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Hi all, I want to know being Under Bortle 7/8/9 how long is the exposure needed or enough to image the stars only say either with RGB filters or OSC camera? I am trying to think about exposing the stars alone with separate equipment/gear so i save time for other data such as SHO and Lum, so i want to know how long is the stars can be exposed under heavy light pollution, and what is a good start exposure time to use, 1 minute or 2 or all the way up to 5 minutes per exposure? |
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I think it is going to depend on: 1) magnitude of brightest stars in your field of view 2) F-ratio of your OTA 3) gain (High? Low? Unity?) for ease of processing it might as well be the same gain you shoot the rest of your lights. 4) type of OTA (reflector vs refractor) and, if reflector, how much you want to try to minimize diffraction spikes… |
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I think it is going to depend on: Hi Noon, Thank you very much for your answers. 1) I don't care about magnitude because people just do imaging without calculating the magnitude of anything, most of them, experts and perfectionists of astro do it, not me, and i already said the stars will be imaged separately from another setup for nebulae or another data. 2) For my dual imaging setup i am mostly imaging between F4 up to F7, depends if i will use a reducer or not, i could go faster too. 3) I will use lowest gain for RGB, depends on which camera i will use, mostly with IMX571 camera they use gain 100, but i live under Bortle 9, so i won't use same light exposure of narrowband with broadband exposure definitely. 4) I will use refractors and reflectors, and for reflectors i really not bother at all with spikes, kinda i love them very much really. |
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Good info Tareq, I will caveat my answer because I do not separately image my stars from my targets. I merely capture my stars in OSC or Narrowband along with my target, then I separate the stars from the target, color correct/process them separately, and recombine at the end. That way I avoid the magenta/weirdly colored stars and I manage to get the same integration time for my stars as for the rest of my target. I mentioned magnitude because if I have a REALLY bright star in your field of view (think Betelgeuse, Rigel, or Sirius), I tend towards shorter subframes (maybe 10-15 seconds even) to try to preserve a little more color/reduce the diffraction spikes more, but you didn't seem to mind those... so you can probably disregard that. That being said, if your desire is to shoot RGB/OSC stars and combine them with narrowband, given your setup, then I would suggest starting with 30-60sec exposures at "unity gain" which is about 100 with your IMX571. The IMX571 has a pretty large file size, so if your file storage/computer will bog down with two many subframes then maybe lean towards 60 sec... This will accomplish two things: 1) Short exposures will minimize tracking errors and should give you rounder stars (assuming collimation/focus is tight). 2) Short exposures will minimize the chance of overexposed star cores, preserving the different star colors Given enough sub exposures, the actual individual exposures won't matter too much. Now for total integration time. A good "test subject" for what you want to do is globular and open clusters. They are pretty much all stars so there are no more faint galactic arms or nebulosity to try to capture. You can concentrate solely on the stars. It is really easy to think that can just jam out about an hour of star subframes and call it good. If you look online you will see a lot of people who talk about just needing 2-3 hours of subframes even. Open clusters are all about not overexposing your star cores and getting your star colors down pat. My personal opinion is that total integration time for stars should be as long as you can stand. If you can get 5 or 10 hours, that is great... 15-20 hours is even better (with diminishing returns), and this is why; 1) There is noise in star data, and the more subframes you can average into your final image the crisper and tighter your stars will appear. you can look at my M38 image (https://www.astrobin.com/full/yptq6g/0/) which is less than an hour of total integration. Zoom in and see what I'm talking about with star noise. It was a good study in stars for me, but I need to go back and add time in the future. Sidenote: this was made from 3 minute ASI2600MC Pro subframes at gain 0, so if you drop the gain, you get some well depth, and preserve color more easily, but now you are subject to more tracking errors and still have to clock in the total integration time... back on topic... 2) You will pick up more and fainter stars than you ever thought previously possible. When I started focusing on M13 (https://www.astrobin.com/r2b3dy/C/) a few months ago, I initially thought I could make do with an hour of integration (original)... and I later added more integration time to get to over 5 hours. You can see the comparison... how many more stars come out after the additional time. My goal... if I ever have a clear night again in my life... is to get up to 10 hours. Theoretically, 2 hours is twice as good as an hour... 4 hours is twice as good as two... then eight... then sixteen... but at some point you are processing a LOT of subframes to get an incremental gain that you can't perceive with your eyes. Generally speaking 24-36 hours of subframes are somehow considered "enough" for heavy light pollution areas, but your mileage may vary and you may