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Being in light polluted cities what is your bare minimum exposure time you will ha e for any given target. Just starting out and I have only been doing short sessions. I would assume the more pictures/exposure time the more data there is for better stacking. Just curious if everyone’s methods. I am using a DSLR on my nexstar 6se. Waiting on my light pollution filter to ship. |
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You have to do it by yourself, anyone will answer you it will be mostly according to their experiences. I don't use DSLR at all although i have many, i am using astro dedicated cameras, and wit that it all depends on targets and scopes type and conditions and filters if been used and settings value such as Gain and Offset, it is all about experience. |
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Hi Petro, I image from a balcony in the center of Geneva, mag 17.80. I usually need around 10hrs of usable exposures to have a decent result whether LRGB or Narrowband + RGB. Of course some brighter targets allow you to do it in less time, but there are not so many of them (globular or open clusters, a couple of planetary nebulas or galaxies). Narrowband + RGB is definitely more rewarding, but needs the same time. I use 3 nanometers filters. So that makes easily 2 to 3 nights per target in the summer, counting overheads and lost exposures. I see no spectacular improvement (if any) with broadband light pollution filters (I used CLS-CCD, UHC and IDAS QRO). Now I use none of them in order to get more signal, have the non scientific impression that it eventually works better in terms of SNR and color balance. Kind Regards, Stevan |
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| Yeah I am still learning all of this and without an eq mount I usually don’t take more than 8-10sec pictures. Still trying to learn the optimal settings for the subs. Like how bright should they be before the data starts to get drowned out. Should I go darker and just get a lot more time overall? I am sure every situation is different but there are just so many variables to play with and so little time lol. |
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Yeah I am still learning all of this and without an eq mount I usually don’t take more than 8-10sec pictures. Still trying to learn the optimal settings for the subs. Like how bright should they be before the data starts to get drowned out. Should I go darker and just get a lot more time overall? I am sure every situation is different but there are just so many variables to play with and so little time lol. Hi Petro, I see you are in Chicago. Can you please tell us your Bortle scale? In a Bortle 8 or 9 is pretty illusory to think you could obtain a decent image in RGB with a DSLR. 5sec exposures are not adapted for your system it is too slow. You can use Sharp Cap for example to establish the brightness of your sky (photon flux/electron flux). Pixinsight also has some estimates based on your sky background flux. Anyway, probably you have to stay under 60s and use an LP filter. The LP will cut through the galaxy signal since they are broadband sources. And integrate a lot of hours 15 to 20h depending on the brightness of your object. Then you could use Pixinsight to extract the gradients. Clear skies Bogdan |
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Yes, it sounds like you are limited by your tracking rather than the camera dynamic range. Per image and total integration times vary wildly by the brightness of the particular target and the amount of light pollution. I image in Bortle 7 or so, and I am typically using image frames of anywhere from 60-300 seconds in length depending on target and total integration times maybe from 5-10 hours. Your 10 seconds are well under that. I do use a light pollution filter; as stated above, there are some benefits and drawbacks, but in very light polluted skies like we are in, they do allow significantly longer exposures before the camera saturates with background light. Another thing to keep in mind is that - assuming your DSLR is still un-modified - it is less sensitive to red so the amount of the red hydrogen you will see in the nebula targets is reduced for any given image duration. I’m sure stacking a huge number of 10second frames is cumbersome, so one suggestion to try is that SharpCap has a “live-stacking” capability. I haven’t really tried it, but it seems promising from what I have read. Basically it does the alignment and stacking in real time for you so you can watch the result getting brighter and clearer in the screen. This might be useful both to reduce the number of files you are dealing with but also to get a real-time sense of what per-frame and total-integration settings do. Until you have an equatorial mount, that might be the best option. As a target recommendation for Bortle 8-9, try the Lagoon nebula this summer. It’s extremely bright and you should see something easily (it was almost over-exposed for me within 30 minutes total time). |
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| Thanks all for the replies. Yeah I am stuck in a bortle 8-9. I am not in the heart of the city but straight across from my deck is a hospital with a very bright sign. I have been able to get results that get me excited to keep going, but I am just trying to learn more that I can do to get better quality without going to dark area and/or spending a lot more money on a new tracking system. I know that will need to happen eventually but for now I just want to try and make due with what I have. I will have to look into SharpCap. I don't currently have a laptop or anything hooked up to my system, but I have need to for the future. |
