Explanation of DIRBE Spectral Energy Distributions

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 ±1.6 deg in Galactic longitude and latitude from this position. Unless the structure of the diffuse infrared emission in the region of interest has unusual features, or another source might be in one of the reference positions, the default selection is probably the best. For generality, the form allows offsets to be deselected for different directions, and the coordinate system for the offsets to be specified.

Ideally reference positions should be chosen after consulting an IRAS or DIRBE map of the region of interest. Of course, a reference position suitable at one wavelength may not be appropriate for use at another. Reference positions should be as close to the on-source position as possible but no closer than 1.6 deg to ensure independence; the DIRBE beam (instantaneous field of view) is 0.7 deg x 0.7 deg and the DIRBE map pixels are approximately 0.32 deg x 0.32 deg squares. (Coordinates are rounded to the nearest DIRBE pixel and intensities are not interpolated within a pixel.)

When you click "Submit Request" the on-source spectrum and the average spectrum of the off-source positions are calculated from Zodi-Subtracted Mission Average (ZSMA) Maps. The on-source and off-source spectra are displayed individually and as a difference (ON - OFF) spectrum. The plotted points are also tabulated and the integral flux of the difference spectrum is estimated.

The Error column in the table lists error estimates that include statistical error and confusion error, calculated from the dispersion of the intensities in the reference positions and the ZSMA Standard Deviation values. These are also the error bars shown in the plot of the (ON-OFF) spectrum, and the errors used to calculate the error estimate for the integral flux. The Disp. Refs. column of the table lists the dispersion of the intensities in the reference positions. Often this dispersion dominates the total error. Note that the error estimates plotted and tabulated do not include absolute calibration errors, which range from 3% for the near infrared bands to 15% for the 25 micron band.

Please also keep in mind that the DIRBE Difference Spectra service is suitable for preliminary investigations only. As is discussed in Section 5.6.6 of the DIRBE Explanatory Supplement, quantitative studies of point sources in the DIRBE data should properly be undertaken with time-ordered data. The DIRBE Point Source Photometry Research Tool can be used for such studies. Point source fluxes derived from the maps systematically underestimate the true values (derived from time-ordered data). The magnitude of the effect at 2.2 and 4.9 microns has been found to be less than ~10%. The primary reason is that the effective beam profiles in DIRBE skymaps are not the true profiles, and in fact the effective profile varies with position in a map depending on the "approach directions" of the scans.

Another point to keep in mind is that the DIRBE intensities are derived on the assumption that nu*I(nu) is constant. Corrections for other spectral shapes can be applied using the System Spectral Response Functions for the various DIRBE bands. Correction factors for various standard power-law and modified black body spectra are tabulated in the DIRBE Color Correction Tables. DIRBE data are well-suited for color correction because the beam size is not a strong function of wavelength, which means that for a given DIRBE pixel the same part of the sky is observed in each of the 10 bands.

Back to the DIRBE Difference 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