Monday, February 22, 2010
After several infusions of liquid helium, FORCAST has been running for the past ten days with its infrared detectors at a chilly 4 K (-453 F, -269 C). Why so cold? Infrared light does not carry much energy so our infrared detectors have to be very sensitive. At room temperature or even at the temperature of liquid nitrogen (-321 F, -196 C) the electrons trapped in the detector still have enough thermal energy that the detector cannot distinguish electrons moving around because they are "warm" from electrons moving around because they have just been hit by infrared light. Once the detectors are cooled with the chillier liquid helium, the electrons are very lethargic unless they are illuminated by infrared. So 4 K is the optimal temperature for running FORCAST. We keep the optical components of the camera, including filters, at 77 K (liquid nitrogen) so that they do not emit infrared light and confuse our observations of warm objects in space. Here's a nice explanation of infrared astronomy.
It's a common misconception that infrared is heat; not so! Infrared is light that our eyes cannot see. Objects that are relatively warm (an astronomer for example) emit a lot of infrared. If you have an infrared camera, you can see the light that people emit--we're walking infrared light bulbs. The image at right is portrait of your's truly taken with FORCAST in the DAOF lab. Notice that my warm skin is bright (high infrared intensity), while my cooler glasses and shirt are darker (low infrared intensity). If you look more closely, you'll see that my nose and beard are cooler than my skin since they appear slightly darker in the infrared image.
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