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Ideal exposure tool


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Hello, 

 

Been through a couple of sites now and watched a few YouTube videos on this topic as it seems to be quite useful. Looks to me as it can save allot of people some head ache and money. 

When I am after is a tool that can calculate the optimal exposure for your setup under your particular sky. 

I know Sharpcap has a sensor analysis tool which is used to calculate the specifics of your camera, but you also need something to calculate your sky background. Then we can go populate the values in http://www.gibastrosoc.org/sections/astrophotography/optimum-exposures-calculator. This calculator does not talk about CMOS cameras which has Gain and offset etc.. So I believe its a bit outdated (only relevant for CCD). Am I wrong is assuming this ? 

A tool that can do all this for you so you dont need to jump back and forth to different calculators and softwares to determine your exposure would be amazing. So if anyone has any idea, please let me know. 

The benefits of knowing this could actually save you allot of money. Say you live under Bortle 4 or 5 sky and invest in a have duty mount for better long exposures which is apparently completely useless with a CMOS camera. In fact (if you use LRGB only you MAY not even need to guide as your ideal exposure times can be reduced dramatically with the same result). 

Any thoughts and help to understand this better would be welcomed 

 

Cheers 

Roger

 

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Any such software tool is bound to be very imprecise.

This is because you can't model every little bit of in order to accurately calculate optimum exposure length - and even then - there is no such thing as optimum exposure length.

Optimum implies best possible value - and in reality best possible value for any setup is single exposure as long as total imaging time. As long as we use cameras that have read noise - optimum / ideal solution is that one - single very long exposure.

What you can do instead is accept some level of tradeoff. What level of tradeoff you accept has much more impact on calculated sub length than other parameters.

Some parameters that you can select also have very large range of "implied" values to be of any use.

For example - you say to take Bortle 4 or 5 sky. That is roughly 19.1 to 21.3 SQM - more than 2 magnitudes of difference. That is more than x6.3 in sky brightness between two extreme points. Consequently - that is more than x2.5 in LP noise levels.

This translates into sub length difference for all other conditions of about x2.5

On one side of spectrum we have for example 1 minute and on other 2.5 minutes exposure - this is just by being indecisive about what exactly is our sky brightness.

Best way to tackle this problem is to actually measure values - measure you background sky flux and your read noise and based on these two parameters alone you can select tradeoff that you are ready to accept.

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48 minutes ago, vlaiv said:

Any such software tool is bound to be very imprecise.

This is because you can't model every little bit of in order to accurately calculate optimum exposure length - and even then - there is no such thing as optimum exposure length.

Optimum implies best possible value - and in reality best possible value for any setup is single exposure as long as total imaging time. As long as we use cameras that have read noise - optimum / ideal solution is that one - single very long exposure.

What you can do instead is accept some level of tradeoff. What level of tradeoff you accept has much more impact on calculated sub length than other parameters.

Some parameters that you can select also have very large range of "implied" values to be of any use.

For example - you say to take Bortle 4 or 5 sky. That is roughly 19.1 to 21.3 SQM - more than 2 magnitudes of difference. That is more than x6.3 in sky brightness between two extreme points. Consequently - that is more than x2.5 in LP noise levels.

This translates into sub length difference for all other conditions of about x2.5

On one side of spectrum we have for example 1 minute and on other 2.5 minutes exposure - this is just by being indecisive about what exactly is our sky brightness.

Best way to tackle this problem is to actually measure values - measure you background sky flux and your read noise and based on these two parameters alone you can select tradeoff that you are ready to accept.

You seem to know what you are talking about so Ill ask some newbie questions if you dont mind.

Lets take my setup for instance. 

QSI 638 - Esprit 120 (This one was at full well max setting when I did the measurement off the sensor in pixinsight) (f5,3 , pixel 5,3 , QE 57% , read noice 10,78e , full well 31800e) 

QHY 268 - Esprit 80 (At what setting should I do measuring for instance ? At what Gain and Offset?) 

