Blogs / Bad Astronomy

It observes the Earth... quite often!

If the Moon is not too bright to see, what about the Earth? On average, it's much more reflective and therefore much brighter. Well, it turns out Hubble not only has observed the Earth... it's done it thousands of times!

One problem with using digital detectors is knowing exactly what you're seeing. If a star looks brighter than another, is the star really brighter, or is the electronic chip just a little too sensitive right there? You have to calibrate the chip to know exactly what it's doing. There are several steps in that process, but one involves using a "flat field", observing a region of the sky that is perfectly evenly illuminated. That way, if one pixel or another is too sensitive, you can see it in the observation.

With Hubble, though, every patch of sky has some object in it, which would screw up the flat field. Some telescopes have internal illumination; little LEDs or some other method, but using them is notoriously difficult to get an evenly illuminated field. So what can you do when using Hubble?

One method is to observe the Earth! As Hubble orbits at 8 km/sec, the out-of-focus Earth screams by. If you observe for a while, objects will actually leave streaks in the image, and these can be treated mathematically to produce a flat field. The image shown here is just such a "streak flat". That's a Hubble observation of our home planet, with objects flying past. It's hard to say what they are, exactly. It depends on where Hubble was when the image was taken, and where it was pointed. They might be trees, hills, valleys, mountains, or even houses!

But don't worry, it can't see people. If the Moon is too fast to track, the Earth is certainly out of the question. But y'know, the company that made Hubble's mirror had an awful lot of those same sized mirrors lying around, and there are no other astronomical telescopes (you know, telescopes that point away from the Earth) with that same mirror. So what could those mirrors have been for?

Hmmm.