Why I'm often unhappy with Hubble images

[Ann]

I got some probably well-deserved criticism for my post regarding today's APOD, a Hubble image of galaxy NGC 2683.

I very much admire the ability of the HST to take sharply resolved images of distant objects. We have learnt so, so much more than we knew before thanks to Hubble. I am, however, very often unhappy with the color balance of the Hubble images. The Hubble people have declared it themselves: They very often don't even attempt to produce true-color images. This is what you can read about the colors in Hubble images on HST's own homepage:

The colors in Hubble images, which are assigned for various reasons, aren't always what we'd see if we were able to visit the imaged objects in a spacecraft. We often use color as a tool, whether it is to enhance an object's detail or to visualize what ordinarily could never be seen by the human eye.

And rarely, in my opinion, are they very successful at showing colors in what I consider a good way. The recent image of globular cluster NGC 6934, which was taken through orange and infrared filters to produce an almost all-blue image, is a perfect example of the Hubble people's lack of interest in color and poor ability to present color in astroimages in a good way.

I have a hypothesis about what might at least contribute to the relatively poor color balance in many Hubble images. My guess is that it may have something to do with the sensitivity of CCD detectors to different wavelenghts. For a long time CCD detectors used to be comparatively poorly sensitive to blue light, whereas they have always been very sensitive to red light.

One thing the HST hasn't got a lot of is time. So many astronomers want to use the Hubble to photograph their favorite objects. If Hubble's CCD detectors are comparatively insensitive to blue light, the telescope is going to have to spend a longer time imaging an object in blue light than it will need imaging the same object in, say, yellow, orange, red or infrared light.

It could be that the Hubble often doesn't have time to acquire an image of an object in blue light. It will be quicker to image the object in two longer wavelengths. Then those two long-wavelength pictures can be used to produce a color image of some sort. To compensate for the fact that the HST didn't image the object in blue light, or didn't image it in blue light for a sufficiently long time to get a good picture, the HST people may decide to ramp up the blue channel to produce the final picture. The result, if it is an image of a galaxy, may be a picture where the yellow populations of the galaxy look strangely blue, whereas the young blue stars don't stand out very well at all, and they may look strangely greenish.

That is why I'm so often unhappy with the colors of HST images.

Ann

[BMAONE23]

The Hubble palette is more interesting though
http://news.discovery.com/space/nasa-co ... ssing.html
than what the naked eye sees. We can take in only so much light in a 30th of a second

[Ann]

Indeed, nebulae look dull to human vision. To me the Orion Nebula looks grey-green through a telescope, not as vividly green as in the link you posted.

But that Hubble image of the Orion Nebula is splendid! The Hubble people really devoted a huge effort to it and photographed the nebula through lots of filters. Read about it here:

http://hubblesite.org/newscenter/archiv ... fastfacts/

So please understand that I'm not saying that all Hubble images are bad-looking or have been acquired through badly chosen filters.

Ann

[BMAONE23]

I often thought it would create an interesting presentation to take a series of images of some celestial object (nebula or such) with each successive image utilizing more light gathering time (depth) then have them stacked into an animated GIF file that would progress, in real time from image to image representing what could be seen by increasing the shutter timing.
Mouse over for 1 second and see what 1 second of light gathering would produce. Mouse over for 10 seconds and see what 10 seconds of light gathering would produce, etc... up to say a minute or two of light gahtering time.