I’ve never quite figured out a way to decide on exposure times. None of the tutorials or tricks ever seemed to quite work for me. Now that I understand it a lot better, I realize it’s because the details of my acquisition are quite a bit different than most people’s. The most common thing I hear is to get the histogram at least 1/3 to the left. In a Bortle 2, that’s hard to do because there’s no light pollution affecting those pixels. 30s and 300s still have me far to the left. So, that doesn’t work.
The other common thing I’ve heard is to “swamp the read noise”. I’ve done all the work there and used the calculators. Before I get into why that’s a problem, I’ll mention the third common thing I’m told to watch out for is oversaturating too many pixels (no more than a few hundred). So, here’s the problem I face: In a Bortle 2, my optimal exposure time is upwards of 4 minutes to swamp the read noise. But with a filterless 8” f/4, I’m saturating stars in less than 30 seconds. So, that leaves me with a dilemma. I can only seem to do one or the other.
For the time being, I’d really just like a way to take a few test shots of my target, look at the metrics on screen, and make at least a generally educated guess as to how long I should expose for. But unless I’m missing something, I’m limited to short exposures to avoid blowing out all the stars.
ASI2600MC
Meade SN8
Bortle 2/Broadband
N.I.N.A.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
I almost always shoot 3 minute (180") subs, mainly because there's less of a chance of airplane or satellite streaks compared to a 5-10 minute sub. Also, depending upon how good your guiding is that night, you might want to increase or decrease exposure time.
I'm using the same camera as you are (2600MC) if that matters. If you're shooting something with a very bright area, i.e. M42, probably best to shoot shorter duration subs to avoid blowing out the core.
I often get good results with 3-minute subs so that has become the default. But as everything in astrophotography, sometimes defaults need to be changed to adapt to changing sky conditions.
Tonight I am imaging M15 and brought the sub exposure time down to 60" to try and avoid the core of the globular from being overexposed.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Are you getting blown out stars? My guiding is fine. I’m capable of going as long as I want. But my stars look terrible.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Terrible in what way? Not sharp? Not round? Trails?
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
They’re completely blown out. No color. Just bright white. I’m saturating thousands and thousands of pixels. Even at 0 gain, I’m limited to 60 seconds. Sometimes 30 or less depending on the star field.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Can you post a sub? It's hard to tell what's going on in the abstract.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
I don’t. I purposely avoid that so I don’t have any images of it. The only time i see it is when taking test subs to figure out exposure times. If I’m saturating 6,000 pixels, I back off. None of those images get saved.
But now that I’m thinking about it, I’m wondering if I’m putting too much weight into what I was told about “no more than a few hundred pixels”. A larger sensor will pick up more stars and, therefore, risk saturating more pixels. So, it doesn’t make sense to hold two different cameras to the same “few hundred” standard. I suspect focal length plays a part as well. Although all stars are pinpoints of light, the atmosphere spreads that light out. So, something like Sadr will cover more pixels in a 1000mm scope than a 250mm scope. I’m mostly just thinking out loud at this point.
i did read something earlier that said, instead of a few hundred, you could saturate .01%-.03% of the pixels. That’s a much better comparison and would allow me, on the high end, to saturate closer to 8,000 pixels. That, in turn, would allow exposure times that get me closer to swamping the read noise.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Good luck figuring it out!
