Explanation of the FIRAS Spectrum Browser

In the form, you are asked to enter the coordinates of the position of interest and specify the offsets of the reference positions. The on-source position may be specified in whatever coordinate system and epoch is convenient. By default, the reference spectrum is derived as the average of the spectra at four reference positions offset by ±10 deg in Galactic longitude and latitude from this position. Owing to the large (7 degree) size of the FIRAS beam, reference positions should be chosen with some care. They should not be so close to the on-source position to overlap it on the sky, but not too far to make them unsuitable as 'background' references. Ideally, reference positions should be chosen after consulting an IRAS or DIRBE map for guidance on the structure of the diffuse microwave/far-infrared emission the region of interest. Of course, a reference position suitable at one wavelength may not be appropriate for use at another.

The form allows offsets to be deselected for different directions, and the coordinate system for the offsets to be specified. If for example the on-source position is close to the Galactic plane, the intensity of the diffuse emission might depend most strongly on Galactic latitude and the best reference spectrum might result from specifying offsets in Galactic coordinates at constant latitude (i.e., E and W directions only).

Optionally, the spectrum of the Cosmic Microwave Background Radiation (monopole and dipole terms) may be specified instead of reference positions.

If the box 'Use no reference positions' is checked, only the on-source spectrum will be shown, and the integrated intensity will be calculated from the on-source spectrum. This box overrides all of the other reference position and CMBR specifications.

A fraction of the pixels, 282 out of 6144, are missing from the FIRAS data file used here (firas_destriped_sky_spectra_lres.fits). The spectra for these positions cannot be retrieved using the browser.

Note: Coordinates are rounded to the nearest FIRAS pixel and intensities are not interpolated within a pixel.

When you click "Submit Request" the spectra of the on-source and the average spectrum of the off-source positions (or the CMBR spectrum) are calculated and displayed individually and as a difference (ON - OFF) spectrum.

In the difference plot, the one sigma statistical uncertainties are shown as green error bars on approximately every tenth point. Keep in mind that if reference positions are used to form the OFF spectrum, the dispersion of intensities between them can be greater than the statistical uncertainties.

Back to the FIRAS Small Source Spectra Form

A service of the HEASARC and of the Astrophysics Science Division at NASA/GSFC
Goddard Space Flight Center, National Aeronautics and Space Administration
HEASARC Director: Dr. Alan P. Smale
LAMBDA Director: Dr. Eric R. Switzer
NASA Official: Dr. Eric R. Switzer
Web Curator: Mr. Michael R. Greason