Bortle 5: gain and exposure length ?

[Edward Dewolf]

Hi,

I started imaging with a ASI294MC-PRO from a Bortle class 5 zone.

Can anyone help me how to find out what the best combination of gain and exposure length would be to image galaxies and nebulaes?

How does one figure that out?

Thx for helping

Edward

[Wyvernp]

If you haven't watched this already it should answer all your questions. 

 

[symmetal]

My method is to expose until the sky background noise swamps the camera read noise, such that the sky background 16 bit ADU value is 10 x the square of the read noise.

Unity gain (1 electron incoming = 1 ADU) is always a good starting gain which is gain 117 for the ASI294. However this camera uses ZWOs HSC (High Conversion Gain) mode at gain 120 and higher which claims to give the same dynamic range as using gain 0 for a much lower read noise and is only slightly above unity gain.

So at gain 120, default offset which I believe is 30, you need to expose until the median 16 bit ADU value of the sky background is around 280 ADU. There is little point in exposing longer as you just end up clipping more stars for little gain in signal to noise. Start another sub when 280 ADU sky background is reached as the noise due to read noise at that point is negligible and won't significantly contribute to the overall noise.

So start off with an exposure of say 2 mins on a plain star field (no large nebula areas), and see what the sky background ADU value is, either by hovering the mouse cursor over the preview image background which should give you an ADU value, or examine the image statistics and note the median value. It should be fairly close to the mean value anyway. If it's more than 280 try again with a lower exposure and if it's less try a longer exposure. As it's a OSC camera the mouse values will depend on what colour bayer pixel your over (assuming the preview is not debayered) so the image statistics median value is probably a better bet for OSC.

For the same sky bortle darkness use this exposure value for all your ASI224 RGB images. It doesn't have to be exactly 280 ADU, between 270 and 290 will be fine. 🙂

That's just my suggestion, others may disagree. 😄

Alan

Edited September 17, 2020 by symmetal

[Adam1234]

15 hours ago, symmetal said:

My method is to expose until the sky background noise swamps the camera read noise, such that the sky background 16 bit ADU value is 10 x the square of the read noise.

Unity gain (1 electron incoming = 1 ADU) is always a good starting gain which is gain 117 for the ASI294. However this camera uses ZWOs HSC (High Conversion Gain) mode at gain 120 and higher which claims to give the same dynamic range as using gain 0 for a much lower read noise and is only slightly above unity gain.

So at gain 120, default offset which I believe is 30, you need to expose until the median 16 bit ADU value of the sky background is around 280 ADU. There is little point in exposing longer as you just end up clipping more stars for little gain in signal to noise. Start another sub when 280 ADU sky background is reached as the noise due to read noise at that point is negligible and won't significantly contribute to the overall noise.

So start off with an exposure of say 2 mins on a plain star field (no large nebula areas), and see what the sky background ADU value is, either by hovering the mouse cursor over the preview image background which should give you an ADU value, or examine the image statistics and note the median value. It should be fairly close to the mean value anyway. If it's more than 280 try again with a lower exposure and if it's less try a longer exposure. As it's a OSC camera the mouse values will depend on what colour bayer pixel your over (assuming the preview is not debayered) so the image statistics median value is probably a better bet for OSC.

For the same sky bortle darkness use this exposure value for all your ASI224 RGB images. It doesn't have to be exactly 280 ADU, between 270 and 290 will be fine. 🙂

That's just my suggestion, others may disagree. 😄

Alan

Hi Alan, I'm interested in this concept and might have an experiment and give it a try, just wondered though, where did you get the ADU value of 280 from?

[symmetal]

1 hour ago, Adam1234 said:

Hi Alan, I'm interested in this concept and might have an experiment and give it a try, just wondered though, where did you get the ADU value of 280 from?

Hi Adam,

There were a mass of topics on this on the CN forums after the ASI1600 came out. It applies to all cameras, but especially CMOS cameras with variable gain and offset.

This one I think got me started. The replies by Jon Rista, (who seems to be their version of vlaiv 😄) from post 3 gives the theory.

Also this post and this one add further discussion and info if you have a couple of spare evenings. 😀

A read noise swamping background ADU is calculated where the background ADU is 10*RN^2. Others use 5*RN^2 or just 20*RN but the first one is most often used.

You need to ensure that consistant units are used in the calculations. The gain values used in the calculations are those read from the GAIN in electrons/ADU from the graphs shown above.