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
@Jaymz Bondurant regarding read noise, I used the following article to understand how to calculate the ideal camera offset to overcome my camera sensor's read noise: https://www.baader-planetarium.com/en/blog/gain-and-offset-darks-flats-and-bias-at-cooled-cmos-cameras/Once the target ADU was calculated for my camera and dialed into NINA (I have the ASI 2600MM Pro), I use the Optimal Exposure Calculator as a starting point to find the desired exposure time. From experience, I've found in Bortle 1-3 locations that the calculator results are about 60s, but I usually increase the time up to 120s if my guiding will allow for it. Subs imaged using unity gain need an offset of 3 to give me an average ADU of ~500, which is well above the 300 ADU's calculated using the Baader article to keep my shot noise above the sensor's read noise. This results in the histogram peak within the left 1/3 of the spectrum and stars retaining their natural colour. If imaging bright targets or galaxies, you may want to decrease your gain to increase dynamic range, but reduce your offset to 0. This will keep the mean ADU of your subs above the target value which was calculated using the Baader article. Someone already mentioned using multiple exposure times to capture regions of varying brightness; that's a great piece of advise which I also use. Clear Skies, Anuraag
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
That is an interesting post. I always struggle to figure out the best exposure times. What I do, and I'm no expert, is I expose for my DSO to get the histogram away from the left side. I use whatever exposure is needed and don't worry about the stars. Then I do a separate run for the stars at a drastically reduced time, like 30 seconds. Take the stars from the DSO image set and replace them with shorter exposures. Otherwise, I don't know how you can do one exposure and capture something extremely faint and bright in the same image.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Anuraag Bhardwaj:
I used the following article to understand how to calculate the ideal camera offset to overcome my camera sensor's read noise: I've (mostly) got that part down. My dilemma is finding a balance between that optimal exposure time and not saturating all the stars. When I used the exposure calculator the other night in N.I.N.A. (which uses the data I gathered from the sensor analysis in SharpCap), it said I needed about 4 minute exposures. As I mentioned, I start saturating stars in less than 30 seconds. So, I guess my issue is the fact that those numbers are so wildly different. If the calculator said I needed 3 minutes and I was saturating stars at 2 1/2 minutes, I could work with that and find a balance. But with one being 4 minutes and the other being less than 30 seconds, it makes me think there's something wrong. Not necessarily with the optics but with the thinking. Here's what I can't wrap my head around: Let's say you and I go to image together. We set up side by side and point at the same target. We both do 180 second exposures. I know from experience that I will saturate thousands of pixels in that situation. What I don't get is why you're not doing the exact same thing. I understand optics may be different. You might have a slower focal ratio and whatnot. But that also means that you'd need a longer exposure to swamp the read noise. So, in the long run, you should be in the same situation as me. How many saturated pixels do you have in that situation?
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Generally speaking, there is enough dynamic range in most astronomical subjects that you are not going to be able to simultaneously swamp read noise and avoid clipping the stars. You will have four choices: expose for the shadows and lose the stars, expose for the stars and lose some shadows to the higher noise floor, find a compromise exposure that balances the two, or take separate exposures for stars and subject.
With narrowband subjects, I would recommend separate exposures for stars and main subject since most people prefer RGB stars along with the HOO or SHO main subject. For broadband, I would recommend the "compromise" approach of not clipping too many stars, while still choosing subexposure durations that let you get deep into the shadows with a minimum of integration time. The particular compromise may vary depending on the subject and how important the faintest areas of the subject are to the overall quality of the image. Also, keep in mind that additional integration time will help even if you don't have "optimal" exposures to swamp read noise.
The usual recommendation is to take subs that are long enough to reach either 5x (read noise)^2 or 10x (read noise) ^2. Under Bortle 2 skies that may be a fairly long subexposure depending on the gain you choose. I would recommend on settling on either 0 gain or 100 gain. 0 gain will (technically) have the highest dynamic range, allowing you to clip the fewest stars while reaching deep into the shadows. Likely, you will find your read noise is somewhere around 3.2 e- and that your gain is somewhere around 0.78 e-/ADU. If that's the case (you'll have to measure it), then 5x rn ^2 would be around 50 e- which should be around 64 ADU above the offset. So, if you take a bias frame (or a dark frame--they will be about the same) and look at the median ADU, you'd want an exposure that has a median around 64 ADU above that value in empty areas of sky at, say, 50 degrees altitude (a compromise value that likely represents the average for your subjects across the night). If you have a monochrome camera, I would measure this with all three filters. If I were shooting with a relatively wide field of view, as you are, I would go with the 5x (read noise) ^2 rather than 10x simply because you are likely to have more bright stars in the field that are going to saturate.
How many pixels are saturating with that subexposure duration on gain 0? If it's thousands and thousands, you may want to consider separate exposures for stars and nebulosity, a la narrowband imaging. If it is 1,200 or fewer (an arbitrary line), I would just go with this as the standard, and only deviate when there is a reason to.
You can run the same calculations at a gain setting of 100. For that, you will likely find a read noise somewhere around 1.3 e- and a gain of around 0.25 e-/ADU. You'll have to measure your camera. So, you would want to expose till you had a mean ADU of around 34 or so above your bias frame median. That would be 5x (read noise) ^2 assuming your camera roughly matches ZWO's specifications. This will result in a much lower subexposure duration to "swamp" read noise, but will likely have slightly more clipped stars (65,535 ADU or so--not every camera quite reaches that value).
Which is the better choice? Gain 100 or gain 0? Assuming similar numbers of clipped stars? I generally prefer the lower gain since dynamic range is usually marginally better, and I'm happier having a bunch of five minute exposures rather than a whole lot of one minute exposures--less storage and easier processing. You'll have to answer that one for yourself, though. I think more people standardize on gain 100 than gain 0.
