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.
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. |
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |