Sony A7S - Star Eater Algorithm

Started Apr 27, 2015 | Discussions thread
sharkmelley
OP sharkmelley Senior Member • Posts: 2,309
Interesting Results of a Star Eating Experiment
2

The UK skies are cloudy, the moon is up but it is still possible to determine by experiment how destructive (or not) the star eating algorithm really is.

All the evidence I've seen so far, points to the fact that the Sony A7S Bulb mode algorithm works in the same way as the original Nikon algorithm as follows:

For every pixel in the image, replace the original V0 value by: Min(V0, Max(V1, V2, ... , V8) )

It is this that removes single hot pixels and leads to "pixel pairing". Note that V1, ... , V8 are not immediate neighbours of V0 but are neighbours of the same Bayer matrix colour.

Now I have some reasonably good data shot with a modified Canon 350D on a Tak Epsilon 180ED. Dithering was performed between each exposure during acquisition. So I stacked 36 exposures of 5min using darks, flats and bias frames as normal.

Then I took the same raw data and applied the above star eater algorithm to the raw lights and raw darks and performed a complete re-process. Looking at the results, it was impossible to tell the stacked images apart without "blinking" one against the other.

Here is a 1:1 scale crop of the original with a log intensity scale (to allow the full dynamic range to be displayed):

Here is the "star eater" version:

Only by blinking the two images can you detect that all the non-saturated stars have been dimmed down.

Here's the difference between the two - again log scaled:

This makes it far more obvious that most stars have had their brightness reduced.

Dividing one image by the other, it also became clear that a great many (non-saturated) stars have had their central peak reduced by a factors of 1.5x - 2.5x

In the "star eater" image every star still had a central peak though in many cases it was severely flattened. I found no cases of an inverted peak (i.e. a central depresssion)

So, does the star-eating behaviour matter? It really depends on your point of view. If your main interest is producing images of the night sky then I really don't believe you will notice the problem (which is probably why it has gone undetected until now). If you have a more scientific interest in star profiles, photometry, variable stars etc. then this is clearly not the ideal camera to use on an undersampled scope such as the one used here.

I should re-iterate that on a longer focal length scope then the "star eating" behaviour only leads to a minor reshaping of the star profile and is almost completely undetectable.

Mark

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Sony a7S Nikon Z6 +1 more
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