Don't feel bad if you find that 5x (read noise) ^2 results in too many clipped stars. That isn't an indication of a problem--it's an indication of really, really dark skies! That's a good thing. If you need to cut back on subexposure duration to avoid clipping too many, so be it. You may not be swamping read noise in each exposure, but that's only because you have so little shot noise! Definitely good. You'll have to use your judgment on how many clipped pixels is too many. It will depend on your taste, processing, and on your subject. You can always take separate shorter exposures for the stars if you want to, but at least figure out what the "optimum" exposure is for the dark portions of the image. That will give you a baseline of, "well, I probably will never need longer exposures than this."
Hope this helps.
- Jared
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
That is an interesting post. I always struggle to figure out the best exposure times. What I do, and I'm no expert, is I expose for my DSO to get the histogram away from the left side. I use whatever exposure is needed and don't worry about the stars. Then I do a separate run for the stars at a drastically reduced time, like 30 seconds. Take the stars from the DSO image set and replace them with shorter exposures. Otherwise, I don't know how you can do one exposure and capture something extremely faint and bright in the same image. I did think of that. But I have concerns about the brighter stars; especially the really bright ones like Sadr and Betelgeuse. Long exposures around bright stars produce halos that will remain in the starless version. I'm afraid putting smaller stars back in that don't match those halos would look very odd. I thought about just doing that and not even making this post. But when I decide to shoot the Sadr region and that trick doesn't work, I'm gonna right back here making this same post anyway. But it may be the way to go for most shots. It's a little bit more work. But I bet the stars are incredible because of it.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Jaymz Bondurant:
Anuraag Bhardwaj:
I used the following article to understand how to calculate the ideal camera offset to overcome my camera sensor's read noise:
I've (mostly) got that part down. My dilemma is finding a balance between that optimal exposure time and not saturating all the stars. When I used the exposure calculator the other night in N.I.N.A. (which uses the data I gathered from the sensor analysis in SharpCap), it said I needed about 4 minute exposures. As I mentioned, I start saturating stars in less than 30 seconds. So, I guess my issue is the fact that those numbers are so wildly different. If the calculator said I needed 3 minutes and I was saturating stars at 2 1/2 minutes, I could work with that and find a balance. But with one being 4 minutes and the other being less than 30 seconds, it makes me think there's something wrong. Not necessarily with the optics but with the thinking.
Here's what I can't wrap my head around: Let's say you and I go to image together. We set up side by side and point at the same target. We both do 180 second exposures. I know from experience that I will saturate thousands of pixels in that situation. What I don't get is why you're not doing the exact same thing. I understand optics may be different. You might have a slower focal ratio and whatnot. But that also means that you'd need a longer exposure to swamp the read noise. So, in the long run, you should be in the same situation as me. How many saturated pixels do you have in that situation? The darker the skies, the more stars you will saturate (regardless of camera and telescope) if you choose an exposure that swamps read noise. This isn't a problem--it's just an indication that you have very little light pollution, so read noise is proportionally higher. In absolute terms, your read noise didn't get any worse, but because other sources of noise are so much better (lower), you need a longer exposure for that better (lower) noise to dominate over read noise. So, you may well choose to use shorter subs than "optimal" because you are saturating too many stars. You'll still get better data in a given integration time than someone with more light pollution. In addition, I have found that shorter focal lengths tend to clip more pixels just because you are increasing the chances that there are some bright stars in the wider field of view. A couple seventh or eight magnitude stars alone can be responsible for a hundred or more saturated pixels.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Jaymz Bondurant:
That is an interesting post. I always struggle to figure out the best exposure times. What I do, and I'm no expert, is I expose for my DSO to get the histogram away from the left side. I use whatever exposure is needed and don't worry about the stars. Then I do a separate run for the stars at a drastically reduced time, like 30 seconds. Take the stars from the DSO image set and replace them with shorter exposures. Otherwise, I don't know how you can do one exposure and capture something extremely faint and bright in the same image.
I did think of that. But I have concerns about the brighter stars; especially the really bright ones like Sadr and Betelgeuse. Long exposures around bright stars produce halos that will remain in the starless version. I'm afraid putting smaller stars back in that don't match those halos would look very odd. I thought about just doing that and not even making this post. But when I decide to shoot the Sadr region and that trick doesn't work, I'm gonna right back here making this same post anyway. But it may be the way to go for most shots. It's a little bit more work. But I bet the stars are incredible because of it. It's less of a problem than you might think. The halos are still there in 30s exposures, it's just that the default stretching will be less aggressive, so halos will look less pronounced and stars will look smaller. If you stretch the star image to the point that it is an appropriate match to the stretch on the nebulosity (judgement call on "appropriate match"), then Betelgeuse and Sadr will swell up in the star image just as they did in the starless image. I think expecting to maintain star color to the core of Betelgeuse and Sadr is not realistic--I wouldn't try to do that.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
I get 180s with a f/2.8 optics and don't saturate that many stars on an average field (e.g. not M45 or M42) and I can't see this being a problem at f/4. In fact I'd have exactly zero concerns abot that. Note that in the main I either shoot at f/4 or f/2.8 or f/2.0.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Jared Willson:
Generally speaking, there is enough dynamic range in most astronomical subjects that you are not going to be able to simultaneously swamp read noise and avoid clipping the stars. You will have four choices: expose for the shadows and lose the stars, expose for the stars and lose some shadows to the higher noise floor, find a compromise exposure that balances the two, or take separate exposures for stars and subject.