SQM reading is 21.70 so Bortle 1,3

Skywatcher EQ8r-Pro mount (stationary) 

How would you go about finding the best exposure? 

 

Cheers 

Roger 

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2 minutes ago, Taraobservatory said:

QHY 268 - Esprit 80 (At what setting should I do measuring for instance ? At what Gain and Offset?) 

Offset is not important for sub exposure length. Use gain setting that you will be using for imaging.

If you want to determine what is best tradeoff for sub length - here are guidelines:

1. How much data you want to stack and process? Shorter subs mean more data. Some algorithms like more data, others like good SNR per sub

2. How likely is it that you'll get ruined sub (for whatever reason - wind, earthquake, airplane flying thru the FOV - whatever makes you discard the whole sub - satellite trails can be easily dealt with in stacking if you use some sort sigma reject). Longer discarded subs mean more imaging time wasted

3. Differences in setup - in general, you'll have different sub length for each filter, but sometimes you will want to keep single exposure length over range of filters (like same exposure for LRGB and same for NB filters) as this simplifies calibration - only one set of darks instead of darks for each filter

4. What is the increase in noise that you are prepared to tolerate?

 

Only difference between many short subs and few long subs (including one long sub lasting whole imaging time) - totaling to same total imaging time - is in read noise. More specifically, difference comes down to how small read noise is compared to other noise sources in the system.

When using cooled cameras and shooting faint targets - LP noise is by far the most dominant noise source, that is why we decide based on it, but it does not  have to be (another thing to consider when calculating). If you have very dark skies and use NB filters - it can turn out that thermal noise is highest component, so this calculation should be carried out against it instead.

In fact - you want "sum" of all time dependent noise sources (which are target shot noise, LP noise and dark current or thermal noise - all depend on exposure length) and compare that to read noise.

Read noise is only time independent type.

Noises add like linearly independent vectors - square root of sum of squares. This is important bit, because this means that total increase is small if you have components that are significantly different in magnitude. Here is example:

Let's calculate percentage of increase if we have LP noise that is same, twice as large, 3 times as large and 5 times as large as read noise.

"sum" of noises will be sqrt( read_noise^2 + lp_noise^2) so we have following:

1. sqrt(read_noise^2 + (1 x read_noise)^2) = sqrt( 2 * read_noise^2) = read_noise * sqrt(2) = read_noise * 1.4142 ... or 41.42% increase in total noise due to read noise

2. sqrt(read_noise^2 + (2 x read_noise)^2) = sqrt(5 * read_noise^2) = read_noise * sqrt(5) = read_noise * 2.23607 = (2 * read_noise) * (2.23607/2) = (2*read_noise) * 1.118 or 11.8% increase (over LP noise which is 2*read_noise in this case)

3. sqrt(read_noise^2 + (3 x read_noise)^2) = sqrt(10 * read_noise^2) = read_noise * sqrt(10) = read_noise * 3.162278 = (3 * read_noise) * 1.054093 = 5.4% increase over LP noise alone (which is 3*read_noise here)

4. sqrt(read_noise^2 + (5 x read_noise^2) = sqrt(26 * read_noise^2) = read_noise * 5.09902 = (5* read_noise) * 1.0198 = 1.98% increase over LP noise alone

From this you can see that if you opt for read noise to be x3 smaller than LP noise - it will be the same as having only 5.4% larger LP noise and no read noise, and if you select x5 smaller read noise - it will be like you increased LP noise by only 1.98% (and no read noise).

Most people choose either x3 or x5 - but you can choose any multiplier you want - depending how much you want to impact final result. Thing is - as you start increasing multipliers - gains get progressively smaller, so there is really not much point going above ~ x5

Ok, but how to measure it?

That is fairly easy - take any of your calibrated subs and convert to electrons using e/ADU for your camera. CCD will have fixed system gain, while gain on CMOS will depend on selected gain. Pay attention when using CMOS cameras if your camera has lower bit count than 16 bits. In that case you need to additionally divide with 2^(16-number_of_bits) - or divide with 4 for 14 bit camera, with 16 for 12bit camera and 64 for 10bit camera.

