Space Telescope Science Institute   6.3 Grism/Prism Spectroscopy  6.3.2 HRC G800L

6.3.1 WFC G800L


The G800L grism and the WFC provide two-pixel resolving power from 69 (at 5500 Ċ) to 131 (at 10,500 Ċ) for first order spectra over the whole accessible field of 202 x 202 arcseconds. Table 6.3 lists the linear dispersion, but a second order dispersion solution provides a better fit. Figure 6.15 shows the wavelength extent and sensitivity for the zeroth, first, and second order spectra when used with the WFC. Figure 6.16 shows the same plot in pixel extent. The 0 position refers to the position of the direct image and the pixel size is 0.05 arcseconds. Note that there is contamination of the 1st order spectrum above 10,000 Ċ by the second order. The total power in the zeroth order is 2.5% of that in the first order, so locating the zeroth order may not be an effective method of measuring the wavelengths of weak spectra. The default method will be to obtain a matched direct image-grism pair. There is also sensitivity of about a percent of first order in the third and fourth orders, and about half a percent in the negative orders. The full extent of the spectrum of a bright source (orders -2, -1, 0, 1, 2, 3) is 1200 pixels (60 arcseconds). The higher orders are not in focus and the spectral resolution of these orders is therefore less than what would be predicted from the nominally higher dispersion.

Figure 6.15: Sensitivity versus wavelength for WFC G800L.

 

Figure 6.16: Sensitivity versus pixel position for WFC G800L.

 

Figure 6.17 shows the full spectrum extent for a 60 second exposure of the white dwarf GD153 (V = 13.35) with the individual orders indicated. When bright objects are observed, the signal in fainter orders may be mistaken for separate spectra of faint sources and in crowded fields, many orders from different objects can overlap. The wavelength solution is field dependent on account of the tilt of the grism to the optical axis, and the linear dispersion varies by ħ11% from center to corner. This field dependence has been calibrated to allow wavelength determination to better than 0.5 pixels over the whole field.

Figure 6.17: Full dispersed spectrum for white dwarf GD153 with WFC/G800L.

 
The numbers indicate the different grism orders.
 

 6.3 Grism/Prism Spectroscopy  6.3.2 HRC G800L
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