RGB L sub times for a ASI 1600mm pro

[simmo39]

Hi all, I know this has been asked before but i cant seem to find the answer. I have been doing Narrow band since I got the camera but with galaxy season coming I treated myself to some RGB and L filters. I have set my gain to 200 for the NB but will go back to unity gain for RGB. Im just wondering where to start my subs at, I thought about 120s for the RGB but what about the L? is that the same and also what kind of ratio am i looking at between RGB and L? 

Thanks in advance.

[eshy76]

Hi there,

I have an ASI1600 - this will depend on your local light pollution. Where I live is Bortle 7-8 and I shoot 15 second L subs, 30 second G and B subs and 60 second R subs.  As far as I understand, your exposure time for each sub should be enough to swamp the read noise of the camera...and then focus on total integration time. So I get lots of subs, but am still imaging for 1-2 hours for each filter basically (more for L). 

If you have darker skies than me, those sub times will be longer for you.

I also keep things simple in terms of proportions of integration time - I shoot 50% L and 50% RGB.

I'm not sure if I can link to other sites here, but if you google "sub exposure time ASI1600", there's a great thread on CN that answers your exact question with tables of exposure time depending on different sky levels. It's an involved thread, so you can dive into all the maths or just stick to the tables in the first post.

I hope this helps!

Edited March 12, 2019 by eshy76

[simmo39]

9 minutes ago, eshy76 said:

Hi there,

I have an ASI1600 - this will depend on your local light pollution. Where I live is Bortle 7-8 and I shoot 15 second L subs, 30 second G and B subs and 60 second R subs.  As far as I understand, your exposure time for each sub should be enough to swamp the read noise of the camera...and then focus on total integration time. So I get lots of subs, but am still imaging for 1-2 hours for each filter basically (more for L). 

If you have darker skies than me, those sub times will be longer for you.

I also keep things simple in terms of proportions of integration time - I shoot 50% L and 50% RGB.

I'm not sure if I can link to other sites here, but if you google "sub exposure time ASI1600", there's a great thread on CN that answers your exact question with tables of exposure time depending on different sky levels. It's an involved thread, so you can dive into all the maths or just stick to the tables in the first post.

I hope this helps!

Hi, thanks for the pointers.  I think my light pollution is classed Bortle 4 - 5. I will have a look at the CN site, once again thank you.

[vlaiv]

That really depends on various factors. For any given setup and conditions, longer subs will always produce better results, but relationship is not straight forward. What happens is: going from 1m to 2m will have significant impact, going from 2m to 3m is going to have noticeable impact, going from 3m to 4m will be barely noticeable and going from 4m to 5m is not going to produce any perceivable difference.

Above numbers are arbitrary and serve just to show you that there is no linear dependence between sub length and improvement - at some point improvement starts to rapidly fall of until it reaches undetectable levels (meaning you can't tell difference to SNR by eye alone and it has virtually 0 impact on image quality as perceived by human eye).

Thing is - above numbers depend on many factors - focal length of scope, aperture, light pollution levels, etc ... With higher sampling resolution (less "/px) - increase sub length. With darker skies - increase sub length. With larger aperture increase sub length. When you have combination of those factors then it's not easy to say without calculations (like using lower sampling rate while moving to darker skies, or other combinations).

As for lum vs RGB, that one is even harder to tell. I usually do it equally for each filter - meaning same time for L, R, G and B. Some people do it like equal time for L as for R, G and B combined and they split R, G and B equally. In theory, given limited time budget there is optimum split - but it is way hard to calculate and one would need much more information than is available prior to imaging (like target brightness in each band - you don't know that until you image target).

One thing that can be useful is: don't be afraid of long exposures if your mount/scope system supports them (guiding and tracking are up to task). You can always get few short exposures at the end to blend in what ever you saturated in long exposures.

[simmo39]

47 minutes ago, vlaiv said:

That really depends on various factors. For any given setup and conditions, longer subs will always produce better results, but relationship is not straight forward. What happens is: going from 1m to 2m will have significant impact, going from 2m to 3m is going to have noticeable impact, going from 3m to 4m will be barely noticeable and going from 4m to 5m is not going to produce any perceivable difference.

