Re: [ATM] Mirrors

[Mark Holm <mdholm@telerama.com>]

Hi Chris,

Yes there are web pages, but honestly, the best introduction is still 
found in books.  The long time standard is Jean Texerau, "How to Make a 
Telescope".  It is getting a bit long in the tooth.  Many of it's 
methods are a bit out of date, but it still gives a clear overall 
picture of the process, and if you were to follow it faithfully, your 
chances of success would be good.  You would only do some things less 
efficiently than you otherwise might.

Another, much more recent book is "*A Manual for Amateur Telescope 
Making" by Karine and Jean-Marc Lecleire.  I haven't read it, so can not 
voice an informed opinion.  Perhaps someone else can say whether it is 
good.  I understand it is aimed at beginners.

Both books can be purchased from Willman-Bell   www.willbell.com  They 
are a reputable firm, in business for at least 20 years, specializing in 
books for amateur astronomers and amateur telescope makers.

There are a bunch of online resources, but none is really complete 
enough to give a good picture of the whole process.

A couple of areas where using more modern methods is a help are:

The grinding tool.  When you have read the basics,  inquire on this list 
about hogging tools and tile in dental stone fine grinding tools.

Pitch lap making.  This is a potentially messy and frustrating task.  
Inquire again after you have read the basics.
*

Foucault and Ronchi testing.  Several online resources as well as feedback from list members are available.  The methods haven't really changed so much, but it is a challenging subject.  The majority of beginners find they need some help.  There are free computer programs that help with various aspects of both tests.

Figuring.  List feedback is very helpful.  Even experienced mirror makers sometimes find figuring challenging, so a beginner should feel absolutely no shame in asking for help.

I know both books give instructions for making a diagonal (flat) mirror.  It really isn't in a beginner's best interest to take this on.  The majority of atm's buy their secondaries, though it is certainly within reason to make them.  A first timer should buy.  A good source is protostar.com.  Reputable and good quality.  There are other good quality sources.

Please don't get aperture fever before making your first mirror.  And don't try to make a very thin one either.  Stick in the diameter range 6 - 10 inches and a thickness ratio of 1:6 - 1:10.  You will greatly increase your chances of success, and, if you really want to make a bigger one after that, the experience on the first will be invaluable.  We have had several people pop up here on the list who tried an overly ambitious project the first time out, and got themselves into difficulties they could have easily avoided with the experience gained from a less ambitious, smaller project.  Many, many atms over the years have said that one can make a smaller first mirror, and a larger second one in the same or less time as it would take to make just one large one.

My own recommendation for the first attempt would be to use a Pyrex mirror blank.  It is worth the few dollars extra cost to have the higher thermal stability.  The thermal stability is a good thing during figuring and testing.  It can help avoid a cause of problems that have plagued some beginners, and some more experienced workers.  You may read about people making mirrors out of candle holders, ash trays and other "cheap" sources of glass.  Don't do it.  Not the first time out.  You have enough to learn without fighting the quirks of non standard glass.

Willman-Bell also sell good quality mirror kits.  The only peculiarity is that they include two Pyrex blanks.  One of these is intended as a tool.   We can show you a better way to make tools, and you can save that very fine second piece of Pyrex for another mirror.  For some reason, W-B do not advertise the mirror kits online.  Ask for a copy of their paper catalogue.

You can also buy a mirror blank from United Lens in Mass. or Newport Glass in Cal.  Newport also sells kits.  You can buy the glass from one of these and then get the rest from gotgrit.com.  Run by an atm, gotgrit is a reputable business.  gotgrit also sells dental stone and tile for making a tile tool.

www.atmsite.org has a lot of online resources, but is not complete or organized for a beginner.

Others on the list can suggest other online resources.

-- 
Mark Holm
mdholm@telerama.com


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