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This is odd... dss problem please help
- Thread starter Sir Canon
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Jammy was close when he said he had problems shooting near the horizon. The problem I atmosphere turbulence, also called seeing by us Astro types, and it blurrs your image.
When you image near the horizon, you are imaging through much more atmosphere, and suffer more turbulence, dispersion and refraction. That is why most imagers don't usually bother below about 30 degrees. Image higher in the sky for better seeing. Also, focus on stars as high overhead as you can for best results.
When you image near the horizon, you are imaging through much more atmosphere, and suffer more turbulence, dispersion and refraction. That is why most imagers don't usually bother below about 30 degrees. Image higher in the sky for better seeing. Also, focus on stars as high overhead as you can for best results.
Sir Canon
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no, i zoom in to 10x on a bright star (say Betelgeuse) then select about as fast of a shutter speed as i can to see the star. this to insure that if it is out of focus it almost disappears. then i use the follow focus function of magic lantern to gently make micro adjustments to the focus. but sometimes autofocus catches the star and you're golden;-)
If you are sure you are focused well, then the only other thing I can think of would be tracking. Low end mounts and even midrange mounts have larger periodic errors. They can be as large as 30-40", which can greatly enlarge the size of a star. You can use guiding to improve that for longer exposures, however guiding itself is a bit of an artform to get the best results. I would look into your tracking capabilities and see what you can do to improve them.
Sir Canon
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i'm using a kenko skymemo mount at 200mm, this is a pretty decent mount. i have access to a celestron cge pro with autoguiding at an observatory im a part of. thats about as accurate as it gets. that bad boy can go 4 min at 3910mm WITHOUT the autoguider.
In my opinion, you are giving incomplete advice at best. Once one reaches a sky histogram of 1/4 to 1/3 histogram on the camera LCD, exposure time is long enough and longer exposures will not improve given a total exposure time. Simply saying sub exposures are not long enough without knowing other parameters is insufficient and could actually be detrimental. Exposing longer so that the sky histogram peak rises above the 1/2 level would decrease dynamic range, saturate more stars and bright areas losing color. For example, below is an image that goes at least as faint as your image, yet is ONLY 1-minute subs and only 9-minutes total exposure time. In another thread you said you do longer than one hour total exposure time and chided me for not going as long. How long is your exposure time? (Note too you have very little h-alpha and lost a lot of red).Swimswithtrout is correct, you just didn't use long enough exposures to stretch this image that deeply. You could overcome that by stacking considerably more sub frames, but because of the way read noise compounds through an integration, you really want to take longer subs to pull out the background details best.
Your longest were 120 seconds, it seems? You only had two of those, which just isn't enough, but even if you had say 16, you probably wouldn't have quite enough to eliminate the banding. You probably need to go longer than that. At f/5, you probably want 360 second subs for your longest subs. I say that, because I used 270 second subs at f/4 with a 150mm aperture on this image:
This was also at a pretty dark site. If you are in a more light polluted zone, then you may be limited in how long you can expose, and if you are in a heavily light polluted zone, your only option will be to integrate LOTS of shorter subs. Based on your 2x120 integration, you seem to have reasonably dark skies, so I would try for a longer set of subs, 270, 300, or 360 seconds, and see how that goes. Also, get a lot more than just 2.
M42 107 mm aperture, 9 minutes total exposure, 1-minute subs.
Exposure is collecting light. Two factors collect light from the subject: aperture area and exposure time. Sub exposure should be such that signal from subject and sky glow raises the histogram above the read noise. For modern DSLRs that occurs at about the 1/4 to 1/3 histogram level. Once you reach that level, read noise is insignificant and increasing sub-exposure time won't improve the image. For example, if histogram is ~1/3 level, then 100 1-minute exposures = ten 10-minte exposures in terms of reaching the faint stuff. But if 1-minute exposures reaches the 1/3 histogram level, a 10 minute exposure is simply blowing out more stars and brighter nebula. The better image will be made from the set with histogram reaching no more than about 1/3 histogram level.
Roger
Let's compare full size images so that we aren't hiding noise in any way, shall we? I would bet good money that your 9 minute image is extremely noisy in the background details. Here's mine:
Nine minutes will get you something, but that something isn't necessarily going to be ideal. It's going to be noisy. It's going to be quite noisy. I don't think I've ever seen a full size or even 50% size image from you. Your conveniently hiding how noisy your images are by scaling them down...a LOT in most cases. So let's be realistic and honest here. Share your unscaled image, and let's see what 9 minutes really gets you.
