IRAC: Software: get_irac_ptg_offsets.pro

Use IRAC astrometric information including distortion parameters to determine pointing offsets for given pixel coordinates, or provide pixel coordinates for a given pointing offset. The pixel coordinates are in BCD orientation, refer to the centers of the pixels and start at (1,1). The procedure uses a set of IRAC BCD FITS headers oriented without rotation in Equatorial coordinates and centered such that the IRAC pointing center is the coordinate origin. As a result, the sky coordinates are the angular offsets from the pointing centers. The standard astrolib procedures are used to calculate sky to pixel and pixel to sky transformations from the image headers. The approximation that the longitude is in units of arc is used as it is practically true for the latitude range (+/- 2.5 arcminutes) covered.

• NAME: get_irac_ptg_offsets
• PURPOSE: Determine row and column offsets for IRAC target position in Spot for a source to be on a given pixel position in the specified channel. If /ROWCOL is set, then return x, y BCD pixel position for input row and column offsets for specified channel.
• CALLING SEQUENCE:
  GET_IRAC_PTG_OFFSETS, CHANNEL, X, Y, ROW, COL[, /SUB]
  --- or ---
  GET_IRAC_PTG_OFFSETS, CHANNEL, ROW, COL, X, Y, /ROWCOL_TO_PIXEL, /SUB
  
• INPUTS:
  • CHANNEL: IRAC channel number 1-4
  • X: Scalar or array of x positions on the array in BCD coordinates. Pixel (1,1) is center of lower left hand corner of array.
  • Y: Scalar of array of y positions on the array in BCD coordinates
  --- or if /ROWCOL is set ---
  • ROW: Offset for input pixel position in arcseconds in row direction as defined in Spot
  • COL: Offset in arcseconds in column direction
• KEYWORD INPUT
  • /ROWCOL: If set, then inputs are ROW and COL and outputs are X and Y
  • /SUB: If set, then X and Y are relative to the subarray rather than the full array. Full arrays 256x256 pixels centered on (128.5,128.5).
    The subarrays are 32x32 pixels with centers located at

    channel	x	y
    1	24	232
    2	24	232
    3	24	24
    4	24	24

    
    
    relative to the full array.
• OUTPUTS:
  • ROW: Offset for input pixel position in arcseconds in row direction as defined in Spot
  • COL: Offset in arcseconds in column direction
  --- or if /ROWCOL is set ---
  • X: Scalar or array of x positions on the array in BCD coordinates. Pixel (1,1) is center of lower left hand corner of array.
  • Y: Scalar of array of y positions on the array in BCD coordinates.
• REQUIREMENTS: IDL v6.0 and higher, GSFC astrolib

This software is not considered part of the pipeline and no installation support from the SSC should be expected. We are however very interested in your feedback in order to improve or replace the algorithm and are glad to work with you in test cases and analysis. Please send your comments to help@spitzer.caltech.edu.

Download get_irac_ptg_offsets.pro.