Subject: if hubble was pointed at earth, what could it resolve? is it too bright?

Contrary to popular belief, the Earth ( or the Moon!) is not too bright for Hubble to see. But it hardly matters: Hubble cannot track the Earth underneath it. It orbits too quickly to compensate for that kind of motion, so objects on Earth leave long streaks across an image. It's just like trying to take a picture from a moving car: nearby objects will streak by, but far away objects appear to be moving slowly. That's why Hubble can track distant astronomical objects better than something right underneath it.

If Hubble could track the Earth, it would be able to see objects as small as roughly 5-10 centimeters across. However, you don't need to keep your curtains closed. From Hubble's point of view, you are moving too fast to see clearly, and an image of you would look like a long, blurry worm.

Still, you may be surprised to find out that Hubble routinely points at the Earth! It uses the bright, daylit Earth to help calibrate one of the cameras on board. The Wide Field Planetary Camera 2 (WFPC2, pronounced ``whiff pick 2'') is an electronic device which detects light, and is similar in principle to a normal digital camera. However, it is far more sensitive, and astronomers are very particular about how well they understand such devices. They want to make sure that every part of the detector is very well calibrated. When they look at some astronomical object they want to know that what they are seeing is actually real, and not some problem with the camera.

One way to calibrate the camera is to look at some bright, evenly lit object. If one part of the detector is more sensitive than another, for example, then part of that object will look brighter, even if in reality it isn't. This can be used to correct any later images. This type of image is called a ``flat field'', because you want the field of view to look as flat as possible.

In space there aren't any flat fields you can use, but we do have one right here: the Earth. To calibrate WFPC2, sometimes Hubble is pointed straight down at the Earth. As the terrain (or water or whatever) streaks across the field of view it forms a very crude flat field image. The image is called a ``streak flat'', and looks really weird. Trees, houses, all sorts of objects blur across the image. It takes a lot of sophisticated computer processing to turn this into a real flat field for Hubble to use, but luckily there are a lot of smart people working on the problem. The results are a camera that is well understood, and images that can be breathtaking.

You can find out more about WFPC2 at WFPC2 Home Page, though some of it is pretty technical. You can always just go look at Hubble images instead.