Re: ATM extremely long focal length parabolic mirror

["Bob May" <bobmay@nethere.com>]

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@nethere.com
NEW! http://bobmay.astronomy.net