If you chose the unity gain value of 117 (in 0.1db units), the value you enter on the camera driver, always a good starting point, the gain in e-/ADU gain is 1.0   The read noise at this gain value from the graphs is about 5.7 e-

The ASI294 HSC mode starts at gain 120 which should theoretically give better noise results so I chose gain 120 which is about 0.9 e-/ADU. Slightly higher than unity gain. The read noise at this value is only about 1.8 e-

10*RN^2 = 10 * 1.8 * 1.8 = 32.4 e-.  To convert to ADU divide it by the gain in e-/ADU. So 32.4/0.9 = 36 ADU

The offset value (default 30 for the ASI294 I believe) effectively just adds a fixed offset ADU value to every pixel so you need to add this to the above figure so it is comparable to the sky background ADU values.  Therefore add 30 ADU to the 10*RN^2 ADU value, 36 + 30 = 66 ADU.

As this is a 14 bit camera the ADU values quoted above are 14 bit values. To display as 16 bit values, as used on most capture programs, you need to convert it to 16 bit by adding 2 zero value LSB (least significant bits). This effectively multiplies the 14 bit ADU values by 4 (ie. 2^2)

So a 14 bit sky background value of 66 ADU becomes a 16 bit value of 66 * 4 = 264 ADU

Whoops! it's not 280. I must have read the read noise and gain graphs a bit differently when I came up with 280. I read them rather quickly when I posted yesterday. 😉

If you want to get the sky background ADU for different gain and or offset values, just read the relevant values from the graphs and do the maths. 🙂

For the offset I assumed that an increase of 1 offset increase the ADU by 1. This is true for the ASI1600 (12 bit) and ASI071 (14 bit) cameras I have. However my ASI6200 (16 bit) increases the ADU by 10 for every 1 increase in offset. This threw me when I first calculated the sky background values assuming ADU/offset was 1 to 1. You can easily check what it is by reading the peak value of the noise in a bias frame, and increasing the offset by say 10, and taking another bias frame. If the noise peak increases by 10 (at native bit depth) it's 1 to 1. Remember, If displaying at 16 bit then it will show an increase of 40 (for a 14 bit camera) or 160 for a 12 bit camera.

Anyway, that's the theory Adam. I hope it helped and hasn't addled your brain. 😁

Alan

Edited September 17, 2020 by symmetal

[Adam1234]

Thank you Alan, a lot of info there but very helpful! I will try calculating what ADU I should aim for. 

Another question then, how do I measure the sky background ADU? FYI I use APT for image for image capture

[symmetal]

2 hours ago, Adam1234 said:

Thank you Alan, a lot of info there but very helpful! I will try calculating what ADU I should aim for. 

Another question then, how do I measure the sky background ADU? FYI I use APT for image for image capture

Look at the image statistics, (I believe most image capture programs have this option) and the median ADU value is the sky background. Also, I don't know if APT has it but normally moving the mouse over the preview image gives a readout of the ADU level under the cursor. Move the cursor around a piece of sky background and get an average value of it. It's best to use an area of sky without a lot of nebula for testing, as they can possibly affect the median value, which doesn't then reflect the true sky background.

Of course the exposure to reach the sky background depends on the filter used. L will be shortest and R, G and B around 3 times longer, assuming you use the same gain setting. For R, G and B it's best to end up chosing the same exposure for each, even though that may not be optimum for some, just to avoid multiple dark exposures to match. If you do use a different gain, you need to recalculate the sky background ADU for that gain setting.

For OSC the mean value may be better to use rather than the median depending on whether it is debayered on preview. They should be fairly close anyway if you chose an area away from the milky way to do the test. 🙂

The swamping sky background value is only really useful for LRGB or OSC imaging. For NB you will unlikely be able to expose long enough to reach the ADU limit anyway unless you expose for an hour or more or you're at a very light polluted site. If you use much higher gain for NB you may well reach it in a shorter time but remember you have to recalculate the limiting ADU value if the gain is changed.

Alan 

Edited September 18, 2020 by symmetal

[Edward Dewolf]

Thank you al for the feedback, lots to look into, but that is what this hobby is all about !

[Adam1234]

Thanks very much Alan. No clear sky tonight, so will see if I can find these image statistics on APT for the 5min Ha, Oiii and Sii subs I've captured this week on the Elephant's Trunk.