I very strongly disagree with the way you advocate very short exposures and very short integration times. I do not think it's good advice. Your the only astrophotographer I know that advocates extremely limited and minimal integration times as a matter of course, and actively argues against deeper integration time. Every other astrophotographer I know advocates the same thing I do...using the longest exposures you can in the darkest skies you can (or by using narrow band filters on a mono CCD), and getting as many sub exposures as possible. This is a fundamental, the most basic thing you teach anyone who is interested in learning astrophotography. I know guys who use 45, 60, 90 minute narrow band subs with CCD cameras using newer Sony ICX sensors with as little as 2.8e- read noise, and their results are phenomenal. Every other serious and skilled DSLR astrophotographer I know, including Scott Rosen and Jerry Lodriguss, use longer exposures and often very extensive integration (as much as 30 hours or more at times) to get the best results. As a matter of course these days, I encourage DSLR imagers to find dark skies, even in a green zone, because the difference between imaging in a green zone and imaging in a white zone is literally orders of magnitude better.
Yes, you are correct, one shouldn't expose past 1/3rd histogram. I agree that going beyond 1/3rd histogram with a DSLR throws away dynamic range. I offered my advice as far as exposure times based on what I know about sky brightness and aperture. I made an assumption about the OP's sky brightness, but I was also clear about the brightness of my skies and that I was recommending 360s relative to that kind of sky darkness at f/5. I was also clear that if he had brighter skies than I was assuming, he would likely be FORCED to use shorter exposures.
However if you do have darker skies, then don't limit yourself to short exposures. Even if you have low read noise, get the longest exposures you can without overexposing. At my dark site on a night with good transparency, I can expose at ISO 800 for 10-12 minutes before I hit 1/3rd histogram. And that's exactly what I do. I also get as much integration time as I can so I can pull out faint details with as little noise as possible.
Nine minutes will get you something, but that something isn't necessarily going to be ideal. It's going to be noisy. It's going to be quite noisy. I don't think I've ever seen a full size or even 50% size image from you. Your conveniently hiding how noisy your images are by scaling them down...a LOT in most cases. So let's be realistic and honest here. Share your unscaled image, and let's see what 9 minutes really gets you.
I very strongly disagree with the way you advocate very short exposures and very short integration times. I do not think it's good advice. Your the only astrophotographer I know that advocates extremely limited and minimal integration times as a matter of course, and actively argues against deeper integration time. Every other astrophotographer I know advocates the same thing I do...using the longest exposures you can in the darkest skies you can (or by using narrow band filters on a mono CCD), and getting as many sub exposures as possible. This is a fundamental, the most basic thing you teach anyone who is interested in learning astrophotography. I know guys who use 45, 60, 90 minute narrow band subs with CCD cameras using newer Sony ICX sensors with as little as 2.8e- read noise, and their results are phenomenal. Every other serious and skilled DSLR astrophotographer I know, including Scott Rosen and Jerry Lodriguss, use longer exposures and often very extensive integration (as much as 30 hours or more at times) to get the best results. As a matter of course these days, I encourage DSLR imagers to find dark skies, even in a green zone, because the difference between imaging in a green zone and imaging in a white zone is literally orders of magnitude better.
Yes, you are correct, one shouldn't expose past 1/3rd histogram. I agree that going beyond 1/3rd histogram with a DSLR throws away dynamic range. I offered my advice as far as exposure times based on what I know about sky brightness and aperture. I made an assumption about the OP's sky brightness, but I was also clear about the brightness of my skies and that I was recommending 360s relative to that kind of sky darkness at f/5. I was also clear that if he had brighter skies than I was assuming, he would likely be FORCED to use shorter exposures.
However if you do have darker skies, then don't limit yourself to short exposures. Even if you have low read noise, get the longest exposures you can without overexposing. At my dark site on a night with good transparency, I can expose at ISO 800 for 10-12 minutes before I hit 1/3rd histogram. And that's exactly what I do. I also get as much integration time as I can so I can pull out faint details with as little noise as possible.
