Spitzer Documentation & Tools
IRS Instrument Handbook

7.3  Low Resolution (SL and LL) Spectral Modules

7.3.1             Low-Resolution Nonrepeatability

The one standard deviation repeatabilities of standard star flux measurements are: 4.6% for SL1, 2.2% for SL2, 3.1% for LL1, and 2.3% for LL2. These are for observations (AORs) that use high accuracy self-peakup. This is based on 129 observations of the standard star HD 173511 (see Figure 7.2). Note that the distributions of flux values are non-Gaussian and that the SL1 distribution in particular has a tail of low flux values. The variations in measured flux are attributed to a combination of inaccuracies in telescope pointing (which can place the source away from the center of the slit) and flat field errors.

All spectrographs are susceptible to pointing errors. Flux calibrations are based on the median of all HR 7341 observations, so individual observations may give fluxes above or below the “truth” value.


Figure 7.2:  129 observations of the standard star HD 173511, processed using the S18.18 pipeline. The observations used high accuracy self-peakup. In each case the spectra have been normalized to the median of all fluxes at nod 1.

7.3.2             Order Mismatches

Depending on telescope pointing accuracy (see Section 7.3.1), there can be mismatch between SL2/SL1, SL1/LL2, or LL2/LL1. See Figure 7.3 for an example of an SL1/LL2 mismatch. Such a mismatch could be mistaken as a broad spectral feature.


Figure 7.3 Order mismatch between SL1 and LL2, which might be mistaken as a broad absorption feature. For each order the two nods are shown.


Mitigation: Critically inspect all order matching. Individual orders that are low may be scaled up to match. Imaging photometry from IRAC, MIPS24, and IRS Peak-Up may be used to derive more accurate order matching corrections. However, see Section 7.3.3 on SL1 curvature induced by wavelength-dependent slitloss.

7.3.3             Curvature in SL1

Spectral curvature (concave upward) may be induced in SL1 when the source is not centered in the slit, due to chromatic PSF losses. The curvature increases with apparent flux deficit.  See Figure 7.4.

Observers should be very wary of broad spectral features at levels <5%, especially at the order boundaries. The effects of the spectral curvature and order mismatch may be mistaken as broad absorption features.



Figure 7.4 IRS Staring observations of 29 Vul, processed with S15 (bksub products). The red traces correspond to IRS observation campaign 7, while the blue correspond to five other campaigns.

7.3.4             LL1 24 micron Deficit

LL1 spectra of faint sources show a dip at 22-28 microns relative to standard models (see Figure 7.5. and Figure 7.6 for examples.)  The amplitude of the effect is 2-3% for faint (<80 mJy) sources. Some sources which show the deficit apparently have nonlinear ramps.


Figure 7.5 Illustration of the 24 micron deficit. The 14 micron teardrop is also visible.

Figure 7.6 Another illustration of the 24 micron deficit. The 14 micron teardrop is also visible.


7.3.5             Incorrect Droop and Rowdroop corrections following peakup saturation

Spurious broad absorption or emission features or incorrect spectral slopes or curvature are seen in some SL1 or SL2 spectra. This may be due to saturation of the peak-up arrays.  SL exposures with long exposure times may saturate in the peak-up arrays. In cases of strong saturation, the droop and rowdroop corrections will be incorrect because the total charge on the chip (which determines the droop correction) and on each row (which determines the rowdroop correction) cannot be measured. The effect becomes worse as more pixels are saturated for a greater portion of the ramp. It may not be obvious from the image that the peak-up arrays are saturated.

7.3.6             Wiggles in Optimally Extracted SL spectra

Sinusoidal oscillations are seen in some high signal-to-noise SL spectra extracted using SPICE's “Optimal” extraction option.

This effect is caused by a mismatch between the source and the standard star spatial profile (rectempl) file. The mismatch may be due to an incorrect “Ridge” percentage or to undersampling of the PSF. The undersampling of the PSF in SL is evident in the BCD images as the source jumps from one column of pixels to the next, along the spectral trace. The location of these jumps must match between source and template to get a clean optimal extraction. SL is the module most affected by undersampling.

To mitigate this effect,

   1. Use Regular extraction for high signal-to-noise data. Generally, if the S/N is high enough to see the wiggles there is little or no gain from using Optimal extraction.

   2. Compare Regular and Optimal SPICE extractions to verify the source of the wiggles.

   3. Manually change the percentage in "Ridge" to minimize the wiggles. In general, automatic ridge finding is preferred for accurate ridge determination.  However, it is possible for the automatic procedure to miss the correct peak.

   4. Generate and use a standard star template (rectempl) that is a better match to the source. The template will match best if the position along the slit is the same (within 0.1 to 0.2 pixels) as the source position, modulo one pixel. For example, if the template is shifted by exactly one pixel, optimal extraction will work well, while if it is off by 0.5 pixel, the match will be poor.

   5.  Try using the optimal extraction algorithm in SMART.

7.3.7             Residual Fringes in SL and LL

SL and LL spectra show residual (non-sinusoidal) wiggles after flat fielding, at the 1-2% level. The detector fringing pattern is sensitive to source location relative to the slit, which can be affected by pointing inaccuracies. In particular, the phase of the fringes changes with source distance from the slit center. The flat field has been constructed to accurately remove the fringes for the mean pointing at the standard nod positions. The residual fringes are non-sinusoidal, so IRSFRINGE does not do a good job at fitting and removing them.  Users should critically assess the reality of spectral features at the 1-2% level.

7.3.8             SL 14 micron Teardrop

Excess emission is found in the two-dimensional SL1 spectral images, to the left of the spectral trace, at 13.5-15.0 microns. We refer to this feature as the "14 micron teardrop", and it is illustrated in Figure 7.7.

Figure 7.7 Example *bcd.fits image illustrating the 14 micron teardrop.  The source was observed using SL1.  The bright continuum can be seen running vertically down the image on the left.  The white circle indicates the teardrop, which appears as a region of excess flux to the left of the continuum.