Spitzer Documentation & Tools
MIPS Instrument Handbook

4.3  Basic toolset for working with MIPS data

MIPS data are produced as standard FITS files, appropriate for use in IRAF, IDL, or your own favorite image analysis software. 


HOWEVER, Spitzer images are in units of surface brightness, MJy/sr, which may be unfamiliar to observers coming from other wavelength regimes.  The main implication is that photometry as performed using the same routines used for, e.g., optical data may not return the correct values.  Section 4.3.1 covers these items. 


Color corrections may be important for some observers; see section 4.3.4. Observers doing aperture photometry will want to make aperture corrections, and, on a related front, observers doing PSF fitting may be interested in the PSF shape itself; section 4.3.6 discusses the PSF.

4.3.1        Units and flux densities

MIPS images are in MJy/sr.  (To convert from MJy/sr to microJy/arcsecond2, multiply by 23.5045.)  When summing up the flux incident on a set of pixels for, e.g., aperture photometry, you need to multiply by the number of steradian per pixel.  See Table 4.9 for average actual pixel sizes; see the BCD header values of PXSCAL1 and PXSCAL2 in your own data for values specific to your program because the pixel scale changes slightly as a function of mirror position.  Note that these values may be refined as our knowledge of the instrument matures.  One square arcsecond is 2.3504x10-11 steradian. 


Note that post-BCD products are resampled images such that the pixels have different sizes from the pixels in the original BCDs.  Please check the post-BCD header keywords CDELT1 and CDELT2 for the pixel sizes (in degrees) in any given mosaic.  Example default mosaic pixel sizes for MIPS products are given in Table 4.9; to double-check that your data have these values, see the values of CDELT* in the header of your data.  Note also that the orientation of the mosaic is often different than the BCD (see header keyword PA in the BCD and CROTA2 in the mosaic).  The SSC tool APEX reads the header keywords properly and reports flux density units of microJy for point sources, not MJy/sr.

Table 4.9: Sizes of MIPS pixels for MIPS BCD and post-BCD products.

MIPS mode/array Average BCD PXSCAL1 (arcsec) Average BCD PXSCAL2 (arcsec) Default post-BCD product pixel sizes (arcsec)
24 micron 2.49 2.60 2.45x2.45
70 fine scale 5.24 5.33 4.0x4.0
70 default scale 9.85 10.06 4.0x4.0
160 micron 15.96 18.04 8.0x8.0
SED 9.8 n/a 9.80