First, where did you EVER see that I say "extremely limited integration times" or argues AGAINST deeper integration time. If you actually read and comprehend what I wrote, you will see that I do not say anything of the sort. I even recently pointed you to my exposure page:
Read the conclusions. Exposure is more than just integration time. Exposure is Etendue * exposure time. Note I say in the conclusions: "Some very faint nebulae and galaxies can benefit from CEF levels of 6000 or more."
Second, regarding sub-exposure time, how long one needs to go depends on read noise. The whole reason for long exposure times is to make read noise insignificant. Once you reach that point, longer sub exposures are no different than even longer ones, given the same total integration time. As read noise decreases with sensor technology, the need for longer sub-exposure lengths decreases. If read noise were zero, it would not matter what sub-exposure one used. In fact, you could do video rates and do lucky imaging on astrophotos. And there are people doing video rate astrophotography.
You advocate long sub-exposures, but your ideas are generations behind. To show you the future, read the LSST paper: http://arxiv.org/pdf/0805.2366.pdf where they say they will be using 15 second sub-exposures and reaching magnitude 24.5 (5-sigma) with 4 exposures. The key there is Etendue, not the sub exposure time you advocate. Same with all imaging. Key is Etendue times exposure time, and what one wants to achieve.
Same with things like binning. Binning on sensor was to reduce effects of read noise. If read noise is not a factor (sub-exposure is long enough) then binning in post processing is no different than in sensor in terms of signal to noise ratio in the image.
Roger
ChrisLX200
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What the future holds? We should be talking here about what amateurs own and use now - readily available tech not some future development (which you seem to be basing your projections on). It's all very well extrapolating from what you get in terms of IQ to what others _should_ be getting if we were being as efficient at turning photons into electrons as you are, but you have nothing on your website - no images that is - that prove you can produce a top-class image using your techniques. Don't try blinding folk with science, just pony up and produce some decent images and someone might listen to you (not me I hasten to add...)
ChrisH
Sir Canon
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i did post the whole image. on the subject of longer integrations i absolutely believe you, honestly its incredible that rogger is getting nearly that good image quality. however to a point more subs aren't helping. the amount noise is decreased is an exponential decay. meaning that if you have 4 subs you will half the noise but you need 16 to reduce it by a factor of 4. so for my intents and purposes over 100 or so frames isnt going to make a difference. i restacked it and learned that i had a wrong combination method on. so here is what it looks like stacked up right. there is still a large lack of detail because of the lack of integration time however.
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I tend to overdo things
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I tend to overdo things
How long you expose depends on read noise AND sky brightness. If you are limited to 60s exposures, and you are reaching 1/3rd histogram in that amount of time, then your limited by light pollution. Your totally swamping your read noise pretty quickly, and the light pollution itself becomes the primary source of noise. That also means that your object signal is fairly weak.
With a 7D II, your FWC (or saturation point) at ISO 1600 is 2230e-, which means the 1/3rd point is around 740e-. To reach 740e- signal in 60s with 59% Q.E. you have a total photon flux rate of ~21 photons per second. That is fairly high. I'd say it's moderate light pollution, yellow zone approaching an orange zone. The faint outer regions of Orion Nebula are delivering only a few photons per second, meaning your object signal is completely dominated not by read noise, but photon shot noise.
In that particular scenario, the recommendation is to get not necessarily to longer exposures, but to get significantly more total integration time. That is the only real option to average out all the excess noise from light pollution once the LP is offset. The deeper into a light polluted area you go, from yellow to orange to red to white zones, the shorter your sub exposures will have to be, and the more total subs you will need in order to compensate.
Why? Because object signal flux is for all intents and purposes a constant. It's the same, whether you are in a city or out at a dark site. If the object flux is 5 photons per second, then it's 5 photons per second in a white zone just the same as in a yellow zone same as at a dark site in a blue zone. In a white zone you might be limited to 10-15 second exposures before reaching 1/3rd histogram. You might have 720e- signal from LP and 20e- signal from your DSO...on a brighter object. Instead of ~4.5e- photon shot noise per sub, you'll instead have over 27e- photon shot noise per sub. That is a lot of noise. It completely dwarfs the ~3e- read noise component. So you stack. You stack not nine subs, but 50, 100, 250 subs.
