8.1	Grism Overview

WFC3 provides a slitless spectroscopy mode in both of its channels. The UVIS channel has a single grism and the IR channel two grisms, enabling low-resolution slitless spectroscopy over the entire field of view of both detectors.

In the UVIS channel, the G280 grism provides spectroscopy over a useful wavelength range of 190–450 nm, at an average dispersion of about 1.3 nm per pixel in the first order. Reduced sensitivity and second-order contamination interfere with grism observations at wavelengths longer than 400 nm.

The two grisms for the IR channel cover the wavelength ranges 800–1150 nm (G102), and 1075 nm to 1700 nm (G141). The dispersions are 2.45 and 4.65 nm per pixel, respectively.

The observing technique of spatial scanning can be used for WFC3 spectroscopic observations. See Section 8.6.

Table 8.1 summarizes the capabilities of the three WFC3 grisms in first-order mode. The resolving power is listed for each grism at a wavelength near the center of its range of coverage. The dispersion differs slightly between the two chips of the UVIS channel, and the mean value is listed. The tilt of the dispersion direction relative to the detector x-axis is also listed.

 

For the latest information on WFC3 spectroscopy, see the WFC3 Spectroscopy Resource page: http://www.stsci.edu/hst/wfc3/analysis/grism_obs/wfc3-grism-resources.html.

Table 8.1: WFC3 UVIS and IR Grisms.

Grism	Channel	Wavelength range (nm)	Resolving power1	Dispersion (nm/pixel)	Tilt (deg)2
G280	UVIS	190–450	70 @ 300 nm	1.3	−3
G102	IR	800–1150	210 @ 1000 nm	2.453	+0.7
G141	IR	1075–1700	130 @ 1400 nm	4.653	+0.5

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1

Resolving power is based on the FWHM of the Gaussian image core measured in direction perpendicular to the dispersion.

2

Tilt of dispersion direction with respect to the positive x-axis of the image frame.

3. The dispersion varies over the field by ±4%; the tabulated value refers to the field center.