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| I think that 5 seconds is way too short. Think of it this way, for 1 hour of integration time - 5 second exposures result in 720 images to stack, calibrate and align vs about 120 for 30 second exposures. This directly translates to more disk space required, more memory and more processing time. A light pollution filter or narrowband filter (for emission nebulas) will help you get longer exposures but then tracking will become your main limitation. The other reason for longer imaging time is that you get much better signal to noise improvements. The noise level will remain the same for each image, but by increasing imaging time you collect more data from the target of interest. |
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Thanks all for the replies. Yeah I am stuck in a bortle 8-9. I am not in the heart of the city but straight across from my deck is a hospital with a very bright sign. I have been able to get results that get me excited to keep going, but I am just trying to learn more that I can do to get better quality without going to dark area and/or spending a lot more money on a new tracking system. I know that will need to happen eventually but for now I just want to try and make due with what I have. I will have to look into SharpCap. I don't currently have a laptop or anything hooked up to my system, but I have need to for the future. If you are not looking to get a filter, the next best thing is to image high in the horizon (~25 degrees or higher) and do as much as you can to block stray light. Put up clothes / light barriers, and use a sun shade/dew shield on your lense if you have one. |
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Thanks all for the replies. Yeah I am stuck in a bortle 8-9. I am not in the heart of the city but straight across from my deck is a hospital with a very bright sign. I have been able to get results that get me excited to keep going, but I am just trying to learn more that I can do to get better quality without going to dark area and/or spending a lot more money on a new tracking system. I know that will need to happen eventually but for now I just want to try and make due with what I have. I will have to look into SharpCap. I don't currently have a laptop or anything hooked up to my system, but I have need to for the future. If you are not looking to get a filter, the next best thing is to image high in the horizon (~25 degrees or higher) and do as much as you can to block stray light. Put up clothes / light barriers, and use a sun shade/dew shield on your lense if you have one. I am waiting on my light pollution filter. It is on back order. Not having an eq mount what is the longest my exposure can be without very noticeable trails. I have been trying to find something to extend off the end of my telescope to help block some like but I haven't found anything yet. I may just be using the wrong terms when searching. |
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There is a scientific method to determine an optimal exposure time. The best explanation I have seen is from Blair McDonald in JRASC feb 2021. It is easy to use and you can measure the parameters yourself for your system. With one sub exposure, a bias frame and a dark. Your camera converts photons to electrons --the ADU (analog-to-digital units, if your camera takes 16-bit images the maximum signal is 65,536 ADU). Your camera sensor responds to photons the same way no matter how long the sub exposure. Your final signal ADU depends on the total time of exposure, not the length of the subs themselves. Think of a bucket in the rain--the rain representing photons, the bucket your sensor. The bucket fills depending on how long you leave the bucket uncovered. It doesn't matter if you cover it occasionally, as long as you leave it open the same length of time. The only thing you lose is the time the bucket is covered, when your sensor is closed. With a digital camera downloading an image takes 2 seconds at minimum, so this is lost time. Note though longer exposures are more prone to problems like clouds, or winds, or satellites, so you have more discarded subs. I think this then is a wash. So, why take longer exposures at all? The only reason is read noise. Every time you take an image (a sub), the camera will add read noise. This depends on your gain--for many cameras higher gain gives less read noise. Fewer subs, the less total read noise. So how long should a sub exposure be? There is a good way to decide. The key point is that if your background sky is bright, in light polluted skies, most of your noise (shot noise) comes from the sky. It overwhelms read noise. So if you sky is bright the read noise rapidly becomes insignificant compared to the sky shot noise. If your sky is very dark, then read noise is a more important contribution. Mr. McDonald provides this estimate for your optimal time for your sub, Tsub, considering your sky brightness, Esky. This equation comes from that fact that for this length of sub, about 95% of the noise is from the sky noise, only 5% is camera read noise. Longer subs reduce the percentage of read noise, but with severely diminishing returns. Tsub = 10 (R^2/Esky) Where Esky=(ADUsky-ADUdark)/Ts - R is read noise in ADU. Measure the standard deviation in the background ADU from a single bias frame taken at the ISO/gain that you will use for your sub. Or use a literature value in electrons for R and divide by the gain to get R in ADU. You can find the R and gain for your camera on-line if you search. But really easy to do it yourself. -ESky is the sky signal in ADU for an arbitrary sub length Ts. Doesn't matter how long, as long as your sky gives you a reasonable signal in ADU and is not saturated. You