With narrowband subjects, I would recommend separate exposures for stars and main subject since most people prefer RGB stars along with the HOO or SHO main subject. For broadband, I would recommend the "compromise" approach of not clipping too many stars, while still choosing subexposure durations that let you get deep into the shadows with a minimum of integration time. The particular compromise may vary depending on the subject and how important the faintest areas of the subject are to the overall quality of the image. Also, keep in mind that additional integration time will help even if you don't have "optimal" exposures to swamp read noise.
The usual recommendation is to take subs that are long enough to reach either 5x (read noise)^2 or 10x (read noise) ^2. Under Bortle 2 skies that may be a fairly long subexposure depending on the gain you choose. I would recommend on settling on either 0 gain or 100 gain. 0 gain will (technically) have the highest dynamic range, allowing you to clip the fewest stars while reaching deep into the shadows. Likely, you will find your read noise is somewhere around 3.2 e- and that your gain is somewhere around 0.78 e-/ADU. If that's the case (you'll have to measure it), then 5x rn ^2 would be around 50 e- which should be around 64 ADU above the offset. So, if you take a bias frame (or a dark frame--they will be about the same) and look at the median ADU, you'd want an exposure that has a median around 64 ADU above that value in empty areas of sky at, say, 50 degrees altitude (a compromise value that likely represents the average for your subjects across the night). If you have a monochrome camera, I would measure this with all three filters. If I were shooting with a relatively wide field of view, as you are, I would go with the 5x (read noise) ^2 rather than 10x simply because you are likely to have more bright stars in the field that are going to saturate.
How many pixels are saturating with that subexposure duration on gain 0? If it's thousands and thousands, you may want to consider separate exposures for stars and nebulosity, a la narrowband imaging. If it is 1,200 or fewer (an arbitrary line), I would just go with this as the standard, and only deviate when there is a reason to.
You can run the same calculations at a gain setting of 100. For that, you will likely find a read noise somewhere around 1.3 e- and a gain of around 0.25 e-/ADU. You'll have to measure your camera. So, you would want to expose till you had a mean ADU of around 34 or so above your bias frame median. That would be 5x (read noise) ^2 assuming your camera roughly matches ZWO's specifications. This will result in a much lower subexposure duration to "swamp" read noise, but will likely have slightly more clipped stars (65,535 ADU or so--not every camera quite reaches that value).
Which is the better choice? Gain 100 or gain 0? Assuming similar numbers of clipped stars? I generally prefer the lower gain since dynamic range is usually marginally better, and I'm happier having a bunch of five minute exposures rather than a whole lot of one minute exposures--less storage and easier processing. You'll have to answer that one for yourself, though. I think more people standardize on gain 100 than gain 0.
Don't feel bad if you find that 5x (read noise) ^2 results in too many clipped stars. That isn't an indication of a problem--it's an indication of really, really dark skies! That's a good thing. If you need to cut back on subexposure duration to avoid clipping too many, so be it. You may not be swamping read noise in each exposure, but that's only because you have so little shot noise! Definitely good. You'll have to use your judgment on how many clipped pixels is too many. It will depend on your taste, processing, and on your subject. You can always take separate shorter exposures for the stars if you want to, but at least figure out what the "optimum" exposure is for the dark portions of the image. That will give you a baseline of, "well, I probably will never need longer exposures than this."
Hope this helps.
