Amateur telescope making

No, but I did give it serious consideration a few years back. Would have been an interesting project. What put me off was not the polishing and grinding but all the other telescope bits and bobs needed.

I ended up buying a second hand 6" scope.

I'd like to make something else now but I can't think of anything practical. Speakers maybe?

Yes. Why?

Nime

How easy was it in the end? I assume you used the Focault test - did you have the help of somebody with that or were you able to figure it out from instructions? Did you use Pyrex or glass?

Deane

I've made mirrors up to 16".
Achromats up to 5".
Optical flats up to 8.5".
Loads of lenses and a few prisms.

I taught myself using elderly ATM books as a guide. Started building telescopes when I was 14. Haven't done much optical work for years now though I sill observe and build kit.

Pyrex is much easier to work with. Though it is much harder and tougher than plate/float. Use a plate/float glass tool and a Pyrex mirror blank. Pyrex can be dunked in water without fear of it breaking due to thermal shock.

Look in the library for suitable books with instructions on mirror making. Or just search online. There's a vast resouce of ATM information online these days.

I used the Foucault test, Ronchi test and others. It's not difficult to make an 8" (20cm) mirror which I would call the preferred size rather than the recommended 6" first mirror.

It could take a couple of weeks or several months depending on your time available. Superb therapy too!

Don't make a mirror to save money. You can buy a good commercial mirror of guaranteed quality these days. Orion(UK) has an excellent reputation and will even test a home-made mirror if you send it to them for aluminising.

If you really want to go ahead buy a mirror kit from an amateur astro instruments supplier. You usually get a Pyrex blank, a plate glass tool, all the abrasives and pitch for polishing using Cerium oxide polishing powder.

I think you are right - with buying a primary rather than making one. I read throught ATM by Ingalls years ago - I am attracted to the meditative discipline of gringing my own mirror - but, a bit like Stephen B, am put off by the rest of the task; and I know from experience that, when all is said and done, a good, sturdy and accurate mount and drive is needed.

I confess I'm completely gobsmacked that it's possible for an amateur to grind a mirror. How accurate can it be? What sort of pictures do you get?

My hat goes off to you folks.

Tom

It is possible, with patience, to grind a mirror with a parabolic curve accurate to a twentieth of a wavelength of light (from memory). (Again, from memory), the Focault knife-edge test will show the slight swelling in the glass/pyrex from the heat left from a fingerprint as an enourmous bulge. The test is so accurate that it is, for all intents and purposes, perfect.

If you get two equally sized disks of glass, put one on top of the other, with the bottom disc fixed in position, and a grinding medium between the disks, and start sliding the top piece around on the bottom piece with an irregular, circular motion then the grinding medium will grind the top one concave and the bottom one convex.

It really is that simple.

Deane

Nime may want to correct me.

Whatever you say Deane, my hat's still off. How about showing us some pics?

Hell, I'm just contemplating it. Nime has actually done it - I hope he does post some pics.

Nime?

Your description is accurate enough Deane. No great skill is involved merely a great deal of patience and cleanliness.

It is the random nature of sliding the mirror back and forth (with only hand controlled stroke lengths) on top of the tool which achieves an accurate curve. Shaving mirror style.

The rough curve is achieved with coarse abrasive then ever finer powders are used in series to smooth the glass to an almost transparent state. Each step of five or six finer powders takes about half an hour. Cleanliness is essential at all times to avoid scratching the surface.

Finally pitch is poured onto the warmed tool and polishing begins in exactly the same way but using a very fine powder called Cerium oxide.

The surface is polished to a perfect sphere first and then slightly deepened to a parabolic curve with slightly longer strokes or W-shaped strokes. This deepening (figuring) is checked visually with a pinhole light source at the centre of curvature using a straight edge to cut the light returning from the mirror to the eye.

While it is true that the Foucault test is incredibly sensitive it is not a null test. So the results are still subject to human error.

You need look no further than Google Image search for <mirror grinding> to get some idea what is involved.

Nime - can you post some pictures of your efforts?