Does a longer exposure shot have more detail than multiple short ones?

[IamLost]

Basically, I was wondering if there would be a difference in detail between, for example, one 15 minute exposure and 1800 0.5 second exposures.

[carastro]

Definitely, why don't you give it a try?

Edited February 29, 2020 by carastro

[IamLost]

7 minutes ago, carastro said:

Definitely, why don't you give it a try?

The reason why I’m asking is because I can’t really take any exposures longer than 0.5 seconds because I don’t have an RA motor drive yet, but when I get one I will compare them.

[carastro]

I think you are pretty stuck for the moment then.  

You could try Moon and planets with a webcam as you can stack the video frames.  Just stop the capture before the target leaves the FOV. 

Carole 

Edited February 29, 2020 by carastro

[carastro]

You might get something faint of M42 with a short exposure because it is bright.

The whole point of doing long exposure is because DSOs are dim.

Carole

[vlaiv]

Depends on what you mean by detail.

If you mean signal or what we call depth - that is rather controversial topic - I'll will put forward my view on it:

- short exposure has as much signal as long exposure, even single short exposure - this is controversial part as many people will disagree with me on this one

- short exposure has much more noise.

- Signal to noise ratio is important for what we call depth of the image. Difference between many short exposures and few long exposures - all adding up to same total time is in read noise of the camera - everything else is the same. If one had 0 read noise camera - there would be no difference between the two (many short vs few long).

Since there is no such thing as camera without read noise - fewer long subs always win in terms of SNR over many short subs - but it does not always win by same "amount". If read noise term is small compared to other noise sources - and that happens when light pollution is very strong (LP noise) or target is very bright (target shot noise) or camera does not have cooling and is running hot - thermal noise. In all of these circumstances there will be very small difference between many short subs and few long subs.

If you have cooled camera and dark skies and going for faint target, then read noise is not negligible and there will be quite a bit of a difference between short subs and long subs.

But there is another meaning to detail - actual resolution or detail captured and here we can argue that more short subs have better detail than few long subs.

Again things are not as simple as that but we have two important factors to consider when we talk about level of detail - atmosphere and mount precision. Both reduce level of detail (in terms of sharpness) of the image, and here shorter subs have advantage because they don't accumulate as much of a blur as long sub. Of course, that depends on atmosphere and mount and respective exposures, but yes - there is a technique called lucky DSO imaging that exploits this and uses many, many very short images - like 0.5s images and tens of thousands of such to produce very good sharp images of DSO objects.

For example this:

This image is made out of 2000 one second exposures with dobsonian telescope mounted on EQ platform

Have a look at more images from Emil here:

http://www.astrokraai.nl/viewimages.php?t=y&category=7

(btw, that is author of planetary stacking software AS2/3!).

[Davey-T]

1 hour ago, IamLost said:

Basically, I was wondering if there would be a difference in detail between, for example, one 15 minute exposure and 1800 0.5 second exposures.

If you're using one of those "modern" CMOS cameras a .5 second file will be the same size as a 15 minute one, something to bear in mind depending how much disc space you have.

Dave

[vlaiv]

1 minute ago, Davey-T said:

If you're using one of those "modern" CMOS cameras a .5 second file will be the same size as a 15 minute one, something to bear in mind depending how much disc space you have.

Dave

I remember when I did a bit of planetary imaging - same approach but instead of using 0.5s exposures - I used x100 shorter exposure at 5ms (or there about), and yes, single recording of 3-4 minutes did eat up couple of gigabytes of memory.

SSDs tend to be cheap these days, you can get decent half a terabyte ssd for price of decent eyepiece

 

[IamLost]

33 minutes ago, vlaiv said:

Depends on what you mean by detail.

If you mean signal or what we call depth - that is rather controversial topic - I'll will put forward my view on it:

- short exposure has as much signal as long exposure, even single short exposure - this is controversial part as many people will disagree with me on this one

- short exposure has much more noise.

- Signal to noise ratio is important for what we call depth of the image. Difference between many short exposures and few long exposures - all adding up to same total time is in read noise of the camera - everything else is the same. If one had 0 read noise camera - there would be no difference between the two (many short vs few long).

Since there is no such thing as camera without read noise - fewer long subs always win in terms of SNR over many short subs - but it does not always win by same "amount". If read noise term is small compared to other noise sources - and that happens when light pollution is very strong (LP noise) or target is very bright (target shot noise) or camera does not have cooling and is running hot - thermal noise. In all of these circumstances there will be very small difference between many short subs and few long subs.

If you have cooled camera and dark skies and going for faint target, then read noise is not negligible and there will be quite a bit of a difference between short subs and long subs.

But there is another meaning to detail - actual resolution or detail captured and here we can argue that more short subs have better detail than few long subs.