Why stack so many subs? Because it's how you average out the noise so you can reveal faint details (click on the image to see animated GIF):
This animation shows the difference in integrating more and more subs, roughly by factor of two increases, in a red/orange zone border with light pollution (measures ~19mag/sq"). By 64 subs the faintest details are visible, it took 240 subs to reduce the noise in those faintest details to a nice low level. Ironically, these 240 subs, which reached 1/3rd histogram and reflect 10 hours of total integration, still wouldn't equal even a 1 hour integration with longer subs at a dark site.
Using short subs is a LIMITATION enforced by imaging under bright, light polluted skies. I know you know this, because I know you often image at much darker skies. I think the people who read these posts, however, need to understand it. Simply exposing to 1/3rd is only part of the story. I agree, don't go past that, as your just throwing away dynamic range. But if you are limited in your exposure length by light pollution, to get the best results...get more subs. Get as many subs as you possibly can, and integrate them all together. That will average out all the excess noise added to your images by light pollution, and allow you to get great images DESPITE the limitations of short exposures.
However my recommendation isn't just to stick it out in a red zone and deal with the LP using lots and lots of short subs with extensive integration times in the tens of hours. My recommendation is to find a nearby dark site, go out there and set up your gear, and get 3-4 hours of integration with longer subs. Instead of 30 second subs or 60 second subs, use four, five, maybe even 10 minute subs if your site is dark enough. Still don't go over 1/3rd histogram...but because of the darker skies, you'll be able to expose longer per sub. The per-sub contrast will be significantly higher than even a 36-64x sub integration in a red zone. Stack a few hours of that stuff...and you will have an AMAZING image on your hands, with low noise throughout the entire image, from the brightest part of the object signal to the darkest parts of the background sky.
You can find good dark skies with this map:
Anything green or darker is good for an "imaging dark site". My own zone is in an area of mottled green, blue and grey. I would offer that you can find a good imaging dark site much closer to home than the average good visual dark site. My own is about 30 minutes away from my home in the city. My visual dark site on the other hand is about an hour and a half away, and it's only marginally darker from an actual sky brightness standpoint (21.5mag/sq" average vs. 21.3mag/sq" average.)
That is, in my opinion, the whole story. I hope this knowledge helps beginners understand the reason imaging from a light polluted back yard can be difficult, and why sometimes people use extensive integration times, and why finding and y
With a 7D II, your FWC (or saturation point) at ISO 1600 is 2230e-, which means the 1/3rd point is around 740e-. To reach 740e- signal in 60s with 59% Q.E. you have a total photon flux rate of ~21 photons per second. That is fairly high. I'd say it's moderate light pollution, yellow zone approaching an orange zone. The faint outer regions of Orion Nebula are delivering only a few photons per second, meaning your object signal is completely dominated not by read noise, but photon shot noise.
In that particular scenario, the recommendation is to get not necessarily to longer exposures, but to get significantly more total integration time. That is the only real option to average out all the excess noise from light pollution once the LP is offset. The deeper into a light polluted area you go, from yellow to orange to red to white zones, the shorter your sub exposures will have to be, and the more total subs you will need in order to compensate.
Why? Because object signal flux is for all intents and purposes a constant. It's the same, whether you are in a city or out at a dark site. If the object flux is 5 photons per second, then it's 5 photons per second in a white zone just the same as in a yellow zone same as at a dark site in a blue zone. In a white zone you might be limited to 10-15 second exposures before reaching 1/3rd histogram. You might have 720e- signal from LP and 20e- signal from your DSO...on a brighter object. Instead of ~4.5e- photon shot noise per sub, you'll instead have over 27e- photon shot noise per sub. That is a lot of noise. It completely dwarfs the ~3e- read noise component. So you stack. You stack not nine subs, but 50, 100, 250 subs.
Why stack so many subs? Because it's how you average out the noise so you can reveal faint details (click on the image to see animated GIF):
This animation shows the difference in integrating more and more subs, roughly by factor of two increases, in a red/orange zone border with light pollution (measures ~19mag/sq"). By 64 subs the faintest details are visible, it took 240 subs to reduce the noise in those faintest details to a nice low level. Ironically, these 240 subs, which reached 1/3rd histogram and reflect 10 hours of total integration, still wouldn't equal even a 1 hour integration with longer subs at a dark site.
