Really slow f-ratio mirrors

Just wondering:

Anyone know where you can get a realy slow f/ratio mirror? Something in the region of f/30?

Cheers,

If you fancied making one it would be the easiest thing in the world. at f/30 you'd hardly be taking any glass out at all and you wouldn't even have to parabolise it. A sphere would do very nicely.

If you fancied making one it would be the easiest thing in the world. at f/30 you'd hardly be taking any glass out at all and you wouldn't even have to parabolise it. A sphere would do very nicely.

I'd have to disagree! I made a 14" f/15 sphere for part of my folded Stevick-Paul scope (coming along nicely, by the way!) and I found figuring such a shallow curve to be very frustrating indeed. A steeper/faster sphere is much easier to produce. At f/15+ the surface becomes super sensitive to hand pressure and stroke pressure during figuring, and removing all trave of primary- and micro-ripple is a cast-iron S.o.B!!! At f/15+ the Foucault test is just incredibly sensitive and shows up the slightest deviation from true as looking like horrible caneons, gouges and divots! Also, you have to construct a full length testing tunnel to stop ALL air currents in the test path or you haven't a hope of getting a good Foucaultgram. My f/15 sphere is "done" but I will probably go back to it with a finer polishing compound like black rouge to try and tackle the residual ripple. I wants an ultra-smooth surface!

Don't let all the above stop you - give mirror making a go, it's great fun!

Phil.

Can't imagine what you would want it for.

Here is a picture of my favourite "long scope" :

amazing-space.stsci.edu/resources/explor...s/hevelius/scope.php

Can't imagine what you would want it for.

Here is a picture of my favourite "long scope" :

amazing-space.stsci.edu/resources/explor...s/hevelius/scope.php

It's just a mad idea i have for a folded optical system. Not too much commericla demand for mirrors of this f-ratio so I guess I would need to make one.

Don't let all the above stop you - give mirror making a go, it's great fun!

Sorry, too late.

Schiefspieglers seem to be very much a "German Thing".

They never really "took off" on other parts of the planet I think.

( www.wolterscope.de/fotogalerien/teleskope/index.html )