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Forum: Lens Clubs
11-07-2020, 08:19 AM
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Strawberry leaves in the woods. Showing the close focus capability of Sigma 4.5mm fisheye.
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Forum: Pentax SLR Lens Discussion
12-05-2018, 01:04 PM
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I suppose the question is about eyepiece projection. Short answer: to change the effective focal length of the telescope.
The eyepiece takes the image created by the telescope objective (the 1000mm f/8.33 in the case of Omni 120 XLT) and projects it on the camera sensor. That projection can enlarge or reduce the image (depending on the FL of the eyepiece and the distances from the eyepiece to the telescope focal plane and to the camera sensor).
For bright objects (Moon, planets), 1000mm is ok, even a bit short for planets. For deep space objects (galaxies, nebulas) it is quite long, unless you have a very good motorized mount, you do a very good polar alignment, and, preferably, you have auto-guiding.
Using eyepiece projection is quite hard to do the mechanical arrangement. Quite few eyepieces have a thread on the side away from the telescope, and you can find an adapter to T thread. And then T thread extension tubes for tuning the distance to the camera, in order to tune the magnification (to the telescope side, the focuser has typically a large travel). Also, you may get vignetting. There is a bit of calculations to be done to figure out the setup.
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Forum: Pentax SLR Lens Discussion
12-05-2018, 11:01 AM
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In astro-photo, the standard way to connect camera equipment is the T thread (M42x0.75, male thread on the lens side, 55mm flange distance to focus plane; as mentioned, do not confuse with M42 mount, which has a thread pitch of 1mm and a shorter flange distance).
So, the normal way to go is an adapter from 2" (I think the Omni XLT has a 2" focuser) or from 1.25" to T thread, and a second from T thread to Pentax K. Each should be around $20. This is by far the most flexible solution, and I see almost no reason to buy a direct adapter (skipping the T thread). The only exceptions I see are if you have very limited backfocus - which is not the case with refractors, or to avoid vigneting - which is not the case with APS-C format of the K200D, plus you need some field corrector as well for large sensors. With refractors you usually have the opposite problem - too much backfocus, because they are designed to work with a diagonal mirror between the telescope and the eyepiece.
BTW, some telescopes already offer a male T thread somewhere (on the outside of the 1.25" barrel or on the 2" to 1.25" adapter).
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