When you prepare your sub - just select empty background and measure mean, or even better median electron value on it (median is better if you select odd star or very faint object that you don't notice). This will give you background value in electrons.

Square root of this value is your LP noise. You need to increase exposure until this LP noise value is your factor times larger than read noise of your camera.

Alternatively, if you want to get exposure from single frame - take your read noise, multiply with selected factor, square it and this will give you "target" LP level. You need to expose for "target" / "measured" longer (or shorter - depending on number you get).

Makes sense?

 

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41 minutes ago, vlaiv said:

Offset is not important for sub exposure length. Use gain setting that you will be using for imaging.

If you want to determine what is best tradeoff for sub length - here are guidelines:

1. How much data you want to stack and process? Shorter subs mean more data. Some algorithms like more data, others like good SNR per sub

2. How likely is it that you'll get ruined sub (for whatever reason - wind, earthquake, airplane flying thru the FOV - whatever makes you discard the whole sub - satellite trails can be easily dealt with in stacking if you use some sort sigma reject). Longer discarded subs mean more imaging time wasted

3. Differences in setup - in general, you'll have different sub length for each filter, but sometimes you will want to keep single exposure length over range of filters (like same exposure for LRGB and same for NB filters) as this simplifies calibration - only one set of darks instead of darks for each filter

4. What is the increase in noise that you are prepared to tolerate?

 

Only difference between many short subs and few long subs (including one long sub lasting whole imaging time) - totaling to same total imaging time - is in read noise. More specifically, difference comes down to how small read noise is compared to other noise sources in the system.

When using cooled cameras and shooting faint targets - LP noise is by far the most dominant noise source, that is why we decide based on it, but it does not  have to be (another thing to consider when calculating). If you have very dark skies and use NB filters - it can turn out that thermal noise is highest component, so this calculation should be carried out against it instead.

In fact - you want "sum" of all time dependent noise sources (which are target shot noise, LP noise and dark current or thermal noise - all depend on exposure length) and compare that to read noise.

Read noise is only time independent type.

Noises add like linearly independent vectors - square root of sum of squares. This is important bit, because this means that total increase is small if you have components that are significantly different in magnitude. Here is example:

Let's calculate percentage of increase if we have LP noise that is same, twice as large, 3 times as large and 5 times as large as read noise.

"sum" of noises will be sqrt( read_noise^2 + lp_noise^2) so we have following:

1. sqrt(read_noise^2 + (1 x read_noise)^2) = sqrt( 2 * read_noise^2) = read_noise * sqrt(2) = read_noise * 1.4142 ... or 41.42% increase in total noise due to read noise

2. sqrt(read_noise^2 + (2 x read_noise)^2) = sqrt(5 * read_noise^2) = read_noise * sqrt(5) = read_noise * 2.23607 = (2 * read_noise) * (2.23607/2) = (2*read_noise) * 1.118 or 11.8% increase (over LP noise which is 2*read_noise in this case)

3. sqrt(read_noise^2 + (3 x read_noise)^2) = sqrt(10 * read_noise^2) = read_noise * sqrt(10) = read_noise * 3.162278 = (3 * read_noise) * 1.054093 = 5.4% increase over LP noise alone (which is 3*read_noise here)

4. sqrt(read_noise^2 + (5 x read_noise^2) = sqrt(26 * read_noise^2) = read_noise * 5.09902 = (5* read_noise) * 1.0198 = 1.98% increase over LP noise alone

From this you can see that if you opt for read noise to be x3 smaller than LP noise - it will be the same as having only 5.4% larger LP noise and no read noise, and if you select x5 smaller read noise - it will be like you increased LP noise by only 1.98% (and no read noise).

Most people choose either x3 or x5 - but you can choose any multiplier you want - depending how much you want to impact final result. Thing is - as you start increasing multipliers - gains get progressively smaller, so there is really not much point going above ~ x5

Ok, but how to measure it?

