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Found 5 results

  1. A friend ask me about filter size, I already had a Excel sheet with that calculation. But I did a new web calculator. It's the first version but it looks to calculate it correct, have a look: http://www.astrofriend.eu/astronomy/astronomy-calculations/minimize-vignetting/minimize-vignetting.html It's not any high precision calculation, but gives knowledge enough I think. Too small filter gives vignetting, too big filter empty the wallet. /Lars
  2. Long time ago I bought a Pentax medium format lens. It's a Pentax SMC 67 165 mm f/2.8. The reason I bought this was to have as low vignetting as possible. Later I bought the Sigma APO 150 mm f/2,8. Very good but a bit high vignetting. After this the Pentax have collected a lot of dust. But why not give it one more chance? Here you can see some graphs over the vignetting where I compare the Sigma with the Pentax: http://www.astrofriend.eu/astronomy/projects/project-sigma-pentax-vignetting/project-sigma-vs-pentax.html Later I shall also do some test how sharp it is, now with a live view camera it's much easier to get it in perfect focus, I think I failed with this earlier. Any other out there using medium format lenses (for astro)? /Lars
  3. Hi, I spend a day here to analyze my master flats and what vignetting I have on my lenses and telescopes. It's very simple done but still interesting to set figures on it. It's done on a full frame camera and I use the values from the center and the corner of the sensor. The corner is 22 mm of from the center on a full frame sensor. I never liked to have strong vignetting in my optics, and in the future I want to go for bigger size than full frame. I have put together a page over the vignetting I got: http://www.astrofriend.eu/astronomy/projects/project-different-lenses-vignetting/01-different-lenses-vignetting.html Maybe interesting for some of you to have a look at. My telescope shall handle a medium format sensor of the size 48 x 36 mm with the setup I have now. My medium format optics already do it. Just missing the medium format camera. /Lars
  4. I've been struggling to find an answer to whether a 2.5" focuser is likely to cause vignetting on a full-frame (36x24mm) camera sensor. This would be for an f/7 130mm scope, probably used in conjunction with a 3" 0.79x focal reducer (stated to have an illuminated image circle of 45mm). Whilst the focal reducer should fully illuminate the sensor (having an M68 connector on the scope side and M48 on the camera side), it is not clear whether any "mechanical" vignetting might occur with a 2.5" focuser drawtube. I imagine this would depend on the backfocus of the scope - i.e. how far the focal plane is behind the end of the focuser. If it's a long way back, I can visualize the light cone potentially being restricted when it enters the drawtube inside the OTA. Hopefully, the scope isn't designed this way, but it's hard to tell. If there is no mechanical vignetting, it does raise the question of what the benefits of larger focusers are. Greater load capacity certainly; improved stability? maybe. But does it actually give a wider unvignetted image circle? I'd like to apply some science to this question, rather than buying a bigger focuser "just in case"! Thanks for any answers! John.
  5. Are there any advantages of using a 2" diagonal with a 1.25" eye piece vs a 1.25" diagonal and 1.25" EP, assuming both are of similar build quality? [Someone asked me this and I gave them my answer, but I wonder what others think]. James
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