Spitzer Documentation & Tools

IRACPC_PMAP_CORR

Purpose

Correct IRAC observed aperture photometry-derived fluxes from the post-cryogenic, or warm mission (IRACPC) for the intra-pixel response, using the pixel maps of the 3.6 micron (channel 1) and 4.5 micron (channel 2) subarray “sweet spots”. The sweet spots are defined relative to the Subarray field of view; if your data were taken in Full Array readout mode (and your centroids are with respect to the full array), use the /FULL keyword.

The correction is derived by interpolating high resolution maps of flux as a function of centroid location over the photometric pixel, which is the nominal subarray center. The quality of the maps varies with position, but in a region of approximately 0.5 x 0.5 pixel around the peak of the intrapixel response (the “sweet spot”) the pixel gain map (“pmap”) is guaranteed to have formal errors in the gain of better than 10-3 (where the average gain across the pixel is 1.0). The effective sweet spot is defined by an “occupation” image which is a weighted sum of the number of photometric data points that contribute to a given grid point in the pmap.

Category

Photometry, corrections

Download

The current version is 1.2. The accompanying fits files for ch1 have been updated using new data, solar flare corrupted data have been removed, and the data have been processed with improved centroiding parameters. This version gives much better results when correcting staring photometry.

Download iracpc_pmap_corr_v1_2.tar.gz, the gzipped tarfile file containing the program file iracpc_pmap_corr.pro and the pmap_fits fits file subdirectory.

Calling Sequence

  IDL> corrected_flux = IRACPC_PMAP_CORR ( OBSERVED_FLUX, X, Y, CHANNEL $
                                           [,FILE_SUFFIX=file_suffix] [,FILE_DIR=file_dir] $
                                           [, /THRESHOLD_OCC ] [,THRESHOLD_VAL=threshold_val] [,/FULL] [,MISSING=missing]

Inputs

OBSERVED_FLUX:array of observed aperture fluxes corresponding to (X,Y)
X:array of X centroids (float), defined such that the center of the bottom left pixel is (0.0,0.0). The bottom left corner of the array is therefore (-0.5,-0.5) and the top right corner is (255.5,255.5). Same number of elements as OBSERVED_FLUX.
Y: array of Y centroids (float). Same number of elements as OBSERVED_FLUX.
CHANNEL:IRAC channel number (1 or 2). Same number of elements as OBSERVED_FLUX, or a single-element array or scalar (if only one element, the program assumes all elements of OBSERVED_FLUX are for the same channel).

Optional Inputs

None.

Keyword Parameters

/THRESHOLD_OCC:Set this keyword to use only data in the pixel gain map for which the "occupation" is greater than THRESHOLD_VAL (see below). The provided gain map images were built by gaussian binning photometric measurements in the vicinity of each bin. The "occupation" is a map giving the gaussian kernel-weighted number of data points that contribute to each grid location in the gain map. We recommend setting /THRESHOLD_OCC, especially if the correction seems to add noise to your data when the keyword is not set. We find that formal errors in the gain map are better than 10-3 when occupation > 20 (this value of occupation partially defines the "sweet spot").
THRESHOLD_VAL:The minimum occupation to accept. If /THRESHOLD_OCC is set, the value THRESHOLD_VAL=20 will be used automatically. Use THRESHOLD_VAL to change this.
MISSING:The value to assign to a photometric data point that has occupancy less than THRESHOLD_VAL (if /THRESHOLD_OCC is set). Default is !Values.F_NAN
/FULL:Set this keyword if your observations were taken in Full Array mode (and your centroids are measured with respect to the full array).
FILE_DIR:Directory where pixel gain map ("pmap") data files are located. The default location is the subdirectory "pmap_fits" of the same directory that holds the iracpc_pmap_corr.pro code.
FILE_SUFFIX:Suffix of pixel gain map ("pmap") data files to be used. For most cases, the default values should not be changed.

Outputs

This function returns the corrected flux, which will be either a scalar or array, depending on the number of elements of OBSERVED_FLUX.

Restrictions

  • This code requires that the pixel map fits files be located on the machine running the program. By default this is the subdirectory "pmap_fits/" of the same directory that holds the iracpc_pmap_corr.pro code.
  • The intrapixel gain map was derived from aperture photometry performed with a 3 pixel aperture (3-7 pixel background annulus), and so is best used to correct data from the same aperture. (A larger aperture will give a less pronounced variation in pixel phase response, in which case this program may overcorrect your data.)
  • This correction is only relevant for warm (post-cryogenic) data.
  • This correction only applies to the "photometric" (subarray nominal center) pixel, where centroid coordinates range from on (x,y) = (14.5,14.5) to (15.5,15.5) in coordinates defined such that the bottom left pixel is centered on (x,y) = (0.0,0.0).
  • As the highest quality portion of the gain map is in regions where occupation > 20, we do not vouch for the effectiveness of this correction outside of the “sweet spot”, and so suggest that you always use the keyword /THRESHOLD_OCC.
  • The sweet spots are defined relative to the Subarray field of view; if your data were taken in Full Array readout mode (and your centroids are with respect to the full array), use the /FULL keyword.

Installation

  1. Download iracpc_pmap_corr_v1_2.tar.gz, the gzipped tar file containing the program file iracpc_pmap_corr.pro and the pmap_fits fits file subdirectory.
  2. Unpack the file in a directory of your choosing.
  3. Ensure that the directory in step (2) is in your IDL path.

Procedure

  1. Measure X and Y pixel centroid position(s) for stellar image(s) (recommended method: first moment for example: http://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/calibrationfiles/pixelphase/box_centroider.pro). X and Y are decimal values defined such that the center of the bottom left pixel of the image has (X,Y) = (0.0,0.0).
  2. Perform aperture photometry on the observation(s) to derive flux(es) (recommended routine: aper.pro from the IDL Astronomy User's Library) NOTE: The present correction is optimized for the 3 pixel radius aperture, and 3-7 pixel sky background annulus.
  3. Correct measured flux(es) using this function.

Example

	    IDL> corrected_flux = IRACPC_PMAP_CORR( OBSERVED_FLUX, X, Y, CHANNEL, /THRESHOLD_OCC)
Last modified: Wed May 2 11:43:58 PDT 2012