Figuring out the offset

In my last post, I said that I needed to figure out what the correct offset for my camera is.   So, how exactly am I going to do that?

Firstly, I’m going to have to do some experimentation, ooo fun.

My ZWO ASI1600-MM-Pro camera will be taking images at -20°C.   So, I’ll make sure to cool the camera to that for all of my experiments.  Then, I’m going to start with the offset set to 25, it’s a starting place.  This is going to take some trial and error.

Next, I’m going to create some bias frames.

After that I’ll have to open these up to analyse the image, and see if there are any 0 pixels in the image.

How exactly am I going to find the minimum pixel value?   Actually that’s much much easier than I thought it was going to be.   Sequence Generator Pro has a statisics panel which contains some analysis of the image.  Amongst the values is the “Minimum pixel value” measured in ADU.   Remember that I’ve set the gain to 139, which is the Unity gain, so the ADU value here is equal to the e- value from the chip itself.  If I can get the camera to register a minimum pixel value of 1 ADU consistently, Then I’ll have figured out the correct offset value.


This turned out to be alot easier than I expected.

Ran a series of bias frames using the offset of 50, and found that there were alot of minimum values of 16, so I raised the biad to 52 and found that the 16 value stopped appearing.  For now, I’m going to run with an offset of 52.


After this, I decided to create a library of bias, darks and darkflats.  This took most of the weekend, however something that I did notice was that the amount of noise increased in the longer exposures (5 min subs)

something else that I found interesting is that the lowest value in the darks raised the mimum value away where I was seeing in the bias frames.


All in all, I think I have a set of calibration frames to run with.  Just need some clear nights now.

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