should subtract the ADU for a dark, though generally this is pretty small. Divide by the length of the sub exposure you used, Ts. Now you have all you need to calculate your optimal subexposure Tsub. You can measure R and ADUsky in many programs. Nebulosity free trial version does it, if you don't haven't purchased software that can do it. The key here for short exposures is find your lowest read noise ISO/gain, generally it is at higher ISO and gain. Generally with short exposures dynamic range is not an issue, but dial back the ISO if you are blowing out bright regions. I don't have experience with filters. The theory above says a filter will reduce Esky. But it will also reduce your signal. So now you are cutting down on your photons. If you have a Ha only target, and an Ha sensitive camera (most DSLRs are not), then an Ha filter would certainly make sense. Other cases would depend on the target and your skies, I suspect difficult to come up with general rules. But due to less light from stars especially with a narrowband filter, you likely need to increase exposure, in some star sparse areas of the sky you might not get enough stars to stack in your subs. I am in a big city, nearly full moon bright, operating at F6.3, my DSLR cameras are low read noise at high ISO--so my optimal exposure is <10 s, more doesn't buy me anything. The darker the site, the longer your subs need to be to be optimal. But remember, in the end the same total exposure will give you the same number of photons, and averaging the subs will improve your S/N, no matter what your sub length was. I use short exposures in my work: @rveregin My early work was with a 6SE and digital camera, just as you have. Regards Rick |
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Not having an eq mount what is the longest my exposure can be without very noticeable trails? That at least should be easy for you to test just by trying different timing until the stars start to look bad to you. It will depend on the weight and balance of your mount, but if it's the 6SE with the included tripod/mount from Celestron, 10 seconds or so does sound like a reasonable limit. My 8SE (I use mostly for visual & planets) is good for about the same. You also might try just using a lens with your camera too if you have anything that came with your camera in the range of a couple hundred millimeters. Just a kit zoom lens (cameras often come with something like a 50-150mm zoom) would be a good learning experiment for you, even if the quality isn't perfect. Photographers use something called the "500 rule" as a rough guess at how long you can expose on a fixed tripod. Most lenses have smaller focal ratios, so it would allow more rapid light collection, plus a wider field-of-view than your 6SE so less subject to star trailing. At least you could compare the results. |
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It depends on the target and the camera sensitivity, etc. When I image with d780 in my Bortle 7/8 skies, my single sub exposure time is anywhere between 3 - 10 mins. Galaxies are usually on the shorter side. Emission nebula are on the longer side, because I use the L-extreme filter, and one needs to collect the faint H-alpha, etc. You have to try a couple exposures and then compare for details to make sure you are getting what you need. Once you nail down the exposure of a single sub, then you should plan on taking many of them. In light polluted skies, that number is really important. If you want a good final picture, then take lots of them. In my experience, only after 30-35 subs is when I start seeing a decent signal to noise ratio. The higher number you can shoot for, the better your final image is. Lastly, I have compared images with and without LP filters. In my backyard, LP filters help significantly. I use the Optolong L-pro for galaxies and planets. For emission nebula, I use L-extreme. Both work really well. Hope this helps. |
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Kapil K.: Hope this helps. Kapil K.: Yeah I have the L-pro on order but it is currently back ordered. |
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| I live in Bortle 8 and use a Canon EOS Ra with the L-Pro and L-Enhance. I have been using the L-Pro extensively this past winter/spring with good results. I do 300s ISO800 subs. Yes, the L-Pro does cut some signal but the colors can be corrected in post. Sometimes I go to a Bortle 4 dark site to image galaxies without a filter. The L-Enhance, however, I can do 600s ISO800 from my yard. I have imaged nebulae like the East and West Veils, Lagoon, Eagle, Pacman, Crescent, and Wizard. |
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Fully agree with comment from @Kapil K.. Let me add that, in case you use N.I.N.A. for imaging, it has a tool that gives to you the optimal minimum sub-exposure duration, based on your sensor data and an exposure taken to the actual sky of the night. This tool is reliable and based on Dr. Glover model https://www.youtube.com/watch?v=3RH93UvP358. Frequently, in heavily polluted sky, the recommended minimum exposure is around few seconds with L-Pro, that would lead to a large number of subs. For this reason I never go below 30 seconds and recently I tend to image faint galaxies at 240 seconds as a minimum, keeping an eye to star cores saturation. For Galaxies I aim to integrate a total of 3-4 hours (60 subs x 240 sec). With L-eXtreme the recommended subs are significantly longer, as predicted by the model. For completeness I use a ASI2600MC Pro camera. Have a nice day! |
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Fully agree with comment from @Kapil K.. Awesome video! Thanks for sharing @Mau_Bard. I will try and internalize this and follow next session 🙂 |
