Data Analysis - MIPS

Overview

Here you will find links to the information required to reduce and analyze your MIPS 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

MIPS "Pocket Guide" (pdf)

MIPS Data Handbook - this page includes a lot of other useful information

Spitzer Observer's Manual (SOM) - see especially Chapter 8: MIPS

Data Reduction Cookbooks - these pages are still under construction but may be helpful to get a feel of how to mosaic your imaging BCDs and extract the source flux densities.

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.

MIPS Interest Group - a forum for MIPS 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 MIPS data reduction

The SSC supported software for the reduction and analysis of imaging data (IRAC, MIPS and IRS-Peak-Up) and MIPS SED data is called MOPEX (MOsaicking and Point source EXtractor). The package includes APEX, Spitzer's Astronomical Point source EXtractor. A GUI version of the software has just been released. The MOPEX page also includes sample namelists for reducing MIPS-24 imaging data. The parameters set in these namelists are a reasonable starting point for most data sets, but not all, so we recommend that users experiment with the namelist settings to find the optimal settings for their dataset. Documentation can be found on the MOPEX Online Manual page, and an example walk-through of how to reduce an IRAC data set can be found on the Data Reduction and Analysis Cookbooks page.

Users should note that, while MIPS data can be reduced with any FITS data analysis tool, geometric distortion is important in Spitzer imaging, and this information is stored in the header rather than in an external distortion table.

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 conventions. For sample PRFs and advice on the mask files to use, see "Calibration Files" below.

Data Features/Caveats

The Data Features page summarizes the most common MIPS image features, including image 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 MIPS Data Handbook.

Calibration Files

MIPS Pmasks (tar file)

The pmask files flag permanently damaged pixels in the arrays. They are updated every 2-6 months, depending on solar activity. Please read the included "pmasks.README" file carefully and note which set of pmasks are applicable to the date when your observations were made.

Users should also use the following DCE status masks (included in the BCD data download) to mask temporary bad pixels in the individual BCD frames:

Basic Data Reduction Steps

Once you have downloaded you data, a basic data reduction will include some or all of the following steps:

MIPS-24 Imaging

1. If you are only concerned with quick photometry, to uncertainties >15%, you can use the SSC-provided Post-BCD mosaics, and just carry out points #6, #8 and #9 in this list to extract the photometry. We do not recommend using the Post-BCD mosaics for science. Users should always create their own mosaics when extracting photometry for publication.


2. MIPS-24 data suffer from the "First-frame Effect", where the first frame of every commanded sequence of observations (data with the keyword DCENUM=0) have a shorter exposure time and are depressed in response by 10-15%. The MIPS IST recommends discarding this first frame. In addition, the second and even third BCDs (DCENUM=1,2) also appear to have a reduced signal (~2%). If you have enough data then you may also want to consider discarding these.


3. Sometimes the pipeline flat-fielding does not completely remove background gradient across the MIPS-24 frames. You may wish to perform an extra flat-fielding of your BCDs before background-correcting them with Overlap (next step). MOPEX includes a command-line script flatfield.pl to do this, or you can use your favorite software package to create the extra flat-field frame from the BCD frames, normalize it, and divide it through the BCDs. This process is also effective in removing latent artifacts.


4. Perform background correction of your BCDs using Overlap in MOPEX. This levels the backgrounds to ensure that the final mosaic is not "patchy". It does not affect the photometry. Overlap does not set the background to zero. If you wish to subtract the background entirely at this point, then you will have to do it yourself.


5. Mosaic your BCDs using Mosaic in MOPEX.


6. Perform photometry on either your mosaic or your individual BCDs using either APEX (distributed as part of MOPEX) or the software package of your choice.


7. Carry out the color correction for your sources. See the MIPS Data Handbook, section 3.7.4, for detailed information and tabulated standard color corrections.


8. 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 2.45" x 2.45". For pixel sizes for the BCDs, see Table 3.7 in the MIPS Data Handbook for more information on pixel sizes. The conversion factor is therefore:
   
   1 MJy/sr = (1E12 microJy)/(4.254517E10 arcsec**2) x 2.45 arcsec x 2.45 arcsec = 141.08 microJy.
   
   To compute magnitudes in the Vega system, magnitude = 2.5*log10(f(0)/f), where f(0) for a 0 magnitude Vega star is listed at http://ssc.spitzer.caltech.edu/mips/calib/.


9. Finally, MIPS photometry is calibrated to an infinite aperture 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 on the MIPS aperture correction page, but the MIPS IST strongly recommends that users derive their own aperture corrections, since the value of the correction is color-dependent (see the MIPS Data Handbook, section 3.7.6, for more information).

MIPS-Ge Imaging (70 & 160 microns)

Most users will find that the BCD products downloaded from Leopard are suitable for their needs, but if re-creation of the BCDs is required, this can be done using The GeRT (Germanium Reprocessing Tool). Common reasons for reprocessing include correction for bad stim calibration, and recovering data from weakly saturated sources. If you decide to reprocess your data then you will have to start from the "Raw" data, downloadable from Leopard. Once you have a set of satisfactory BCDs, the photometry can be extracted by following the basic steps for MIPS-24, but with the following differences:

1. For extraction of point sources, users should start with the filtered BCDs (*fbcd.fits) that have had a median background subtracted, or carry out their own background subtraction. Most Ge data benefit from re-filtering after masking out the bright objects and extended sources in the field of view. Care must be taken when re-filtering data with extended fluxes, particularly when this flux is diffuse.


2. The default pixel size for MIPS-70 mosaics is 4.0" x 4.0", so the conversion factor from MJy/sr to uJy is therefore:
   
   1 MJy/sr = (1E12 microJy)/(4.254517E10 arcsec**2) x 4.0 arcsec x 4.0 arcsec = 376.067 microJy


3. The default pixel size for MIPS-160 mosaics is 8.0" x 8.0", so the conversion factor from MJy/sr to uJy is therefore:
   
   1 MJy/sr = (1E12 microJy)/(4.254517E10 arcsec**2) x 8.0 arcsec x 8.0 arcsec = 1504.27 microJy


To compute magnitudes in the Vega system, magnitude = 2.5*log10(f(0)/f), where f(0) for a 0 magnitude Vega star is listed at http://ssc.spitzer.caltech.edu/mips/calib/.

MIPS SED

Interpolation, coaddition and extraction of MIPS SED data can all be done using mosaic_sed, distributed with MOPEX. Follow the steps outlined in the mosaic_sed documentation (pdf).