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v1_0 (18-Oct-2009) run with idlutils rev. 2196 (roughly v5_4_8)

DISCLAIMER: The data products described below are not an official LAT
data release, nor has the procedure used been endorsed by the LAT
collaboration.  

The Fermi Science Support Center web page is:
http://fermi.gsfc.nasa.gov/ssc/

The following briefly describes the Fermi LAT maps used in
"The Fermi Haze: A Gamma-Ray counterpart to the Microwave Haze,"
G. Dobler, et al. 2009, arXiv:0910.4583

See the arXiv paper for more details. 

INSTRUMENT RESPONSE FUNCTIONS: 
P6_V3_DIFFUSE
P6_V3_DIFFUSE::FRONT
P6_V3_DIFFUSE::BACK

DATA FORMAT:
FITS files with 2 HDUs:
HDU0: Gamma-ray skymaps [counts/cm^2/s/sr]
HDU1: Exposure map [cm^2 s]

PIXELIZATION:
Maps are in HEALPix NSide=256 pixelizations (786432 equal-area pixels).
The maps are "ring-ordered."

ENERGY BINS:
For convenience, we present the LAT maps in two sets of energy bins.

 SPECBIN: 12 log-spaced energy bins used for spectral fits
000.3-000.5GeV
000.5-000.9GeV
000.9-001.7GeV
001.7-003.0GeV
003.0-005.3GeV
005.3-009.5GeV
009.5-016.9GeV
016.9-030.0GeV
030.0-053.3GeV
053.3-094.9GeV
094.9-168.7GeV
168.7-300.0GeV

 IMBIN: 8 somewhat wider energy bins used for studying images
000.5-001.0GeV
001.0-002.0GeV
002.0-005.0GeV
005.0-010.0GeV
010.0-020.0GeV
020.0-050.0GeV
050.0-100.0GeV
100.0-300.0GeV

FILENAME CONVENTIONS:
The 20-50 GeV map, for example, is named
fermi-allsky-020.0-050.0GeV-fwhm000-0256.fits

Front- and back-converting events may be found in
fermi-allsky-020.0-050.0GeV-fwhm000-0256-back.fits
fermi-allsky-020.0-050.0GeV-fwhm000-0256-front.fits

Because these events convert in different parts of the LAT, the front
and back maps are generated from disjoint sets of gamma-ray events
with somewhat different systematics.  In general, the angular
resolution of the "front" maps is a factor of 2 better than the "back"
maps.  

The exposure map for front and back maps is generated by gtexpcube
using the appropriate instrument response functions. The combined map
is the exposure-weighted mean of front and back maps, NOT the sum. 

POINT SOURCE SUBTRACTION:
The 3-month 10-sigma bright source catalog has been masked in these
maps.  This masking appears in the exposure maps.  Maps with no point
source subtraction have the -nopsc suffix.  Because the front- and
back-converting maps have different PSFs, the point source masking is
also different.  The mask circle radius is chosen to contain 95% of
the flux for the lowest energy event in the energy bin.


SMOOTHING:
Both unsmoothed maps (fwhm000) and smoothed maps (fwhm120,180,etc) are 
provided. The smoothed maps contain an initial FWHM and target FWHM
in the header.  If the initial FWHM is LARGER than the target FWHM, 
then no smoothing has been performed. 

The combined smooth maps are produced by spherical harmonic convolution of 
the front and back maps, and then a weighted average is taken.

FITS HEADERS:
The primary header of each file contains useful information about
the PSF size.  The target PSF is the desired PSF; in cases where the
initial PSF is smaller than the target PSF the maps have been smoothed
to the target PSF with spherical harmonic convolution.  If the initial
PSF is larger than the target, no smoothing has occurred.  For combined
maps (weighted average of front- and back-converting event maps) an 
average value is used for the initial PSF. 
The mean energy EMEAN is taken to be the geometric mean of EMIN and 
EMAX, for simplicity.
PSCSUB is 1 for maps with point-source subtraction. 
The other HEALPIX-related header cards are provided for compatibility
with HEALPix software used by the WMAP team. 

As an example, for the file
fermi-allsky-001.0-002.0GeV-fwhm120-0256.fits
the header looks like:

SIMPLE  =                    T / Written by IDL:  Sun Oct 18 14:26:09 2009
BITPIX  =                  -32 / Number of bits per data pixel
NAXIS   =                    1 / Number of data axes
NAXIS1  =               786432 / 
EXTEND  =                    T /Extensions may be present
DATE    = '2009-10-18T18:26:10' / Creation date
COMMENT FITS (Flexible Image Transport System) format is defined in 'Astronomy
COMMENT and Astrophysics', volume 376, page 359; bibcode 2001A&A...376..359H
BUNIT   = 'counts/cm^2/s/sr'   /
FWHM    =              120.000 / Target FWHM after smoothing [arcmin]
FWHMINIT=              51.0692 / Original FWHM [arcmin]
PIXTYPE = 'HEALPIX '           / Pixel algorithm
ORDERING= 'RING    '           / Ordering scheme
NSIDE   =                  256 / Resolution parameter
NPIX    =               786432 / # of pixels
FIRSTPIX=                    0 / First pixel (0 based)
LASTPIX =               786431 / Last pixel (0 based)
EMIN    =              1.00000 / Energy bin Emin [GeV]
EMAX    =              2.00000 / Energy bin Emax [GeV]
EMEAN   =              1.41421 / Mean energy of event in bin [GeV]
PSCSUB  =                    1 / Point sources subtracted?
HISTORY  Reprojected to HEALPIX by D. Finkbeiner, Harvard CfA
END


SOFTWARE:
All software available from dfinkbeiner@cfa.harvard.edu upon request. 
Please tell me if you use the software for any published work.
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