Quote:Thanks for the responses. I still haven't got my mind completely wrapped around F ratio being the only thing affecting brightness. I'm getting there, but I'll have to chew on it for a while. I'll doodle some diagrams and I'll get there. So for Saturn, for example, is it correct to say that the image brightness will be the same in a 2" F/5 as a 4" F/5, but the size of the image will be 2x in the 4" (because of 2x increase in focal length)? Is that what you mean by "more detail"? Thanks for the sample image, Jim. I saw something similar: 2 brighter areas. In fact, I thought the cores looked more like globular clusters, and that's what I see in your pic, though a little denser than what I saw. I see some of the spiral arms of M51 in your pic, something I didn't see in mine. I had to give up when my battery tank started to get low and I lost good tracking, otherwise I may have played around more with camera settings. Brian
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Quote:Brain,Your focal ratio question usually would have generated tons of replies in an imaging forum, so let me try if I can reduce the traffic a little bit.1. The answer will be the same, be it using a (long exposure) videocam in near realtime mode, an astro CCD imager, or a DSLR.2. The Answer will be different depend on if the study subject is a point light source (a star) or an extended object (e.g., a galaxy, such as M51.)3. This one is the one that usually triggers the apples vs. orange debate:Assuming the same imaging device is used:3a. (more degrees of freedom:) choice of two OTAs, one is 320mm f/5 (fl=1600mm) and the other is 80mm f/5 (fl=400mm). Note that not only the aperture is different (can collect more photon flux) but the sky coverage is different (larger fl sees less sky coverage -- FoV.)This comparison assumes both are perfect APO refractors to avoid OTA design type debate/tradeoffs.3b. (one degree of freedom) only one optical system (e.g., a camera lens), manually adjust lens's aperture (f-number ring in camera lens or change aperture mask in an OTA), and compare. Note that in this case, fl never changes. Reducing the f-ratio effectively reduces the aperture size proportionally. 3c. (one degree of freedom:) only one OTA is used but this time use auxiliary optics to adjust formula. Adding a focal reducer to reduce the f-ratio (now fl is lowered but aperture is still the same). Adding a Barlow to increase the f-ratio (now fl is higher but aperture value stays the same.)Hope this set up the reference framework properly.Clear Skies!ccs_hello
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