Astronomical object emit light over a broad range of wavelengths, not
just those visible to the human eye. Infrared light constitutes
light that is redder than the reddest visible light. Light with
wavelengths between 0.9 and 2.5 um is generally considered as the near
infrared. Light at these wavelengths is primarily produced by
objects that are familiar to visible light astronomers: stars, gas,
etc. At longer wavelengths (mid- to far-infrared), light is
produced by much cooler objects, i.e., dust grains, cool gas,
etc. The fact that cooler objects emit strongly at infrared
wavelengths means that infrared astronomy is very different from
optical astronomy. In particular, the night sky, the telescope,
and the instrument itself emit light in the infrared. This means
that infrared instruments must be cooled to low temperatures and be
carefully designed to avoid the stray light from the telescope and it's
surrounding structure. The figures below show some familiar
objects at infared wavelengths. Note how the infrared light traces heat.
Infrared Imaging Surveys
Astronomers analyze the light from faint astronomical sources.
This means that telescopes are built with ever increasing
size in order to allow astronomers to probe deeper into the mysteries
of the universe. However, because of design limitations, the
largest telescopes can only image a
small region of the sky at a given time. The largest
infrared telescopes are optimized for
high resolution imaging over small fields of view. As a result,
moderate sized telescopes, such as WIRO, can still serve a crucial
support role via wide field imaging. For example, a wide field
survey can identify the interesting, but rare, astronomical objects
that can be studied in more detail with the larger telescopes.
One measure of the efficiency of imaging surveys is given by the product of
the collecting area of the telescope, i.e., the aperture squared, and
the area of the sky imaged onto the detector, i.e., the field of view
squared. Since the aperture size of a given telescope is fixed
the only option
available for optimizing the survey efficiency of a given telescope is
to minimize the telescope focal length in order to maximize the field
of view. This need drives the optical design of a survey instrument.
Once this is established and the instrument is built the survey can begin.
Now the depth of the survey is
primarily determined by the exposure time spent on each field.
Thus the survey can be designed as either a relatively narrow field but
"deep" survey or a wider field, "shallow" survey. The figure
below shows the depth vs. field for several near infrared surveys and
illustrates how a WIRO survey might complement these.
As astronomers strive to build ever-larger telescopes, the future research
productivity of WIRO will likely involve a wide-field imaging capability.
Recently, UW astronomers have successfully proposed to the National Science
Foundation's Major Research Instrumentation program to construct just such
an instrument for WIRO. The WWF-Cam promises to provide UW faculty and
students with one of the largest field of view of any infrared camera to date
and will thereby enable a number of survey orientated research projects.
WWF-Cam Design (internal use only)
University of Wyoming Dept. of
Physics & Astronomy