Visible light is only one type of electromagnetic radiation. You have probably heard of many other types as well: radio waves, microwaves, infrared (heat) radiation, ultraviolet, x-rays, and gamma rays. The only difference between all these types of radiation is the radiation’s wavelength. Our eyes happen to be sensitive to a certain section of the electromagnetic spectrum – a wavelength of about 5000 Angstroms
(5 x 10-7 m), the range we call visible light. This part of the spectrum is also the peak of the Sun’s thermal radiation curve – our eyes are best adapted to detect sunlight!
The SDSS is not strictly a visible survey, because it extends slightly into the ultraviolet and slightly into the infrared. Today, other astronomy projects are surveying the sky using different parts of the electromagnetic spectrum
One such survey is the Two Micron All Sky Survey (2MASS), which uses two 1.3 meter telescopes, one in Arizona and one in Chile, to map the entire sky in the infrared wavelength band, at a wavelength of about 2 microns (1 micron = millionth of a meter). Cool objects, such as small dwarf stars, give off most of their thermal radiation as infrared light. Therefore, 2MASS can see these cool objects, which are invisible to visible-light surveys like SDSS.
Scientists from 2MASS are releasing their data on the web just like SDSS scientists are. 2MASS currently has about half the sky in its data release.
Like SDSS, 2MASS produces its color images by combining pictures taken through three different filters. 2MASS’s filters (called the J, H, and K filters), see radiation at 1.25 microns, 1.67 microns, and 2.17 microns in the infrared part of the spectrum. When you call up an image in 2MASS’s web interface, you will see the raw images in those three filters.
Several objects in SkyServer’s Famous Places list are also visible in the 2MASS survey. Let’s look at some images from 2MASS and see how they differ from SDSS images.