T n = Optical transmission (due to surface coatings taking into account the number of surfaces)ĬO n = Central obscuration ratio (= D secondary/D primary) I personally find it more useful to simply compare the ratio of sensor signal strengths for two systems to see how they compare. In the link tkottary shared, John Hayes explains with details his formula: F-ratio is all you need to know for extended objects, assuming the same camera on each scope.Įdited by Jon Rista, 18 July 2019 - 12:39 AM. If you know f-ratio, you do not need to know aperture, because f-ratio is focal length/aperture. Your previous post said you needed to know "aperture and f-ratio", which is what I was responding to. When comparing two telescopes of different apertures and different focal ratios, both have to be accounted for in exposure time for extended objects. In order to have both telescopes at the same focal ratio the 200 mm aperture telescope would have to have a focal length of 2,000 mm.įocal ratio can only be calculated if both the focal length and the aperture are known. Then they will be F/5 and F/10 respectively with F/5 being faster. When comparing exposure times for two telescopes you have to know both the aperture and focal length to be able to calculate the focal ratio of each.įor example, if two telescopes both have a focal length of 1000 mm but one has an aperture of 200 mm and the other 100 mm. It won't be absolutely perfect.there are other things like transmission and filter bandpass to consider, as well as the more true nature of quantum efficiency (which is an integration over the band), but the formula above will usually work well enough for most comparisons.įocal ratio only accounts for aperture when two different telescopes have the same focal length which they do not in the example given. Where A is aperture, S is image scale and Q is quantum efficiency. But for more general comparisons, the following formula will give you a simple "performance" factor that can then be compared across systems: A lot of factors come into play, so overall if you want exact results it can get quite complex. When you get into comparing different scopes AND different cameras, then things get more complicated. Now, these apply for a given camera, used across different scopes. Stars are not necessarily true point sources all the time, and exactly how they cover pixels can make how quickly they saturate may depend just on aperture, maybe both aperture and f-ratio.įor extended objects, aperture is actually accounted for in f-ratio, and since the signal of extended objects is distributed over the field only f-ratio really matters. The exposure time for the 6.1" telescope is 81% of the exposure time of the 5.5" telescope.įor extended objects such as galaxies both the aperture and focal ratio need to be considered.įor an extended object, to gather an equal amount of photons, the exposure time for the 6.1" F/5.5 telescope will be 68% of that of the 5.5" F/6 telescope.įor a true point source, only aperture would matter. It is actually a little more complicated than that.įor point sources such as stars, only aperture matters so for a star:
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