Using short subs is a LIMITATION enforced by imaging under bright, light polluted skies. I know you know this, because I know you often image at much darker skies. I think the people who read these posts, however, need to understand it. Simply exposing to 1/3rd is only part of the story. I agree, don't go past that, as your just throwing away dynamic range. But if you are limited in your exposure length by light pollution, to get the best results...get more subs. Get as many subs as you possibly can, and integrate them all together. That will average out all the excess noise added to your images by light pollution, and allow you to get great images DESPITE the limitations of short exposures.
However my recommendation isn't just to stick it out in a red zone and deal with the LP using lots and lots of short subs with extensive integration times in the tens of hours. My recommendation is to find a nearby dark site, go out there and set up your gear, and get 3-4 hours of integration with longer subs. Instead of 30 second subs or 60 second subs, use four, five, maybe even 10 minute subs if your site is dark enough. Still don't go over 1/3rd histogram...but because of the darker skies, you'll be able to expose longer per sub. The per-sub contrast will be significantly higher than even a 36-64x sub integration in a red zone. Stack a few hours of that stuff...and you will have an AMAZING image on your hands, with low noise throughout the entire image, from the brightest part of the object signal to the darkest parts of the background sky.
You can find good dark skies with this map:
Light pollution map
Interactive world light pollution map. The map uses NASA Black marble VIIRS, World Atlas 2015, Aurora prediction, observatories, clouds and SQM/SQC overlay contributed by users.
www.lightpollutionmap.info
Anything green or darker is good for an "imaging dark site". My own zone is in an area of mottled green, blue and grey. I would offer that you can find a good imaging dark site much closer to home than the average good visual dark site. My own is about 30 minutes away from my home in the city. My visual dark site on the other hand is about an hour and a half away, and it's only marginally darker from an actual sky brightness standpoint (21.5mag/sq" average vs. 21.3mag/sq" average.)
That is, in my opinion, the whole story. I hope this knowledge helps beginners understand the reason imaging from a light polluted back yard can be difficult, and why sometimes people use extensive integration times, and why finding and y
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There have been a lot of discussion regarding exposure max referencing 1/3 to 1/4. Just so I, and maybe others, are clear as to what is being stated here - is the attached what you would consider the max 1/3 exposure (such that read noise is met and so as to not loose DR)?
Jon, you missed critical points. The histogram on the back of the LCD is a tone curve variable gamma plot. The 1/3 point on that histogram is at about the 3% level of full scale. If full scale is 2230 electrons, that means the sky peak is about 67 photons, over 11 times lower than your number, or less than 2 photons per second. That is not hard to achieve, even from airglow at a dark site.
So with sky + dark current at ~2 photons per second, and 1 minute exposures, that is ~120 photons. Noise with 2.4 electron read noise would be: sqrt (120 +2.4^2)= 11.21 electrons. If read noise were zero, noise would be sqrt (120) = 10.95. I would say that difference is insignificant. Stack 100 images and you get noise = 1.1 versus 1.0, again you couldn't see the difference in the final images.
And that is with a sensor available today.
This says nothing about sub-exposure length. It just says more total exposure improves S/N. We knew that.
So what is the point? If you had 10 hours of integration with ten 1-hour subs versus 240 2.5 minute subes, you wouldn't see a difference (assuming the 1-hour subs didn't clip). And with a camera with under 3 electron read noise and 1/4 to 1/3 histogram, 240 1-minu8te subs would show the same result. The KEY is total integration time, not total sub exposures.
That is also incomplete. Short subs, even from dark sites, as long as you can make read noise insignificant compared to the noise from airglow (even when no light pollution) and dark current, give more dynamic range, saturating fewer bright stars. Again, if read noise were zero, sub exposure could be 1/100 second (video). Total integration time is what matters.
But I get to 1/3 histogram from dark sites with f/2.8 optics. I also image with f/1.4 optics and from very dark sites and even at 30 second subs, the histogram peak is high. You recommendation of longer subs is out of date, based on old concepts and not on actual science. If you have a low read noise system, shorter and shorter subs are fine, just keep total integration time the same. If you have a camera with under 3 electron read noise, 1/4 histogram peak sky is plenty.
Advantages of short subs: 1) Minimizes tracking errors, 2) more subs mean better rejection of airplanes and satellites, 3) with dithering, better rejection of fixed pattern problems and hot pixels.
The key to astro imaging faint stuff is not simply long exposure time. It is Etendue * exposure time.
