Conflicting gain values in FITS header (ZWO ASCOM driver)


#1

Currently with my ASI294MC Pro I am using SGP v3.0.3.140 and the ASCOM driver. After having captured with unity gain (setting: 120) for a while, I now for the first time tried to capture at a higher gain (setting: 200). Whereas I got matching values in the FITS header with unity gain (GAIN 120 Camera gain, EGAIN 1.0022 Electrons per ADU), now the information is sometimes conflicting. I am getting images that show all possible combinations of the two GAIN and EGAIN values, right and wrong:

GAIN     EGAIN     comment
120      1.0022    matching
200      1.0022    not matching
200      0.3989    matching
120      0.3989    not matching

I would like to know why that happens. If I understand it right, the GAIN is the setting that I set, and the EGAIN is the response from the camera. There are two possibilities to set the gain: either in the ASCOM driver or in the Control panel/camera for each binning setting.

How can it occur that EGAIN does not correctly correspond to GAIN in the FITS header?
Which information is the correct one, if the values don’t match?

Bernd


#2

I’m not 100% on this but I believe the control panel values are informational and don’t actually set the gain.

So I think if you set the gain in the ASCOM driver and the e/ADU in the control panel to correspond to the ASCOM gain, you will get what you expect.


#3

I don’t think so. Cited from the manual (PDF, pages 36, 37) for the Control panel / Camera dialog box:

Gain: These controls allow you to change gain values for your camera for multiple binning levels. If your camera does not support these options these will be grayed out:

  • Gain: This sets the gain for the camera given the binning level.

Special notes for ASCOM camera that support gain:

  • SGPro cannot tell if an ASCOM camera supports setting gain until it is connected. Prior to connection, the gain drop box control will simply read: “Link Cam”. Once the camera is connected, the gain drop box control will either disable these controls (gain not supported) or populate allowable values for camera gain (gain is supported).
  • If gain is supported, it must be set for each binning mode or the first gain entry in the list will bw used for all images (probably not desired).
  • These notes are also true when defining equipment profiles. A camera’s gain should not be defined in the profile unless the camera is connected.

However even if I did not follow these notes correctly, my questions remain:

How can it occur that EGAIN does not correctly correspond to GAIN in the FITS header?
Which information is the correct one, if the values don’t match?

Bernd


#4

@Jared
@Ken

I would like to receive an answer from one of the developers.

Bernd


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