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.


3 Responses to “Raw Data, Straight Dope”

  1. Thanks for posting this, you’ve given me a better idea for my dissertation 🙂 As an artist it’s interesting to see how heavily processed the raw images that you deal with are.

    Also, if you look at a lot of art history there is a lot about the gaze of the artist and the gaze of the viewer and how they affect the way a particular work is viewed and interacted with. It would be interesting to look at the “gaze of the astronomer” and see how this differed to, say, the gaze of the lay viewer or even that of the artist. I’m sure it all has an impact on how astronomy is generally viewed and perceptions of space art by the public and the like.

    • Thanks – I’m glad this has been useful! I do find it interesting to compare the difference between the processed output and ‘what we’d actually see’. In the case of the infrared images above, obviously, we wouldn’t actually see anything, because the eyeball doesn’t work at those wavelengths!

      As for all those Hubble press-release images and so on, apparently they always put the colour contrast on them as high as they’ll go. to make them prettier. I have mixed feelings about that, but I can understand why they do it! (It’s just as well for us that we can get pretty pictures out of our work, or probably no-one would fund us…)

  2. […] as all of the images in question were taken in the near-infrared. (For an idea what they look like, this gives some indication.) It’s kind of strange, in a way – I work on these things every day, and yet I […]

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