Above numbers are arbitrary and serve just to show you that there is no linear dependence between sub length and improvement - at some point improvement starts to rapidly fall of until it reaches undetectable levels (meaning you can't tell difference to SNR by eye alone and it has virtually 0 impact on image quality as perceived by human eye).

Thing is - above numbers depend on many factors - focal length of scope, aperture, light pollution levels, etc ... With higher sampling resolution (less "/px) - increase sub length. With darker skies - increase sub length. With larger aperture increase sub length. When you have combination of those factors then it's not easy to say without calculations (like using lower sampling rate while moving to darker skies, or other combinations).

As for lum vs RGB, that one is even harder to tell. I usually do it equally for each filter - meaning same time for L, R, G and B. Some people do it like equal time for L as for R, G and B combined and they split R, G and B equally. In theory, given limited time budget there is optimum split - but it is way hard to calculate and one would need much more information than is available prior to imaging (like target brightness in each band - you don't know that until you image target).

One thing that can be useful is: don't be afraid of long exposures if your mount/scope system supports them (guiding and tracking are up to task). You can always get few short exposures at the end to blend in what ever you saturated in long exposures.

Hi Vlaiv. Thanks for the reply. I'm going to be using my new SW 72ED. Think it's about F5. Comparing to my SW 130 PDS is F4.8 I think. I have been using 240s subs for narrow band with that and was hoping that 240s for NB would be good for the 72ED.  As for the RGB I was thinking about 120s, do you think that would be a good starting point or should I see if I can push it to 240s?

[vlaiv]

2 minutes ago, simmo39 said:

Hi Vlaiv. Thanks for the reply. I'm going to be using my new SW 72ED. Think it's about F5. Comparing to my SW 130 PDS is F4.8 I think. I have been using 240s subs for narrow band with that and was hoping that 240s for NB would be good for the 72ED.  As for the RGB I was thinking about 120s, do you think that would be a good starting point or should I see if I can push it to 240s?

What is your light pollution like? Do you know average SQM reading for your site? (or maybe data from lightpollution.info)?

I think numbers that you mention are good for average to strong LP (120s). Only if you have very dark skies it makes sense to go longer.

[cjdawson]

Hiya.

I'm also getting myself up and running with LRGB imaging using a ZWO ASI1600MM-Pro.  Everything I understand on this subject is that there is no easy answer.  That said, I've got a plan.   Turns out there's an idea exposure time statistic in SGPro, I'm not expecting this to be the perfect answer by any means.

 

As an experiment, I'm going to take a sub next time I'm out, and will see what this statistic says.  From there, I'll adjust the sub time and see what happens.   I'm going to try this in L only to begin with, then if it works well, I'll try the same for each of the other filters on the same target.   If it all works out, then I'll should have some idea of the ratio of exposure times for each filter (yes, I know this is nieve, it's a start and not ment to be a complete answer)

 

From there on out, I'll have a starting point to tweak from.

[eshy76]

2 hours ago, simmo39 said:

Hi, thanks for the pointers.  I think my light pollution is classed Bortle 4 - 5. I will have a look at the CN site, once again thank you.

No probs - I forgot to mention I'm using an f5.9 scope - as vlaiv says there are lots of factors to take into consideration....

...but if there is one thing I am aiming for - it is a certain level of median background ADU per sub (you can see this in your capture software or Pixinsight). Based on my use of unity gain (139), default bias offset (50) and the associated read noise of the camera at unity (1.8 e-), the theoretical minimum median ADU I need per sub is about 1400 to swamp the read noise (by about 20x). You'll see this in the CN thread I mentioned.

That number of 1400 (for me) governs the length of the subs I mentioned....the R, G and B sub lengths get me in the 1500-1700 ADU range...the 15 second L sub actually delivers about 2500 ADU on average, more than I would want, but I can't really go much shorter than 15 seconds from a practicality perspective.

So I didn't just pluck those numbers out of thin air in case you were wondering! The theoretical stuff is a nice baseline for me to hang my hat on and know I am being efficient - not under or over exposing, although there is some leeway on the latter.

Ultimately, this low-noise camera allows you to take shorter subs and use stacking and sigma rejection benefits, but there's a trade off between being efficient and practicality (hard disk space as you'll need a lot of subs using my approach) and also your own preference on whether you would like deeper individual subs.

Good luck with it!

