Bias frame gradient - Forums

Topic starter

Mar 25, 2023

I have a new ASI294MM Pro (monochrome). I took 100 bias frames at gain 120, offset 20, -10C, and 1x1 binning. The statistics are consistent across the raw frames I examined. However, I extracted the gradient across several of the unstacked frames using ABE in PI with default params and got results similar to the example below. Each frame has a slightly different pattern, but all consistently show low brightness on the left/right edges and higher brightness in the middle. I took additional bias frames with the same settings except I changed the offset to 30. This resulted in the same general pattern, however the gradient is more gradual and extends closer to the edges.

I know I can subtract this gradient in my subs, so am not concerned about imaging impacts (although I haven't taken light subs yet). However, I'm curious if this is "normal" behavior or whether there is a sensor / electronics issue.

Appreciate any thoughts on this.

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Arun H

Mar 25, 2023

Especially with the 294MM, a bias frame should be used as is. You can read more about the peculiarities of that sensor here. The long and short of it is that, for very short exposures, the circuitry on the chip causes it to heat up to an extent and on a timescale that the TEC cannot handle, leading to anomalous results. Calibration should be done with darks from which bias has not been subtracted and such calibration frames should exceed 3s in duration.

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Mike Harris

Topic starter

Mar 25, 2023

Thanks, Arun. I knew that there are peculiar bias frame issues with the 294MC sensor, but had not read definitively that the 294MM had similar issues. I'll follow your suggestion about darks exceeding 3s and will continue researching and run some comparisons.

Appreciate your response!

Mike

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andrea tasselli

Frequent contributor

Mar 26, 2023

Mike Harris:
Thanks, Arun. I knew that there are peculiar bias frame issues with the 294MC sensor, but had not read definitively that the 294MM had similar issues. I'll follow your suggestion about darks exceeding 3s and will continue researching and run some comparisons.

Appreciate your response!

For the record I never encountered any such issue on my 294MC, see master bias below:

Jonny Bravo

Mar 26, 2023

andrea tasselli:
Mike Harris:
Thanks, Arun. I knew that there are peculiar bias frame issues with the 294MC sensor, but had not read definitively that the 294MM had similar issues. I'll follow your suggestion about darks exceeding 3s and will continue researching and run some comparisons.

Appreciate your response!

For the record I never encountered any such issue on my 294MC, see master bias below:

A bias frame is _NOT_ a 0.1" or 0.2" exposure. That's a dark flat. It's a short one, but it's a dark flat nonetheless.

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andrea tasselli

Frequent contributor

Mar 26, 2023

Fair enough, here is one: