Incorrect bias reduction?

[jjongmans]

Hi all,

I have a problem with bias reduction.

I created a master bias frame out of 100 frames:



I calibrated a set of frames with the master bias and I left the other set uncalibrated, all four images identically processed:



Top Left: Master Luminance with Bias reduction
Top Right: Master Red with Bias Reduction
Bottom Left: Master Luminance without Bias reduction
Bottom Right: Master Red without Bias reduction

Why is the vertical banding still showing up in the Bias reduced stacks (banding is even worse in calibrated stack)? The master bias frame should take care of the read noise. It seems like the Master Bias isn't doing a correct reduction of the light frames; undercorrection?

[jjongmans]

A second overview:

Bias Frame
Bias reduced Red
Non Bias Reduced Red

[Alejandro Tombolini]

Hi jjongmans

How the MasterDark looks like?

I believe that your banding has another cause than read noise, may be temperature.

Saludos, Alejandro.

[bitli]

Both the biased reduced and non biased reduces images look identical to me (seen on the screen here), and the bands in your bias seems to be inverted with the bands of the image, but I do not know if the bias aligned on your script dump.
In principle the calibration with the bias is just a subtraction of the bias from the image.  You can try it with PixInsight to see if this work.  You may also want to move the cursor around and look at the values of the pixels to see of the bands have the same absolute value difference between light and dark area in the bias than in the image. 

Other users may have more expertise in this, there are scripts like CanonBandingReduction which may or may not be useful.

I am in the process of accumulating information on all the thinks that can go wrong with calibration, I would be interested if you could post the bias and one frame on DropBox or similar (maybe just a crop but of the same area).

-- bitli

[jjongmans]

How the MasterDark looks like?
I believe that your banding has another cause than read noise, may be temperature.

I don't use dark frames, so I have to shoot one first.
All frames are created at -25°C

Both the biased reduced and non biased reduces images look identical to me (seen on the screen here), and the bands in your bias seems to be inverted with the bands of the image, but I do not know if the bias aligned on your script dump.
In principle the calibration with the bias is just a subtraction of the bias from the image.  You can try it with PixInsight to see if this work.  You may also want to move the cursor around and look at the values of the pixels to see of the bands have the same absolute value difference between light and dark area in the bias than in the image.

Images are aligned. Indeed, banding is inverted in light frames, also in the Non Bias Reduced Light frames, that's strange. There is only 2/3 ADU difference between bands.

[bitli]

Banding could have a lot of causes, including interference inside the camera (like defective filtering at some point, just to mention one possibility). It may not be a constant value independent of exposure time.

Looking again at your image, the 'border glow' seems to be correctly corrected by the bias (may be even slightly over-corrected?). If this is the case the bias will not help correct the banding (unless you accept a poorer correction of the glow). As suggested by Alejandro you may want to take darks instead (or in addition) with the same duration and temperature and see if they have the same banding. If this is the case they will also correct the glow and warm pixels.
-- bitli

[georg.viehoever]

Looks a bit like this one http://pixinsight.com/forum/index.php?topic=6028.msg40846#msg40846