest an 8" to 10" mirror first. Even better, make an 8" AND a 10" and then the 20". Get the glass for the 20" and put it in a prominent place so you can look at it while your working the smaller mirrors. Seriously, make a smaller mirror first!!! It will take AT LEAST twice as long making a 20" first then it will making an 8" and then the 20". I know this to be true through hard won experience.
Dave
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gpelf
super member
Reged: 12/28/11
Loc: Kentucky
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Re: 20" mirror blank question
[Re: 73pitch]
#5600934 - 01/02/13 10:16 AM
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"DITTO". Speaking from the perspective of a Newbie, I am just finishing up a 8" F8.6 as a first mirror project and while the "ATM`rs are right, an 8" is doable for a first time mirror project. It is the "finesse" factor that holds the learning curve (Learning what your own style produces in the way of Grinding, Figuring, Reading tests and e.t.c.)
One of the most fun exercises in futility that I have ever done !!! I dont mean to scare or sway your decision, I am NOT saying you cant do it, But I dont think I could right now after finishing my 8" 
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Mirzam
Post Laureate
Reged: 04/01/08
Loc: Lovettsville, VA
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Re: 20" mirror blank question
[Re: gpelf]
#5601127 - 01/02/13 11:52 AM
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That blank is too thin for a 20" mirror. Maybe could be used as a tool.
JimC
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Pinbout
Post Laureate
Reged: 02/22/10
Loc: Montclair
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Re: 20" mirror blank question
[Re: Mirzam]
#5601222 - 01/02/13 01:01 PM
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make a sphere then flex it parabolic. 
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JohnH
Carpal Tunnel
Reged: 10/04/05
Loc: Squamish BC Moved!!!!!
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Re: 20" mirror blank question
[Re: Pinbout]
#5601459 - 01/02/13 03:45 PM
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The old adage for mirror making is that it takes the same amount of time to make a 6" mirror THEN a 12" mirror, rather than making a 12" mirror straight off.
The reason being, you learn so much doing the first few, that it actually saves time later on as you waste less time
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sopticals
sage
Reged: 03/28/10
Loc: New Zealand
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Re: 20" mirror blank question
[Re: JohnH]
#5602002 - 01/02/13 09:36 PM
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It is a doable project. Better to gain some experience by making a 10"-12" first. John Dobson made at least one 24" that was only 3/4" thick (plate glass), and others have done the same. I have usable 22" and 25" mirrors made from 3/4" float glass. Admittedly as the "skinnyness factor" increases difficulty to establish a satisfactory figure increases. A 20" mirror is about 8x as difficult to complete as a 10".
Stephen.(44deg.S.)
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Gray
professor emeritus
Reged: 01/31/11
Loc: Hixson, TN
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Re: 20" mirror blank question
[Re: sopticals]
#5602425 - 01/03/13 06:58 AM
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Thanks for the wisdom folks. I'll put it on the burner and let it settle down some. I won't start with a 20", I promise. Thanks again. Graham
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Achernar
Postmaster
Reged: 02/25/06
Loc: Mobile, Alabama, USA
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Re: 20" mirror blank question
[Re: Gray]
#5607191 - 01/05/13 07:39 PM
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Definitely try making an 8-inch mirror first, and work on your skill by making a couple of truly good 12-inch mirrors before trying a 20-inch. A one inch blank is very thin, possibly too thin for a 20-inch, and definitely way too difficult for a first time mirror maker on the basis of the size and the thinness of the blank. Think blanks can be used to make excellent mirrors, but you must be careful how you grind and polish them to avoid severe astigmatism. Even if you made it successfully, you will have to support it evenly in its cell for it to perform well. I considered taking up mirror making, but I decided to pay an optician to make the mirror for my 15-inch, which is a thin, lighterweight mirror. It was a wise decision to leave that job to an expert.
Taras
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