Picking the correct exposure for Deep Sky
Forum rules
One tutorial per topic please.
Please only post content that you have created in this form.
Tutorials should be either in PDF format as an attachment or written in the first post of the topic. Links to video tutorials you created are also acceptable.
One tutorial per topic please.
Please only post content that you have created in this form.
Tutorials should be either in PDF format as an attachment or written in the first post of the topic. Links to video tutorials you created are also acceptable.
-
- Posts: 156
- Joined: Mon Mar 01, 2021 5:24 am
Re: Picking the correct exposure for Deep Sky
You've mentioned the 10/20 min stack would be a total integration time of 15 hours or so.
The deep image would be (6x1) + (36x0.67) for a total of 30 hours.
If that's right, yes the deep image should in theory have an SNR of 1.4 times the short subs.
The subs time calculation is for a given total integration time.
If possible, you could integrate 15 hours worth of the deep subs, and then the two sets would be more comparable.
The deep image would be (6x1) + (36x0.67) for a total of 30 hours.
If that's right, yes the deep image should in theory have an SNR of 1.4 times the short subs.
The subs time calculation is for a given total integration time.
If possible, you could integrate 15 hours worth of the deep subs, and then the two sets would be more comparable.
-
- Posts: 34
- Joined: Wed Nov 03, 2021 8:33 am
Re: Picking the correct exposure for Deep Sky
Ahh - thank you zerolatitude - yes, at last it is now beginning to make some sense
Greg
Greg
Re: Picking the correct exposure for Deep Sky
Hi Robin
I have watched your excellent youtube video on this topic. Marvellous.
I wonder if it would be possible to extend the calculator on https://tools.sharpcap.co.uk/ to also allow the user to enter the camera info to take them all the way to the ideal sub exposure?
I have watched your excellent youtube video on this topic. Marvellous.
I wonder if it would be possible to extend the calculator on https://tools.sharpcap.co.uk/ to also allow the user to enter the camera info to take them all the way to the ideal sub exposure?
- admin
- Site Admin
- Posts: 13520
- Joined: Sat Feb 11, 2017 3:52 pm
- Location: Vale of the White Horse, UK
- Contact:
Re: Picking the correct exposure for Deep Sky
Hi,
the idea is rather to persuade people to buy the SharpCap Pro license, which will calculate that for you
cheers,
Robin
the idea is rather to persuade people to buy the SharpCap Pro license, which will calculate that for you
cheers,
Robin
Re: Picking the correct exposure for Deep Sky
Do you have a formula for stack dynamic range? Seems vital.
Cheers
Tony
- admin
- Site Admin
- Posts: 13520
- Joined: Sat Feb 11, 2017 3:52 pm
- Location: Vale of the White Horse, UK
- Contact:
Re: Picking the correct exposure for Deep Sky
Hi Tony,
yes, the SNR of the stack is one of the key things - I don't think the formula is listed in the posts at the top of this thread, but it's relatively simple :
Stack SNR = number of subs * sub FWD / stack noise (in electrons)
The stack noise is the only thing there that is complex, but the formula for that is given in post #8 of this thread.
Hope this helps,
Robin
yes, the SNR of the stack is one of the key things - I don't think the formula is listed in the posts at the top of this thread, but it's relatively simple :
Stack SNR = number of subs * sub FWD / stack noise (in electrons)
The stack noise is the only thing there that is complex, but the formula for that is given in post #8 of this thread.
Hope this helps,
Robin
Re: Picking the correct exposure for Deep Sky
Um, that's not quite right. I was asking about dynamic range of a stack, not SNR. Did you mean
Stack DR = number of subs * sub FWD / stack noise (in electrons)
or
Stack DR = number of subs * sub FWD / stack read noise (in electrons)
I used the second formula and the results of the sensor analysis for my camera and I did not get for any value of 'number of subs' a point where one red outline is higher than the other red outline.
