Irreplaceable Astronomy

Contributed by
Jul 12, 2007

From Astronomy Buff I learned of an effort to digitize a century's worth of astronomical plates.

Back in the day, before digital detectors, astronomers used glass plates to take images of the sky. Film is too flexible, and cannot be used to make accurate measurements of the positions of astronomical objects. Instead, companies like Kodak would spray the light-sensitive emulsion onto glass plates, which could be loaded into a large chamber on the back end of a telescope. The plates were big; 5x7 inches, 8x10 inches, and some were monsters, 18 inches on a side or more.

Using plates presented many problems. One is, they were heavy. Another was, duh, they were glass, and tended to break easily. A third one, weird as it may sound, is knowing which side had the emulsion on it! You can't look -- they're light sensitive! All this has to be done in the dark, in a dark room. If you rub the plates, you mess up the emulsion and leave oil from your fingers. One classic way is to taste a corner! The emulsion has an odd, flat taste. Another way (invented by me -- or at least I came up with it myself in grad school when we used plates as part of an imaging class) was to tap a corner of the plate on your front teeth. The glass side went tink! and the emulsion side went tunk! That worked every time.

A big problem is that plates aren't very sensitive to light. They absorb only about 2-5% of the light that hits them, as opposed to modern digital detectors, which can detect well over 95% of the light that hits them. So you might think that old plates are worthless, right? We can just retake the data with better cameras now!

Bzzzzt. Nope. The sky changes. Galaxies change brightness, stars move (slowly, but some plates are 100 years old), supernova remnants expand. Old astronomical plates are literally a priceless, irreplaceable treasure.

And there lies the big problem. The Harvard Observatory has a collection of something like 500,000 plates -- yes, half a million. These fragile slices of the past are hard to store. Imagine just how much they weigh in total! So the Observatory is embarking on a project to digitize all those plates.

If that sounds like a big job, well, yeah. We're talking petabytes of data! A petabyte is a thousand terabytes, and a terabyte is a thousand gigabytes. Getting the picture (harhar)? This is millions of gigabytes of data.

Yeah. It's a big job.

They were able to get enough money to buy the digitizer and start the project, but they'll need a few million bucks to get the job done. If there are any very wealthy BABloggees out there, now's the time to make yourself heard. You can pay per plates!

I was able to look at an old plate taken at the University of Virginia's telescope when I was in grad school there. I don't remember when it was taken; probably in the 1930s or so. It was of the globular cluster M13, a glorious beehive of stars about 25,000 light years away. Had the plate not absorbed those photons, they would have streamed on their way, and they'd be, oh, some 70 trillion kilometers past the Earth by now (actually, they would have fallen to the floor of the observatory, but give me this poetic image). That old photo represented the cluster as it was seen more than half a century ago, a view we will literally never see again.

I hope Harvard is able to complete this noble project. The Universe goes on with or without us, but we have a chance here to freeze it, if only for a moment, and see what it was like in days gone by.

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