I would like to discuss the "information content" of today's APOD (or rather, the information content inherent in the exposures used for today's APOD) as well as the information content of the RGB image that Geck posted.
I think the central star stands out more in the RGB image because of its bluish color. Others might disagree. In today's APOD, the central star, even though it has an orange halo, is still whiter in color than the other stars, again suggesting that it is different from them.
In the RGB image, much of the nebula is green. I would guess, although I don't know, that the green color corresponds to OIII emission. Other parts of the nebula are reddish-brown, which I interpret as dust.
In today's APOD, the innermost part of the nebula (actually the central "hole" of it) is faintly orange, while the outer parts of it are slightly blue. The parts in between are generally purplish-white. I wouldn't know how to interpret the colors, although I must say that the structure of the bright inner part of the nebula is much better revealed in today's APOD than in the RGB image. Why is that?
This is actually one of the few planetary nebulas in Hubble's archive that has wideband Red, Green, and Blue filtered data available. Of course, no one processes it with just those three because the narrowband imagery is just so much nicer to look at since it brings out the nebula much more clearly
I think you are saying that today's APOD was made from narrowband images. But is that true? I followed the above false color image
link of the caption and came to the Fast Facts page
of Hubble Heritage. The way I interpret the information there, NGC 2440 was imaged through just two filters, F555W
. To the best of my understanding, F555W would detect but not single out OIII emission, and neither filter would detect Ha. Both filters are broadband filters.
So the way I understand it, today's APOD has not been made from narrowband exposures. Why is it, then, that it reveals a lot more detail than the RGB image? I would have guessed that today's APOD was made from fairly recent Hubble exposures, but that is apparently not true. The exposures appear to be from 1995. Perhaps the exposure time was longer. I have no idea.
In any case, it seems to me that today's APOD was made from images taken through two broadband filters. The colors chosen for the final image appear to contradict the established practice to use blue (or cyan) color for the shortest wavelength exposure and red (or orange) color for the longest wavelength exposure. I expect, for example, that the all-but-empty central "hole" of a planetary nebula will be green from OIII emission. I don't expect to see a lot of dust in that "hole" at all. If today's APOD had mapped the exposure through the F555W filter as cyan and the exposure through the F814W filter as orange, then the innermost part of the nebula should have been cyan in color. There should probably be almost no trace of orange in there due to a lack of dust.
Interestingly, in the RGB image part of the "hole" of the nebula is bluish in color. Another part of the "hole" was greenish. In today's APOD, the part of the "hole" that is blue in the RGB image is orange here, and the part of the hole that is greenish in RGB is slightly bluish here.
Could it be that Forrest Hamilton has mapped the image taken through the infrared filter as blue and the one taken through the visible filter as orange? If so, there is most certainly no law forbidding anyone to map the longest wavelength exposure as blue and the shortest wavelength exposure as orange. But I think it would be slightly unexpected, and it would have been helpful if this kind of mapping had been spelled out clearly in the caption of today's APOD.
Or perhaps, indeed, the coloring of today's APOD is even more creative. Perhaps Forrest Hamilton has indeed colored his processed image according to his own sense of aesthetics. Again there is nothing wrong with that, but again, it would have been helpful to be told about it.