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IR Compendium: Background: IR Background
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This is the page on Infrared Background; also see
Resolution,
Diffraction, and
Confusion
Sources of background noise in the infrared can be very different than
those for optical astronomy. The structure of the background effectively
increases its noise contribution above what would be calculated based on
the flux contributed by all of the sources in a region. The structure
contributes directly to the variance of the background as it would be
measured by aperture or PSF-fitting photometry extraction software, for
example.
- Source Confusion
- Source confusion itself is a source of
background noise, and this is covered on a different page.
- Zodiacal Light
- Optical astronomers do not often worry about Zodical light, but
thermal emission from what is in essence our own remnant dust disk is a
source of IR background. Scattered sunlight dominates this component of
background at wavelengths shorter than 3.5 microns; at longer wavelengths,
thermal emission from the particles themselves dominates this component.
See the plot below for an indication of sky brightness as a function of
wavelength; see the Solar System Science
Overview for images of the zodiacal light.
- Interstellar cirrus
- Emission from the interstellar medium dominates the sky brightness at
wavelengths longer than 70 microns at high galactic latitudes, shifting
to 45 microns at galactic latitudes around 10 degrees. In the galactic
plane, the interstellar medium dominates at all wavelengths with a
minimum around 20 microns. See the plot below for an indication of sky
brightness as a function of wavelength.
- Cosmic Background
- This is not the IR portion of the CMBR, but rather the integrated light
of unresolved galaxies.
- Nice summary plot of sky brightness as a
function of wavelength
- This plot is from Leinert
et al, 1998, A&ASS, 127, 1 This is an overview of the sky
brightness outside the lower terrestrial atmosphere at high ecliptic and
galactic latitudes. Of course, Spitzer will not be affected by airglow, and
of course the brightness of the non-terrestrial components are a function
of galactic and ecliptic latitude.
IR Background and Spitzer
At mid and low ecliptic latitudes, zodiacal light is the dominant
source of background for IRAC, IRS, and MIPS 24 microns. The zodiacal
background is very significant for the 8 micron channel in IRAC, and in
fact sets the upper limit to exposure times by consuming the detector
well depth. At mid and low galactic latitudes, the interstellar
medium dominates, especially for the MIPS 160 microns. Use Spot (see
below) to interactively calculate background estimates for targets. Source
confusion is more difficult to quantify, but could be important, depending
on where you are looking, which wavelength you use, and what exposure time
you select. See the confusion page for some
additional references for confusion limits as they apply to Spitzer.
List of pages in the Spitzer hierarchy related to
backgrounds:
