Foucault tester

[earth titan]

Before buying a mirror blank, I've decided to build test equipment. Just so I can drop the mirror in during the grinding and test, and cos I'm in no hurry.

I've seen lots of drawings and sketches, even photographs but still unclear as to exactly what is required. The photos and diagrams in the NE Howard book are quite poor and don't seem to help too much.

I think I have the basic principles in my head, but does anyone have a good sketch, description or diagram? Being an engineer, I like engineering drawings, so these would be good.

[Merlin66]

How to Make a Telescope

There are good illustrations in Texereau's book.

I added a small 9micron fibre to give an artificial star. I also use the Surplus Shed variable slit.

You can add a dial indicator to the knife table to make the readings easier to record. A piece of Ronchi grating also helps.

[ponytale]

these links will provide most of the info you will need to know about making and testing mirrors.

hope this helps . ps, pm me your address and i will post you some ronchi film which is easy to use ,

Mirror testing

Stellafane ATM: Build an Adjustable Mirror Test Stand

my you tube videos: http://www.youtube.com/my_videos?feature=mhum

[Glasspusher]

I would recommend a slitless tester, easy to make and works well. If you have a look here....Stellafane Links Page scroll down the page to 'Testers' you will find a lot of good stuff. Nice to see people are still making there own mirrors, good luck with the project and please keep us updated.

John

[BinocularSky]

I've made a couple. The first (moving knife-edge only) had a sliding base I made from perspex, with a Barr scale inscribed on it. The second was a slitless tester; the carriage was driven by an M6 threaded rod, which has a pitch of 1mm, with a pie-chart protractor (circle divided into 100, mere pence from WHSmiths at the time) as a poor-man's micrometer (but, in practice, the actual precision was nearer 0.05mm than 0.01mm, but still adequate for the job). No photos, I'm afraid, as I've long since given both away, since I've not made a mirror for about 15 years.

However, there's a pretty good page on them here. There are lots of images of various kinds on the interweb: try a Google image search.

[FraserClarke]

Lots of suitable links already provided. I'd agree that you should go with a slitless tester rather than the classical slit/point source tester a la Texereau. The principles of the stage in Texereau are still good though. I recently built one along those lines, and it works fine. I will at some point build a much more integrated system though, so make it easier to get the alignments between the mirror axis / knife edge / motion axis right.

Just to note though, that you won't be using a Foucault tester until well into polishing. During grinding, a spherometer will be your bestest friend ever. At the risk of self-promotion; here's one I made earlier -- http://stargazerslounge.com/diy-astronomer/101050-spherometer.html#post1409698. I possibly even have the engineering drawings for it somewhere . Not that it really needs drawings.

[earth titan]

Thanks everyone. I hear what you say TeaDwarf regarding the need for the Foucault later on, but the Howard book (which I shall be mainly following) talks through using it to check sphericity (is this a word?). I shall need one anyway and as I have most of the parts kicking around, I shall lash one together.

Got to wait for the funds (wait for the accountant to finish the books) before I buy the mirror blanks.

Here is the one I shall be making:

Looks nice and simple.