- Jared Jared, Thank you so much for the detailed response. While I don't quite get all the math, you've definitely put my mind at east a bit. It's been frustrating reading so many different things. I'm used to that with this hobby. But when Joe, Bob, and Sally have three different answers, at least they're usually in the ballpark of each other. Upon further reading this morning, I've found answers ranging anywhere from 200 to 2% which would equate to about half a million on my camera. Understandably, I'm not quire sure what to do with that information. I mentioned that I don't quite get all the math. But I have run the sensor analysis in SharpCap and applied it to the exposure calculator in NINA. So, I should be ok there. I've only tinkered with it so far. But all the numbers are in there. All I have to do is point and click to get a number. Ideally, I prefer 0 gain for the same reason you do. I'd never tried it until the last few weeks. I'm not experienced enough to say for sure that it's not the target itself, but the first thing I noticed in that shot were the different shades of red I'd never seen before. I also saw the same thing in the Elephant Trunk shot I just did this morning. There seems to be a lot more detail and contrast. That being said, there's a reason I still use 100 whenever possible. Shooting broadband, I only shoot from a dark site with no moon. That limits how often I can shoot and, therefore, limits my integration time. I have no desire to spend months on one target. So, whatever I get that single night is it. Of all the shots in my gallery, I think the highest integration time is about 5 hours. With so little integration time, I need all the help I can get in reducing noise. 100 gain does that beautifully. But I've been toying around with broadband from my Bortle 6 backyard. For that, I can't imagine not doing 0 gain. The only reason I did Elephant Trunk at 0 gain the other night was because I was saturating the stars at 100 gain and didn't want to reduce to 30s exposures. The less subs to stack, the better. If I recall correctly, my first test shot at 100 gain/120 seconds saturated about 6,000 pixels. 100 gain/60 seconds saturated about 2600. And 0 gain/60 seconds saturated about 1300; which is what I went with. I did consider the separate exposure idea. But my recent experience with Sadr and one of the bright stars in Cygnus led me to realize the problem I described in an above post. And that is the large halos those bright stars leave in the long exposure starless image. Although not ideal, it still looks "normal" putting that bright star back in there. But putting a small star back into a large halo wouldn't look quite right. I know there are ways to fix that. But with my current processing skills, I think that would present a bigger problem than the one I'm trying to solve. But I'm definitely considering it for the other 90% of targets I plan on shooting. I'm unfortunately restricted to doing all of my processing on a rather slow laptop. So, I'd love nothing more than to be able to set every sequence to 300 seconds and let it go to save processing time. But I'm also to a point where I think my images are finally getting good and I don't want to sacrifice that just because me and/or my laptop are lazy. If I have to take 20 second exposures every night to keep the saturated pixels under 1200, so be it. But if there's a compromise that allows me to go longer without sacrificing quality, then that's the route I want to go.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Jared Willson:
read noise is proportionally higher. In absolute terms, your read noise didn't get any worse, but because other sources of noise are so much better (lower), you need a longer exposure for that better (lower) noise to dominate over read noise. THAT just made a light bulb go off! Thank you! All of your responses have been very insightful!
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
andrea tasselli:
I get 180s with a f/2.8 optics and don't saturate that many stars on an average field (e.g. not M45 or M42) and I can't see this being a problem at f/4. In fact I'd have exactly zero concerns abot that. Note that in the main I either shoot at f/4 or f/2.8 or f/2.0. Are you judging "saturated stars" visually or are we talking about saturated pixels? Looking back through some of my images, it doesn't seem the stars are visually saturated. I think I have good color. But I'm told to watch out for saturated pixels. And that's the number that's getting too high. I shot Elephant Trunk the other night and saturated over 6,000 pixels in 120s.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
The only way to judge saturated stars is by visually inspecting them, certainly not by looking at how many saturated pixels you get per shot (and that's assuming there are zero hot pixels, which isn't the case). 6000 pixels over 26 millions is just a statistical irrelevancy.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Jaymz Bondurant:
Jared Willson:
Generally speaking, there is enough dynamic range in most astronomical subjects that you are not going to be able to simultaneously swamp read noise and avoid clipping the stars. You will have four choices: expose for the shadows and lose the stars, expose for the stars and lose some shadows to the higher noise floor, find a compromise exposure that balances the two, or take separate exposures for stars and subject.
With narrowband subjects, I would recommend separate exposures for stars and main subject since most people prefer RGB stars along with the HOO or SHO main subject. For broadband, I would recommend the "compromise" approach of not clipping too many stars, while still choosing subexposure durations that let you get deep into the shadows with a minimum of integration time. The particular compromise may vary depending on the subject and how important the faintest areas of the subject are to the overall quality of the image. Also, keep in mind that additional integration time will help even if you don't have "optimal" exposures to swamp read noise.
The usual recommendation is to take subs that are long enough to reach either 5x (read noise)^2 or 10x (read noise) ^2. Under Bortle 2 skies that may be a fairly long subexposure depending on the gain you choose. I would recommend on settling on either 0 gain or 100 gain. 0 gain will (technically) have the highest dynamic range, allowing you to clip the fewest stars while reaching deep into the shadows. Likely, you will find your read noise is somewhere around 3.2 e- and that your gain is somewhere around 0.78 e-/ADU. If that's the case (you'll have to measure it), then 5x rn ^2 would be around 50 e- which should be around 64 ADU above the offset. So, if you take a bias frame (or a dark frame--they will be about the same) and look at the median ADU, you'd want an exposure that has a median around 64 ADU above that value in empty areas of sky at, say, 50 degrees altitude (a compromise value that likely represents the average for your subjects across the night). If you have a monochrome camera, I would measure this with all three filters. If I were shooting with a relatively wide field of view, as you are, I would go with the 5x (read noise) ^2 rather than 10x simply because you are likely to have more bright stars in the field that are going to saturate.
