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Discussion in 'Miscellaneous Equipment' started by Marc Leest, Apr 16, 2009.
Why has the aperture on a mirror lens to be fixed ? Is this a fundamental design issue ?
Hmm, I am interested in this answer as well. Looking at the catadioptric design...
...I am guessing that it'd be very tough (=expensive) to change the sizes of the primary and secondary reflectors on the fly.
With this design I think you really want to avoid overfilling the primary and secondary reflectors, or else you'll get all kinds of stray light and kaleidoscopic entertainment. So it'd be tough to design a system that allows you to change the total amount of light in to the primary while also changing the secondary. You could probably swap out both mirrors but that'd be a pain.
Perhaps a limiting aperture could be placed at the node of the focuser....
Aside from the mechanical difficulties, part of the problem with stopping down a Cassegrain variant or Maksutov is that the central mirror becomes a larger percentage of the aperture, causing diffraction problems in the same way as stopping down a regular lens causes diffraction limited resolution at smaller apertures.
Astronomers do make their own aperture masks to place in front of Cassegrain and Maksutov telescopes. They are typically off-center, and stop the 'scope down pretty far. An aperture mask with two, three, or four holes is often helpful in fine focusing for astrophotography. A single hole offset aperture mask can provide a helpful increase in practical (as opposed to theoretical) resolution for viewing bright objects like planets and the moon. I use one with a solar filter for viewing sunspots.
A single, offset circular aperture mask will also eliminate the doughnut shaped out-of-focus highlights that many find annoying. Maksutovs are normally already pretty slow, as are some Cassegrains, and there's typically not a lot of room to play with making a smaller aperture on one of these optics designed as a camera lens, but if you have a large enough aperture and can afford the loss of lens speed, a mask might work for you. It's very cheap to make one and try it.
I wonder if something like a Imagon set of adjustable holes would be usefull?
I think the bottom line with these is that you pretty much want to focus at infinity and have everything in focus, and f/8 seems to be the optimal aperture for most purposes, with the smaller mirror lenses intended for SLR use.
The bokeh is bubbly.
They would work more like Waterhouse stops, but they would work. The Imagon apertures were designed to allow differing percentages of abberations from the outer parts of the lens to affect the image for controllable softness. Modern mirror lenses won't have much of this present, and mirror lenses don't have the same chromatic aberration of refractive lenses when properly made.
With the focal lengths typical of these lenses, f:8 isn't nearly hyperfocal, and many people use these lenses for closer photography, like birding from a few meters away.
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There may be an adjustable CAT somewhere. But I?e never heard of any that have adjustable apertures. F/8 and be there.
I once had a reflector (Hama, I believe, or Hanimex), 500mm f/8, which came with a set of neutral density filters that were screwed onto the back of the lens.
Good morning, Marc;
I have several mirror lenses. For "f stop" adjustment, they all use neutral density filters. Most of them screw into a provided filter mount in the lens mounting flange where they are accessible. One of them has a small turret assembly that puts an ND 2X or an ND 4X filter into the light path just in front of the lens mount so that it has "f stops" of f 8.0, f 11, and f 16. Because the glass of the neutral density filter will also affect the light rays, there is also a "normal filter" that is made of clear glass that is screwed into the place for a filter to provide a common element of refraction to insure that the light rays all go to the same place on the film plane.