Idiots' guide to grinding?

[dav1d]

I'm buying a single 6" mirror blank in a few days, from which I intend to fashion a primary which will form the core of a f10 /f11 reflector.

I've read around and have come up with an incomplete picture of what I need to know/have/do.

What I know :

I need a tool (made of cement with tiles embedded) and a sturdy table or barrel on which to work.

I need to make strokes towards the centre and rotate regularly.

Polishing is involved.

What I don't know:

How to measure the curve of the mirror

How to grind in a way which will result in a particular F ratio.

Everything else.

I have a fair amount of free time over the summer and want to start as soon as I can, but not if it means I ruin the mirror because I don't actually know what I'm doing.

Is there a step by step 'how-to' which can tell me all the necessary steps, in which order to do them and basically just fill in the huge gaps in my knowledge. Oh, and tell me how to start.

[pixelsaurus]

Something like?

http://stellafane.org/tm/atm/index.html

[JB80]

Stellafane is a really good resource and you can find loads of info there.

Also the two books that I'm reading before starting are http://www.amazon.co...d/dp/006181394X

and http://www.willbell.com/tm/tm3.htm with the second book being maybe the most highly regarded of them.

Having the books handy will probably be better as you can have it next to you without the need to keep running back to the computer, or at least that's my theory anyway.

Sorry I can't help you beyond that as I'm pretty much at the same point you are.

[Glasspusher]

Have a look at ny thread on grinding a 12 inch mirror, hopefully you will find some useful stuff there. Good luck with the project.

John

[Beulah]

Another book to recommend is one that a good friend on SGL let me borrow for a bit. Yep, it's a bit old, coming from the days where there was real excitement for ametuers being able to grind their own optics but I love it and has all the info you need.

http://www.amazon.co.uk/Handbook-Telescope-Making-N-Howard/dp/0571046800

[SamAndrew]

I found this video useful

You should be able to find some other useful videos on youtube,

I need to get learning as my 6" kit should be arriving very soon

[dav1d]

Thanks guys. A great deal of stuff to look at there. No doubt I'll be back on once it's underway

[nytecam]

Follow the guides given above. Just a couple of points - if you can afford it a second glass disk same size as the mirror would be good. Concrete+tiles ??? At f/10 a spherical mirror ok - no need to parabolise. If you wet the mirror [as you proceed] with the grinding you'll get a reflection image back of say a LED torch - the fl is half mirror radius as measured. The coarse git used until correct curve is ground. Cleanliness is the key to a scratch-free mirror as you work through the various finer gits prior to polishing. Keep us posted with your progress

[fwm891]

If your intending an f10 then avoid the coarse grits and start with 120 or 320 grit as your central depth is only 37.5 thou ( mirror radius^2 / four times the focal length) 3^2 / 4x60 = 9/240 = 0.0375

I think thats right...

[nytecam]

If your intending an f10 then avoid the coarse grits and start with 120 or 320 grit as your central depth is only 37.5 thou ( mirror radius^2 / four times the focal length) 3^2 / 4x60 = 9/240 = 0.0375 I think thats right...

Good point !