![]() You can image DSOs at any size from tiny objects that are highly magnified, to medium size objects, to large objects, to giant complex fields full of many objects and stars or parts of the milky way. However, aperture does play a role in how the signal gets distributed by seeing, and to get the best resolution possible, you need to pair the right pixel size with the right aperture, which tends to be somewhere between 8-10" unless your seeing is usually excellent (in which case, you could use a larger aperture scope).īut high resolution imaging is not the only kind of imaging. If you want the best resolution possible for a given amount of seeing, then the optimal pixel size in angular terms (arcseconds) will be around 1/3rd the seeing conditions in arcseconds. The thing about "optimal" is when it comes to DSO, unlike SSO, it often depends a lot on what you want to do. For DSOs you also want to sample sufficiently so that your stars are recorded with at least two pixels (well, you can go below that but you don't want to approach one pixel stars as one pixel can't make a round looking star). In any case, there is no law that says you have to stay exactly at these recommendations, but if you deviate too far from the above then you are most likely not getting all of the resolution that is possible for your setup or you are penalizing yourself by requiring unnecessarily long exposure times (to compensate for the slower f-ratio). ![]() In this case, you go by the focal length, not the f-ratio, since focal length and pixel size determine your image scale. Thus for DSO imaging people usually try to estimate their typical seeing conditions (which might be around 2 arc seconds) and then take one half to one third of that for their image scale. That said, you generally don't want to image DSOs at those kinds of image scales since your seeing conditions are going to limit what you can actually resolve. Furthermore, if you are using a one-shot-color camera you probably want to use an even bigger multiplier, which I've often suggested might be about 7.5X. So, if you have a camera with a pixel size of 3.37um you'd want to use an f-ratio of 5 x 3.37 ≈ f/17. For so-called critical sampling as used for planetary imaging the generally accepted "rule" that relates pixel size to f-ratio is that for a mono camera you want to use an f-ratio that is equal to five times the camera pixel size as measured in microns.
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