Raw Data, Straight Dope
Actually I lie, just a little. These images are not quite what raw astronomical data looks like. The two images below, from the WFCAM Science Archive, have already been through the WSA pipeline. Bad pixels, flat-fielding and all those kinds of stuff have already been done, so most (if not all) of the ‘dodgy detector crud’ has already been removed. (A genuinely raw image would look, well, almost unrecognisable. You’d have trouble picking out even the brighter stars.)
However, these are close enough to raw data to give you some idea what we work with.
Briefly, both of these images are in what’s called the J-band – that is, near-infrared light with a wavelength of ~1.25 microns, or about twice as long as what the human eye can see. They’re both 3×3 arcminutes in area (the full Moon is ~30 arc minutes wide). The first image is taken in the galactic plane, through the UKIDSS Galactic Plane Survey. Here it is:
The second image shows you an image from the UKIDSS Large Area Survey:
As you can see, the galactic plane field contains vastly, vastly more stars. Also, hidden away in this image is UGPS J0722-0540, one of the closest known brown dwarfs, and also the current record-holder for being the coldest.
You can see that there are far fewer stars in the LAS field. This is because the LAS is oriented out of the galactic plane – essentially, you’re looking out into the void between galaxies. (This was done deliberately, to minimise background star contamination. As you can see from the GPS image, this can be quite a problem!)
Now, I deliberately picked this image, because it is less than perfect. You’ll notice a line of odd, donut-shaped features. These are called ‘cross-talk’. Cross-talk happens when an overly-bright star dumps too many photons onto the chip. Rather like dropping more water into a full bucket and it spilling over, the chip ‘spills’ electrons onto neighbouring chips, so you get this odd, repeating feature that spreads some way over the image.
And cross-talk can be a right pain, because it creates spurious objects in the databases. (This was a big problem during my Master’s … half my ‘objects’ turned out to be cross-talk. Gah!) On a day-to-day basis, a lot of what we do in astronomy is trying to control for things like this.