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Hey fellow astrophotography enthusiasts,I hope you're all doing well! I recently had the opportunity to capture an image of the Sadr region, and I'm eager to improve my skills and learn from your expertise. https://www.astrobin.com/avyvgu/ I'm particularly interested in hearing your thoughts on:
Please don't hesitate to be candid in your feedback. I'm here to learn and grow as a photographer, and I truly value your input. If you have any suggestions for resources, tutorials, or techniques that might help me enhance my skills, I'd be more than grateful! TIA for taking the time to help me improve! Your expertise means a lot to me... CS, Harlan |
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| A super image Harlan! 👏🏻 great dedication. Framing is great. I love the bold saturation, my personal preference. I pixel peeped at full res...nice stars corner to corner. The only think I could critique if I had to is the star color, not any red or gold, but that's the nature of NB stars, (but you have to pixel peek to notice). So I give this two big thumbs up. -john |
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John Favalessa: Thanks John! I've been reading about using a different filter to get RGB stars - something like the Antlia Triband RGB Ultra. Is this the proper way to do it, or is there a cheaper solution with similar results? Thanks again, Harlan |
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| I actually just bought and used that filter on my latest post. (in hindsight I should have used zero gain instead of 100...to get better star color) It seems to be a good light pollution filter which gives good RGB balance. So far I'm happy with it. The Optolong L Pro is also a good one. With a 4 panel mosaic it would add difficulty although RGB wouldn't need long integration as the NB. You'd essentially have to different 4 panel mosaics and swap out the stars...lots of work. |
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John Favalessa: I'm doing mono + filters so my question may or may not be germane for me but why would a lower gain give better color (unless you are over-saturating the stars in which case just reduce exposure time?)? |
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| exactly, over saturated. the full well is 4x larger at zero gain. you are right about just reduce exposure time...but with the large file size of the 6200 it's a nightmare. |
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I have it as well. That's actually an aspect I still do not quite understand. The spec sheet says that 0 vs 100 gain has a dynamic range in stops of about 13.8 vs 13.5 (eyeball estimate from the graph). Yet the FW(e-) does look like a large change (can't read it well, but will accept it is 4x). I'm unclear how the dynamic range can vary so little with full well varying so much. But the read noise takes a dramatic drop from about 3.6 to 1.5 or so while gain goes from about 0.8 to 0.28. So you get 2.8 times the gain with 2.4x the noise. I just don't know how to interpret the DR change, but in experimenting I stopped imaging broadband at zero and just do everything at 100, and have seen no degradation and significantly shorter imaging. Can you (or someone) explain the DR vs FW? |
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| I've been doing 100 gain and 300sec for my Narrow Band. but for RGB stars, 0 gain and 60 or 120 secs. seems to help the star color. -john |
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Linwood Ferguson: Dynamic range is a tricky thing. It is defined as the ratio of the full well to the read noise. So what you have to do is look at the effect of gain on both of these. As you increase gain, full well will decrease. No way around this unless you increase the bit depth. So if you have a 16 bit output file and the gain increases, the signal adu/e increases, so for the same photon input you get more signal into your 16 bit value. The full well in terms of electrons, the number of photons captured, must go down, since the signal per electron increases. As you go up in gain many cameras decrease the read noise. And often when the camera switches modes there is a big jump to lower noise. So dynamic range around that gain value may be about the same, as the full well decrease is matched by the big noise decrease. At higher gain, both full well and dynamic range decrease, as the full well decrease is faster than the noise decrease at those higher gains. This means one should really only go to higher gain to reduce read noise, and thus also to reduce the sub exposure time. If you don't reduce the exposure time you will loose full well, even though your dynamic range at some higher gain may not be reduced. Of course, this only matters if you are saturating the stars, if you are not saturating you could even increase gain further to get more read noise decrease, as long as you keep within the full well. Personally I look at the histogram on my subs to check if my stars are saturating. Of course, brighter stars will saturate the core no matter what low gain and short exposure. So I also use Nebulosity (free version) to actually check what is saturating, if it is just the cores of a few bright stars that is fine, as long as other star cores are not saturated, and thus will have good color. Hope this helps Rick |
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| Thank you Rick, from your description then on the 6200 is there any reason to do broadband at gain zero versus 100? And adjust exposure down so as not to saturate things you care about? |
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Your camera has similar behavior to mine, I have a 2600. Generally I would never recommend running this camera at Gain 0. There is a huge difference in read noise going to gain 100, noise is bad in an image, so why not take advantage. The dynamic range is the same, so yes, if you are saturating you just decrease your sub exposure time. If you are not saturating you are good to go, or even to increase your gain until you are getting close to saturation. The only disadvantage is more subs to process, but this actually helps using rejection methods in your stacking, to remove satellites, etc. But also shorter exposures are less prone to errors in tracking, wind, seeing and satellites. There are some subtle things too, there is something called quantization error. So by expanding the scale of adu/electron, you get more levels. So at 1 electron adu, if the system adds or subtracts say 0.6 adu equivalent noise, you can only get 0 or 1 as outputs. If you are at 10 adu/electron, then the 0.6 adu of noise is now 6 adu. So now you get 4, 10, and 16. So at least for the fainter signals you can get a more representative signal out, which is very important if you want the noise to cancel, as quantization reduces the randomness. Rick |
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Excellent description. I love it when I discover a good reason to do what I'm already doing.  |
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Lol! That's why I love the science and math behind what we are doing: it is so nice to know that one is doing the best with what one has, and what setup might improve over it. What is the current pinch point? There is nothing worse than wondering if somehow you could do something better. Now that I'm retired, I'm happy to be able to use my 35 years experience in imaging science at Xerox and applying it in astrophotography. CS Rick |
