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There is still a few gaps to fill in.
Most of this is recap, however I have added thoughts about Full Well.
Also have reached (for me) a definitive conclusion about the ideal GAIN setting.
Once again, please contribute. It has been through all of your help that I have got this far.
The ZWO1600 camera uses a 12 bit ADC.
The maximum number it can output is 4095.
It doesn’t matter what GAIN setting, the max. output is still 4095.
The Full Well of the camera is 20k electrons. How does this work?
Example:
At unity GAIN (Sharpcap GAIN setting of 139) it takes one photon to generate 1 electron for the ADC to output the number 1.
At GAIN of less than 1, ie: 0.5(Sharpcap GAIN setting of 75), it would take 2 photons/electrons to output the number 1.
This interpretation can be seen by looking at the FW(e-) graph in the ASI1600 specifications.
Another way of saying this is:
At GAIN = 139 (unity) we gather 1 photon/electron for 1 ADU. => 1e/ADU => 4095 electrons Full Well
At GAIN = 75 we gather 2 photons/electron for 1 ADU. => 2e/ADU => 8200 electrons Full Well
At GAIN = 0 we gather 5 photons/electrons for 1 ADU. => 5e/ADU => 20,000 electrons Full Well.
So what does this mean, and how do we use this information?
But before I try and answer that, there is the issue about the 12 bits being made a 16 bit number by Sharpcap (or other software).
12 bits is the maximum resolution that can be had. It doesn’t matter if this 12 bit number is then converted into a 16 bit format. It is still the same information, just ‘scaled up’ in a linear way (multiplied by 12).
It does matter when we use Shiraz’s formula. He calls for a BIAS value that is supplied by the manufacturer. However, by taking a BIAS image (cap on, fast exposure) the Statistics tool can give a value.
For example:
I set the BIAS (BRIGHTNESS) in Sharpcap to a value of 20.
The camera is at -20C (most important).
I take a BIAS IMAGE and then analyse it using the Statistics tool in Pixinsight.
If I set the tool to 16 bits, the K (ADU) value is 300 (the BIAS setting has been multiplied by 16 due to the 4 bit shift).
If I set the tool to 12 bits, the K (ADU) value is 20 (the same as the BIAS setting).
It doesn’t matter what you choose, but the tool setting must be the same for later evaluation.
Personally I will use the 12bit setting in the PI tool. It just seems logical.
Now back to how many photons/electrons to use (or gather) to make up one ADU.
I think this is now the Crux of the whole matter.
It all comes back to the best SNR (Signal to Noise Ratio).
I know that this is basic information, but I have to spell it out…
Noise is random in nature. It will tend to spread itself throughout all the pixels in the camera array.
The Signal (hopefully) will tend to gather in specific pixels.
The more Signal we get, the better or the more photons/electrons we gather the better.
And there is a point where the amount of noise will over ride everything. Ie: if we exposed for 2 hours, all you would get is noise. The signal would get lost.
This is where the likes of Shiraz’s formula kicks in. (Target_ADU=BIAS + 10*RN*RN/GAIN).
Remember that GAIN in this formula is e/ADU (Not the GAIN setting found in Sharpcap), and is found in the ASI1600 specifications.
Next I used the results of this formula for Sharpcap GAIN settings of 0, 50, 100, 139(unity), 150, 200.
Values of e/ADU and RN were taken from the ASI1600 specifications for each GAIN setting.
The BIAS was set to 50 in all cases.
See the attached image.
The graph clearly shows a knee at GAIN 100 (Sharpcap setting) which equates to about 1.5 e/ADU and also equates to a Full Well of about 6500 electrons.
It can also be noted at this point, the RN (Read Noise) starts to climb more quickly for lesser values of the Sharpcap GAIN setting.
So what conclusion to draw from all this information?
I am going to set my BIAS (BRIGHTNESS) to 50, and forget all about it. Never touch it again.
I am going to do trial images at varying exposure times with Sharpcap GAIN set to 100 .
At Sharpcap GAIN = 100 the Target_ADU = 53 ADU.
Evaluate the images using the Statistics Tool in Pixinsight (with the tool set to 12 bits).
And then see what results
This might take some time because of the resident cloud that sits over my house…
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