Roger
Imaging efficiency is severely limited by light pollution. More than even read noise, which is why I wasn't talking about read noise in the quote above. Read noise compounds as you stack more and more subs, so it's less efficient to stack lots of short subs than to stack fewer long subs...but that pales in comparison to imaging under light polluted skies in the first place. Light pollution will limit your maximum exposure length, effectively forcing you to use more shorter subs to overcome all the extra noise from the light pollution itself, which will be left behind after subtracting the LP signal out of the image. (That in turn will increase the total amount of read noise in your integration, and as you increase sub count, read noise just keeps compounding. It's not as big a problem as the LP, but it keeps growing...so it could become a problem. But that is a secondary concern while your still imaging with light pollution.)
Your missing the link between sub length and sky darkness (or brightness). This is the critical point I think is lost in most conversations on this subject. I used 10 hours worth of 150 second subs with an IDAS LPS-P2 filter. The filter gives me about an extra stop and a third in terms of maximum exposure length. Without the filter, I'd have been limited to around 60-75 seconds of exposure.
In a light polluted zone, I couldn't get one hour subs. Even with the 150 seconds subs I did get, as much of the signal is from LP as it is from the object, at the very least. If I tried to get one hour subs in my light polluted back yard, I'd have totally clipped white frames.
However, go from an 18.5mag/sq" light polluted back yard (average red zone), to a 21.5mag/sq" dark zone (green/blue zone border on into blue zone, where your average astronomical society dark site is likely to be), and light pollution is ~16x less (2.512^(21.5-18.5)). That's four stops less. Assuming you had fully transparent skies, then you would probably be able to get around 960 second subs. (To get hour long subs, you would need even darker skies...and I am not even sure you could get 1-hour subs without narrow band filters.) Those 960 second subs are mostly object signal, with very little signal from artificial light (light pollution and airglow).
Those 10 960-second dark site subs are going to be RADICALLY superior to the light polluted subs. That actually has very little to do with read noise as well...it has to do with what the signal in the image is. Instead of it being mostly light pollution, which must be offset (subtracted out), it's mostly your deep space object. That has a MASSIVE impact on the difference between 10 960 second subs at a dark site, and 240 150 second subs in a light polluted back yard.
You don't even need to take it to such extremes. My dark site which averages around 21.3mag/sq", is usually tainted by classic Colorado inversion layers, so it's not highly transparent, but it is quite a bit darker than my back yard. I get up to around 10-12 minute subs on more transparent nights, or about 5-7 minutes on nights when an inversion layer is in place. The difference is still massive, though, because I don't have the extra light pollution.
It's not just that simple, though. Read noise wasn't even what I was referring to, it was the noise intrinsic to the extra light your getting from light pollution that I was referring to. Light pollution signal doesn't matter, it's not a signal you want, your going to eliminate it by offsetting the black point and subtracting out any gradients. However the noise in the light pollution signal does matter. Once you offset the LP signal, your left with the object signal...and all the various noise terms. One of those terms is the noise that was left behind after you subtracted out the light pollution, and in yellow or brighter zones on a Bortle scale map, LP tends to be the larger signal, and it's noise tends to be the largest noise term.
For a given exposure length, object signal is going to be the same regardless of whether you are imaging in a light polluted city or at a nice dark site. If your object photon flux is 10e-/m/px, then it's 10e-/m/px regardless of whether you are also getting additional photons from LP or not. In a red zone, you could be getting as many as 25 times or more the photons from LP as from your object, all being reflected off of the atmosphere back into your scope. So while your object signal is 10e-/m, your LP signal (skyfog signal) might be as much as 250e-/m. In one minute, you might have a 260e- signal...but most of it is unwanted. Your object is still just 10e-. If you were at a pristine dark site (22mag/sq") that was only limited by airglow and got that same signal, you might have only 1e- signal from airglow, and the rest would be your object signal. Your signal strength would be 11e-. If your read noise is 3e-, then your SNR is 10/SQRT(10 + 1 + 3^2) = 2.24:1. In the red zone, your SNR is 10/SQRT(10 + 250 + 3^2) = 0.61:1.
