The image above is a "rollover": by moving your mouse pointer in and out of the image you can switch from the infrared (mouse out) to visible (mouse in) images of the Hubble Ultra Deep Field. I have tried to scale and align these press images so you can "blink" from one to the other.
A version of these images suitable for higher resolution screens is also available.
Notice the large number of faint red spots that appear on the infrared image but not on the visible image. These are probably very distant, high redshift galaxies. The expansion of the Universe has shifted the starlight emitted by these galaxies into the infrared. Infrared radiation is emitted by warm objects, and the Hubble Space Telescope mirrors are kept warm deliberately by electrical heaters in order to preserve their precisely [incorrect] shape. As a result, the HST infrared image only goes to a longest wavelength of 1.6 microns. A telescope designed for infrared observations, like the JWST, will have cold mirrors and will be able to observe much longer wavelengths and hence see much more distant objects.
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© 2004 Edward L. Wright. Last modified 11 Mar 2004