This guy only has it half right - you have to make the hole off axis
so it is not in line with the central obstruction of the secondary
mirror as I stated in my previous post. This increases the effective
f ratio and will increase the DOF - it has to since the f ratio is
increased. This works well for larger astro scopes, but the
practicality is limited in the camera lens due to the small space
between the secondary mirror and the edge of the corrector lens.
--- Jeff Keller <jrk_om@xxxxxxxxxxx> wrote:
> 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
=====
Don Shedrick
http://groups.msn.com/firstlightimaging
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