I'm in a Bortle 8 sky, so quite a bit of LP so these calculations will probably be very useful.

@Edward Dewolfsorry to have hijacked your thread, but probably been useful for the both of us!

 

Adam

[symmetal]

3 hours ago, Adam1234 said:

Thanks very much Alan. No clear sky tonight, so will see if I can find these image statistics on APT for the 5min Ha, Oiii and Sii subs I've captured this week on the Elephant's Trunk.

I'm in a Bortle 8 sky, so quite a bit of LP so these calculations will probably be very useful.

It's best to wait an hour or so after astro dark begins before doing the tests to get a more realistic sky background value as the sky gets darker for a while after astro dark 'officially' begins. 🙂

If you tell me what camera you're using Adam, and the gain and offset values you've used, I can calculate the sky background ADUs for you if you don't fancy doing the maths. 😁

Same for you Edward, if you want to use a gain other than 120, but that looks a good value to use, at least for starting out.

For narrowband there is some merit in using higher gain, twice unity is often used though I tend to use unity gain all the time. Just being lazy to avoid having to have multiple sets of darks. 😁

Alan

[Adam1234]

4 hours ago, symmetal said:

It's best to wait an hour or so after astro dark begins before doing the tests to get a more realistic sky background value as the sky gets darker for a while after astro dark 'officially' begins. 🙂

If you tell me what camera you're using Adam, and the gain and offset values you've used, I can calculate the sky background ADUs for you if you don't fancy doing the maths. 😁

Same for you Edward, if you want to use a gain other than 120, but that looks a good value to use, at least for starting out.

For narrowband there is some merit in using higher gain, twice unity is often used though I tend to use unity gain all the time. Just being lazy to avoid having to have multiple sets of darks. 😁

Alan

HI Alan, I'm using the ASI1600mm pro. 

For RGB I'm using gain 75 and NB gain 200. I think I've got it set to use the default offset, which I think was 25 (although on my filenames it says offset N/A so not sure whether it actually uses the offset?)

I did some rough calculations myself (assuming it doesn't use the offset) and came up with 896 for NB and 320 for RGB. Or 1296 (NB) and 720 (RGB).

Would be useful to know if I'm in the right ballpark figure.

[symmetal]

1 hour ago, Adam1234 said:

HI Alan, I'm using the ASI1600mm pro. 

For RGB I'm using gain 75 and NB gain 200. I think I've got it set to use the default offset, which I think was 25 (although on my filenames it says offset N/A so not sure whether it actually uses the offset?)

I did some rough calculations myself (assuming it doesn't use the offset) and came up with 896 for NB and 320 for RGB. Or 1296 (NB) and 720 (RGB).

Would be useful to know if I'm in the right ballpark figure.

Hi Adam,

I assume you're using the Ascom driver for the 1600 as your filenaming didn't know what it was. It's a limitation of Ascom that offset is not a parameter that is available to the capture program using the driver. If you use the native Zwo driver then the offset parameter is available to the capture program to be read and written to. If using the Ascom driver then it can only be set in the driver control panel itself. The capture program will give you the option to access the Ascom driver and set the offset on the camera, but the capture program has no way of knowing what you set it to. To view the Ascom offset value you have to click the 'Advanced' box on the driver shown here

When it first came out they was much debate on what offset value is optimum to use for different gain settings, but it became such a pain having to alter it all the time, and the fact that altering the offset has an insignificant effect on dynamic range, it's not worth worrying about and just set it to a high enough value where it works at all gain settings without black clipping data. In later driver updates Zwo set it to be 50 and hid it behind the Advanced box so it wasn't easily changeable by accident. I have mine set to 56 above but have now had to increase it to 64 (the same as vlaiv has his 1600 set to) as I was still getting slight black clipping on dark flats of about 1 sec duration.

You may be wondering why seemingly adding fixed ADU values using offset works and what it actually does. If it just added a fixed number of digital ADU units it wouldn't really do anything useful. But offset and gain work in the analog domain and not the digital. The analog voltage signal value read from each pixel is processed by an amplifier before it does to the A-D converter to create the digital ADU value. You want the min and max signal voltages read, to lie between the A-D converter reference analog signal voltage limits which correspond to ADU values of 0 and 4095 (for a 12 bit camera like the 1600). The A-D converter may be designed to accurately convert an analog input signal between say 0.5 and 1.5 volts for example. If the A-D converter is powered just from say 5V, you don't want the input signal too close to the 0V power rail as things may get non-linear so a value above 0 volts is taken as the minimum value into the converter.

