WFC3 Data Handbook V. 4.0

help@stsci.edu

10.2	IR Scanned Data

10.2.1 IR Scanned Spectra

For IR data, the flt.fits file is useless, because to produce one, the calwf3 pipeline fits a straight line to the slope of accumulating charge within each pixel, but for spatially scanned data, the accumulation occurs all in a very short moment, so that rather than a *ramp* of charge accumulation, there is a *step* instead, which the pipeline (figuratively speaking) trips over: the pipeline flags many pixels as cosmic rays and fits others very poorly.

Figure 10.1: Schematic diagram of charge accumulation in a pixel sampled with the RAPID sequence for a source observed in staring mode (diamonds, in a ramp) or in spatially scanned mode (squares, in a step function).

Instead we recommend starting with the ima.fits files, which contain the individual readouts of the detector, adjusted for the appropriate dark and non-linearity corrections. The next step is to calculate differences of the Nth readout and the (N-1)th readout to form a set of images, each one representing the accumulated charge in each pixel in the given time interval. Those difference images can be subsequently analyzed with custom procedures. In order to compare given difference images, it may be useful to know the effective exposure time; this can be derived from the relative times of each sample (keyword SAMPTIME associated with each extension's header in the ima.fits file).

For the SPARS sample sequences specifically, the time interval between the 0th readout and the 1st readout is much shorter than the intervals between subsequent readouts; hence for SPARS data, the first interval probably should be discarded in subsequent analysis. For example, with a GRISM512 subarray and SPARS10 readout, the first interval is 0.85 seconds, whereas subsequent intervals are 7.92 seconds (Petro & Wheeler 2006). Additional meta data related to spatial scans is available in the _spt.fits file; an example block of keywords associated with spatial scans is listed in Table 10.1.

10.2.1 IR Scanned Spectra

Deming et al. (2013) detail one such analysis customized for spectroscopy of exoplanet host stars. Additional tips on data analysis of spatially-scanned IR spectroscopy are in WFC3-2012-08.

Table 10.1: Spatial Scan Keywords in the _spt.fits files

Keywords	Description
PROPOSAL INFO	Spatial Scan Info
SCAN_TYP= 'C’	C:bostrophidon; D:C with dwell; N:N/A
SCAN_WID=0.000000000000E+00	scan width (arcsec)
ANG_SIDE=9.000000000000E+01	angle between sides of parallelogram (deg)
DWELL_LN=0	dwell pts/line for scan pointing (1-99,0 if NA)
DWELL_TM=0.000000000000E+00	wait time (duration) at each dwell point (sec)
SCAN_ANG=2.248278045654E+02	position angle of scan line (deg)
SCAN_RAT=5.000000074506E-02	commanded rate of the line scan (arcsec/sec)
NO_LINES= 1	number of lines per scan (1-99,0 if NA)
SCAN_LEN=2.969249963760E+00	scan length (arcsec)
SCAN_COR='V'	scan coordinate frame of ref: celestial,vehicle

Figure 10.2: A spatially-scanned spectrum labeled with its 0th and +1st order light, and compared to a nominal staring-mode slitless spectrum of the same field (red outlined inset); reproduced from McCullough & MacKenty (2012).