How Much Gain and Sub Exposure is Enough?

hello this is joe and welcome back to the channel you know i haven't had very good weather lately in the last three weeks i've only got to shoot a couple times a couple nights and and then uh it was working on another project so i really didn't have anything fancy or any wonderful pictures to show in today's video but what i did want to talk about was that i get asked a lot about my portal class here and what i what kind of sub-exposure links i take total integration stuff like that and so i thought well i would take a few minutes in a video to answer some of these questions but unfortunately to answer the questions and make them sound sensible you have to kind of go a little deep down the rabbit hole when it comes to your camera and the gain and the exposure time and the full well depth and the thermal noise and the shot noise i mean it just there's a lot to it and there's a a lot of reasons why you want to do certain things the way that you do now i know for a lot of you this is just going to be a review but for i'm hoping that this will help a lot of newcomers to the hobby who are trying to understand how to take the best sub length for each sub exposure and then the total integration time needed for a particular target so let's get started on talking about some of this so first of all i want to mention that um astropharsography uh had captured uh a talk by robin glover who is the creator of sharpcap and he put a video out and it's it's been a few years back now but i remember watching that video and a lot of the stuff that i use or talk about you could probably see more in depth if you want to even go deeper down the rabbit hole in that video so i'm going to leave a link in the description below to that video and then also robin glover did a follow-up video on all about gain and stuff and i'm going to put a link in the description below to that one as well because it kind of does follow up on what he was talking about uh in the first video that you watch these are long videos and they're pretty in-depth and pretty mathematical heavy so i'm going to try and just cover some of the basics as to why i shoot the sub lengths that i do and hopefully uh you can just that's enough for you to at least get started until you have a chance to go study this or research it a little bit more in depth so really in order to get an optimal sub exposure length you kind of need to at least know what the read noise is of the camera and it also the thermal noise affects this as well but but not to a great extent and if you've got a cooled camera you could just cool your camera down and it it really doesn't matter much about the thermal noise but it does play a part the other thing you need to find out is your light pollution because the more photons that are out there in the sky the less of a sub-exposure length you can take before you saturate your full well in your camera when i look at the clear sky tool my light pollution comes up as a bordel class 3 but it also gives me a magnitude and that's really what you want to plug in now sharpcap has this cool calculator and i'll leave these links in the description below as well so you don't have to try and find them but they've got a really cool calculator and you could put in what your limiting magnitude is in the area that you're at and it will tell you basically it'll give you what portal class you are or vice versa now the easiest way to find that is to go to the clear sky app or website and i'll leave that in the description as well and you'll be able to see right you could actually put in your latitude and longitude if you want or you can just put in the town that you live in or the address and it'll do its best to get you the correct limiting sky magnitude value that you're looking for let's just took a quick second to go over the the calculator here on the sharp cap website so your sky brightness is 21.85 which is the value that i got from clear outside after i put in my latitude and longitude now the program here shows as a bortle one i don't think that's correct i think that i'm really a portal three or maybe a two and a half but it doesn't really matter i don't think because the i believe that the calculation is being done based on the actual sky magnitude number so the telescope f ratio is what your your f-stop is for your telescope now currently mine is uh i've got the my z81 in there with the focal reducer and flat neuron which comes out to f 5.2 now the camera i'm using is the asi 2600 mm pro you have to be careful here because the quantum efficiency is actually 91 but that's not across the entire bandwidth of the color spectrum uh but for just to make it easy i went ahead and put in 90. i don't you can't really put in 91 but you most likely if you wanted an average if you were going to do luminance i would probably do more of uh 75 or 80 because it does drop off quite a lot as the further into the color spectrum and then for your filter you're going to pick the filter that you're going to use if you're a mono user you'd pick you know luminance and if that's the one that you were trying to calculate for and luminance bandwidth is 300 nanometers and if you were going to be doing like ha at 3 nanometers then you would put the bandwidth of three nanometers in there and if you're doing color or you have a color camera you would put in the basically 100 nanometers for each color on mono or leave it at 300 nanometers if you've got a color camera and if you're using a light pollution filter on your color camera what i would do is just add the two bandwidths together if it's like a dual band filter i see a lot of those i would add those together and use that as your bandwidth and what it's going to spit out for you is the result and that's going to be used in the formula coming up here in a minute that we're going to talk about which actually gives you your minimum sub length that you would need in order to swamp your read noise and in my case i'm getting 0.86 electrons per pixel per second and that would go into the formula 10 times the read noise squared divided by this value so then what you want to do is just take a small little formula and it's really 10 times your read noise squared divided by your light pollution i know that sounds really complex but it's it's not very difficult at all if you go to the your camera manufacturer's website you'll get some charts you've seen them in my videos before if you've watched any of my camera reviews and they basically tell you what your read noise is and at what game so going to gain with unity gain on the the later cmos zwo cameras and that's pretty much all i'm talking about because that's what i own and so that's what i'm really looking for and it's really the sensor so the sensor should be the same whether you have a qhy or a zwo or an altair astro most likely if you've got that particular sensor the graphs are going