How many pixels are saturating with that subexposure duration on gain 0? If it's thousands and thousands, you may want to consider separate exposures for stars and nebulosity, a la narrowband imaging. If it is 1,200 or fewer (an arbitrary line), I would just go with this as the standard, and only deviate when there is a reason to.
You can run the same calculations at a gain setting of 100. For that, you will likely find a read noise somewhere around 1.3 e- and a gain of around 0.25 e-/ADU. You'll have to measure your camera. So, you would want to expose till you had a mean ADU of around 34 or so above your bias frame median. That would be 5x (read noise) ^2 assuming your camera roughly matches ZWO's specifications. This will result in a much lower subexposure duration to "swamp" read noise, but will likely have slightly more clipped stars (65,535 ADU or so--not every camera quite reaches that value).
Which is the better choice? Gain 100 or gain 0? Assuming similar numbers of clipped stars? I generally prefer the lower gain since dynamic range is usually marginally better, and I'm happier having a bunch of five minute exposures rather than a whole lot of one minute exposures--less storage and easier processing. You'll have to answer that one for yourself, though. I think more people standardize on gain 100 than gain 0.
Don't feel bad if you find that 5x (read noise) ^2 results in too many clipped stars. That isn't an indication of a problem--it's an indication of really, really dark skies! That's a good thing. If you need to cut back on subexposure duration to avoid clipping too many, so be it. You may not be swamping read noise in each exposure, but that's only because you have so little shot noise! Definitely good. You'll have to use your judgment on how many clipped pixels is too many. It will depend on your taste, processing, and on your subject. You can always take separate shorter exposures for the stars if you want to, but at least figure out what the "optimum" exposure is for the dark portions of the image. That will give you a baseline of, "well, I probably will never need longer exposures than this."
Hope this helps.
- Jared
Jared,
Thank you so much for the detailed response. While I don't quite get all the math, you've definitely put my mind at east a bit. It's been frustrating reading so many different things. I'm used to that with this hobby. But when Joe, Bob, and Sally have three different answers, at least they're usually in the ballpark of each other. Upon further reading this morning, I've found answers ranging anywhere from 200 to 2% which would equate to about half a million on my camera. Understandably, I'm not quire sure what to do with that information.
I mentioned that I don't quite get all the math. But I have run the sensor analysis in SharpCap and applied it to the exposure calculator in NINA. So, I should be ok there. I've only tinkered with it so far. But all the numbers are in there. All I have to do is point and click to get a number.
Ideally, I prefer 0 gain for the same reason you do. I'd never tried it until the last few weeks. I'm not experienced enough to say for sure that it's not the target itself, but the first thing I noticed in that shot were the different shades of red I'd never seen before. I also saw the same thing in the Elephant Trunk shot I just did this morning. There seems to be a lot more detail and contrast. That being said, there's a reason I still use 100 whenever possible. Shooting broadband, I only shoot from a dark site with no moon. That limits how often I can shoot and, therefore, limits my integration time. I have no desire to spend months on one target. So, whatever I get that single night is it. Of all the shots in my gallery, I think the highest integration time is about 5 hours. With so little integration time, I need all the help I can get in reducing noise. 100 gain does that beautifully. But I've been toying around with broadband from my Bortle 6 backyard. For that, I can't imagine not doing 0 gain. The only reason I did Elephant Trunk at 0 gain the other night was because I was saturating the stars at 100 gain and didn't want to reduce to 30s exposures. The less subs to stack, the better.
If I recall correctly, my first test shot at 100 gain/120 seconds saturated about 6,000 pixels. 100 gain/60 seconds saturated about 2600. And 0 gain/60 seconds saturated about 1300; which is what I went with.
I did consider the separate exposure idea. But my recent experience with Sadr and one of the bright stars in Cygnus led me to realize the problem I described in an above post. And that is the large halos those bright stars leave in the long exposure starless image. Although not ideal, it still looks "normal" putting that bright star back in there. But putting a small star back into a large halo wouldn't look quite right. I know there are ways to fix that. But with my current processing skills, I think that would present a bigger problem than the one I'm trying to solve. But I'm definitely considering it for the other 90% of targets I plan on shooting.
