Buying Glass

JJDOBBER79 (Jas) · #1 27-07-2013, 08:55 AM

Hi There,
I am thinking of getting started on my first scope. In regard to buying glass, can I just go to my local glass merchant and buy a piece of glass or does it have to be a certain type/quality/grade etc. I just want to start on a 6" but I dont want to do all the work and find out that I used the wrong type of glass. Thanks in advance.

cfranks (Charles) · #2 27-07-2013, 09:18 AM

3/4" Plate glass will be OK for your first telescope. I suggest you go for an 8" scope rather than a 6", the cost is just about the same, the work is just about the same but you get twice as much light at the eyepiece. You will need 2 pieces of glass, one for the mirror and one for the tool.

Charles

Rod · #3 27-07-2013, 09:32 AM

Hi Jas

Pyrex Or plate glass that is about 1 inch thick is the traditional size and type for a 6 inch first mirror. Many people start with an 8 inch of 1 to 1.25 inches thickness.

Most glaziers will get glass up to 3/4 inch thickness and that should be OK too as long as it is properly annealed. Most that I have tried seem to have difficulty cutting the glass to a disk that small. I would recommend purchasing a blank from an astronomy club or an ATM supplier. In that size they are not that expensive. You only need one good blank Make the tool from ceramic tiles and plaster or cement. Or if you can get a disk of plate glass about 1/2 inch thick that will make a good tool too.

There is a mirror making thread that has been stickied in this forum. You may want to post your questions there.

Make sure you check the prices of new mirrors against the cost of making your own. I enjoy making mirrors but I started doing it when the cost saving was significant. That's not really the case now. So do it because the process interests you or because you want the skills to tackle more complex projects later.

Good luck.

Rod

Satchmo · #4 29-07-2013, 06:03 PM

I concur that 8" is a better choice - fits the hands better - and if you are careful 3/4" glass from the local glass merchant is fine . It is usually pretty well annealed . A second disc say 1/2" with ceramic tiles epoxied on makes a good grinding tool . F8 would be a good choice .

Obviously with coating costs at around $200 and that you can get a finished Chinese mirror for $250 - you are in it for the pleasure of viewing the sky with an optics fashioned with your own hand !

JJDOBBER79 (Jas) · #5 29-07-2013, 07:32 PM

Thanks Guys, your advice is great as always and I will no doubt have a million more questions if I decide to go down this road. Is there a simple formula for working out the focal length from the diameter of the primary mirror?

Rod · #6 29-07-2013, 08:01 PM

Hi Jas

I suggest you have a read through the stellafane site on mirror making to learn the basics. Here is their section on determining the focal length:

http://stellafane.org/tm/atm/grind/measure.html

You can have any focal length you like with any diameter mirror. In practice most mirror makers with 6 to 8 inch mirrors got for a focal length 6 to 8 times the mirror diameter. That gives a good general purpose telescope and is relatively easy to make.

Rod