10.2 IR Scanned Data

Normally, calwf3 constructs calibrated and cosmic-ray cleaned flt.fits files for IR data by fitting a straight line to the slope up-the-ramp of accumulating charge within each pixel. This method, however, is inappropriate for spatially scanned data where the charge accumulation in any given pixel crossed by a source occurs within a very short period of time and thus appears as a step rather than a ramp (see Figure 10.1).

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).

As a consequence, analysis of IR scanned data in the past has typically been done using the ima.fits files, which contain the individual readouts of the detector, adjusted for the appropriate reference pixel, dark and non-linearity corrections. However, starting with calwf3 version 3.3 in early 2016, the flt.fits files for scanned data are constructed without the fit up-the-ramp (CRCORR is set to OMIT). The result is a calibrated science data file (flt.fits) consisting of the first-minus-last science extension from the MULTIACCUM ima file (where the individual reads are stored in reverse time order), a more reasonable representation of the image than the up-the-ramp fit. Observers with calibrated scan data predating 2016 may re-retrieve their files from the MAST archive to obtain the improved flt.fits files. Since CRCORR is OMIT, these calibrated files will contain cosmic rays that will require removal via traditional routines e.g., stacking of dithered images, Laplacian cosmic ray identification (e.g., LACosmic from van Dokkum), Astrodrizzle, and so on.

Alternatively, observers can of course continue using the standard ima files. In that case, an additional processing step is required: manually calculating differences of the Nth readout and the (N-1)th readout to form a set of images. Those difference images can be subsequently analyzed with custom procedures. As reported in Section 3.3.9 and in the note in Section 3.3.10, depending on the values of the UNITCORR and FLATCORR switches, the ima file will report the average count (or electron) rate up to the given read or the total accumulated counts (or electrons) up to the given read. Given that WFC3/IR sampling sequences may have unevenly spaced readouts, converting from one to the other requires using the values of the readout timings (recorded in the ima image as either the PIXVALUE keyword of the TIME extension of each imset, or as the SAMPTIME keyword of the SCI extension of the same imset).

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 (see WFC3 ISR 2006-06 or Phase II Proposal Instructions). 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. With calwf3 version 3.3 and later, all the UVIS and IR scan-related keywords formerly accessible only via the engineering file headers (*spt.fits), are now present in the calibrated science data headers (e.g., *flt.fits).

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




C:bostrophidon; D:C with dwell; N:N/A


scan width (arcsec)


angle between sides of parallelogram (deg)


dwell pts/line for scan pointing (1-99,0 if NA)


wait time (duration) at each dwell point (sec)


position angle of scan line (deg)


commanded rate of the line scan (arcsec/sec)


number of lines per scan (1-99,0 if NA)


scan length (arcsec)


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 WFC3 ISR 2012-08.

10.2.1 IR Scanned Spectra

MAST provides the user with calibrated images taken in scanned mode. However the analysis of spatially scanned WFC3/IR spectroscopy (see Figure 10.2 for an example) involves ad-hoc post-processing steps that are left to the users. Some guidelines for the analysis of spatially-scanned IR spectroscopy can be found in WFC3-2012-08. Additionally, papers exist in the literature that illustrate custom data reduction procedures for the analysis of scanned data. The WFC3 team does not endorse of any specific paper, however the interested reader should search the literature for such examples and use their own judgement in adopting the reduction strategy best suited for their purposes