I found some more info about stopping down a mirror but no comments
about how well it works for increasing DOF.
***
Another good idea was a simple variable aperture system taken from
amateur astronomers studying the moon. The moon is often so bright that
you need a way to reduce the amount of light coming into your telescope.
The trick used is a large cardboard cutout, over the front of the
telescope, which acts to reduce the diameter of your mirror telescope's
opening. That trick effectively reduces your telescope's aperture. The
Sigma 500mm f/4 mirror lens comes with a special lens cap that has a
removable inner section. Leaving the lens cap in place, but removing the
inner section, cuts the mirror's aperture to f/5.6.
http://medfmt.8k.com/third/cult.html
***
Moose's comment
>> I can't imagine that some manufacturer would
>> not have tried to get a feature jump on the competition (and an
actually
>> useful feature, at that) if they could figure out how to do it.
is probably right on but even if there are limitations, being able to
increase DOF of a mirror after focusing could be worth fighting the
problems ... at least for me. The cost and weight of a 600mm f4 Ni*on
nor Ca*on don't make them very appealing. I haven't ever seen a Sigma
500/4.5 apo nor Sigma 1000/8 apo.
-jeff
----- Original Message -----
From: "Skip Williams" <om@xxxxxxxxxxxxxxxx>
> The typical way that astronomers reduce the amount of light coming in
a folded-light-path telescope like a Schmidt-Cassigrain or a Maksutov is
to construct off-axis masks that reduce the amount of light coming in
the front of the scope. Since the telescope is so slow anyway, the only
typical use is for solar observations. The mask is usually in the form
of a round circle offset to the side so that it fits clear between the
secondary mount and the tube edge. This will increase the focal ratio
of the lens, but I don't know whether it will alter the DOF.
> > >>>
> The net-net is that it's usually not worth it to to cut down the light
transmission from Cat lenses, as they're so slow already. I really
don't like the OOF doughnuts either. The only real use IMO is for
astronomy.
>
> Skip
>
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