Data Analysis - IRS

Overview

Here you will find links to the information required to reduce and analyze your IRS data. Your Spitzer data can be downloaded from the archive through Leopard. We assume that you are starting with the Basic Calibrated Data (BCD) products.

User's Guides

IRS "Pocket Guide" (pdf)

IRS Data Handbook

Spitzer Observer's Manual (SOM) - see especially Chapter 7: IRS

Data Reduction Cookbooks - these pages are still under construction, but may be helpful to get a feel of how to extract your IRS spectra and Peak-Up photometry.

Spitzer Synthetic Photometry - guide to predicting the calibrated flux densities that would be reported in the various Spitzer bands (e.g. for deriving photometric redshifts)

MOPEX Online Manual - online documentation for MOPEX - the SSC software package for photometric reduction and analysis.

IRS Interest Group - a forum for IRS users to discuss data analysis issues

Contributed Software page - useful reduction software contributed by the Spitzer community. Users are advised to check this page before writing their own data reduction programs.

Software for IRS data reduction

Filenaming Convention and Pipeline Products

Basic Calibrated Data downloaded from the archive consists of many data products for each frame. Here you can find a description of all the files and explanations of the filenaming convention, including advice on which files you should be using for your data analysis.

Data Features/Caveats

The Data Features page summarizes the most common IRS detector features, including data artifacts. We describe these features, show images with representative examples, and provide a recommended mitigation method for their removal from the data. More information can be found in Chapters 5 and 6 of the IRS Data Handbook.

Calibration files

All required mask files are included in the BCD data downloaded from Leopard. PSF images for Peak-Up Imaging are available for use with APEX or other PSF-fitting software.

Example namelists for use with MOPEX for analysing Peak-Up Imaging data can be found on the MOPEX page. More information about IRS calibration can be found in the IRS Data Handbook.

Basic Data Reduction Steps

A detailed version of the reduction steps is given in Chapter 10 of the IRS Data Handbook. Please read the Data Caveats/Features page, especially if you are intending to carry out high-precision data analysis. Warm/bad pixels are transient on the IRS detector. A number of mitigating steps are included in the data reduction process. Still, if there is any doubt about the reality of a given spectral feature seen in an extracted spectrum, we recommend that the observer always examines the 2D BCD image to confirm that the feature show the expected spatial and spectral dispersion on the array.

Spectroscopy

These instructions refer to staring observations. If you have mapping observations, you can use CUBISM to combine them, sky-subtract them, etc.

High resolution:

1. Subtract the sky from your data (bcd.fits files). You should have taken sky observations. If you did not do so, the Data Handbook provides some alternatives.


2. Combine all the cycles within an exposure in a single 2D image.


3. Clean images, using IRSCLEAN if necessary


4. Extract spectra using SPICE or SMART. If the target is extended, refer to the documentation on extended source extraction and calibration.


5. Combine the two nods for each staring position, using your favorite software.


6. Extracted, calibrated spectra are provided in the PBCD directory for coadded images (see the tune.tbl products). These spectra are not sky subtracted.

Low resolution:

1. Subtract the sky. You can use the other nod in a given staring observation, or the other order, if you have multi-order observations in a given AOR. If your integration is long (>1 hour) refer to the report on ultradeep IRS spectroscopy of faint sources.


2. Clean images (mainly Long-Low) using IRSCLEAN if necessary.
3. Combine all the cycles within an exposure in a single 2D image.


4. Extract spectra using SPICE or SMART. If the target is extended, refer to the documentation on extended source extraction and calibration.


5. Combine the two nods for each staring position, using your favorite software.


6. Within the PBCD directory provided with the data, you will find files with the suffix bksub.tbl. These are extracted, calibrated spectra of observations with one nod subtracted from the other. This is a very effective way of doing sky subtraction and cleaning the data. If you can use these (if you have isolated point sources), you are done.

Peak-Up Imaging

1. If you are only concerned with quick photometry, to uncertainties >15%, you can use the SSC-provided Post-BCD mosaics (b_mos.fits and r_mos.fits in the pbcd directory), and just carry out points #3, #5 and #6 in this list to extract the photometry. The namelist used to produce these files is provided with the data download (mopex.nl). We do not recommend using the Post-BCD mosaics for science. Users should always create their own mosaics when extracting photometry for publication.


2. In the BCD directory, your data will have suffixes bcdb.fits (16 microns) and bcdr.fits (22 microns). You can use MOPEX to create a mosaic from these individual images.


3. You can extract photometric information from your mosaics using APEX (distributed as part of MOPEX), or your favorite photometric tool (e.g. IRAF). PRFs are provided to use within APEX.


4. Users should apply the appropriate color correction to their data. See the IRS color correction page for more information


5. The units of photometry from APEX are automatically given in microJy. If you are using another package, e.g. IRAF, the fluxes will be output in the BCD units of MJy/sr. In order to correct to microJy, you should convert to steradian per arcsecond, and then multiply by the pixel area. The default pixels in the mosaics are 1.8" x 1.8" = 3.24 square arcseconds per pixel (the BCDs have a pixel area of 3.367 square arcseconds per pixel). The conversion factor for the default mosaics is therefore:
   
   1 MJy/sr = (1E12 microJy)/(4.254517E10 arcsec**2) x 3.24 sq arcsec = 76.1536 microJy.


6. Finally, the calibration of IRS PU Imaging is done with apertures of 12 and 13 pixels in the blue and red arrays respectively. In order to properly calibrate your photometry, you therefore need to correct from the aperture you used to extract the photometry out to the calibration aperture. Standard aperture corrections (good to ~5%) can be found in Section 6.3.2 of the IRS data handbook, but the IRS IST strongly recommends that users derive their own aperture corrections, since the value of the correction is color-dependent.