Again things are not as simple as that but we have two important factors to consider when we talk about level of detail - atmosphere and mount precision. Both reduce level of detail (in terms of sharpness) of the image, and here shorter subs have advantage because they don't accumulate as much of a blur as long sub. Of course, that depends on atmosphere and mount and respective exposures, but yes - there is a technique called lucky DSO imaging that exploits this and uses many, many very short images - like 0.5s images and tens of thousands of such to produce very good sharp images of DSO objects.

For example this:

This image is made out of 2000 one second exposures with dobsonian telescope mounted on EQ platform

Have a look at more images from Emil here:

http://www.astrokraai.nl/viewimages.php?t=y&category=7

(btw, that is author of planetary stacking software AS2/3!).

Thanks a lot for the info  I didn’t know you could take images like that with only 1 second exposures!

[Davey-T]

3 minutes ago, IamLost said:

Thanks a lot for the info  I didn’t know you could take images like that with only 1 second exposures!

As long as you don't plan on imaging the Squid Nebula 😂

Dave

[vlaiv]

3 hours ago, Davey-T said:

As long as you don't plan on imaging the Squid Nebula 😂

Dave

With a large scope - why not? Granted, it is very large object so mosaic will be necessary, but I think it can be done given enough imaging time.

[Davey-T]

18 minutes ago, vlaiv said:

With a large scope - why not? Granted, it is very large object so mosaic will be necessary, but I think it can be done given enough imaging time.

How large ? I tried 10 minutes O3 with 110 f/7 refractor and saw no sign of it.

Dave

[vlaiv]

2 hours ago, Davey-T said:

How large ? I tried 10 minutes O3 with 110 f/7 refractor and saw no sign of it.

Dave

I think OP will use 12" reflector and above image of M51 was done with 16" scope. In any case, if you have detail on surface brightness of that object - we can calculate needed exposure. Both scopes will capture more than 7-8 times the light 110mm aperture captures (in case of 16" aperture, difference is about x12 or so once you account for reflectivity of the mirrors and central obstruction)

For example, above mag 26 source is captured in total of 2000 seconds, or just a bit more than half an hour. And that at resolution of about 0.9-1"/px (just a guess, I did not measure).

[dan_adi]

On 29/02/2020 at 21:40, vlaiv said:

I think OP will use 12" reflector and above image of M51 was done with 16" scope. In any case, if you have detail on surface brightness of that object - we can calculate needed exposure. Both scopes will capture more than 7-8 times the light 110mm aperture captures (in case of 16" aperture, difference is about x12 or so once you account for reflectivity of the mirrors and central obstruction)

For example, above mag 26 source is captured in total of 2000 seconds, or just a bit more than half an hour. And that at resolution of about 0.9-1"/px (just a guess, I did not measure).

How can you calculate exposure time based on surface brightness of the object of interest? I did try google but could not find a formula or an example. Vlaiv you should write down a tutorial for beginners with all the equations you know  it would be pure gold (at least for me anyway)

[vlaiv]

2 minutes ago, dan_adi said:

How can you calculate exposure time based on surface brightness of the object of interest? I did try google but could not find a formula or an example. Vlaiv you should write down a tutorial for beginners with all the equations you know  it would be pure gold (at least for me anyway)

Not an easy answer, I'm afraid.

Nevertheless, let's take it step by step. First we establish some base "rules" for SNR calculation.

We model process by 4 types of noise and one type of signal - target signal we are interested in. Noise types are:

- read noise - given in electrons for particular camera model - added per exposure

- thermal noise - given as dark current at certain temperature as e/px/s (electrons per pixel per second). We actually want thermal noise and it is modeled like Poisson noise related to thermal signal/dark current. Magnitude of that noise is square root of dark current signal (which depends on exposure length)

- light pollution noise - here we need to know sky brightness in magnitudes per arc second squared. Again Poisson process - associated noise is square root of accumulated signal in exposure

- Target noise - same as light pollution noise - but this time we take target brightness in magnitudes per arc second squared - and use square root.

Read noise and thermal noise are straight forward - we have that from camera specs (or we can measure ourselves).

Sky brightness can be measured or read off from websites like lightpollution.info . Be aware that this info depends on conditions on particular night - so we use approximate value for our calculations.

We need to know our sampling rate in arc seconds per pixel. We need to know atmospheric extinction. Telescope aperture and reflectivity of mirrors / transmission of glasses also.

We start by taking that mag 0 source produces 880.000 photons on top of the atmosphere per 1cm squared per second. We adjust target magnitude by atmospheric extinction. We find clear aperture of our telescope - aperture surface - central obstruction surface time reflectivity / transmission of each glass/mirror component. For example newtonian will have (aperture_radius^2 - co_radius^2) * pi * 0.94 * 0.94. For enhanced mirrors reflectivity is about 0.94, for standard it is 0.91, starbright 0.97 and so on ...