You can now see what the offset value does. It adds a fixed DC voltage to the analog input signal such that the lowest signal value input to the A-D converter is say 0.5V, using the example above. A covered sensor pixel may give an output between plus or minus a few microvolts, depending on the noise present. You want all those values to be correctly converted so add a fixed positive DC value (corresponding to the offset) to all the pixel readings so they will always be within the range of the converter.

The gain just amplifies the signal so more digital units can be used to represent the smaller analogue value, so getting a higher digital resolution. Of course, if the gain is too high you get excess white clipping and loss of signal information. Bright stars will always end up getting white clipped so gain setting is a compromise. Similarly, too low an offset will give black clipping and loss of signal or noise information.

Back to your post, 😁 your ADU values are a bit higher than mine. You have to plot a curve to fit the graphs so values between the plotted points are a bit lower that those shown by the straight lines. 

At zero offset and camera gain 75, I got 2.00 read noise and 2.15 gain giving ADU 298. At camera gain 200 I got 1.45 read noise and 0.50 gain giving 673 ADU.

Your adding of 400 to each value for offset 25 is correct.

As it's most likely you are using default offset 50, your sky limit ADU values to aim for become 1098 ADU at gain 75 and 1473 ADU at gain 200.

Hope that helps Adam. 🙂

Alan

[Adam1234]

That's brilliant thanks Alan, I was more or less in the right ballpark then, the read noise and gain values I used for my calcs were off from yours by a decimal place as I very crudely read the graphs for my camera. 

Come to think of it I think it is the default of 50 offset I'm using as I didn't change it.

Thanks for your help!

[Phillyo]

Good evening all,

Just reviving this post as I'm very confused (nothing new there). I've just taken receipt of my lovely shiney new ASI533 and I plugged it into sharpcap to run the sensor analysis thing and to try and calculate my sub exposure lengths. My back garden has an SQM of approx 20.50 (according to the lightpollutionmap.info website). I've used Dr Robin Glovers calculations here http://tools.sharpcap.co.uk/ with an F ratio of 2 (Samyang 135/2 lens), pixel size of 3.76 and a QE of 80% to give a result of approx 6 e/pixels/s. Once I put that calculation into his 10x(R^2/P) where R = Read Noise (electrons) and P = Light pollution rate (e/pixel/s) I get a calculation of 10x(1.5^2/6) = 3.75...I'm assuming that's 3.75 seconds. 

Now according to his presentation, for an RGB sensor you would then multiply that by 3 to give a grand total of 11.25. So that means I need to take photo's for approx 12 seconds. That can't be right surely!? That's at gain 100, which I think is unity gain?

I know I can pay to get the pro version which will do the calculation for me when I point it up at the sky but a) it's cloudy (again) and b) my Samyang lens hasn't arrived yet so I can't do that.

Help? Please?

[symmetal]

The 10x(R^2/P) equation seems to calculate the exposure when the sky electron flux equals 10xR^2 at unity gain.

Using the website calc you linked I get 1.21 -e/pix/s with my ZS61 scope and ASI1600 at unity gain. Inserting that in 10x(R^2/P) gives me 24 secs.

The exposure I need to achieve a 16 bit camera output sky background ADU = 10*R^2, (as per the formula I use, mentioned above) is around 75 seconds, three times the exposure.

At unity gain the sky electron flux should be the sky background ADU contribution. The camera output sky background 16 bit ADU will be the sky background ADU contribution plus the camera offset, and adjusted for the bit depth of the camera when converted to 16 bit. Assuming the calculation for the sky background electron flux is correct, I'm trying to see where the discrepancy is occurring at the moment. 🤔

Alan

Edited October 15, 2020 by symmetal

[noah4x4]

There is an easy way in Sharpcap that is so good I can get close to the right gain/exposure without even looking at my DSO object.

1.  First focus on any star. Then Goto your target. 

2.  Next open Smart Histogram in logarithmic mode.

3.  Adjust gain until the two upper green bars in Smart Histogram  are at their maximum length and square ends are aligned left.

4.  Adjust exposure until the steep slope at the left edge of the histogram is a fraction to the right of the left end of the green bars. 

Now look at your target. You will be within a notch of perfect gain and exposure. Tweak that (and focus) from there.