That is fairly easy - take any of your calibrated subs and convert to electrons using e/ADU for your camera. CCD will have fixed system gain, while gain on CMOS will depend on selected gain. Pay attention when using CMOS cameras if your camera has lower bit count than 16 bits. In that case you need to additionally divide with 2^(16-number_of_bits) - or divide with 4 for 14 bit camera, with 16 for 12bit camera and 64 for 10bit camera.

When you prepare your sub - just select empty background and measure mean, or even better median electron value on it (median is better if you select odd star or very faint object that you don't notice). This will give you background value in electrons.

Square root of this value is your LP noise. You need to increase exposure until this LP noise value is your factor times larger than read noise of your camera.

Alternatively, if you want to get exposure from single frame - take your read noise, multiply with selected factor, square it and this will give you "target" LP level. You need to expose for "target" / "measured" longer (or shorter - depending on number you get).

Makes sense?

 

Thank you so much for this. This makes sense and I see what you mean when you say a calculator might not work for all these factors :)

 

Cheers 

Roger 

 

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On 14/06/2022 at 20:18, Taraobservatory said:

Hello, 

 

Been through a couple of sites now and watched a few YouTube videos on this topic as it seems to be quite useful. Looks to me as it can save allot of people some head ache and money. 

When I am after is a tool that can calculate the optimal exposure for your setup under your particular sky. 

I know Sharpcap has a sensor analysis tool which is used to calculate the specifics of your camera, but you also need something to calculate your sky background. Then we can go populate the values in http://www.gibastrosoc.org/sections/astrophotography/optimum-exposures-calculator. This calculator does not talk about CMOS cameras which has Gain and offset etc.. So I believe its a bit outdated (only relevant for CCD). Am I wrong is assuming this ? 

A tool that can do all this for you so you dont need to jump back and forth to different calculators and softwares to determine your exposure would be amazing. So if anyone has any idea, please let me know. 

The benefits of knowing this could actually save you allot of money. Say you live under Bortle 4 or 5 sky and invest in a have duty mount for better long exposures which is apparently completely useless with a CMOS camera. In fact (if you use LRGB only you MAY not even need to guide as your ideal exposure times can be reduced dramatically with the same result). 

Any thoughts and help to understand this better would be welcomed 

 

Cheers 

Roger

 

I had to write my own software for this. Install python, synphot and astropy, then run ETC.py. All files should be in the same folder.

Didn't get a chance to make a standalone app. I'll get to it some time. Tested on IOS and Ubuntu, windows not yet. If you can't compile and run it, PM me and I'll calculate the total exposure time for your target. 

Clear skies!

 

CCD.csv compute.py device.py ETC.py utility.py

Screen Shot 2022-06-16 at 2.00.22 AM.png

Edited by dan_adi
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6 hours ago, dan_adi said:

I had to write my own software for this. Install python, synphot and astropy, then run ETC.py. All files should be in the same folder.

Didn't get a chance to make a standalone app. I'll get to it some time. Tested on IOS and Ubuntu, windows not yet. If you can't compile and run it, PM me and I'll calculate the total exposure time for your target. 

Clear skies!

 

CCD.csv 6.05 kB · 0 downloads compute.py 7.47 kB · 0 downloads device.py 1.86 kB · 0 downloads ETC.py 21.19 kB · 0 downloads utility.py 16.33 kB · 0 downloads

Screen Shot 2022-06-16 at 2.00.22 AM.png

 

Oh my, How amazing! And thank you for sharing. This is what I am after! 

The season is over for me here in Norway due to the sky brightness but ill write down my measurements and send you as soon as the skies get dark again. . Is the calculator available for Windows ? im not familiar with any of the programs you mention. 

Kind regards

Roger 

 

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It could be run on windows too, with python and the dependencies installed. But first I must run it on my windows machine to check. As a side note the calculator computes the total integration time needed for a desired SNR. The individual subexposure length is not computed.  The  thing to remember is that the total SNR and total integration time is important. You can reach the desired SNR with lots of small subexposures or fewer longer exposures. What dictates this, is the type of camera you use, CMoS or CCD, your light pollution and read noise. 