We also need quantum efficiency of sensor. From these we can calculate signal per pixel per exposure - from both target and sky. That gives us noise from target and sky (square root), we have read noise and dark current noise to add. Noise adds in quadrature - like linearly independent vectors (square root of sum of squares).

If you want to be pedantic - add in calibration frames and their noise and "stack" wanted number of subs and calculate total SNR.

Or rather - put everything in spreadsheet and solve how much time / exposures you need for target SNR.

You'll find example of such spreadsheet in attachment here. It does not include calibration frames noise.

SNRCalc-english.ods

[dan_adi]

2 hours ago, vlaiv said:

Not an easy answer, I'm afraid.

Nevertheless, let's take it step by step. First we establish some base "rules" for SNR calculation.

We model process by 4 types of noise and one type of signal - target signal we are interested in. Noise types are:

- read noise - given in electrons for particular camera model - added per exposure

- thermal noise - given as dark current at certain temperature as e/px/s (electrons per pixel per second). We actually want thermal noise and it is modeled like Poisson noise related to thermal signal/dark current. Magnitude of that noise is square root of dark current signal (which depends on exposure length)

- light pollution noise - here we need to know sky brightness in magnitudes per arc second squared. Again Poisson process - associated noise is square root of accumulated signal in exposure

- Target noise - same as light pollution noise - but this time we take target brightness in magnitudes per arc second squared - and use square root.

Read noise and thermal noise are straight forward - we have that from camera specs (or we can measure ourselves).

Sky brightness can be measured or read off from websites like lightpollution.info . Be aware that this info depends on conditions on particular night - so we use approximate value for our calculations.

We need to know our sampling rate in arc seconds per pixel. We need to know atmospheric extinction. Telescope aperture and reflectivity of mirrors / transmission of glasses also.

We start by taking that mag 0 source produces 880.000 photons on top of the atmosphere per 1cm squared per second. We adjust target magnitude by atmospheric extinction. We find clear aperture of our telescope - aperture surface - central obstruction surface time reflectivity / transmission of each glass/mirror component. For example newtonian will have (aperture_radius^2 - co_radius^2) * pi * 0.94 * 0.94. For enhanced mirrors reflectivity is about 0.94, for standard it is 0.91, starbright 0.97 and so on ...

We also need quantum efficiency of sensor. From these we can calculate signal per pixel per exposure - from both target and sky. That gives us noise from target and sky (square root), we have read noise and dark current noise to add. Noise adds in quadrature - like linearly independent vectors (square root of sum of squares).

If you want to be pedantic - add in calibration frames and their noise and "stack" wanted number of subs and calculate total SNR.

Or rather - put everything in spreadsheet and solve how much time / exposures you need for target SNR.

You'll find example of such spreadsheet in attachment here. It does not include calibration frames noise.

SNRCalc-english.ods 14.94 kB · 1 download

Thanks for the help. I ran a test with the following parameters in the spreadsheet:

• aperture 175 mm
• central obstruction 0% (a refractor)
• focal length 1400 mm
• pixel size 6 um
• read noise 10 e
• dark current 0.08
• QE 0.6
• Light pollution 20.37 mag/arsec
• target brightness 18 mag/arcsec

I tested with a total exposure of 2 hours with various subs length and a total exposure of 50 hours.

the results are in the table below

It seems 600 sec exposure would be optimum, also increasing total integration time boosts SNR. Do you agree? 

Thanks again for the spreadsheet. It takes out the guess work and trial and error ...

 

 

[vlaiv]

26 minutes ago, dan_adi said:

It seems 600 sec exposure would be optimum, also increasing total integration time boosts SNR. Do you agree? 

Thanks again for the spreadsheet. It takes out the guess work and trial and error ...

I agree, 600 sec is about right for 10e read noise camera

For single sub duration - you did not have to go thru all that trouble. It depends on other noise sources compared to read noise on your given resolution.

What target has brightness of 18 mag/arcs2?

That is very bright stuff - as bright as bortle 8-9 sky. Actual surface brightness of objects is hard to find. Usually you get average surface brightness and for a good image, you want faintest parts of your target to have SNR of about 5.

On average galaxy, faintest parts are mag28-29 or there about and most galaxies have higher average surface brightness.

For example:

Stellarium lists M51 at being mag21.45. But that is average magnitude of object. For actual profile, look here:

Faintest parts are at about mag27

[dan_adi]

I got a fast result from google for a brightness of a galaxy's core. (that is why I put 18 mag, I should change that)

 So the faintest part of the object of interest should have a minimum SNR of 5, good to know. Even with a 1 second exposure you get a total SNR stack of 49.3 for the 2 h total exposure test. I guess this explains why DSO 'lucky imaging' works. Could a 7-8 inch refractor do DSO lucky imaging or is the aperture too small?

