Spitzer Documentation & Tools
IRS Instrument Handbook

Chapter 9.    IRS Enhanced Spectrophotometric Products

9.1  Introduction

This document describes the IRS Enhanced Spectrophotometric Products.  The Enhanced Products consist of two elements:

(1) A collection of 16,986 low-resolution,  Merged Spectra

(2) A Catalog of extracted source positions, synthetic photometry in several bands, PSF profile widths, and other useful quantities.  See Table 9.5 in Section 9.4 for a listing and brief description of all data columns in the Catalog.

These products were produced starting with the final SSC pipeline  (ver. 18.18) bksub.tbl SL and LL spectra. The bksub.tbl spectra (see Table 6.7 in the IRS Instrument Handbook) were extracted from the nod two minus nod one and nod one minus nod two background-subtracted basic calibrated data, using an aperture that expands linearly with wavelength (see Section 5.7.3 of the IRS Instrument Handbook).  The calibrated fluxes are consequently strictly valid only for point sources.

9.2  Merged Spectra

All low-resolution spectra of a given target in an AOR were combined to give a single low- resolution spectrum. For AORs with spectra from all 4 low-resolution slits (SL2, SL1, LL2, LL1), the resultant spectra span the wavelength range 5.21-38 microns.

The median spectrum was created for all exposures at a given nod position. The two nod spectra were then averaged to give a single spectrum per spectral order (Fig. 9.1). The available orders were then concatenated with the wavelength clipping ranges shown in Table 9.1. The standard flux calibration for each order was maintained. No additional scaling was applied to the flux densities of the orders to make them match.  The flux uncertainty (‘error’) was computed by adding the flux uncertainty from the two nod spectra in quadrature.

Each Merged Spectrum is an ASCII IPAC table, containing a header followed by a number of data rows, one for each spectral sample.  The meaning of data rows and columns is identical to that in the bksub.tbl files except for the values in the "order" column.  See Table 9.2 for details.

Table 9.1 – Orders and wavelengths in a Merged Spectrum file

Order Number

Module Name

Minimum Wavelength

Maximum Wavelength






















Figure 9.1 - Example IRS Merged Spectrum (black line).  The IRAC 8 (blue), IRAS 12 (cyan), IRS PU 16 (green), IRS PU 22 (orange), MIPS 24 (red), and IRAS 25 (blue-green) transmission curves are superimposed on the merged spectrum.  The synthetic flux in each band is indicated by a colored horizontal line. For clarity, the MIPS 24 and IRAS 25 photometry have been shifted up or down by 0.002 Jy, respectively.





Table 9.2 - Data columns in a Merged Spectrum table file:

Column Name





1 for SL2;  2 for SL1;  3 for LL2; 4 for LL1



Wavelength of spectral sample. The data rows are in strict order of ascending wavelength and there is no order overlap.



Flux density at a wavelength sample in the Merged Spectrum.



Estimated uncertainty of the flux density. This is computed by adding the two nod "error" values in quadrature. When doing this computation, if any nod "error" value is less than 0.0001, it is assumed to be 0.0001 .


Integer representation of binary number

Status bit-flags for this sample -- This is computed as the logical “OR” of bit-flags from the two input nods.  Section 6.5.5 of the IRS Data Handbook gives the meanings of the individual bits.

  NOTE:  There are never any NaN values in the table.  If a value is not available for a data row, that data row will simply be left out of the table.