also be comfy with a third of that for stars alone. My skies are between Bortle 7 and 8, depending on the amount of dust in the air. With Bortle 9, you are probably close to needing double what I'm happy with... it just takes patience and time. With your double rig, if you are taking RGB data with one camera and narrowband with the other, it may work out great. I've personally never used a rig like that. Honestly, I'd probably say 5-10 hours would be enough...to start... That is 300-600 60-second subframes and 600-1200 30-second subframes. That is a lot of storage and computer resources, so if those are limited, you can hedge the lower end of that... If you are patient and/or you can combine multiple years worth of data over time... you know your star data will only get better. Start with an hour of star data... then add 4 more hours and compare... then add 5 more hours and compare... did you notice a difference? I'm sure others will recommend more... or less... an any will give you a result and more time will always give you a better result. Hope this helps? Is that what you are looking for? The answer is always "it depends," but to my eyes, 5 hours is around the bare minimum. You can dig into histogram analysis and signal to noise ratios... or go with the old WAG (wild ass guess) that has been a ballpark solution for me. |
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Thanks again for your detailed answer, so here i can add for clarifications. I am planning to use RGB stars not from narrowbanding but a separate setup, so in this case i don't worry about color correction or coma or whatever, so i leave stars away from my actual narrowbanding data, and if i shoot broadbanding then simply i just capture OSC or RGB filters and Lum is always by mono, so what i am talking about is RGB stars only to add for narrowbanding data using my 90mm f6 triplet and 0.8x reducer and IMX571 camera, because i have two 90mm triplets and reducers and also two IMX571 cameras then i can just go for third setup, and this third setup must match FOV of the two setup, and i am planning to buy a third IMX571 camera, but the options of two are either IMX571 color so i don't change filters for RGB or IMX571 mono then i must use RGB filters but i won't use a filter wheel for 2", so changing filters is a bit out of plan, and changing with filter drawer isn't practical much. I will use gain 100 with my IMX571 either broadband or narrowbanding, but with narrowbanding i use 2min-10min, so i have no problem exposure with that, but with broadbanding it is always affected by LP more, so i want to make sure i use enough correct exposure, 10sec or 30 or 60sec or even 120, but i see many rested with 60sec mainly, so maybe this is what i will use, or maybe lower it and go for 30sec if that is enough, and definitely many frames, but many are doing RGB for like 1-2 hours mostly, or say like 20-30minutes per filter/band, and i like the idea of doing it shorter so i get less guiding error and more rounder stars because that is what i need to use. I have enough storage, plenty, too much, so this is never an issue for me, i can take even 20000 frames with my IMX571 50mb file with ease, that is say 1GB data, i have 4-5TB harddrives and i even will buy maybe 8-12TB external drive for total whole storage, and use my small portable external drive for imaging file collect, even my computer is provided with 1TB SSD for operating system and another 1TB SSD for storage, and it is fast transfer so i can always transfer when data reach around 500GB, i never imaged like 500-1000 frames per night, so i am always safe every night with enough storage, i just worry about perfect nice setup equipment to use so i don't have issues later in processing, many people wasted time in correcting issues with processing, if i made things correct by setup that will save my processing time at least, and,....... AND AND millioin AND, it is ALWAYS about STARS, so manufacturers making filters and scopes only to make best corrected stars in color or halo, so why not make a perfect setup separate only for stars and leave my other data with different setup, NB is easy to do with any setup, so even if the stars are bad i can easily remove them and use the stars from another setup which i ask about here for better exposure with gain 100 under B7/8/9. Hope i clarified enough for you so you understand my point and question, and if you need any more details i am willing to provide you or add those to better answers and solutions and help. |
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Also in future i have to think about my reflectors scopes ideas for multiple imaging setup, those are the main reflectors i have now for DSO: 1. 10" F/8 RC Truss 2. 8" F/5 Newt solid tube 3. 6" F/4 Newt solid tube So if i will use those then i must try to find backups or pair to those to make things easier faster for me, let's say RC is mainly for galaxies and clusters so 90% broadbanding, if i use it as a main it means i have to find another scope cheap for Lum to add to RGB and Ha using mono camera, i think i might go with OSC over mono for RGB while my mono is for LSHO, many said that RGB is for coloring mainly and can go with shorter exposures, while Lum and SHO are holding the main strong details, so RGB mostly is to get colors and Lum will give the details, this is good idea. Later i can think about another backups for 8" and 6" too, Newt are cheaper, RC is expensive and i was lucky to save and buy one, and in future if i buy a bigger mount to carry it then i better think about a second scope next to 10" RC but not another RC, not 10" or smaller or even larger, but the