[Adam J]

9 minutes ago, eshy76 said:

No probs - I forgot to mention I'm using an f5.9 scope - as vlaiv says there are lots of factors to take into consideration....

...but if there is one thing I am aiming for - it is a certain level of median background ADU per sub (you can see this in your capture software or Pixinsight). Based on my use of unity gain (139), default bias offset (50) and the associated read noise of the camera at unity (1.8 e-), the theoretical minimum median ADU I need per sub is about 1400 to swamp the read noise (by about 20x). You'll see this in the CN thread I mentioned.

That number of 1400 (for me) governs the length of the subs I mentioned....the R, G and B sub lengths get me in the 1500-1700 ADU range...the 15 second L sub actually delivers about 2500 ADU on average, more than I would want, but I can't really go much shorter than 15 seconds from a practicality perspective.

So I didn't just pluck those numbers out of thin air in case you were wondering! The theoretical stuff is a nice baseline for me to hang my hat on and know I am being efficient - not under or over exposing, although there is some leeway on the latter.

Ultimately, this low-noise camera allows you to take shorter subs and use stacking and sigma rejection benefits, but there's a trade off between being efficient and practicality (hard disk space as you'll need a lot of subs using my approach) and also your own preference on whether you would like deeper individual subs.

Good luck with it!

The longest Luminance sub I have ever felt the need to do from my 5/6 location is 30 seconds. Higher than that then I would think 15 seconds is not a bad idea. All this talk of 3 or 4 min exposures only makes sense in a very dark site.

[vlaiv]

5 minutes ago, Adam J said:

The longest Luminance sub I have ever felt the need to do from my 5/6 location is 30 seconds. Higher than that then I would think 15 seconds is not a bad idea. All this talk of 3 or 4 min exposures only makes sense in a very dark site.

Or when one is using very long focal length, high resolution (and maybe binning afterwards in software). Even in red bordering with white zone, I benefit from couple of minutes subs at 0.5"/px (this gets binned afterwards as it is oversampling). Skyglow also gets "diluted" over large number of pixels and LP noise is smaller in given time when imaging at high resolution.

[Adam J]

3 minutes ago, vlaiv said:

Or when one is using very long focal length, high resolution (and maybe binning afterwards in software). Even in red bordering with white zone, I benefit from couple of minutes subs at 0.5"/px (this gets binned afterwards as it is oversampling). Skyglow also gets "diluted" over large number of pixels and LP noise is smaller in given time when imaging at high resolution.

Not sure about very long focal length unless that is also implying very slow F-ratio in which case longer exposures will be required.

[vlaiv]

1 minute ago, Adam J said:

Not sure about very long focal length unless that is also implying very slow F-ratio in which case longer exposures will be required.

Indeed, that is what I meant

[simmo39]

1 hour ago, vlaiv said:

What is your light pollution like? Do you know average SQM reading for your site? (or maybe data from lightpollution.info)?

I think numbers that you mention are good for average to strong LP (120s). Only if you have very dark skies it makes sense to go longer.

from the numbers i would say my light pollution is average. I have no idea about SQM sorry.

[simmo39]

1 hour ago, cjdawson said:

Hiya.

I'm also getting myself up and running with LRGB imaging using a ZWO ASI1600MM-Pro.  Everything I understand on this subject is that there is no easy answer.  That said, I've got a plan.   Turns out there's an idea exposure time statistic in SGPro, I'm not expecting this to be the perfect answer by any means.

 

As an experiment, I'm going to take a sub next time I'm out, and will see what this statistic says.  From there, I'll adjust the sub time and see what happens.   I'm going to try this in L only to begin with, then if it works well, I'll try the same for each of the other filters on the same target.   If it all works out, then I'll should have some idea of the ratio of exposure times for each filter (yes, I know this is nieve, it's a start and not ment to be a complete answer)

 

From there on out, I'll have a starting point to tweak from.

Hope it works, I use APT and im not sure if it works the same.

[simmo39]

BTW, I think the SQM value here is: 20.4–21.3

[simmo39]

56 minutes ago, eshy76 said:

No probs - I forgot to mention I'm using an f5.9 scope - as vlaiv says there are lots of factors to take into consideration....

