Really slow f-ratio mirrors

I'd have to disagree!

I would certainly bow to your superior knowledge and experience. I can imagine that finding somewhere to test an f/30 mirror would be next to impossible anyway.

Quick question. If you had a reference flat of very high quality could you use this in the testing to reduce the length of the testing tunnel?

My Opinion:
Easier to recruit..... Nature...or Commerce.... than to waste time grinding your own glass for months.

Nature:

At that f ratio a small vacuum chamber behind the mirror should pull it into a perfect paraboloid.

In theory at least.
(They tell us that glass is not a real crystal at all, it’s just a liquid which flows infinitesimally slowly at room temperature, so it should “slump” into a paraboloid with vacuum pressure (or lack therof.) from behind.

Commerce:

But why waste months of your time and money?

Meade and Celestron will provide you with far better optics for the price of a couple of weekends overtime.

P.S. What sane person would build their own motorbike when there are Hondas and Yamahas in the shops?

OK, slight change of plan.
What I have in mind is a slightly unusual project, hence some slightly unusual questions below:
I'm looking for a 6" singlet or achro lens of f/30.
Is it possible to get this anywhere?

Also, I'm wondering is it possible to make a diagonal with a low level of reflectivity but accurate figure?
If you have a piece of glass which does not have a mirror coating applied to it, when it is positioned in a strong beam of light, will it reflect any light at an angle of 45degrees?
I have heard of people using a large aperture newtonian with an uncoated mirror and using it to observe the sun (with further filters). I'm wondering if this priniciple of reflectivity of uncoated glass applies to flats at 45degrees as well.

I have heard of people using a large aperture newtonian with an uncoated mirror and using it to observe the sun (with further filters). I'm wondering if this priniciple of reflectivity of uncoated glass applies to flats at 45degrees as well.

Madness which could strike you blind.

Buy something from commercial operators who have studied the EXTREME risks.

Coronado is one.

P.S.

Four percent of the light striking a piece of uncoated glass will reflect off it.

If this 4 percent hits your eyes you can be blinded

For a full explaination read Richard P. Feynman's book:

"QED The Strange Theory of Light and Matter ".

Chapter 1.

www.amazon.com/QED-Strange-Theory-Light-Matter/dp/0691024170

P.S.

Four percent of the light striking a piece of uncoated glass will reflect off it.

If this 4 percent hits your eyes you can be blinded

For a full explaination read Richard P. Feynman's book:

"QED The Strange Theory of Light and Matter ".

Chapter 1.

www.amazon.com/QED-Strange-Theory-Light-Matter/dp/0691024170

Thanks Peter.
Don't worry. I'm only doing a hythetical design of an optical system at this stage. I'm aware that we need to protect the eyes to the equivalent of ND5.