If we use StackFullWell = n * FullWell, StackReadNoise = sqrt(n * ReadNoise.^2) and StackDR = log2(StackFullWell ./ StackReadNoise)
then we get
StackDR(n) = 0.5 * log2(n) + StackDR(1)
so we don't get that bump in DR from stacking. Stacking just shifts the whole curve up by 0.5 * log2(n)
Stack DR = number of subs * sub FWD / stack noise (in electrons)
or
Stack DR = number of subs * sub FWD / stack read noise (in electrons)
I used the second formula and the results of the sensor analysis for my camera and I did not get for any value of 'number of subs' a point where one red outline is higher than the other red outline.
If we use StackFullWell = n * FullWell, StackReadNoise = sqrt(n * ReadNoise.^2) and StackDR = log2(StackFullWell ./ StackReadNoise)
then we get
StackDR(n) = 0.5 * log2(n) + StackDR(1)
so we don't get that bump in DR from stacking. Stacking just shifts the whole curve up by 0.5 * log2(n)
- admin
- Site Admin
- Posts: 13520
- Joined: Sat Feb 11, 2017 3:52 pm
- Location: Vale of the White Horse, UK
- Contact:
Re: Picking the correct exposure for Deep Sky
Hi,
so you need to use the full noise of the stack (including the shot noise of the background, which should dominate the read noise because that is how we choose the sub-exposure length).
Once you have picked a gain value, the camera read noise and sky background brightness allow you to calculate the recommended sub exposure length (t). Additionally, you get the FWD as a function of the gain value being used.
The total exposure time is T, so the number of frames captured is T/t. That means that the brightest thing you can capture in time T without loss of information to oversaturation is T/t * FWD (measured in electrons). We assume that the dimmest thing you can detect is of the order of the stack noise (in fact for a single pixel it would be a few times the stack noise, but for an extended object it may well be less than the stack noise...). Anyway, now we have the brightest and dimmest things that can be properly represented which gives us the dynamic range (being basically the SNR for a saturated signal).
Hope that's clearer,
Robin
so you need to use the full noise of the stack (including the shot noise of the background, which should dominate the read noise because that is how we choose the sub-exposure length).
Once you have picked a gain value, the camera read noise and sky background brightness allow you to calculate the recommended sub exposure length (t). Additionally, you get the FWD as a function of the gain value being used.
The total exposure time is T, so the number of frames captured is T/t. That means that the brightest thing you can capture in time T without loss of information to oversaturation is T/t * FWD (measured in electrons). We assume that the dimmest thing you can detect is of the order of the stack noise (in fact for a single pixel it would be a few times the stack noise, but for an extended object it may well be less than the stack noise...). Anyway, now we have the brightest and dimmest things that can be properly represented which gives us the dynamic range (being basically the SNR for a saturated signal).
Hope that's clearer,
Robin
Re: Picking the correct exposure for Deep Sky
My apologies if this has been answered somewhere.
Presuming that your guiding can handle it, and you care not concerned about spoilage of individual frames (satellites, airplanes, etc.), why would you ever trade dynamic range for higher gain and reduced read noise? As long as you are swamping the read noise with the appropriate exposure length, the total noise in the final image is determined by the total integration time, no? Why would you ever give up dynamic range?
Thanks.
Presuming that your guiding can handle it, and you care not concerned about spoilage of individual frames (satellites, airplanes, etc.), why would you ever trade dynamic range for higher gain and reduced read noise? As long as you are swamping the read noise with the appropriate exposure length, the total noise in the final image is determined by the total integration time, no? Why would you ever give up dynamic range?
Thanks.
- admin
- Site Admin
- Posts: 13520
- Joined: Sat Feb 11, 2017 3:52 pm
- Location: Vale of the White Horse, UK
- Contact:
Re: Picking the correct exposure for Deep Sky
Hi,
because although you might give away dynamic range in the individual frames, you can often win on dynamic range for the final stack because the you can take more frames in the same amount of time because you don't need such long exposures.
cheers,
Robin
because although you might give away dynamic range in the individual frames, you can often win on dynamic range for the final stack because the you can take more frames in the same amount of time because you don't need such long exposures.
cheers,
Robin