There's anecdotal evidence that the Nikon D5300 can suffer from coloured concentric rings in stacked astro-images. They become apparent after calibration with flats. So I thought I'd investigate further. I have a Nikon D5300 which I bought as a backup camera but it's been sitting around gathering dust because my testing uncovered the green stars issue. Out of the cupboard it came!
Here's a series of flats taken at ISO 100 in 14-bit mode with a legacy 50mm f/1.8 Nikon lens, which has no electronic chip and is unrecognised by the camera firmware:
Going left to right, top to bottom each exposure is half the previous one. The exposure in the middle of the 3x3 montage has its back-of-camera histogram peak 1/4 from the left which is a typical recommendation for shooting light frames. The above shots are the out-of-camera JPGs but for the rest of the analysis I use the raws.
Each flat is divided by the bright one at the top left, using PixInsight as follows:
- Debayer using SuperPixel mode to preserve the original raw pixel values
- Subtract the bias level of 600
- Perform 8x8 binning (to reduce noise) by using IntegerResample using the "average" downsample mode
- Divide by the 8x8 binned, bias subtracted, superpixel debayered top left flat
- Apply an unlinked screen transfer function
Here's the result:
The coloured concentric rings are pretty clear. It's the kind of thing you might see appearing in ISO 100 calibrated astro-images. Sometimes it will be the light frames that contain the rings and sometimes it will be the flat frames. Sometimes it will be both.
Anecdotally images taken at higher ISOs don't show any problem, so I thought I'd test this. Here's an equivalent 3x3 montage generated from ISO 800 flats. Again each flat differs by one stop and the central image had its back-of-camera histogram peak 1/4 from the left:
So the rings are slightly different and generally much fainter but they don't disappear.
At the present time I have no idea what is causing these rings except that it must be caused by some kind of internal camera processing of the raw data. Also I've no idea how many other Nikon models might show the same artefacts.
My test procedure is quite clear (I think) so try it on your own cameras and feel free to add your results to this thread.
Edited by sharkmelley, 19 December 2020 - 06:46 PM.