Clear skies

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My goal is to take as short exposures as possible without adding too much noise. I like the stars to be very tight. But I didn't want to miss out on faint details due to noise from my CCD. 

Here are some parameters if you dont mind. 

Read noise 10,78

Dark current 0.018 

Pixel Size 5,3

QE 57% 

A 120mm FL 630mm (f5.25) 

SQM (Sky brightness) 21.78mag (Bortle 1.4) 

What would this look like ? 

 

Kind regards and clear skies :)

Roger 

 

 

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36 minutes ago, Taraobservatory said:

My goal is to take as short exposures as possible without adding too much noise. I like the stars to be very tight. But I didn't want to miss out on faint details due to noise from my CCD. 

Here are some parameters if you dont mind. 

Read noise 10,78

Dark current 0.018 

Pixel Size 5,3

QE 57% 

A 120mm FL 630mm (f5.25) 

SQM (Sky brightness) 21.78mag (Bortle 1.4) 

What would this look like ? 

 

Kind regards and clear skies :)

Roger 

 

 

When I get to my computer I will input your parameters. Don't worry so much about noise, with stacking the signal will "grow" faster than the noise, that is why we stack multiple photos. I also have a CCD with 10 electron read noise and 300 sec exposure for LRGB, in Bortle 4 zone, is a good compromise. If you have such dark skies you can expose for longer, but one limitation would be your mount and guiding performance, wind gust, and also satellite trails and such. 

 

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Posted (edited)

Indeed, 

I have very dark skies and allot of potential to get some good photos. I noticed that my 60 and 120sec exposure of M13 was perfect for stars at least. I also did a another with M92 where I exposed for 180sec where the stars are sharp but not as tight as M13. This leads me to believe that my guiding could be better its but good for my mount (around RMS 0.4 which is great). Its a Skywatcher EQ8r-Pro with a 40kg load. See examples in these links https://www.astrobin.com/53lqze/B/https://www.astrobin.com/fbphrz/

The reason for the 40kg load is because I have a dual setup which I have now dismantled in favour of single telescope configuration because it gives me more freedom to experiment with different telescopes without having to unload a 40kg rig every time. And hopefully my guiding will improve too if only slightly. 

Not sure I gave you full well but it is 31268 from my measure in pixinsight. I believe this also has to be in your calculations ? 

Thanks for your help! Really appreciate it :)

 

Kind regards

Roger 

Edited by Taraobservatory
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3 hours ago, Taraobservatory said:

Indeed, 

I have very dark skies and allot of potential to get some good photos. I noticed that my 60 and 120sec exposure of M13 was perfect for stars at least. I also did a another with M92 where I exposed for 180sec where the stars are sharp but not as tight as M13. This leads me to believe that my guiding could be better its but good for my mount (around RMS 0.4 which is great). Its a Skywatcher EQ8r-Pro with a 40kg load. See examples in these links https://www.astrobin.com/53lqze/B/https://www.astrobin.com/fbphrz/

The reason for the 40kg load is because I have a dual setup which I have now dismantled in favour of single telescope configuration because it gives me more freedom to experiment with different telescopes without having to unload a 40kg rig every time. And hopefully my guiding will improve too if only slightly. 

Not sure I gave you full well but it is 31268 from my measure in pixinsight. I believe this also has to be in your calculations ? 

Thanks for your help! Really appreciate it :)

 

Kind regards

Roger 

For example galaxy M33, surface brightness 23 mag/arcsec^2, and a desired SNR of 20 in each band:

Luminance: 1.55 hours

R: 5.38 hours

G:5.72 hours

B: 6.3 hours

For a SNR of 40 in each band:

Luminance: 5.98 hours

R: 21.7 hours

G: 22.6 hours

B: 24.9 hours

As a side note I don't know how the SNR adds in each band, I've had a conversation with Vlaiv some time ago about this, but I could not find some equations in this regard. The conclusion beeing the SNR in each band is not additive, like the final SNR is not the sum of SNR in L + RGB.

