[pvelez (Pete)]

Stop me if you've heard this before. I can't get my flats right.

It presents as an excess of blue in one side of the image - the left, particularly the lower left corner.

I had thought it was LP, a dodgy filter, a dodgy lightbox, poor calibration technique or something else. So far, I can't pin it on anything.

The colour gradient is almost exclusively in blue - I don't think LP is wavelength sensitive. I'm happy to be told that I am wrong on this.

The lightbox is excellent - one of Exsfo's. I've rotated it and the flats and results are the same.

I have reduced the lightbx brightness, thereby avoiding shutter artifacts. No change.

Internal reflections? Nuh. I have the same issue with the Tak FSQ106 and the RC8.

The filters are ok I think - I plan on rotating them between sets of images to see if there is any difference. When the clouds part I'll try this out.

I'm very new to PI and have still to learn the ropes. Having said that, I have the same result in Maxim. The flats have master bias and darks subtracted. The darks are scaled.

I've looked at the raw flats - the dynamic range of the B flat is no different from the R.

So what else might it be?

To give you an idea, I've attached a screenshot of 2 subs. On the left is B, the right G. Both are 180 seconds and have been calibrated. This was last night near a full moon so there is a gradient obvious.

If you look closely, the dark section to the left in the corner is much smaller than the corresponding corner of the G sub. In other words, there is more signal in the B than the G. When combined, I end up with and indigo background here.

I have stretched both images to the same extent using the STF in PI. The images were taken consecutively.

So what am I doing wrong?

I'm open to suggestions - very open.

Pete