Space Telescope Science Institute   6.2.9 Choice of Filters for Coronagraphic Observations  6.3.1 WFC G800L

6.3 Grism/Prism Spectroscopy


The ACS filter wheels include four dispersing elements for low resolution slitless spectrometry over the field of view of the three ACS channels. One grism (G800L) provides low resolution spectra over the 5500 Å to 10,500 Å range for both the WFC and HRC. A prism (PR200L) in the HRC covers the range 1700 Å to beyond 3900 Å (although reliable wavelength and flux calibration is only guaranteed to 3500 Å). In the SBC a LiF prism covers the wavelength range 1150 Å to ~1800 Å (PR110L) and a CaF2 prism is useful over the 1250 Å to ~1800 Å range (PR130L). Table 6.3 summarizes the essential features of the four ACS dispersers in the five available modes. The grism provides first order spectra with almost constant dispersion (about 40 Å/pixel) as a function of wavelength but with second order overlap beyond about 10,000 Å; the prisms however have non-linear dispersion with maximum resolution at shorter wavelengths but much lower resolving power at longer wavelengths. The two-pixel resolution is listed for each grism or prism at a selected wavelength in Table 6.3. The pixel scale for the prism spectra is given at the selected wavelength. The tilt of the spectra to the detector X axis (close to the spacecraft V2 axis) is also listed.


Table 6.3: Optical parameters of ACS dispersers.
Disperser
Channel
Wavelength range (Å)
Resolving power
Å/pixel
Tilt1 (deg)
G800L
WFC
1st order:
5500 to 10500
100 @ 8000 Å
39.82
-2
G800L
WFC
2nd order:
5000 to 8500
200 @ 8000 Å
20.7 2
-2






G800L
HRC
1st order:
5500 to 10500
140 @ 8000 Å
23.9 3
-38
G800L
HRC
2nd order:
5500 to 8500
280 @ 8000 Å
12.03
-38
PR200L
HRC
1700 to 3900
59 @ 2500 Å
21.3
-1






PR110L
SBC
1150 to 1800
79 @ 1500 Å
9.5
0
PR130L
SBC
1250 to 1800
96 @ 1500 Å
7.8
0
1Tilt with respect to the positive X-axis of the data frame.
2The dispersion varies over the field by ±11%; the tabulated value refers to the field center.
3The dispersion varies over the field by ±2%; the tabulated value refers to the field center.

 6.2.9 Choice of Filters for Coronagraphic Observations  6.3.1 WFC G800L
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