I'm unfortunately restricted to doing all of my processing on a rather slow laptop. So, I'd love nothing more than to be able to set every sequence to 300 seconds and let it go to save processing time. But I'm also to a point where I think my images are finally getting good and I don't want to sacrifice that just because me and/or my laptop are lazy. If I have to take 20 second exposures every night to keep the saturated pixels under 1200, so be it. But if there's a compromise that allows me to go longer without sacrificing quality, then that's the route I want to go. With regard to 200 pixels saturated vs. 2% of pixels, neither is right or wrong. It will depend on the subject, the processing, and what you care about in your astrophotos. I am not at all surprised to hear that you have seen such wide ranging answers. Personally, with a 24 megapixel camera, I don't think I'd go with either of those responses. They both seem too extreme. At 200 pixels, I suspect I would be giving up too much in terms of SNR in the fainter regions of my image. In your case, given you skies, I would definitely want to be going after subjects that benefitted from the dark skies, and that means faint stuff. Likewise, 2% seems waaay too high. I'd be clipping almost every star in the image. My guidance would probably be something in the high hundreds for clipped pixels with your camera and scope, but I'd want to see what that meant in terms of "optimal" exposure recommended by NINA or SharpCap. If getting down to fewer than a thousand saturated pixels meant 30s vs. a recommended 3 minutes, I think I'd learn to tolerate a couple thousand saturated pixels instead. In any event, there is no way to calculate a "right" answer. There is only "right" for your skies, images, equipment, and preferences. You'll have to experiment. Totally fine if you want to stick with 100 gain. A lot of people do. But your listed reasons for doing so are probably not the best. Your SNR is going to be determined almost exclusively by the total integration time with only slight advantages to either gain 0 or gain 100 based on the improved/worsened dynamic range of the camera. The edge is slightly in favor of gain 0. Just look at the graph on the ZWO website and you will see that dynamic range is about 13.9 stops at gain 0 and more like 13.8 at gain 100. When you go with gain 100 the read noise drops by a factor of about 2.4, but the full well capacity drops by a slightly larger factor of perhaps 2.6. Slight advantage to gain 0 overall for any given integration time. Gain 0 would add to that advantage slightly because you would likely spend less time dithering since you would be taking fewer overall exposures. The only time gain 100 has a distinct advantage is if you are shooting narrowband where you otherwise won't be able to come close to swamping read noise and clipping stars is all but irrelevant, or if you would be limited to so few exposures at gain 0 that the error rejection (and Drizzle if you are using it) can't do a good job. For those two situations, I would stick with gain 100. For everything else, gain 0 is likely a better fit. Honestly, though, the difference is so slight that, if you just like gain 100 or know how to get the most out of it, it's probably not worth switching. For me, it's more about the total number of subs than anything else. I wouldn't want to deal with too many subs from a given target, but I will often spend an entire month of clear, moonless nights on one image in which case 60s subs would add up to a whole lot of subs. One way or another, the differences between gain 0 and gain 100 will be very small in terms of final image quality--it's more a question of storage and processing than it is image quality, even for a single night of integration.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Jared Willson:
With regard to 200 pixels saturated vs. 2% of pixels, neither is right or wrong. It will depend on the subject, the processing, and what you care about in your astrophotos. I am not at all surprised to hear that you have seen such wide ranging answers. Personally, with a 24 megapixel camera, I don't think I'd go with either of those responses. They both seem too extreme. At 200 pixels, I suspect I would be giving up too much in terms of SNR in the fainter regions of my image. In your case, given you skies, I would definitely want to be going after subjects that benefitted from the dark skies, and that means faint stuff. Likewise, 2% seems waaay too high. I'd be clipping almost every star in the image. My guidance would probably be something in the high hundreds for clipped pixels with your camera and scope, but I'd want to see what that meant in terms of "optimal" exposure recommended by NINA or SharpCap. If getting down to fewer than a thousand saturated pixels meant 30s vs. a recommended 3 minutes, I think I'd learn to tolerate a couple thousand saturated pixels instead. In any event, there is no way to calculate a "right" answer. There is only "right" for your skies, images, equipment, and preferences. You'll have to experiment.