 

Very interesting stuff ! 

 

Edited April 29, 2020 by dan_adi

[dan_adi]

I hope I am not asking too many questions off topic, but everytime I ask Vlaiv something, I learn something and then more questions come to mind ...

 

[vlaiv]

19 minutes ago, dan_adi said:

Even with a 1 second exposure you get a total SNR stack of 49.3 for the 2 h total exposure test. I guess this explains why DSO 'lucky imaging' works.

You should really check for mag26 for example - what 1s stack of two hours brings with 10e read noise camera.

Then switch to 1.5e modern CMOS camera and check the same thing .

I think it is modern low read noise cameras that enable DSO lucky imaging approach - as only difference in total SNR of the stack vs single exposure as long as total time of the stack - is in read noise.

If you by any chance had camera with 0e read noise - you could in theory do "planetary type" imaging of galaxies - meaning exposures in milliseconds. There would be other issues - like amount of data, or problem with aligning frames as you need enough signal from stars in single exposure to align subs properly - but exposure length would not be an issue.

23 minutes ago, dan_adi said:

Could a 7-8 inch refractor do DSO lucky imaging or is the aperture too small?

With appropriate camera it is very much possible. By appropriate I mean - large enough pixels to get you decent sampling rate like 1"/px and very low read noise.

[dan_adi]

12 hours ago, vlaiv said:

ou should really check for mag26 for example - what 1s stack of two hours brings with 10e read noise camera.

Then switch to 1.5e modern CMOS camera and check the same thing

I did compared this morning. Here are the results:

[dan_adi]

The surface brightness was changed to 26 mag/arcsec. The ASI 183 was binned 2x2.

If the minimal SNR for detection is above 5 none of the cameras can detect detect the signal in reasonable amount of time. We will need above 400 hours of total integration time (16.6 days). But.... with some dedication, and 600 hours of total integration time its possible

 

[vlaiv]

2 minutes ago, dan_adi said:

The surface brightness was changed to 26 mag/arcsec. The ASI 183 was binned 2x2.

You say you have 1400mm focal length scope?

When you bin camera 2x2 - you need to increase read noise x2 for CMOS cameras and leave it the same for CCD cameras. You also want your sampling rate to be higher at about 1"/px. You can go lower than that - but that requires large scope.

Let's for example take ASI1600 binned x2. Same as above 1s exposure, total 2h

Total SNR of stack = ~0.12052 for mag26 target.

SNR5 is to have the thing comfortably displayed after processing. SNR2-3 is for detection. If you don't mind having noisy hinted outer parts of a galaxy then you can go for SNR 3 instead.

Key here is sampling rate and keeping read noise low. If you get good x0.5 focal reducer, then you don't need to bin and read noise is back at 1.7. SNR after stacking is ~0.2184

[dan_adi]

2 minutes ago, vlaiv said:

You say you have 1400mm focal length scope?

It is a hypothetical 175 mm Lzos APO with 1400 mm focal length. Lately I was agonizing whether or not to buy one  (I like the no fuss, low maintenance of refractors, but in large aperture -7-8 inch- these instruments are sooo expensive).

5 minutes ago, vlaiv said:

When you bin camera 2x2 - you need to increase read noise x2 for CMOS cameras and leave it the same for CCD cameras. You also want your sampling rate to be higher at about 1"/px. You can go lower than that - but that requires large scope.

I did not know about the need to increase the read noise (I' ve made a note in the spreadsheet). Bin 2x2 is only 0.7 "''/pixel indeed below 1'''

[vlaiv]

@dan_adi

Let's do validation of spreadsheet on above image of M51 by Emil

I would say that zone marked by arrow is barely detectable. He used 16" F/5 scope and ASI1600. His image is scaled down 50% - so we can use x2 in our calculations (actual binning will yield a bit better result than scaling down, but they are rather close).

From this diagram we can see that this particular part has magnitude of about 22-23 (core is at mag17 and each color is one mag step). Let's approximate it with mag22.5 in our example.

I used 400mm aperture, 26% central obstruction and enhanced coated mirrors - 0.94

2000mm FL (F/5) - I doubled pixel size and read noise (CMOS binned). I put target brightness at 22.5 and sky brightness at about 20.5 (we don't have info on this, but we can take a guess).

SNR is about 3.3 - which is right there in range to be detectable. No nice features rendered and you need denoising to make it look smooth, but it will show in the image like this:

(I just cropped that part at 1:1 zoom)

I guess that is pretty good calculation?