Re: [ATM] lensmaker for a very long focus objective

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

Putting the lens on a flagpole would be a poor thing to do.  Better to put
it on a pintle or something else much more solid.  The image scale would be
huge as the standard is basically 1" for a 1 deg. FOV for a 108" Focal
Length.  100' would end up with about 1/12th degree FOV for a 1" eyepiece
aperture.  IN addition, a black background around the lens would be a wise
thing to do so that contrast will be higher - without a black background
around the lens, contrast can be so low as to make anything but bright
lights hazy and hard to see.  The telescope may not become really useful
until night falls if you're outside.  Thus you may have to end up with a
tube for several feet to limit the view with a black surround of the lens of
several feet in diameter.
Next you run into a problem with magnification where the max magnification
of the objective aperture would be about 200x (4" lens) which means a 6" FL
EP.  This will, for a Hygunens EP will probably produce about 3" eye relief
which will mean that most viewers won't really see the full view as they
won't know to put their eye there.  The option does exist of making the EP
so that the user's eye is forced to that distance but then you run into
sanitary issues which will need to be addressed on a regular basis, probably
hourly.
I don't know how far the Sears Tower is from the Museum but I'd suspect that
something a fair bit shorter will be a lot better to use than 100' of FL for
the objective.  For my 200" refractor, anything within two miles or so is
close enough to change the true focal position from that of infinity.
I'd also more consider that the objective be a fair bit larger than your
suggested aperture in order to obtain better magnification possibilities as
well as better access to the view.
As to the actual FL of the lens, I could consider making one but will prefer
that you not start assembling the rest of the scope until you have the
actual optics in hand as focal lengths of this length are hard to do to any
really exact length as the curves are very gentle, almost being flat on both
sides of the lens.  Any little error in the surface will end up changing the
lens from a nice spherical lens to something much more strange.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net


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