glass for secondary mirror

[dark star]

I am planning to start making a 20 inch mirror for a reflector, as soon as I have finished the 14 inch (350mm) mirror I am making (hopefully in a few weeks-I will post pictures when I am finally finished).

l would like to make the secondary mirror also. Reading on line the best approach seems to be to grind 3 mirror blanks together in turn to get a flat surface. Does anyone know of a good source of glass for secondary mirrors?

I was planning to buy 3 6 inch blanks, however I now realise blanks meant for primary mirrors may be too thick. The secondary mirror I bought for my 350mm is around 14mm thick, would 25mm be too thick for a secondary mirror for a 20 inch?

Could I just buy window glass and cut it up? Presumably normal window glass would be too thin?

Obviously cutting the glass to the shape of the secondary mirror, once ground, would present a challenge. There is something on line involving a drill press and concrete mold that looks pretty scary (in tems of breaking the blank)! I have been told that a hexagon shape secndary mirror is easier to cut and does not lose much extra light/cause much more obstruction.

David

[Daniel-K]

14mm for a primary blank seems really thin how deep is the sagitta? And what FL are you going for?

[Astrobits]

12mm float glass should be O.K. for the secondary of your 20".  That will give you a thickness to diameter ratio about 8-10 to 1, probably similar to your primary T/D ratio if you are going for 2" thick primary. I used 12mm for the secondaries of the 20" mirrors that I have made with complete success. Just make sure that the glass you get is not toughened if you try to source it from scrapped items ( glass table tops etc. ) which can be a good source of glass in the 12-18mm thickness range

The three surface method of making flats works very well. Just remember that there are six combinations to grind as equally as possible: A on B; B on C; A on C; B on A; C on B; C on A. I made an 8" reference flat that way. A spherometer is very useful for making flats.

Nigel

[fwm891]

I have cut a 45° secondary with a drill press (no concrete!) Make up a wooden box that will support the secondary blank at 45° use plaster of paris (or similar) to encase the secondary within the wooden box. Leave to set for a few days. Mine was 3 inch minor axis and I used a piece of 3 inch copper tube as my cutter. I made a mandrel to fit the half inch chuck on the drill press and support the copper tube then started slowly grinding away using #60 grit carborundum as the cutting medium. Things get hot so keep retracting the copper tube and flood with water to cool and lubricate things. DO NOT RUSH this.

Once you have this cut you can then work the elliptical blank against two other disks (they can be round) 1 on 2, 1 on 3, 2 on 3 etc etc...

12 mm will be fine

[Peter Drew]

Although using discs of glass in the method described by Astrobits, bear in mind that the final disc would need cutting to shape for a Newtonian diagonal. Apart from the obvious damage potential there is also the risks of releasing strains and altering the figure. 

[Astrobits]

The trick is to cut your blank and then mount it with some smaller bits of the same

thickness on a plaster tool. Then you work that with thicker glass of the same diameter. Use Francis's tube method to cut the glass but don't forget to make the tube long enough.

Nigel

[dark star]

The 350mm primary is 25mm thick, it is the secondary which is around 14mm thick.

I will start looking for a source of scrap glass-scrap yards, recycing centres? How do I check if the glass is strengthened or not? I guess there is no commercial source for small quantities of 12mm-14mm thick glass?

I am glad someone has managed to cut the glass for a secondary mirror on a drill press, so at least it is possible!

I am going to a telescope making class (Camden Telescope Making Society in London) which is very useful for getting advice  on interpereting the Foucault test-the shadows I find a challenge for a beginner. Although it is getting easier with experience. I have taken my 350mm mirror to the class a few times. But as I do not have a car I will not be taking a 20 inch mirror!

So I intend to grind the secondary mirror at the class, once I get the glass.

I have made a bath intereforemter, but have not got good enough interferograms for analysis yet. I am concentrating on the Foucault test at the moment. When I get closer to the required parabola for my 350mm mirror I will test with the interferometer as well.

David

[Glasspusher]

Hi David,

Lots of folks have difficulty with the Foucault test, it has probably resulted in a lot of people abandoning their mirrors over the years due to the steep learning curve...it is very much a skill based test.

Can I recommend the matching Ronchi test to you? Here are some notes that I put together on the test...http://www.nicholoptical.co.uk/pdf/The%20Ronchi%20Test.pdf

A combination of the matching ronchi test and star testing is well capable of producing an excellent mirror, this testing regime does not have the learning curve of the Foucault test and makes mirror making accessible to all.

Good luck and keep us updated on your progress.

John

[Sfarndell]

Hi David

I second what John says about the Foucault vs Ronchi. The results I get from the Ronchi test match what the experienced guys see at my telescope class using a Foucault test. However, I can't come close to replicating their results - it's a steep learning curve and requires a lot of patience and time. 

You're in good hands at CATS. Terry is a master.  Simon was also making 3 secondaries of +-100mm minor axis a few years ago and can give you lots of advice (or sell you one  if he finished them  ). 

Good luck! I'm looking forward to following your progress.

Scott