In the UVIS channel, the G280 grism provides spectroscopy over a useful wavelength range of 200-400nm, at a dispersion of ~14√Ö per pixel in the first order. The two grisms for the IR channel cover the wavelength ranges 800-1150nm (G102) and 1075-1700nm (G141). The dispersions are 24.5 and 46.5√Ö per pixel, respectively. The primary aim of the reduction of WFC3 slitless spectra is to provide one-dimensional wavelength- and flux-calibrated spectra of all objects with detectable spectra. The reduction presents special problems because of the dependence of the wavelength zero-point on the position of the object in the field, the blending of spectra in crowded fields, and the need for flat-field information over the whole available wavelength range. The aXe
software package is designed for the automated extraction, calibration, visualization, and simulation of spectra from slitless spectroscopic instruments. It was originally developed by the Space Telescope - European Coordinating Facility (ST-ECF), a former unit of the European Space Agency (ESA). As of January 1, 2011, STScI has taken responsibility for support and distribution of the software.
Because of the high sensitivities of the WFC3 grisms, observations of moderately crowded fields can produce many instances where spectra overlap. It is important to know if a given spectrum is contaminated by that of a neighbor. This can be determined by performing simulations of the grism image with software such as aXesim
using a catalog of positions, brightnesses, and angular extents of sources in the field. The simulations can be used to choose a telescope roll angle which eliminates or minimizes contamination for specific sources of interest. Contamination can also be mitigated by obtaining grism observations of the same field at different telescope roll angles, which improves the chances of cleanly extracting the spectrum for a given target.
The direct image of a direct-plus-grism image pair can be fully reduced by calwf3, including bias subtraction, dark subtraction, flat fielding, and computation of the photometric zero-point in the header. In contrast to direct images, no single flat-field image can be correctly applied to grism images, because each pixel contains signal arising from different wavelengths. Flat fielding must therefore be applied during the extraction of spectra once the wavelength corresponding to each pixel is known in post-pipeline processing with aXe
. With aXe
, the user can apply flat-field corrections which are dependent on the wavelength falling on each pixel, as specified by the position of the direct image and the dispersion solution. So during calwf3 processing the FLATCORR step is still performed, but the division is done using a special flat-field reference file that only contains information on the relative gain offsets between the different detector amplifier quadrants. This allows the FLATCORR step to still apply the gain correction (converting the data to units of electrons for UVIS or electrons per second for IR) and thus also corrects for offsets in gain between the various quadrants of the detectors.
The calwf3 flt products should then be the starting point for all subsequent reduction of slitless data with aXe
or other software. The units of the data in the SCI and ERR extensions of these files are electrons for UVIS and electrons per second for IR. The primary output of aXe
is a file of extracted, flux calibrated spectra, provided as a FITS binary table with as many table extensions as there are extracted spectra.
task, which is normally used to correct for the geometrical distortion of WFC3 and combine dithered exposures, is not generally applicable to grism observations. This is due to the fact that the spatial distortion correction would only be applicable to the cross-dispersion direction of grism images. For similar reasons, the combining of dithered grism images before extracting spectra is not generally recommended. Every detector pixel has a different spectral response, which has not yet been corrected in the calibrated two-dimensional images (see Section 9.4.2
on flat fielding). Combining dithered grism images before extraction will combine data from different pixels, making it difficult or impossible to reliably flat field and flux-calibrate the extracted spectra. Extracted spectra from dithered images can be properly combined into a final spectrum using the aXedrizzle
task in the aXe
processing of dithered grism exposures can, however, be useful for simple visual assessment of spectra in a combined image and for the purpose of flagging cosmic-ray (CR) hits in the input flt images. When AstroDrizzle
detects CR’s in the input images it inserts flags to mark the affected pixels in the DQ arrays of the input flt files. The aXe
spectral extraction can then be run on these updated flt images and utilize the DQ flags to reject bad pixels. This is very useful for rejecting the large number of CR hits that occur in long UVIS G280 exposures. It is not as necessary for IR grism images, because the IR flt files have already had CR’s rejected by the calwf3 up-the-ramp fitting process.
As an example, Figure 9.3
shows a G280 image of the Wolf-Rayet star WR-14, which is used as a wavelength calibrator. Superimposed on the dispersed image is a F300X image, which illustrates the relative location of the direct image of the source (circled in Figure 9.3
). The full 4096-pixel x-axis extent of the detector is shown, which is completely filled by the positive and negative orders of this bright source.
shows a zoomed view of the first several positive spectral orders of this source, where wavelength increases to the left. Notice how the blue end of each order curves upwards, and that at longer wavelengths (greater than ~400nm) there is significant overlap of adjacent orders. Very bright sources produce spectra in which orders up to 6-8 can be detected. These spectra, which in principle can be analyzed (although dispersion solutions have been determined for only the first few orders), provide a strong source of contamination for the spectra from fainter objects. In addition, the higher order spectra are increasingly out of focus and thus spread in the cross-dispersion direction.
