From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


For a start, the time it takes to rough grind a mirror is a fairly small
part of the grinding process.  For a typical amateur mirror, slightly less
than 1/2 of the grinding time is coarse grinding.  Polishing time is
approximately the same for all focal ratios and the figuring time is a wild
variable depending upon the system used for figuring and the accuracy of
the worker.
I'll also not that the 40 meter focal length 1 meter mirror is going to be
1/4 wave accurate with just a spherical surface so you aren't going to be
having to figure very much.
The focal length doesn't affect the "quality" of the surface so
it doesn't matter what focal length you use.
I would suggest that you look at the Airy disc formula and the angular
scale of that mirror when you look at the resolution that you are getting. 
The Airy disk will set the size of the spot that you are going to get for a
star point while the focal length will set the actual angular resolution. 
It does not good to set the angular resolution to a quarter arcsecond and
have a 2 arcsecond Airy disc.
Also, I'd not worry too much about the construction of standard optics as
most of it goes fairly quickly with the final angstrom level figuring the
only really difficult part of the process. Set the secondary's focal length
to what you feel is right according to the details of what you are trying
to see and let the glass get cut according to that without fear of it being
anything particularly difficult. I will note that 40 meter focal lengths
are a bit difficult to test as the long tunnels really aren't part of the
opticans supply of tools.  Going from there to a flat that you will stress
to a paraboloid probably wouldn't be that far anyway.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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