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.
You could also put ND filters in the focal path, but unless it's designed for
them, they will degrade the optical path. For solar observation, DO NOT put
filters anywhere except over the front of the scope. The heat buildup from
those department store filters that screw into the back of the eyepieces are
enough to crack them and blind an unsuspecting observer.
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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>Subject: Re: [OM] Sigma 600mm F8 Mirror Lens
> From: Jim Brokaw <jbrokaw@xxxxxxxxxxx>
> Date: Wed, 27 Aug 2003 14:04:21 -0700
> To: <olympus@xxxxxxxxxxxxxxx>
>
>on 8/26/03 9:51 PM, Moose at olymoose@xxxxxxxxxxxxx wrote:
>
>> However, this is not a Vivitar or Tamron, etc. The Sigma 600/8 has a
>> slide-out filter holder on the side of the lens almost back at the
>> mount. Easy to change filters with lens on the camera. With an extra
>> holder or 2, switches would only take a couple of seconds.
>>
>> My suspicion is that only ND filters will work in this posititon (on any
>> brand) as the image is close enough to formed that waterhouse stops
>> would cause vignetting. I think this partially for vaguely formed
>> theoretical reasons, but mostly from the fact that nobody has ever done
>> it on a commercial product. 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.
>>
>> Moose
>>
>> Jeff Keller wrote:
>>
>>> Unfortunately the filter on a Vivitar Series 1 resides inside the T-mount so
>>> changing the filter involves taking the camera off
>>>
>
>Seems like in order to limit the aperture of a mirror lens, you would have
>to restrict the diameter of the front aperture -without- blocking so much
>that you close off the entire area between the edge and the central mirror.
>This probably will reduce the light entering the lens, but it might not have
>the same effect as a traditional lens design's aperture with regard depth of
>field. It might also simply cause vignetting to reduce the light level in
>aggregate, without changing the light level in the non-vignetted areas.
>Further, it probably will reduce the resolution of the lens since that is
>somehow related to the total aperture opening, at least for astronomical
>mirror telescopes. I think the only way to reduce the light on a mirror lens
>is with ND (or other) filters, either in front or at the smaller end of the
>light path. These won't change the depth-of-field, unfortunately. My
>Celestron 300/5.6 uses screw-in 35.5mm (I think) filters in the back of the
>lens, and in order to put them in place you have to remove the T-mount. My
>Vivitar 600/8.0 Solid Cat uses a similar setup. To use a polarizer
>practically it would have to go in front, so it would be big and
>expensive...
>--
>
>Jim Brokaw
>OM-'s of all sorts, and no OM-oney...
>
>
>
>
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