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Since I got my ASI533MC-Pro, I've usually been using 300s exposures, although I have seen many other people using shorter exposures, and getting low-noise images in a fraction of the time I am (With similar sized telescopes, a 130mm F5 reflector). I'm using a gain of 100, which I verified is in the region of low noise. Tonight I performed a test on M81 and Horsehead. 10x30s stacked frames vs 1x300s frame. On both, I found that (As expected), the 30s stack had significantly less noise than the single 300s. But what was unexpected was that I appeared to have the same amount of image detail in both if you looked though the noise. I expected the 30s stack to lack the fine tips of Bodes arms, or some of the dimmer dust around the horse, but it was all there. Is there a certain brightness floor where long exposures really come into play? Given that these objects range from magnitude 6.9 to 7.3, they're not what I would consider to be 'dim' objects. Thanks! |
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If your subs are shot noise-limited, then stacks of equal time will have the same limiting magnitude (or same SNR and thus same faint detail visibility). When read noise is made essentially irrelevant by exposures than are long enough, only total time matters. And because full well capacity is not infinite, you would tend to prefer the shorter shot noise-limited subs in reaching that total time because you would otherwise just be wasting dynamic range with unnecessarily long subs i.e., you will lose less data to clipping stars and highlights. https://smallstarspot.com/shiny/StackSNR/ |
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Hi Kyle, as you mentioned yourself, the fainter details seem to come out a bit better on the long exposure. Thats the key. You have to lift those faint details out of the noise of one single exposure. Then you are able to see things in your stack that were not there if you take short exposures only. The question is, how much of a difference this means for your target. If you try to image narrowband data with exposure times of 30s your program may not be able to stack them, because the light is so faint, that it causes problems. You need longer exposure times to even see the details. An image of a star cluster, on the other hand, may not benefit from long exposures. Because of the full well capacity, you may blow out the stars and their colors. I am not an expert in this topic, but I did some experiments in the past and decided, that the longer exposures may create some benefits for image processing (in which I am also no expert in by far). At the end of the day, it depends on the target and on your personal interest. A lot of us are used to look for the fainter things in images, because we are living this hobby quite seriously. But the common people may not even recognize some of the missing stuff in the final images, because they only see the obvious. What I mean is, that there are a lot of images out there, that were taken with really short exposures and look fantastic. There are also some images with long exposure times that would improve from missing processing skills (like mine). But my impression is, that the detailed images I personally like the most are taken with longer exposure times. The equipment question and its precision is not taken into account here. Long exposure times of course need a good working setup as well. CS Christian |
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Total integration time is the key. A 1 hour exposure vs. 60 1 minute exposures should have roughly the same amount of data. I also have the ASI533MC-Pro and I typically stick to 5 minute exposures, unless the target is very bright (M42, M45). Having shorter exposures can compensate for bad tracking (mount, seeing, etc...). The longer the exposure, the greater the odds that you will capture something you don't want (airplane, satellite). I'd much rather throw out a 5 minute exposure than a 10 minute exposure that had multiple trails. <opinion>Honestly, I think that doing 10 minute exposures is a bit of a "look how good my mount is" brag...</opinion> Here are some videos that I've found informative: https://youtu.be/sn97Ja7wIJM?si=2u4S1IM52Pvl5EA6 https://youtu.be/rQAXVwE_nvY?si=IDjpn2zK88rTGHL3 https://youtu.be/8DhRy1MT1Qs?si=stuHajcEacWTpTNc https://youtu.be/zRXjD4r_3To?si=_5f-ebmAxIm3_Gqc |
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Christian Großmann: That's not what they found. They said: Kyle Cerniglia: Faint detail visibility (and I'm assuming that's what they're referring to rather than resolution) is a matter of total exposure time, not sub-exposure time if there is not impact from time-independent noise sources like read noise. Now, strictly speaking, there's always some impact of read noise, but with long enough subs, it can be made practically irrelevant. What counts as long enough will depend on the system used and the sky brightness. So if what they say is true, then we can assume the system and sky brightness was such that 30s was long enough. Now, they also said: Kyle Cerniglia: This is somewhat contradictory. You can't have more noise (with the same signal) and, at the same time, have "the same amount of image detail" or limiting magnitude. The amount of visible detail just is a matter of the noise (and the signal of course). That said, there are smoothing benefits from registering multiple exposures that the single sub could not have taken advantage of, and this will have an effect on limiting magnitude (but also resolution). |
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Hi Kyle, I can recommend this video: Deep Sky Astrophotography With CMOS Cameras by Dr Robin Glover (youtube.com) . It points at a few really important aspects of your question. Cheers Manuel |