The dispersion of the G102 grism is high enough that only the positive 1st and 2nd order spectra generally lie within the field of the detector. For the lower-dispersion G141 grism, the 0th, 1st, 2nd, and 3rd order spectra lie within the field for a source that has the 1st order roughly centered. The IR grisms have the majority (~80%) of their throughput in the +1st order, resulting in only faint signals from the other orders. The trace of the observed spectra are well described by a first-order polynomial, however the direct-to-dispersed image offset is a function of the source position in the field. The tilt of the spectra relative to the image aXe
s are small, being only 0.5-0.7 degrees. Typical filters used for obtaining companion direct images are the F098M and F105W for the G102, and the F140W and F160W for the G141. Other medium- and narrow-band filters can be used when necessary to prevent saturation of very bright targets. The image centroids of sources at a given telescope pointing will depend on the filter used. These filter-dependent systematic variations, which are documented in WFC3 ISR 2012-01
, are generally at the sub-pixel level, but must be taken into account during the spectral extraction and reduction process, if the filter that is used to take the direct images is different from the filter in which the trace is calibrated.
The software package aXe
provides a streamlined method for extracting spectra from WFC3 slitless spectroscopy data. aXe
is distributed as part of the STSDAS software package. More information can be found at http://axe-info.stsci.edu/
There is a detailed aXe
manual and a cookbook specific to WFC3 grism data reduction, both of which are available from the aXe
webpages, so only a brief outline of its use is presented here.
has two different strategies for removal of the sky background from the spectra. The first strategy is to perform a global subtraction of a scaled ''master sky'' frame from each input grism image at the beginning of the reduction process. This removes the background signature from the images, so that the remaining signal can be assumed to originate from the sources only and is extracted without further background correction in the aXe
reduction. Master sky frames are available for download from the aXe
The primary output of aXe
is a file of extracted and calibrated spectra, which is provided as a multi-extension FITS binary table with as many table extensions as there are extracted spectra. The table contains 15 columns, including wavelength, total and extracted and background counts and their errors, the calibrated flux and error, the weight and a contamination flag. The primary header of this "SPC" table is a copy of the header of the frame from which the spectrum was extracted
also creates a 2-d “stamp” image for each beam. The stamp images of all spectra extracted from a grism image are stored as a multi-extension FITS (STP) file with each extension containing the image of a single extracted spectra. It is of course also possible to create stamp images for 2-d drizzled grism images.
The output products from aXe
consist of ASCII files, FITS images and FITS binary tables. The FITS binary tables can be accessed using the tasks in the stsdas.ttools package and wavelength-flux plots, with error bars, can be plotted using stsdas.graphics.stplot.sgraph.
When there are many detected spectra on a single image, a dedicated task aXe
2web is available. aXe
2web creates html pages consisting of direct image cut outs, stamp images and 1-d spectra for each extracted beam. This enables convenient browsing of large numbers of spectra or the publishing of aXe
spectra on the Web with minimal interaction.
The WFC3 grism dispersion solutions were established by observing both astronomical sources with known emission lines (e.g., the Wolf-Rayet star WR-14 and the planetary nebula Vy2-2; see WFC3 ISR 2009-17
) and ground-based monochromator sources (see WFC3 ISR 2009-01
and ISR 2008-16
). The field variation of the dispersion solution was mapped by observing the same source at different positions over the field. The internal accuracy of these dispersion solutions is good to ~0.25 pixels for the IR grisms (~6Å and ~9Å for the G102 and G141, respectively), and to ~1 pixel (~14Å) for the UVIS G280.
The sensitivity of the dispersers was established by observing a spectrophotometric standard star at several positions across the field. The sensitivity (aXe
uses a sensitivity tabulated in ergs/cm2/sec/√Ö per detected electron) was derived using data flat fielded by the flat-field cube. Results for the IR grisms show 4-5% differences in the absolute flux of spectra located near the center of the field as compared to those near the field corners. This is clear evidence for a large-scale variation in the overall illumination pattern in the grism flat-field data cubes. Additional field-dependent flux calibration observations are planned, which will enable such corrections to be implemented.
The sensitivity of the G280 grism was established by observing a spectrophotometric standard star at several positions across the field. The sensitivity (aXe
uses a sensitivity tabulated in ergs/cm2/sec/√Ö per detected electron) was derived using data flat fielded by the flat-field cube. Please check the WFC3 Instrument Science Reports for updates.