You might need to perform a Sensor Analysis before step (2), but this has already been done in Sharpcap for most popular ZWO cameras. Few people seem to embrace Smart Histogram, yet I find it brilliant. I still don't know my gain from my ISO, but with Smart Histogram it's sorted for me. 

[Phillyo]

9 hours ago, noah4x4 said:

There is an easy way in Sharpcap that is so good I can get close to the right gain/exposure without even looking at my DSO object.

1.  First focus on any star. Then Goto your target. 

2.  Next open Smart Histogram in logarithmic mode.

3.  Adjust gain until the two upper green bars in Smart Histogram  are at their maximum length and square ends are aligned left.

4.  Adjust exposure until the steep slope at the left edge of the histogram is a fraction to the right of the left end of the green bars. 

Now look at your target. You will be within a notch of perfect gain and exposure. Tweak that (and focus) from there.

You might need to perform a Sensor Analysis before step (2), but this has already been done in Sharpcap for most popular ZWO cameras. Few people seem to embrace Smart Histogram, yet I find it brilliant. I still don't know my gain from my ISO, but with Smart Histogram it's sorted for me. 

Hi, thanks for the reply. I don't have the lens yet so I can't do any actual imaging (plus it's cloudy) so I was just trying to do the maths to get a rough idea first.

Thanks.

9 hours ago, symmetal said:

The 10x(R^2/P) equation seems to calculate the exposure when the sky electron flux equals 10xR^2 at unity gain.

Using the website calc you linked I get 1.21 -e/pix/s with my ZS61 scope and ASI1600 at unity gain. Inserting that in 10x(R^2/P) gives me 24 secs.

The exposure I need to achieve a 16 bit camera output sky background ADU = 10*R^2, (as per the formula I use, mentioned above) is around 75 seconds, three times the exposure.

At unity gain the sky electron flux should be the sky background ADU contribution. The camera output sky background 16 bit ADU will be the sky background ADU contribution plus the camera offset, and adjusted for the bit depth of the camera when converted to 16 bit. Assuming the calculation for the sky background electron flux is correct, I'm trying to see where the discrepancy is occurring at the moment. 🤔

Alan

Thanks Alan. I gues I wait until all my kit is here then try doing some actual measurements, see what it comes back with.

[Phillyo]

So I went out last night and set everything up (with some hiccups but that's an issue for another day). I polar aligned nicely with sharpcap then moved the lens towards the east where I'm going to be imaging M45. I ran the Histogram/brain function in sharpcap and it did some tests and told me that for my set up my optimal exposure time is 3.2 seconds. What?

System setup = ASI533, Gain 100. Samyang 135/2 @F2. No filter other than UV/IR. 

Surely 3 seconds isn't going to really catch anything useable is it? I ended up setting it to 2.8 for rounder stars and leaving it doing 1min exposures but the clouds came over and I packed up and went to bed. I'll have a quick check of what I got last night (not much) but apparently 60 seconds is WAY too much. I've made a post on the sharpcap forums too so hopefully Robin will be able to confirm that 3.2s exposure time but it seems very fast! I know the 533 is sensitive and at F2 I'm catching lots of photons but still!! 

Interestingly the exposure calc in NINA gave pretty much the same result. 

Phil

[noah4x4]

I often use 2 second exposures on Hyperstar at f/2, so I am not surprised at your 3.2 seconds. Remember at f/2 you are capturing photons 25x faster than at f/10. Hence a 2 second exposure is the equivalent of almost a minute at f/10.

I do use live stacking, so 100 frames x 2 seconds will capture a huge amount of data.

[Viktiste]

I have read the posts above from @symmetal numerous times, as well as the posts linked to on the CN forum, trying to wrap my head around this.

I realized I have a knowledge gap, so did some reading. I found this 4 part blog very informative to help me understand the basics:

https://cloudbreakoptics.com/blogs/news/astrophotography-pixel-by-pixel-part-4-the-adu-and-you

 

So, for my ASI1600mm camera (which is 12 bit, hence the x 4 factor), does it all boil down to that the minimum exposure is this? 

So for example :

Unity gain, 139 -> Gain in e = 1 ->  Rnoise about 1.7 (from the red ZWO spec graph below)

Offset = 50

So I must expose at least until my background ADU = 316?

 

This will give very impractical short exposures in a light polluted area? Say I image one target for one whole night (or two nights for that matter) the amount of files will make stacking next to impossible?

