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Hi All I'm Looking for some advice on integrating calibration frames into a pure Ha imaging stack please. I've read somewhere (but cant exactly remember where) that there is no need to integrate darks, bias and flats into a Ha imaging stack if using Ha data as a luminance layer combined with RGB in Photoshop. Ill be honest, this doesn't make sense to me, but I would be really interested to hear more experienced peoples thoughts / input on this. Bias and flats aren't really an issue as I use a light box for my flats so can take my flats and bias straight after my imaging session, but obviously darks can be a real burden, especially with a DSLR (sensor temp) and when its 3am in the morning in the back garden. I appreciate a cooled camera is the way forward, but unfortunately for me, that just isn't an option at the minute. If it transpires that there isn't a need for calibration frames when stacking Ha, that would be a huge help, but Im not sure if that's correct or not?? Any input on this will be greatly appreciated. Many thanks Stuart |
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Well to be honest with you that also doesn't make sense to me. I guess you don't need to take calibration frames the same way as you don't need to take more than a single sub, but your image quality will probably be compromised one way or another. As long as you are taking calibration frames properly then your data quality will always benefit by using them, making your images much much easier to process. As for darks with DSLR, I agree. They are a pain and hard to get right also. Just make sure you sufficiently dither between frames and that will help alot. Flats are probably the most important so I will never recommend skipping them. Bias frames are good also. |
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Hi, From context I deduce you are taking bias and flats after *every* session? This is kind of an overkill. Bias frames are pretty much the only kind of frames you can keep for almost as long as you have the camera. And if your optical train doesn't change, you only need to take flats once in a while to correct vignetting. I know flat frames are also useful for reducing the effect of dust motes but realistically speaking dust motes fall under three categories: easy ones on the telescope which you remove by cleaning with a suitable cloth, easy ones on the sensor which you remove with the embedded ultrasound cleaner and slightly more challenging ones on the sensor which you have to use a blower or a cleaning kit to get rid of. Either way, the best treatment of dust motes is to clean them (or ignore them if they are at the edges). A flat just won't do the same job. Bottom line: create a master dark and a master flat that match your ISO settings, and you can reuse them for weeks, months or years (in the case of the bias). Darks are a different matter because they have to do with two factors that are dynamic: exposure and sensor temperature. In my experience darks are largely unnecessary with a reasonably modern DSLR when ambient temperature is below 15 degrees centigrade (this tends to keep the sensor cool enough for thermal noise to be pretty much a non factor), unless there is virtually no skyglow (skyglow tends to saturate your sensor more than any thermal or random pattern would) _and_ you are exposing for several minutes. I am afraid the last two factors are definitely at play when doing Ha: the sky is ultra dark because of your filter and you have to expose for at least five minutes because of your filter. There are basically two things you want to get rid of with darks: one is several hot pixels which will inevitably pop up in a long exposure. Those you can easily get rid of by dithering, by creating a bad pixel map or even with software. The other is thermal noise manifesting most prominently as a glow around the edges. This is generally very low on DSLRs but I would be very surprised if nothing showed up in a 5 minute exposure. It is quite prominent on my D7500 once you go above 4 minutes or so. This kind of noise needs darks in order to be eliminated or at least reduced, but is also the trickiest since the duration and sensor temperature must match your lights quite accurately (not 100% accurately but say within a couple degrees plus or minus, YMMV of course). If your darks are not good, they end up making more harm than good. So I guess the only way to know for sure is to experiment. Simply try integrating without darks (and the various different methods me and Janco outlined to get rid of hot pixels). If you find that you like the result (or that at least it is better than what you are getting with darks), by all means go on without darks. Otherwise perhaps you should start thinking about purchasing a cooled camera with low amp glow for doing your Ha, and use the DSLR for more traditional wide band targets where it really excels at. Cheers, Dimitris |
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Thank you Janco and Dimitris Yes, you have both confirmed what I was thinking and added weight to my understanding for sure. I do take calibration frames after pretty much each imaging session, especially darks as it is very difficult to gauge when the outside temperature will next match that particular nights worth of imaging. I also take flats as I have to strip my rig after each session, so I can never guarantee that my imaging train has stayed exactly where it was once I've stripped it down and moved it all indoors. It makes perfect sense to integrate calibration frames regardless of what filter your using and I do also use dithering, so it sound as though I'm on the right track. I suppose I was looking for corner to cut, but you both have reaffirmed that there isn't a corner to cut without detriment to the final image. Thanks again guys, really appreciate you sending me your thoughts and advice. Cheers Stuart |
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| Some great ccds with low noise, will not need darks. Does not apply to you with a dslr. |
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| Some great ccds with low noise due to cooling, will not need darks. Does not apply to you with a dslr. |
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Actually, CMOS sensors (which includes DSLR cameras) tend to have much lower read noise than CCD sensors because of fundamental differences in design which cause the CCD readout to take longer. For example the cooled Atik 4120 EX has roughly twice the read noise of the uncooled ZWO 178MC, despite the 4120 being a very high quality camera that costs 4+ times as much as the somewhat low end 178MC. The fact that CCD cameras have worse read noise than DSLRs doesn't mean much though, because both kinds of cameras have insignificant read noise compared to 10-15 years ago. And it doesn't mean anything about the need to take darks: darks are not for readout noise, that's what bias frames are for. |
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from the Atik website... Cheerio! Cmos have a lot pixel, dead or alive, amp glow and noise. CCD's have much less noise. dark frames allows you to minimize the amount of noise in your long exposure images that were created by the sensor, dslr, cmos. An average of dark frames can be subtracted from your final image, as the fixed-pattern noise and amp glow will be isolated in a master dark. Features of the 4-Series (from the atik web site 460 ex series) Our 4-Series is designed in such way as to combine power and control with flexibility and ease of use. The narrow body makes them ideal for use on Hyperstar and Fastar systems whilst maintaining suitability with a huge range of other telescopes. They feature fully regulated set-point cooling which makes for easy and consistent image calibration, though they feature such low noise that dark frames are no longer even a requirement, meaning you can spend more time imaging the things that matter. |