...but if there is one thing I am aiming for - it is a certain level of median background ADU per sub (you can see this in your capture software or Pixinsight). Based on my use of unity gain (139), default bias offset (50) and the associated read noise of the camera at unity (1.8 e-), the theoretical minimum median ADU I need per sub is about 1400 to swamp the read noise (by about 20x). You'll see this in the CN thread I mentioned.

That number of 1400 (for me) governs the length of the subs I mentioned....the R, G and B sub lengths get me in the 1500-1700 ADU range...the 15 second L sub actually delivers about 2500 ADU on average, more than I would want, but I can't really go much shorter than 15 seconds from a practicality perspective.

So I didn't just pluck those numbers out of thin air in case you were wondering! The theoretical stuff is a nice baseline for me to hang my hat on and know I am being efficient - not under or over exposing, although there is some leeway on the latter.

Ultimately, this low-noise camera allows you to take shorter subs and use stacking and sigma rejection benefits, but there's a trade off between being efficient and practicality (hard disk space as you'll need a lot of subs using my approach) and also your own preference on whether you would like deeper individual subs.

Good luck with it!

So what is your average RGB sub lenght, L is 15s that seems v short. I think with number crunched by my poor head 60s for L but that could be off as I guessing the bits I dont understand ! lol.  At least I have got a starting point all I need now is the weather to cooperate.

[vlaiv]

30 minutes ago, simmo39 said:

BTW, I think the SQM value here is: 20.4–21.3

If it's 21.3 - that would be considered fairly dark sky in terms of imaging. You would benefit from longer exposures then.

[eshy76]

1 hour ago, simmo39 said:

So what is your average RGB sub lenght, L is 15s that seems v short. I think with number crunched by my poor head 60s for L but that could be off as I guessing the bits I dont understand ! lol.  At least I have got a starting point all I need now is the weather to cooperate.

My L subs are 15 seconds, G and B are 30 seconds and R subs are 60 seconds. With my filters, my R needs longer to get the same ADU as G and B.

The formula I used was Jon Rista's:

Minimum ADU per sub = ((20 x read noise/gain in electrons)+(bias offset)) x 16 

So at unity gain, associated read noise of 1.8e- and using offset of 50 that would be:

((20x1.8/1)+50)x16 = 1376

The 16 multiplier is to gross up from the 12 bits of the ASI1600 to 16 bits which is displayed in SGP etc.

The 20 x read noise can also be 3 x read noise squared or 10 x read noise squared....there's some discussion about that.

It's a starting point as you say! My skies are light polluted which is why my subs are so short!

Edited March 12, 2019 by eshy76

[vlaiv]

1 minute ago, eshy76 said:

The 20 x read noise can also be 3 x read noise squared or 10 x read noise squared....there's some discussion about that. 

There is no definite value that one should use here. It comes down to when you can consider read noise contribution too small to matter. Best way to "visualize" this is by right angle triangle. If one side is much smaller than other - longer side approaches hypotenuse in length. Total noise is hypotenuse and read noise and LP shot noise are sides of right angle triangle. When LP shot noise "side" becomes much larger than read noise "side" - total noise (hypotenuse) comes close to that LP shot noise "side".

a = b implies c>b

O<<A implies H and A almost equal in length (O here being read noise almost has no impact on total noise - it's dominated by LP noise - here A)

[simmo39]

1 hour ago, eshy76 said:

My L subs are 15 seconds, G and B are 30 seconds and R subs are 60 seconds. With my filters, my R needs longer to get the same ADU as G and B.

The formula I used was Jon Rista's:

Minimum ADU per sub = ((20 x read noise/gain in electrons)+(bias offset)) x 16 

So at unity gain, associated read noise of 1.8e- and using offset of 50 that would be:

((20x1.8/1)+50)x16 = 1376

The 16 multiplier is to gross up from the 12 bits of the ASI1600 to 16 bits which is displayed in SGP etc.

The 20 x read noise can also be 3 x read noise squared or 10 x read noise squared....there's some discussion about that.

It's a starting point as you say! My skies are light polluted which is why my subs are so short!

 

1 hour ago, vlaiv said:

There is no definite value that one should use here. It comes down to when you can consider read noise contribution too small to matter. Best way to "visualize" this is by right angle triangle. If one side is much smaller than other - longer side approaches hypotenuse in length. Total noise is hypotenuse and read noise and LP shot noise are sides of right angle triangle. When LP shot noise "side" becomes much larger than read noise "side" - total noise (hypotenuse) comes close to that LP shot noise "side".

a = b implies c>b

O<<A implies H and A almost equal in length (O here being read noise almost has no impact on total noise - it's dominated by LP noise - here A)

Thank you, That does help. Now if you can calculate the clouds away  that would be brilliant!