You don't have much option in the red zone but to suck it up and get lots and lots of shorter subs. However if you could handle 260e- worth of signal in the red zone, why couldn't you handle it at the dark site? You might as well go for 24 minute subs, in which case you would have 240e- from your object, and 24e- from airglow, and your SNR would be 240/SQRT(240 + 24 + 3^9), or 14.53:1. The long dark site sub is a whole different ball game. That one single sub is better than 24 subs from the red zone. A stack of 24 subs from the red zone would actually only have an SNR of 3:1: (10 * 24)/SQRT(24 * (10 + 250 + 3^2)) = 2.99:1! To get the same final SNR of ~14.5:1 from a light polluted zone, after subtracting out the light pollution, you would have to stack nearly of 600 subs!!!!
As for read noise itself. If you stuck with the 1 minute exposures at the dark site, you would need to stack more than 24 subs to get the same SNR as with a single 24 minute sub. I know you know that, Roger, as you noted it in your comment above...but for other readers here. You would need around 42 subs, because of the compounding of read noise. SNR would be (10 * 42)/SQRT(42* (10 + 1 + 3^9)) = 14.5:1. A single 24 minute sub or 42 one minute subs. The choice there should be pretty easy.
Personally, if your willing to image for 42 minutes, you might as well go for two 24 minute subs and your SNR will be up to 20.5:1. Having plenty of first hand experience imaging in both my 18.8-19mag/sq" red zone back yard and a 21.3mag/sq" green zone dark site, I have to disagree that getting longer subs under darker skies doesn't matter. Even if you have a camera like the 7D II...it matters. It matters a lot! It also doesn't really have anything to do with read noise...even with the 7e- read noise of a KAF-8300 CCD sensor, read noise isn't the differentiating factor here. Once your at dark skies, depending on the exposure length choices you make, read noise might then become a factor, as the more subs you stack, the more read noise you have, and that will affect your imaging efficiency while at the dark site...but it's not the reason you would move out of the red zone back yard and into a green, blue or gray zone dark site in the first place.
This is really only viable if you are either A) using fast camera lenses, or B) using specialized ultra fast telescopes like the Celestron with Hyperstar, where you can get around f/2 optics. Not everyone who does astrophotography uses camera lenses, and not many can handle tracking and guiding something like an 11" f/2 EdgeHD Hyperstar. Not many camera lenses that get you nebula-sized fields of view are f/2.8 and none to my knowledge are f/1.4. If your doing ultra wide field imaging, it's kind of a different ballgame...gathering signal isn't really a problem. You face other problems...I won't get into those here.
For those who are using legitimate telescopes (or as in my case, a camera lens that basically is a telescope), f-ratios for common refractors tend to be around f/4, f/5.5, maybe f/6.8. If you get into RCs or SCTs, then your looking at f/8 or f/10, and with reducers anywhere around f/5.5-f/7.1 or so. Most of the deep sky astrophotographers I know...well, the majority of them...use short refractors, around 400-600mm focal length, with native f-ratios of around f/4-f/5, maybe in a few cases with ultra fast refractors you might have f/3.8 or around there. Slower f-ratios require longer exposures. Even with an efficient sensor.
Tracking errors are an interesting thing as well. They are easy to handle with very short exposures. However they are also easy to handle with much longer exposures, assuming you can overcome the PE of your mount (takes some skill, but it's more than possible and most astrophotographers can do it once they learn). It's the midrange exposure lengths that actually get tricky. When I expose at around 200-300 seconds, that's when I tend to have the worst eccentricity in my stars. However 600-720 seconds (720 is about the limit with my EF 600mm f/4 lens setup before differential flexure starts causing problems) and the handful of momentary tracking errors that might mess up a five minute exposure are just absorbed and either have no visible impact or minimal visible impact. Throw in 20, 30, 60 minute narrow band subs, and tracking is actually less of a problem than it is with those medium length exposures. (At least...in my experience, and the experience of a number of more hard core narrow band imagers I know. YMMV.)
If I'm to offer advice, I have to offer such that any kind of astrophotographer can benefit from it. I cannot make the assumption that everyone is using an f/2.8 lens. I also cannot make the assumption that everyone is using a nice new 7D II under fairly dark skies, either. Most of the beginners I encounter are looking at the $175 used astro-modded Canon Rebel XSi DSLR on AstroMart or CNC. They aren't looking to spend $1500 on a 7D II...they probably aren't even looking to spend as much as $1500 in total at all!
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