Quote:
Originally Posted by multiweb
Good stuff. I'll email you some raw fits.
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I received Marc's images and did the following. (Please be aware the image files are quite big).
This is the image I got from Marc.
First I created a light pollution map. I used StarWipe, but you might want to use something else;
StarWipe --in=bin_test.tif --out=bin_test_lpmap.tiff --scale=5 --window=30 --mode=global --maponly
That resulted in
this light pollution map.
Next I subtracted the light pollution map from the original image. Again, I used StarWipe for this as well, but importing the original image in PhotoShop, then importing the light pollution map in a separate layer and setting that layer to 'subtract' will do the same thing;
StarWipe --in=bin_test.tif --inmap=bin_test_lpmap.tiff --mode=global --out=bin_test_wiped.tiff --nonormalize
Now we're left with
a dimmer version of the original, but with the light pollution removed. However, now I'm no longer using the full dynamic range I have at my disposal - I can still crank the brightness up without clipping my histogram to the right.
We should increase the signal in the image so that the image is bright again, but not so bright that my histogram starts clipping to the right.
At this point, I use (fractional) software binning (approx 1.41 x 1.41 in this case) to crank up the brightness again, instead of stretching the brightness levels of the pixels.
StarBright --in=bin_test_wiped.tiff --scale=71 --mode=cap
With
this as the final result. (notice though that the image is 71% of the original size in X and Y directions)
So what's the difference between stretching and binning?
If I stretch the brightness levels, I trade off precision for signal
If I bin the image, I trade resolution for signal.
or
If I stretch the brightness levels, gaps will start to appear in the histogram. Stretch the levels enough and you'll start to see banding and noise will become more apparent.
If I bin the image, the histogram will stay intact and appear smooth. I can bin as much as I want without ever seeing any banding (though my image will get smaller and smaller and uselessly bright).
That's how (and why) I use software binning in a nutshell!