[Starwiz]

If all goes to plan I'll be starting imaging with an ASI1600mm-Pro next month.  The exposure times seem incredibly short compared to the 10 - 15 minutes I was doing with my modded Canon 1200d.  Is that because the sensor is much more sensitive on the ASI?

BTW, I've just emigrated to Malta, so looking forward to the cloudless skies.  Light pollution where I am is slightly worse than my UK location (Bortle 5 compared to Bortle 4), but I'll also be doing narrow-band as well as LRGB.

[vlaiv]

4 hours ago, Starwiz said:

If all goes to plan I'll be starting imaging with an ASI1600mm-Pro next month.  The exposure times seem incredibly short compared to the 10 - 15 minutes I was doing with my modded Canon 1200d.  Is that because the sensor is much more sensitive on the ASI?

BTW, I've just emigrated to Malta, so looking forward to the cloudless skies.  Light pollution where I am is slightly worse than my UK location (Bortle 5 compared to Bortle 4), but I'll also be doing narrow-band as well as LRGB.

No, sensitivity does not play a part in that. Here we are talking about splitting one very large exposure into smaller ones. You can look at it that way - instead of doing 1 grand exposure lasting multiple hours, you break it into shorter ones and add up those shorter ones. All things that are important to final result add up with time - signal adds up, more you image more there is of it. Light pollution behaves the same - longer exposure you accumulate more light pollution. Thermal noise is also the same - more you image, more it builds up. Everything except the read noise - it is the same regardless of exposure length. 10 minute sub will have same read noise as 1 minute sub.

And that is the difference - more subs you take, more times you add read noise. Level of read noise determines how much it will impact final result - lower the read noise, less impact it will have compared to other sources. Most often read noise is compared to LP noise - look above at triangle explanation, but other noise sources also participate. With narrowband imaging you eliminate most of LP - this is why you need to go longer in NB - read noise becomes important factor.

ASI1600 has very low read noise compared to other sensors. Most CCDs and DSLRs have it at about 7-9e range, some low read noise models have it at 4-5e, but that is still x3 compared to ASI1600 (and other CMOS models). With DSLRs it's often recommended to go for ISO800 or similar instead of ISO100 - this is because CMOS sensors tend to have lower read noise at higher gain settings.

[Starwiz]

4 minutes ago, vlaiv said:

No, sensitivity does not play a part in that. Here we are talking about splitting one very large exposure into smaller ones. You can look at it that way - instead of doing 1 grand exposure lasting multiple hours, you break it into shorter ones and add up those shorter ones. All things that are important to final result add up with time - signal adds up, more you image more there is of it. Light pollution behaves the same - longer exposure you accumulate more light pollution. Thermal noise is also the same - more you image, more it builds up. Everything except the read noise - it is the same regardless of exposure length. 10 minute sub will have same read noise as 1 minute sub.

And that is the difference - more subs you take, more times you add read noise. Level of read noise determines how much it will impact final result - lower the read noise, less impact it will have compared to other sources. Most often read noise is compared to LP noise - look above at triangle explanation, but other noise sources also participate. With narrowband imaging you eliminate most of LP - this is why you need to go longer in NB - read noise becomes important factor.

ASI1600 has very low read noise compared to other sensors. Most CCDs and DSLRs have it at about 7-9e range, some low read noise models have it at 4-5e, but that is still x3 compared to ASI1600 (and other CMOS models). With DSLRs it's often recommended to go for ISO800 or similar instead of ISO100 - this is because CMOS sensors tend to have lower read noise at higher gain settings.

Thanks Vlaiv, that's very helpful information.

John

[knobby]

How's life in Malta ? looking for envious emoji 🙂

 

[tooth_dr]

On 13/03/2019 at 11:27, Starwiz said:

With narrowband imaging you eliminate most of LP - this is why you need to go longer in NB - read noise becomes important factor.

This has puzzled me. Why would this be any different to imaging in an area of low light pollution with a luminance filter?