In your case I would aim for a SNR of 40 in Luminance, and a SNR of 20 in each RGB, so I would expose for 5.98 hours in Luminance, 5.38, 5.72,6.3 in RGB. Round that to 6 hours in each band, so 24 hours in total exposure time. Hope this gives you an idea about your setup and imaging time.

Another note, for extended objects like galaxies and nebulae, you need the surface brightness for computation, not the simple magnitude. Here is a simple explanation: https://rasc-vancouver.com/2020/08/23/surface-brightness-vs-magnitude/

Clear skies!

Edited by dan_adi
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6 hours ago, Taraobservatory said:

My goal is to take as short exposures as possible without adding too much noise. I like the stars to be very tight. But I didn't want to miss out on faint details due to noise from my CCD. 

Here are some parameters if you dont mind. 

Read noise 10,78

Dark current 0.018 

Pixel Size 5,3

QE 57% 

A 120mm FL 630mm (f5.25) 

SQM (Sky brightness) 21.78mag (Bortle 1.4) 

What would this look like ? 

 

Kind regards and clear skies :)

Roger 

 

 

You have very dark sky and very high read noise - optimum exposure length is 10+ minutes for you.

 

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I just ran some basic calculations and it will actually take about an hour of exposure in those conditions to swamp the read noise x3

If you really want shorter exposures - look at CMOS sensors with very low read noise.

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3 hours ago, vlaiv said:

I just ran some basic calculations and it will actually take about an hour of exposure in those conditions to swamp the read noise x3

If you really want shorter exposures - look at CMOS sensors with very low read noise.

I also have a QHY 268M which I will use with my Esprit 80 for nebula. That has a very low read noise. What other CMOS cams can you recommend. One without  amp glow and and large enough pixels for a 1300mm ? 

 

Kind regards 

Roger 

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7 hours ago, dan_adi said:

For example galaxy M33, surface brightness 23 mag/arcsec^2, and a desired SNR of 20 in each band:

Luminance: 1.55 hours

R: 5.38 hours

G:5.72 hours

B: 6.3 hours

For a SNR of 40 in each band:

Luminance: 5.98 hours

R: 21.7 hours

G: 22.6 hours

B: 24.9 hours

As a side note I don't know how the SNR adds in each band, I've had a conversation with Vlaiv some time ago about this, but I could not find some equations in this regard. The conclusion beeing the SNR in each band is not additive, like the final SNR is not the sum of SNR in L + RGB.

In your case I would aim for a SNR of 40 in Luminance, and a SNR of 20 in each RGB, so I would expose for 5.98 hours in Luminance, 5.38, 5.72,6.3 in RGB. Round that to 6 hours in each band, so 24 hours in total exposure time. Hope this gives you an idea about your setup and imaging time.

Another note, for extended objects like galaxies and nebulae, you need the surface brightness for computation, not the simple magnitude. Here is a simple explanation: https://rasc-vancouver.com/2020/08/23/surface-brightness-vs-magnitude/

Clear skies!

Thank you so much for this! Great help! 

I will et you know how it goes :)

 

Clear skies 

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2 minutes ago, Taraobservatory said:

I also have a QHY 268M which I will use with my Esprit 80 for nebula. That has a very low read noise. What other CMOS cams can you recommend. One without  amp glow and and large enough pixels for a 1300mm ? 

 

Kind regards 

Roger 

Small pixels are not really a problem, as you can bin your camera and make larger pixels and optimize you image scale. I guess the new ASI 6200 or 2600 series don't have amp glow. But given your top notch skies the cameras you have will do a great job as is. I use a CCD KAF 16200 sensor and it's ok in Bortle 4. Bellow is a work in progress, about 20 hours worth of subs in luminance with a CCD. I'm ok with it.

MasterLightAbell2218_ABE.tif

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3 minutes ago, Taraobservatory said:

I also have a QHY 268M which I will use with my Esprit 80 for nebula. That has a very low read noise. What other CMOS cams can you recommend. One without  amp glow and and large enough pixels for a 1300mm ? 

