WFC3 Instrument Handbook for Cycle 24

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8.2	Slitless Spectroscopy with the UVIS G280 Grism

The G280 grism is a WF/PC1 spare. Figure 8.1 shows a spectrum of the wavelength calibration star WR14 observed as part of the Cycle 17 calibration program 11935. The circled spot shows the location of a direct image of the source obtained with a separate (undispersed) F300X filter exposure, but superposed on the grism image for illustrative purposes only. The prominent star-like feature near the center of the picture is the zeroth-order grism image, and the positive and negative higher orders extend towards the left and right of the zeroth order, respectively. The +1st order is defined to be the order with the higher throughput (due to the grating blaze), even though it falls at lower x-pixels than the position of the zeroth order. The +1st order extends to the left of the zeroth order a distance of about 1/4 of the image size. Further left there is heavy overlap with higher orders. Some prominent emission lines can be seen along the spectral trace.

Figure 8.1: Appearance of the G280 spectral orders on the detector. The circled source is the position of the direct image formed by summing an F300X image with the grism image. The stronger 1st order is to the left and the 0th order is in the center. Above the 1st orders, much weaker 2nd and 3rd orders are barely visible. The image shows the full extent of the detector in the x-axis and about 500 pixels in the y-axis.

There are several features of this grism that differ, for example, from the G800L grism on ACS. There is an offset of about 175 pixels in the y-direction between the direct image and the spectra, the zeroth-order is relatively bright due to a lower grating efficiency and clear substrate, and there is curvature of the spectra at the blue ends of the first orders (nearest the zeroth order). The amplitude of the curvature is about 30 pixels in the detector y-direction. Figure 8.2 shows a close up view of the first few positive orders of the WR14 spectrum, which illustrates the curvature at the short-wavelength end of each order.

Figure 8.2: Close up view of the first few positive orders of the WR14 spectrum

Figure 8.3: The absolute throughput of the G280 grism as a function of wavelength for the lowest orders.

The spectral trace and dispersion have been measured both during ground calibration and on-orbit using the wavelength calibration star WR14. The trace is well fit with a 5th order polynomial, with a trace amplitude of about 25 pixels in the y-direction due to the curvature at short wavelengths. The dispersion is well fit with a 4th order polynomial, with a standard deviation of 2.5 Angstroms. The mean dispersion in the 1st order at 2200 е is 12.2 е/pixel, varying from 10.8 е/pixel at 1850 е to 14.4 е/pixel at 4000 е. The dispersion per pixel in the higher orders is higher by the approximate ratio of the orders; for example, in +3rd order it is 48 е/pixel.

Note that the current trace and dispersion solutions are only valid for three locations within the UVIS field of view that have been sampled. The three locations are the center of chip 1, the center of chip 2, and the nominal center of the UVIS channel (near the bottom of chip 1). A coarse sampling of other locations during ground calibration showed that the trace and dispersion are complex 2-dimensional functions of the field location. Observers having a single target of interest are therefore encouraged to place the target near one of the well-calibrated positions in the field. The center of chip 2 is preferred, due to the higher QE of chip 2 at very short wavelengths.

Figure 8.3 shows the total 1st-order transmission for the WFC3 G280 mode (including the instrument and telescope) for observations on chip 2, as determined from observations of the HST standard star GD-71 during Cycle 17. The +1st order is more sensitive than the zeroth order at wavelengths less than 320 nm, but longward of this wavelength the zeroth order dominates. On deep exposures, orders out to at least -8 and +8 have been detected. The current absolute flux calibration of the G280 is estimated to be accurate to better than 10%. Due to the relatively high throughput of the 2nd order, 1st order spectra longward of 400 nm are likely to be overlapped by 2nd order light longward of 200 nm, at least for sufficiently hot sources.

Grism exposures of a given target field should always be accompanied by a direct image. The direct image provides source sizes and identifications, from which the corresponding locations of spectra in the grism images are determined. Knowledge of the direct-to-grism exposure source offsets is necessary to set the wavelength zero-point for each extracted spectrum and the source size measurements enable the software extraction slit to be tuned to each object (see Section 8.5). The 0th-order trace in G280 images is slightly dispersed (making it difficult to centroid) and often saturated (making it impossible to centroid), therefore it cannot be used in place of a direct image.

The natural choice of a direct-imaging filter to provide the reference image for G280 exposures is the F300X, because its response matches most closely the +1st-order grism response. The broader F200LP filter may be preferable for fainter objects. The shape of the spectra in G280 exposures varies significantly across the field of view but has been calibrated. The sensitivity of the +1 order also varies, but its field variation cannot be handled by the aXe package and is only calibrated at the center of chip 2. Small dithers of G280 exposures and accompanying direct image exposures is recommended to remove the effects of bad pixels. In addition, it is recommended that CR_SPLIT be used in both the direct and dispersed exposures to allow for cosmic-ray identification and removal.

G280 exposures can only be obtained using the “UVIS” aperture selection, which places the target at the reference point of the UVIS field of view, about 10" above the inter-chip gap. Accompanying direct images should use the G280-REF aperture selection, which places the target at the same location as in the dispersed exposures. If your observations are of a single primary target, it is best to specify a POS TARG Y of -50", for both the dispersed and direct exposures, to place the target at the center of chip 2, where the spectral calibrations are best determined and the near-UV sensitivity is somewhat higher than on chip 1.

For sufficiently bright objects, the multiple spectral orders may extend across the full field of view of the camera. This leads to overlapping of fainter spectra in the vicinity of the bright-object spectrum. Therefore, a careful determination of the optimum telescope roll angle is required to obtain non-overlapped spectra of faint objects in the vicinity of much brighter objects. (i.e., the observer needs to set the orientation of the detector on the sky by using the Visit Orientation Requirements parameter “ORIENT” in the phase II proposal; e.g. ORIENT ~135 degrees aligns the Y axis of the UVIS detector with North.)

CTE losses can be large for faint spectra on faint backgrounds. They should be taken into account when planning UVIS exposures. See Sections 5.4.11 and 6.9 for information on CTE and observing strategies.