From: 3D{at}taconic.net
To: atm{at}shore.net
Reply-To: 3D{at}taconic.net


When a short focal length mirror is being made, more glass needs to be
removed from a flat than a longer focal length mirror. The shorter focal
length increases the
time it takes to grind and polish. Very long focal length mirrors ought to
be less time
consuming to make, but the tube needed for longer focal lengths is less convenient,
because primary focus is not near the center of gravity, and rigid structures are
difficult to maintain at great length to say nothing of the many problems associated
with pointing them. Folding mirrors can bring the focal point back to a
more convenient location, but these add to the optical train and introduce
losses and
distortions. Even with folding mirrors, the benefit in tube length is
typically 2:1, a
relatively small gain.

I am designing a telescope that uses a diffraction grating as the primary
collector. An
ordinary parabolic mirror telescope with a spectrometer forms the secondary. This
design is detailed in three papers that are on line as PDF files. They are
hyperlinked
at:

http://home.earthlink.net/~scan3d/html/Dittoscope.html

Currently I am making a model using Oslo LT. Being a computer program, Oslo lets
me model a parabolic secondary mirror with an extremely long focal length. Since the
primary grating would be long and narrow and rest on the ground, and the device has
no moving parts, it seems feasible to make the parabolic mirror with a
focal length as
long as the grating. The initial computer model has a 1 meter mirror with
focal length
of 40 meters.

I am asking the list to advise me on a couple of issues. Firstly, I need to know what
would happen if the grinding operation was shortened so dramatically as to make a
40 m focal length for a 1 meter diameter mirror. Would this imply a poor figure?
Secondly, I am interested in any gain in acuity. What there be a meaningful increase
in the angular resolution using scopes with such long focal lengths?

Please refrain from flames. I know this is theoretical. If it doesn't interest you to
speculate on radical concepts, just let me work out the numbers on my own.

Tom Ditto

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