ISSA Explanatory Supplement
IV. ANALYSIS RESULTS
D. Photometric Consistency

1. Point Sources
2. Extended Sources
3. Absolute Photometry

Comparison of the IRAS calibration to other measurements is a separate issue and is briefly addressed in §IV.D.3. No adjustments were made to the IRAS photometric calibration based on the results from these comparisons.

D.1 Point Sources

Figures IV.D.1 and IV.D.2 display the plots and the encircled flux density curves for the four ISSA wavelengths. The shape of the encircled flux density curve is consistent with the measured size of about 4.5' (FWHM) for the ISSA point spread function. A circular Gaussian PSF would have 90% encircled flux density at a radius of 1.1 times its FWHM. The level of the flat portion of the curve gives the ratio of ISSA to PSC flux densities as 1.11± .09 at 12 µm, 1.05± .10 at 25 µm, 0.92± .05 at 60 µm and 0.93± .05 at 100 µm. Two known effects contributing to this difference are point source tails at 12 and 25 µm, which add about 6% to the flux within a 7' radius aperture, and noise induced by the convolution filter, about 4%, which was used to resample the full resolution IRAS data to 2' samples (Appendix B). The 60 and 100 µm ISSA fluxes are less than 1 by about 1.5 sigma. We know of no reason why they should be less than 1.

Figure IV.D.2 Flux Density vs. Aperture Diameter. Reference Diameter was 10' larger largest

Figure IV.D.1 (cont'd) Scatter Plots of PSC vs. ISSA Point Source Fluxes Measured with a 9' Aperture larger largest

D.2 Extended Sources

Comparison with the ZOHF reveals the effects of the ISSA destripers and binner at spatial scales larger than about 1°. No cross-check of the ISSA data was possible at spatial scales between 1° and 5'. ISSA and the ZOHF were compared by preparing all-sky maps with both data sets and producing scatter plots of a pixel by pixel correlation of the two maps. The maps used the Aitoff projection in Galactic coordinates with 0.5°× 0.5° square pixels at the projection center. The ZOHF has a 0.5°× 0.5° square beam and required no additional averaging to produce the map. ISSA data were smoothed to a 0.5° circular beam before binning into the Aitoff map. Both the ZOHF and ISSA maps were further smoothed by convolution with a 1.5°× 1.5° rectangle function to reduce differences in the point spread functions. The comparison was done with the first ISSA release which covered only the high-ecliptic-latitude, || > 50°, sky.

Plots of the ISSA vs. ZOHF were produced for the northern cap of the ISSA map. Trend lines were fit subjectively to the data, with care being taken to assure that the overwhelming number of low surface brightness points would not bias the trend line away from the data at high surface brightness. The slopes of the best-fit trend lines for the data are 0.99± .01, 0.98± .01, 0.97± .02 and 1.00± .02 for the 12, 25, 60 and 100 µm data, respectively.

The slopes from the data indicate that calibration scale factors of ISSA and the ZOHF are the same to within 2% at all IRAS wavelengths. The destriping and binning procedures used in ISSA production had little or no effect on the gain calibration of the data. Examination of the polar mosaics revealed no evidence of field boundary discontinuities larger than 0.1 MJy sr^-1, indicating satisfactory performance of the local destriper.

D.3 Absolute Photometry

The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer satellite (COBE) has made absolute surface brightness measurements of the sky in a 0.7° square beam at wavelengths from 1.3 µm to 240 µm (Hauser et al., 1991). Since the IRAS mission was primarily designed to measure point sources and DIRBE was primarily designed for measuring extended emission, we believe the COBE-DIRBE calibration is valid and the DIRBE results provide a check on the large-scale performance of IRAS. The COBE/DIRBE Explanatory Supplement (19 July 1993) [Present version 2.2, 5 May 1997] presents a preliminary linear transformation between IRAS and DIRBE data, which is shown in Table IV.D.1 below. This linear transformation was derived based on carefully selected DIRBE data compared to IRAS (HCON-1 and HCON-2) scan data. The constants given below are average values for the duration of HCON-1 and HCON-2. The transformations are applicable to spatial scales on the order of an IRAS scan but there is no certainty as to their applicability at smaller spatial scales down to 0.7°. The IRAS point source calibration is not affected by these numbers.

The offset term is relevant to the total intensity IRAS product, the IRAS Zodiacal History File (ZOHF). It is not applicable to the ISSA since the zodiacal emission was subtracted. See Appendix G for details on the zodiacal model. The offset term is known to have a systematic variation with time at 12 and 25 µm. The offsets given below are simply mean values and do not reflect this trend. The user should refer to the COBE-DIRBE Explanatory Supplement to better understand the offsets at 12 and 25 µm.

The gain term is applicable to both the ISSA and the ZOHF and shows the IRAS measurements at 60 and 100 µm were too bright relative to DIRBE at large spatial scales. There is no simple prescription for unraveling the varying spatial response of the detectors to effect a more accurate calibration for the ISSA images. The data given in Table IV.D.1 are representative of the comparison of IRAS HCON-1 and HCON-2 scans with DIRBE data based on the current DIRBE calibration. The gains at 12 and 25 µm are consistent with IRAS. The IRAS brightness appears to be 13% and 28% high at 60 µm and 100 µm, respectively. The 60/100 µm color is about 12% too low in IRAS data relative to DIRBE data.