 

Kind regards 

Roger 

Pixel size is not important, or rather, small pixels can be used on long focal length.

You can reuse the same camera on 1300mm of FL as long as you bin your pixels to get adequate sampling rate.

I use 3.8um ASI1600 camera on 1600mm FL.

Sometimes I can use bin x2 - for 1"/px but most time I'll bin x3 for 1.5"/px. I bin in software after capturing, when I asses FWHM of the capture (binning in software also allows for more flexibility than binning in firmware of camera).

Do be careful - binning with CMOS sensors does not keep the same read noise, so above calculation (or measurement) for exposure length is always performed at native resolution. This might be an issue for you as higher native resolution of the system - longer subs are needed to swamp the read noise.

This is because with higher resolution - LP signal is also spread over more pixels and each pixel gets less LP signal and thus has lower LP noise (read noise per pixel remains the same).

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16 hours ago, dan_adi said:

Small pixels are not really a problem, as you can bin your camera and make larger pixels and optimize you image scale. I guess the new ASI 6200 or 2600 series don't have amp glow. But given your top notch skies the cameras you have will do a great job as is. I use a CCD KAF 16200 sensor and it's ok in Bortle 4. Bellow is a work in progress, about 20 hours worth of subs in luminance with a CCD. I'm ok with it.

MasterLightAbell2218_ABE.tif 62.57 MB · 5 downloads

Reassuring to say the least. I can honestly not see any noise in your picture. What is your exposure pr sub here? I have never been able to put my gear to proper use yet. The images I have done is no more than max 10subs pr filter at max 300sec for M101 and 180 for the clusters. Not nearly as much time as I actually need. Just waiting to get started again :)

 

Clear skies 

 

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6 hours ago, Taraobservatory said:

Reassuring to say the least. I can honestly not see any noise in your picture. What is your exposure pr sub here? I have never been able to put my gear to proper use yet. The images I have done is no more than max 10subs pr filter at max 300sec for M101 and 180 for the clusters. Not nearly as much time as I actually need. Just waiting to get started again :)

 

Clear skies 

 

300 sec per sub, - 10 degrees C. Don't worry, it took me 3-4 years to have a setup that works with minimal effort. By the time I was finished  the new gen CMOS sensors came out :))

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On 14/06/2022 at 20:43, Taraobservatory said:

How would you go about finding the best exposure? 

As I have always done. Take a test exposure using an educated guess of what is suitable. Adjust subsequent exposure times such that the peak of the histogram is about one-third of the way up from zero.

It doesn't matter too much if the histogram peaks at one-quarter or one-half. A "ball park" exposure time is all that is necessary.

This works no matter what camera, telescope, filters or sky quality you have.

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3 minutes ago, pete_l said:

As I have always done. Take a test exposure using an educated guess of what is suitable. Adjust subsequent exposure times such that the peak of the histogram is about one-third of the way up from zero.

It doesn't matter too much if the histogram peaks at one-quarter or one-half. A "ball park" exposure time is all that is necessary.

This works no matter what camera, telescope, filters or sky quality you have.

Sorry to say, but that is completely useless way to determine correct exposure length for astrophotography.

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What is the definition of optimum exposure time?

In my mind, it is collecting as much signal as possible without blowing out the stars.

That makes me think, why not just take looong exposures, let the stars blow out, and then take some short exposures of just the stars and amalgamate the two.

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1 hour ago, vlaiv said:

Sorry to say, but that is completely useless way to determine correct exposure length for astrophotography.

Practice beats theory every single time.

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30 minutes ago, pete_l said:

Practice beats theory every single time.

Don't see how practice relates to this.

Just change offset or gain and you will move histogram peak (considerably).

Say your histogram peak is at 1/3 and you raise gain so that e/ADU is half of the original - now histogram peak is suddenly at 2/3.

Does this mean you need half of exposure time or if you change gain so that e/ADU is double the original - now peak is at 1/6 - must be twice as long exposure.

Which one is it?

Changing gain by factor of 2 in either direction might change read noise by minimal amount, and read noise is only thing that dictates needed sub length.

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