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andrew s

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andrew s last won the day on June 8

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About andrew s

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  1. Indeed. I was trying to be amusing. Regards Andrew
  2. But that is why I was confused. I got a refractror for solar and you said " not for solar surely " Maybe you were being ironic. Regards Andrew
  3. My advice would be to start by adding 1mm. If this proves unsatisfactory experiment about +/- 2mm of the optimal distance Regards Andrew
  4. @david_taurus83 I don't doubt you experimental results. If you change the distance from the objective to the reducer then you will change its performance. If a filter is thin enough not to add aberration then it will just push out the focus. I don't think optical v physical distance is the issue it is the simultaneous move ment of focuser and back distance change due to the filter that confuses the situation. If you have a built in corrector (I.e. before the focuser) then I belive you would have to push our the camera to obtain focus with a filter compared to without. However, I would always recommend experimenting to find what works with your particular set up. Without detailed ray tracing of the entire system it is impossible to predict exactly what will happen if you change the objective to corrector and correctror to CCD distance. Regards Andrew
  5. I don't agree as per my diagram in the linked thread you need to increase the back focus distance by 1mm. Regards Andrew
  6. I found both in the 80mm F15 refractor I just got for white light solar. It confirms why I love reflectors. Regards Andrew
  7. As a Cheshire as long as its stable in the focuser length does not matter. Using it as a sight tube to place the secondary under the focuser you should adjust it so the the end of the tube just encloses the secondary. Length might matter. Using the cross hair to adjust the secondary tilt is more accurate with a longer distance between the peep hole and the cross hairs but I doubt it would be significant. Regards Andrew
  8. I think that captures the reason why "small" refractors have such a strong following. Couple that with ease of use and stable collimation then you have a winner. Especially with modern examples with little or no chromatic aberration. Regards Andrew PS I still prefer reflectors.
  9. If Olbers paradox were true this is what it might look like. Rather abstract. Regards Andrew
  10. Thanks, @Geoff Barnes just seen it. The explanation has gone from dust to spots and now dust again. It will be interesting to see where the consensus finally settles. Regards Andrew
  11. @John I agree In the end visual astronomy has to consider the human visual system even as its power wains. Regards Andrew
  12. Spot on @chiltonstar the Airy disk is not the image of a star but the convolution of a point source with the telescope aperture function. I sometimes wonder what these debates would be like if the "natural " form of a lens had been square or hexagonal rather than round. Regards Andrew
  13. Your first sentence quoted above seems to be a logical contradiction. Even a mirror can have a Strehl that depends on wavelength. However, I think both are true. Since frequency is related to wavelength Strehl depends on wavelength. I suspect we agree but for wording Regards Andrew
  14. Sorry I intended to say the Airy disk depends on aperture so you can have a larger aperture with a small PSF than a smaller one even if its Strehl ratio is not as good. Hence I don't think you can use it other than comparing like with like if real world performance is the consideration. It does depend on wavelength indeed Takahashi quote it as a function if wavelength for some of its scopes. You can easily have a higher Strehl ratio in the IR compared to the UV. Regards Andrew
  15. Strehl ratio also has issues. It refers to actual v ideal, however ideal is a function of both wavelength, and aperture (and possibly design depending on the definition you use) . I think it's fine if you compare like with like say 5 " APOs but comparing a 1" APO with a 5" APO would make no sense. Regards Andrew
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