Comments from astronomer Olivier Hainaut.
Quote:
Dear Steven,
Impressive data processing! It is very hard to evaluate it in details on the re-scaled JPG (which loses lots of information wrt the FITS), but here are some comments:
- The noise texture in the background, and the large-scale flatness of the background suggests you have used a sky-subtraction algorithm (which is OK) using a sampling window that is too small (probably 100pix, while you could push it to ~500 or 1000)?
- the overall depth of the image is really good!
- combining the data over a huge wavelength range UBVI can be counter productive, even if your only goal is to go deep. For instance, the individual U images tend to be much shallower (because the camera is less sensitive in U, and bc the stars are fainter in U), so if you average them with the others without a different weight, you actually increase the noise more than the signal. You could consider to use the following weight for each image:
w = F/t
with t = exposure time (i.e. normalize by exposure time) and
F(U) = 0.2; F(B) = 0.4; F(V) = 1; F(R) = 1.5; F(I) = 0.8
An other way would be to pick a star that is not saturated in any of the images and that has neutral colours in our image, and normalize the frames by the (sky-subtracted) flux of that star.
If you have the time/the patience, you could give it a try, and I would not be surprised if the end result is a little deeper.
Note also that for the astronomers, combining all the filters together is usually not useful. It can help to create a catalogue of positions of the starts, and then go back and measure the flux in the single-filter recombinations U, B, V, R, I.
- the stretch function you apply is very good for displaying the image - but for us the only useful one is the original linear pixels. The stretch functions are really just for display. Anything that screws up the linearity of the data is a problem (and actually, we sometimes have problems with the raw data being not perfectly linear).
Cheers
oli
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The image qualifies as nice "pretty picture" but doesn't carry much scientific value.
An astronomer would not be able to use the image and calculate the surface brightness of the galaxy due to the non linear stretching of the data.
Interesting however is that while the pixel mapping function is non linear over the entire pixel range (0-65536), it is in fact close to linear in the low range which corresponds to the brightness of the Carina Dwarf.
It is great that an expert on image processing has taken the time out to give a professionals perspective on the subject.
Steven