Totally fine if you want to stick with 100 gain. A lot of people do. But your listed reasons for doing so are probably not the best. Your SNR is going to be determined almost exclusively by the total integration time with only slight advantages to either gain 0 or gain 100 based on the improved/worsened dynamic range of the camera. The edge is slightly in favor of gain 0. Just look at the graph on the ZWO website and you will see that dynamic range is about 13.9 stops at gain 0 and more like 13.8 at gain 100. When you go with gain 100 the read noise drops by a factor of about 2.4, but the full well capacity drops by a slightly larger factor of perhaps 2.6. Slight advantage to gain 0 overall for any given integration time. Gain 0 would add to that advantage slightly because you would likely spend less time dithering since you would be taking fewer overall exposures. The only time gain 100 has a distinct advantage is if you are shooting narrowband where you otherwise won't be able to come close to swamping read noise and clipping stars is all but irrelevant, or if you would be limited to so few exposures at gain 0 that the error rejection (and Drizzle if you are using it) can't do a good job. For those two situations, I would stick with gain 100. For everything else, gain 0 is likely a better fit. Honestly, though, the difference is so slight that, if you just like gain 100 or know how to get the most out of it, it's probably not worth switching. For me, it's more about the total number of subs than anything else. I wouldn't want to deal with too many subs from a given target, but I will often spend an entire month of clear, moonless nights on one image in which case 60s subs would add up to a whole lot of subs. One way or another, the differences between gain 0 and gain 100 will be very small in terms of final image quality--it's more a question of storage and processing than it is image quality, even for a single night of integration. Again, great info. I feel better going forward. I'd never paid any attention to the saturated pixels until recently. When I did and saw that my numbers were nowhere close to what I'd heard they should be, I was concerned. I'm still not totally settled. But I feel better knowing that's more wiggle room. It seems I may not have a choice but to use 0 gain considering the saturated pixel issue vs. optimal exposure time issue. I played around with the calculator the other night and the time was over 3 minutes. That was before the moon had completely set. I expect it would go up afterwards. But even at 0 gain, I can't get anywhere near 3 minutes without going over a thousand saturated pixels. In fact, 1 minute puts me over a thousand. So, if I'm going to strive for optimal exposure times, it looks like I'm going to have to live with the fact that there will be 10,000 saturated pixels. But, it's definitely another reason to migrate away from 100 gain.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
If it were me, 10,000 is a little higher than I would want. I'd probably live with not quite swamping read noise and trying to keep it down around 2,500 pixels. That's me, though, and it's not because I am "right" but just a general feeling of what I tend to see after processing with 1,200 to 2,000 saturated pixels in my own imaging. Your focal length is different, your camera is different, and your processing will be different, though, so I am not at all confident about whether 10,000 is the right compromise--it might well be. In any event, I would stick with zero gain if you are otherwise looking at one minute exposures. Exposures that short work just fine, but they do build up a lot of data and the need for a lot of CPU cycles very quickly, even with just five hours of integration time. In any event, the difference in final results between gain 0 and gain 100 is likely to be minimal, and even 10,000 clipped pixels will probably not result in much loss of star color. Most stars--the brightest few pixels--have very little color after processing no matter what just because they are stretched above 60,000 ADU. It's the surrounding halos that are responsible for the color. Unless you are heavily undersampled (you aren't), stars cover quite a few pixels in area.
- Jared
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
I use a C9.25 Edge at f10 in Bortle 3/2 skies. So, not f4, but at 10 min for R, G, & B subs, and 250 sec for Lums, I saturate a handful of pixels at most. Could be I'm wrong, but wouldn't light polluted skies add to the pixel values of a star and saturation would be more likely, all else equal (exposure, gain, etc.), than at a dark sky site. Dark skies provide more contrast between background and stars/target, but how would the stars themselves be brighter? Where saturation can become a problem is at longer exposure times intended to swamp the read noise under dark skies. Anyhow, here's a presentation by Dr Robin Glover (sharpcap) and at about 50 min he shows an exposure chart relative to focal ratio and Bortle value. He doesn't show Bortle 2 (just 3 and 1), and your camera may have less read noise than the 2.5e he charts, but extrapolating I would guess the optimum exposure would be around 60s for a mono sensor and 180s for OSC. https://www.youtube.com/watch?v=3RH93UvP358As has been mentioned, posting an image would be helpful. Cheers, Scott
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
Jaymz Bondurant:
They’re completely blown out. No color. Just bright white. I’m saturating thousands and thousands of pixels. Even at 0 gain, I’m limited to 60 seconds. Sometimes 30 or less depending on the star field. That doesn't sound right. I am using the 183MC Pro at unity gain (111) and it has a laughably low Full well at this gain, and I can still do 120s or even 180 seconds if the sky is dark enough fairly well. I suggest stretching stars and object seperately, it helps tremendously, and even before I started doing this, my star colors were fine and only the brightest cores got blown out a little. With a 2600MC Pro and 0 gain, you shouldn't run into any of those issues.
|
You cannot like this item. Reason: "ANONYMOUS".
You cannot remove your like from this item.
Editing a post is only allowed within 24 hours after creating it.
You cannot Like this post because the topic is closed.
Copy the URL below to share a direct link to this post.
This post cannot be edited using the classic forums editor.
To edit this post, please enable the "New forums experience" in your settings.
