IRSKY Background Estimation

Estimating the background intensity

The Background Estimator outputs a table with 8 columns of data and possible warning messages.

The background region around your target position may be thought of as including two components: The zodiacal background intensity (eg.,MJy/sr), and the combined intensity of all other contributions (astronomical sources, cirrus, etc.).

The Zodiacal Background column lists a model value computed from the smooth model prepared by J. Good. Details of the model are found in the ISSA Explanatory Supplement, Wheelock et al. 1994, Appendix G. The value was computed at a fixed solar elongation angle of 90 degrees and at the point in the Earth's orbit where the orbit crosses the ecliptic plane. When the actual solar elongation angle is different from 90 degrees and/or the Earth is slightly out of the ecliptic plane (inclination is 1.5 degrees), the true zodiacal contribution may be different from the values listed by a factor of as much as two (See also ISOCAM Observer's Manual, Appendix A).

The zodiacal model was used with its full set of parameters, taking into account the earth's position and the observational solar elongation angle, to subtract the zodiacal contribution from the IRAS survey data to produce the Low Resolution Product (LRP), and the Infrared Sky Survey Atlas (ISSA). Data values from these products appear in their respective columns in the table.

The intensity left after the subtraction of the zodiacal background is represented in each of the columns: 30' LRP, 4' ISSA, and 30' x 30' mean. The 4' ISSA column represents the intensity of the central pixel in the image, at the target coordinates, with the pixel being 1.5' on a side, but representing about 4' in actual resolution. At some positions the subtraction of the zodiacal model has resulted in regions of negative fluxes in the ISSA and LRP data.

The 30' x 30' Mean is taken from the ISSA data, and all pixels greater than 5-sigma from the mean are removed before averaging. Thus the central source and bright point sources in the field do not contribute to the mean. Similarly the LRP has been processed to exclude the contribution of bright discreet sources. Thus the 30' x 30' mean ISSA values and the 30' LRP values are likely to be similar. If there are significant differences between these values, it may indicate that one or the other of the products had trouble removing the contribution of bright and/or extended sources.

COBE Data: An attempt is made to provide a background estimate using the COBE data. This estimate may be useful if the observing wavelength falls outside the IRAS range (7-120 microns). The algorithm for producing a background estimate from the COBE data:

The intensity values provided are merely estimates, or educated guesses. They are not predictions of what you will see at the position and wavelength chosen. They are meant to guide you in planning observations, but it is up to you to decide how to use them.

The COBE datasets were developed by the NASA Goddard Space Flight Center under the guidance of the COBE Science Working Group and were provided by the NSSDC.

INTERPOLATION TO THE SPECIFIED WAVELENGTH: The background flux at the wavelength of interest is listed in the 5th row of the table, and is estimated by decomposing the ISSA 4 point spectrum at the selected location into the best-fitting combination of two far-infrared emission models taken from the literature. The models were drawn from the emission models representing different physical regions (cirrus, dark clouds, reflection nebula, stars and HII regions), given by Desert, 1990 The flux density shown is the sum of the two contributing models' values at the selected wavelength. With this method, the continuum background within the range 12 to 100 microns can generally be estimated to within 0.5 MJy/sr at a given wavelength. Outside this range, background estimates may not be as robust.

INTERPOLATION WARNING MESSAGES: (These messages may apply to a product including Zodiacal, LRP, ISSA, COBE)

Warning: "Insufficient [product] flux for accurate interpolation" Meaning: one or more of the IRAS Band (12, 25, 60, 100 um) fluxes are below the three sigma noise level, making interpolation difficult.

Warning: [product] flux too spikey for interpolation Meaning: One of the four IRAS bands has over 5 times the flux of any other band, or the spectrum is double-peaked, making modeling unreliable.

Warning: [product] fluxes not well modeled. Meaning: The interpolation routine was not able to consistently model the IRAS bands themselves with its two component model, so extrapolating to other bands is unreliable.

Warning: too few pixels .... Meaning: There are too few ISSA pixels for an accurate flux estimate.

Warning: "Position outside COBE data set " Meaning: The only COBE data currently available between +10 and -10 degrees from the galactic plane.

Desert, F-X., Boulanger, F., and Puget, J., 
1990, Interstellar Dust Models for 
Extinction and Emission, A & A 237, 215.

Van Buren, D., 1992, Spectral Decomposition 
of ISSA Fluxes, IPAC InternalMemorandum.
[contains information on interpolation method used]

This paper is online in 
PostScript form on IPAC's 
publicftp machine:

Look for the file:

Wheelock, S. L. et al., 1994 
IRAS Sky Survey Atlas Explanatory
Supplement, JPL Publication 94-11 
(Pasadena:JPL). [contains information 
on the attributes of the ISSA images 
and the zodiacal model]

Information on the COBE data is 
available via anonymous ftp from
in the COBE directory tree.
Last update: August 1998