to pretty much be identical for the most part i know there's a little bit of changes based on the software and the hardware that they use to put the cameras together but for the most part that sensor data is going to be the sensor data no matter what camera manufacturer you have so you're going to go in there and you're going to look up the read noise and you'll you'll notice that there's a unity gain and dr glover actually says that the unity game doesn't really matter and and he's right in theory however the usually the unity gain is where the camera changes mode so you have what they call a low conversion mode and you have a high conversion mode and in high conversion gain it's it's h gc height or hcg high conversion gain mode you'll notice that your dr stops and your full well both go back up a little bit not on all cameras but on most of the cameras sensors the you'll notice that when you hit that that one electron past the the hcg your dynamic range goes back up but your read noise drops drastically now that's where i use that's where i have my gain set it's it's not for any other reason for me personally other than i want to take advantage of as much dr as i can as much full well as i can and as low of read noise as i can so that's probably where i would say you should start and then you could adjust your sub length time in between based on the magnitude of the object that you're shooting so if you're shooting m42 you would probably have that pretty low but if you're shooting some faint little you know planetary nebula then you're gonna kind of want a very long sub exposure and in that case you may want to adjust the gain up a little higher now what gain does not do is give you more photons it does not collect more photons the higher the gain is it basically just lowers that read noise and the read noise is really where you you get you lose detail because the noisier your photo is the less detail you could really see that you've captured so basically by upping your gain you're you're reducing the read noise and the higher your gain goes the lower your read noise is but if you look at your chart by the camera manufacturer the sensor you are going to notice that at some point on that chart the read noise is just barely dropping off and that's the point of diminishing returns now unfortunately the higher your gain also the lower your full well goes and the full well capacity really affects your dynamic range as well and if you notice again on these charts you'll see that the dynamic range and the full well capacity are pretty linear on the graph they're not exactly linear but they're very close and it depends on the camera but the majority of them they're very linear you'll you'll watch them both drop off at the same exact rate based on how much gain you add and the goal is is to get as much dr as you can and have as much full well as you can to take that longer exposure in which case you really don't want to up the gain too much so it's up to us to figure out how long of a sub-exposure we need to take and at what gain can we get away with that in order to swamp the read noise of the sky now by saying swamp to read noise i just mean that you get enough signal to noise ratio to to wash out the the reed noise and the thermal noise that's based if you've got a cooler or not and if you don't that's okay but you do need to take even more sub-exposures the more sub-exposures you take the lower your noise is going to be and the higher your signal is going to be when you stack them also your dynamic range should go up when you stack them as well so you really want to take as many sub exposures as you can but also at some point you don't want to take like a hundred thousand two second sub exposures because now we've got limitations on our processing power on our computer and how much space we have so for me what i found i like five minute subs and i like ten minute subs depending on the target that i'm shooting in narrowband and in color i range anywhere between 90 seconds for rgb up to 180 seconds for rgb and when i shoot in luminance i range anywhere from 30 seconds sometimes even 15 all the way up to 2 minutes and anywhere in between and it all really depends on the magnitude of the target that i'm shooting and most of the time when i'm shooting in rgb it's usually a galaxy so it's almost going to always be 180 seconds of rgb and 90 seconds in luminance that's that's what i shoot at the unity gain when the unity gain puts the camera into the high conversion gain mode the hcg i know this is a lot to take in and if you go through those videos that i mentioned uh you'll you'll get a lot more understanding he's got a lot of great charts and graphs and shows you some examples of you know what the noise looks like and why you want to swamp that read noise to get more detail in the images but i've just i've heard a lot of people with some misconceptions and there's some myths that you need to take long exposures in order to gather all those photons and you're not completely wrong because our cameras aren't perfect and we have read noise and we have thermal noise and then there's a thing called shot noise and where the photons don't always land in the exact same spot on the sensor every time per pixel so there there's a lot to it and unfortunately what i found is is that you do need to take a little bit longer exposure so a lot of people talk about lucky imaging they're not wrong but you would you would seriously need to do um when you're doing like one or two or three second exposures you would have so many files that you would have i don't know if there's a hard drive large enough to fill them all up especially if you're using something like the 294 mm pro and bin one mode and you've got a 93 megabyte files each one and you're looking at taking thousands of them so it makes a lot more sense to just take a longer sub exposure to the point where your other gear can handle it your mount any tracking areas you might have and and your sky pollution i hope this made some sense and i hope it helps some people if so please go ahead and hit that like button and if you or if you are a beginner and you're new to the hobby then please check out this video right here about some of the equipment that you need to get started

Channel: Joe's Astrophoto

Views: 7,856

Keywords: astrophotography, Astronomy, Astro, Astroimages, Photography, Art, Images, Space, Space art, Night sky, Night time, Dark skies, Deep Space Objects, DSO, Nebula, Nebulae, Narrowband, filterwheel, Telescope, EQmount, Eqmod, Pixinsight, William optics, Sky Watcher, EQ6 R Pro, Guiding, Phd2, Photoshop, Lightroom, Stacking, light frames, bias, darks, darkflats, flatdarks, calibration frames, integration, calibration, NINA, APT, iphone astrophotography, mobile phone astrophotography, Timelapse, starlapse, exposure time

Id: kxEHcRghhUM

Length: 15min 53sec (953 seconds)

Published: Fri Jun 10 2022