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I’m sort of new to Astrophotography and was curious how to go about processing data taken over multiple nights? My current process would be to take my lights and calibration frames and then stack those to make a final photo. But if I wanted to get more integration time over several nights, would I need to hold on to all those pictures and stack everything at once, or would I need to stack my finals from each night? Thanks in advance!
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Hi Steven, the way to go depends a bit on the software you use. If you reuse your calibration frames over multiple nights (as I do typically over a span of 3 to 4 nights depending on the time I have), you can stack all the images at once. But this is unusual to most of the astro photographers. With darks and bias frames it is usually no problem to use them for longer periods of time. Many of us create dark libraries for example. With flats its a bit of another story. They should be taken before/after each session, but personally I do flats not every night, because of my permanent setup. But thats not the way to go. Anyway, if your software does not offer a way to order your flats to the right light frames, you have to calibrate your subs with the correct calibration frames individually (I mean session/night wise) and then register and stack all the calibrated output files as if they were taken within the span of one night. That for me is the easiest way to go. Maybe other users have different opinions. If you use PixInsight, the WBPP-script gives an opportunity to order the frames by keywords. So I create one in the filenames of all flats and lights and every time I do flats, I change the ID I give to the next number. This way, the script knows which flat frames to use for which lights. There is a description by Adam Block on youtube, where he explains this process in his WBPP script description series. (By the way, even if you do not use PI, it is really interesting and you learn a lot about calibration frames and stacking in general.) I am quite sure, that similar methods are offered by other stacking software as well, but personally I don't use other software than PixInsight for stacking. I hope this helped a bit to clear thigs up. CS Christian |
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Steven Shook: I stack each session's lights separately and then stack all the sessions in one go. Makes life so much easier. |
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Like Christian, I use the keywords feature of WBPP associate different calibration masters with different groups is lights, letting me process everything in one go of WPBB. I use the keyword feature for other attributes than which flats go with which batch of lights. I've added add to projects in subsequent years, and the new data needs different darks as well, so I can use a darks keyword to associate with different groups of lights. I even switched the sensor temp at some point, so I used a temp keyword to associate lights taken at one temp with lights at another. Keywords can be used for stuff unrelated to calibration frames. When I shoot a mosaic, I group my panels with a panel keyword on both the pre and post phases of processing, so WBPP won't try and integrate the subs from all the panels into a single image. I also recommend the Adam Block videos. He has a whole series on WBPP, but I believe this is the one discussing using WBPP keywords. https://www.youtube.com/watch?v=J6tif1v8hmY |
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It depends on your software, might help to include your preferred stacking suite to get usable answers. Pixinsight - use keywords and folders to match your cal's to the lights from each night and stack all at once. AstroPixelProcessor - make sure multi session is click and load your lights and cals into sessions. Stack all together. Siril - Modify a stacking script (available on CN forums) to stop the process after calibration and use an index for nights 2,3,4 etc. calibrate each night, copy the pp_lights from each night to a new folder and register and stack. Alternatively you can run the unmodified script for each night then use a long command in a terminal to combine the pp_ sequences together into a new folder and sequence, register and stack. I believe that Sirilic can accomplish all of this in a nice interface but I have never used it. |
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Thank you everyone, that helps a lot!
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The software-specific suggestions are useful. But it's good to understand the principles. Regardless of your software - the key idea is that you calibrate each light sub with the correct calibration masters, then you register (align) and stack them all together. Are you using a dslr or a cooled astro-camera? If astro-cam: If you left everything in place, so the flat frames from night 1 are still valid for night 2, then you can calibrate them all together just like they were collected on one night. If you disassembled the imaging train (or if you're just diligent and always take new flats for each session), then you calibrate night 1 lights with night 1's flat, etc. If dslr: then you need to also calibrate each light with a reasonably temperature-matched dark. |
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If you want to make the process as smooth as possible, I would also second the idea of stacking on a single session. Please consider not changing your rig between imaging nights ,so you can use the same flats (critical), darks and bias. Keep the same sensor temperature in every night. The moon or any other offending light source might provide gradients. You may easily fix the background using Graxpert, Siril or Pixinsight but in case you are getting very different gradients between imaging nights, you will want to preprocess separately every session. That is why, when imaging on different nights, I would try to pick a target at a fairly far angular distance from the moon.
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Another vote for using keywords on the folder names for WBPP. Then using the pre-processing keywords (to match the folders) it can group everything and handle all the calibration and stacking as you want. Gradient removal is going to depend somewhat. In my area most gradients are very small because light pollution is fairly even through the sky. The biggest change is with the moon. That adds a big gradient. NormalizeScaleGradient can be helpful there as well as PixInsight's Local Normalization. I know APP has a normalization but it wasn't as good to me as the others when I tried it. I'm sure other tools have equivalents or near equivalents.
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