Nikon D750 for Astrophotography

Started 7 months ago | Questions thread
sharkmelley
sharkmelley Senior Member • Posts: 2,114
Re: Nikon D750 spatial filtering causes magenta stars

sharkmelley wrote:

I agree with you that there is a filtering issue but only on the Nikon D750. If you look at the D750 star colours in the images above, there is strong tendency for the stars to turn magenta. The reason for this is that there is something really weird about the Nikon D750 spatial filtering that I haven't seen before. It's something I'm still analysing in an attempt to work out what's going on. I see the same effect happening in D750 dark frames.

The headline summary is that the Nikon D750 spatial filtering algorithm preferentially attacks the green pixels in a star, thus altering the colour balance towards magenta. But why it does this, I'm not yet sure. It could potentially be a firmware bug.

The same magenta effect on the Nikon D750 can be seen in Alan Dyer's comparison - look at the "De-bayering Star Artifacts" section at the bottom of this article:

https://www.diyphotography.net/in-depth-testing-of-the-sony-a7iii-for-astrophotography/

If you are worried about the colour of small tightly focused stars then the Nikon D750 certainly looks like a camera to avoid.

Mark

As evidence, here's one of my plots of the raw values in a Nikon D750 dark frame:

Every pixel in the raw data has been plotted against the maximum value found in the 24 neighbouring pixels in the 5x5 surrounding block.

You can immediately see that no green pixel is brighter than its neighbours. On the other hand there are a significant number of red and blue pixels that are significantly brighter than their neighbours. This is purely a result of the spatial filtering algorithm.

In the context of a small tightly focused star this means that on average the red and blue pixels will tend to dominate, leading to a magenta/pink star colour.

Mark

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Takahashi Epsilon 180ED
H-alpha modified Sony A7S
http://www.markshelley.co.uk/Astronomy/

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