only scope i can think about is either 10" Newtonian or 12" Newtonian, in any case the stars will be taken from one scope so spikes are fine, the other backup scope is for either Lum or Ha or OIII or both [if i do planetary nebulae]. I wasted like 3-4 years with only one setup and always short time and issues here and there with setup, so when i get successful setup i end up with really short data, so it means i need to live forever to finish so so many targets in the sky every year, i hate to spend many or several nights only for 1 target, and being under Bortle 8/9 i know i always need like 5-50 hours, even that 5 hours sometimes i need like 2-3 nights to complete, imagine if i need like 2-4 hours per filter, this is too much, that is why i keep thinking about multiple setup, i already have several cameras and scopes and i can add few more later, so why not, i don't drive at all to darker skies, and i don't want to miss with so so many steps in processing mainly because of stars alone. |
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I understand your dilemma. I used to jump around from target to target as well. Now I sometimes spend a month on a target until I get at least 24 hours of integration to work with. it is frustrating because I can’t capture all the different targets that interest me, but if I have it my way, I’ll be pursuing this hobby for many many years to come, and I trust that I will get there eventually. Plus, the lessons I learn with processing deep integration times will only help me as I get more experienced and attempt more challenging targets. as I get older I find that patience makes up a lot for speed and enthusiasm. I personally think gain 100 is a good choice for you. Perhaps one final consideration, if you are open to the suggestion; maybe try RGB instead of LRGB. These days I really only use the L filter for focusing and plate solving. I have attempted Lum subframes many times but generally, if I have to shoot in heavy light pollution, my Lum subframes are less beneficial and tend to mess with my final color balance… most of my colors end up getting washed out even if I get a little more detail. The RGB filters each do a pretty good job of isolating out parts of the light pollution together. Now a 6 hour summertime window for image capture can be split evenly between R, G, and B (or SII, Ha, OIII) with 2 hours each… if you can manage that over four nights, that’s 24 hours of integration. while I have definitely used OSC, and I have an IMX571 color camera, I find it is difficult to successfully integrate OSC and RGB without some sort of pixel math, which isn’t really taking advantage of any of the noise reduction of multi subframe pixel averaging. Unless you have a solid gameplan (that works well) for combining OSC and mono data, I might recommend choosing one or the other and sticking with it… although if you are truly just using Mono RGB or OSC for stars… you can probably ignore this last paragraph. regardless, your setup sounds more complicated than anything I could successfully manage, so I wish you the best of luck and all the clear skies. |
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I understand your dilemma. I used to jump around from target to target as well. Now I sometimes spend a month on a target until I get at least 24 hours of integration to work with. I have IMX571 color and IMX571 mono, so if i use that color for RGB then i use the mono for Lum and SHO, most likely targets are part of that, i mean either LRGB only or HaRGB or LHaRGB or SHO+RGB, so if i leave OSC for RGB alone then i can have all time with mono to do Lum and SHO, but i asked here for exposure of RGB so then i can decide for that third IMX571, should it be second color IMX571 or second mono IMX571, i also was planning to get that dual NB filter for color camera soi can image Ha and OIII together for whole time available, then i can have second color camera for RGB, and that leave the mono for SII, but if short exposure is enough for RGB then maybe another mono, but in this case i better buy a quad so i never worry about back focus and make sure it is a good Quad scope so i don't need to refocus each RGB filter. I also have two 60mm doublets, both with old mono cameras of Panasonic 4/3 [QHY163M and ASI1600MM], so i might think for third 60mm in future but then which camera, color or mono again, or, i just pass the 60 and use my 72mm [FRA400] with its QHY294Mto be main scope for Ha and leave two 60s for OIII and SII, i can think of so many options with my whole setup, i spent like 3 years only to collect gear instead of collecting photons, and later once i am done with gear then i will be busy with photons whole life i hope. |
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| Assume i will have like3-4 hours per night, with 3 same setup i will end up with about 9-12 hours of data, with one setup this is like 3-4 nights instead of one night, and not always every night is perfect, so i need to have more and more setup as much i can afford and get mounts to handle that, then i can collect as much data in few nights per target, so i keep going, otherwise i might use one or two setup fortwo years ands then i give up with only several targets i captured and not all of them are great as well, that is not a good sign for me if i want to keep going, and the life here is also affecting me, so need to find a way to accelerate things in Astro imaging. |
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| Sounds like a good plan. Clear skies! |
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Sounds like a good plan. Clear skies! Thank you, clear skies too! |