Is there any downside to exposing much longer (apart from guiding, satellite trails etc.), as long as the target and most stars do not saturate?

 

ZWO 1600mm spec for reference:

 

Edited October 26, 2020 by Viktiste

[symmetal]

On 25/10/2020 at 22:53, Viktiste said:

I have read the posts above from @symmetal numerous times, as well as the posts linked to on the CN forum, trying to wrap my head around this.

I realized I have a knowledge gap, so did some reading. I found this 4 part blog very informative to help me understand the basics:

https://cloudbreakoptics.com/blogs/news/astrophotography-pixel-by-pixel-part-4-the-adu-and-you

 

So, for my ASI1600mm camera (which is 12 bit, hence the x 4 factor), does it all boil down to that the minimum exposure is this? 

So for example :

Unity gain, 139 -> Gain in e = 1 ->  Rnoise about 1.7 (from the red ZWO spec graph below)

Offset = 50

So I must expose at least until my background ADU = 316?

 

This will give very impractical short exposures in a light polluted area? Say I image one target for one whole night (or two nights for that matter) the amount of files will make stacking next to impossible?

Is there any downside to exposing much longer (apart from guiding, satellite trails etc.), as long as the target and most stars do not saturate?

Hi Viktiste,

Your calculation is correct apart from the x4 multiplication at the end. Converting from 12 bit (1600 camera) to 16 bit (usual format for capture program info and fits files) you append 4 zero value bits to the least significant end of the 12 bit data.  So a 12 bit white pixel in binary of 1111 1111 1111 becomes 1111 1111 1111 0000 in 16 bit. Every bit you append multiplies the result by 2, so appending 4 bits multiplies the result by 16.

You need to replace the x4 in your equation by x16 so your optimum sky background ADU is 1262 and not 316. This should give you more useful exposure durations. In my bortle 3 sky I use 60s for L and 180s for R, B and B to achieve around a sky background of around 1262 ADU. With higher LP you may find 45s for L and around 135s for R, G and B gets you nearer to 1262 ADU.

For narrow band imaging, expose for as long as is practical, as you won't achieve that sky background ADU unless you expose for a few hours. I normally use 600s for narrow band imaging with the 1600 at unity gain. You can opt to use higher camera gain for narrow band to enable shorter exposures and you need to put the new read noise figure and gain in -e/ADU in the equqtion for your new camera gain. At 2 x unity gain (camera gain 200) your sky background level works out at 1473 ADU.

Hope this helps you out. 🙂

Just for info, in computing, for normal mathematical working, when changing the bit depth used to represent integer numbers, you append or remove bits from the most significant end of the number, to avoid the actual number changing in value. If this was done with astro images the image would appear very dim when viewed in 16 bit and the stars would be clipped at 1/16 of the full 16 bit histogram range.

Alan

[Viktiste]

Ah! Thanks for the clarification.

Since I use Pixinsight, I guess  the easiest option is to set the factor to 1 and evaluate BG noise as 12 bit since this is possible in Pixinsight. Like this:

 

This is from imaging Bode last week, at a fairly dark site. Luminance, 600s, Gain=0, offset=0 (because I did not know any better).

Reality check for gain 0 offset 0:

(((10*(3.6)^2)/5) + 0) *1 = 26   (there should have been an equation utility on the forum, really hard to read as text 😉)

So I probably could have gone for 300s exposure in this example.

 

Thanks for your inputs in this thread, it made me learn something new!

 

 

[symmetal]

13 hours ago, Viktiste said:

Ah! Thanks for the clarification.

Since I use Pixinsight, I guess  the easiest option is to set the factor to 1 and evaluate BG noise as 12 bit since this is possible in Pixinsight. Like this:

This is from imaging Bode last week, at a fairly dark site. Luminance, 600s, Gain=0, offset=0 (because I did not know any better).

Reality check for gain 0 offset 0:

(((10*(3.6)^2)/5) + 0) *1 = 26   (there should have been an equation utility on the forum, really hard to read as text 😉)

So I probably could have gone for 300s exposure in this example.

Thanks for your inputs in this thread, it made me learn something new!

Yes, if it's easy to display the info in 12 bit you might as well do it that way. 🙂 As you used 0 offset above there's a direct correlation between ADU level and exposure so your optimum exposure would have been 26/65 * 600 = 240s.

I agree it's awkward displaying equations on the forum in a readable format. 😐

Alan