Space Telescope Science Institute   6.3.5 SBC PR130L  6.3.7 Extraction and Calibration of Spectra

6.3.6 Observation Strategy


The normal observing technique for all ACS spectroscopy is to obtain a direct image of the field followed by the dispersed grism/prism image. This combination allows the wavelength calibration of individual target spectra by reference to the corresponding direct images. For WFC and HRC, the scheduling system automatically inserts a default direct image for each specified spectroscopic exposure, which for G800L consists of a 3 minute F606W exposure, and for PR200L, a 6 minute F330W exposure. If the observer wishes to eliminate the default image, the supported Optional Parameter AUTOIMAGE=NO can be specified. Then a direct image with a different filter and/or exposure time can be specified manually, or no direct image at all in the case of repeated spectroscopic exposures within an orbit, or if no wavelength calibration is required. For the SBC prisms, there is no default direct image because of the Bright Object Protection requirements ( Section 7.2); the direct image must always be specified manually, and it must satisfy the BOP limits, which will be more stringent than for the dispersed image.

Because of the offset between the direct imaging and prism aperture definitions, the SAME POS AS option will generally not have the desired effect for prism spectroscopy (PR110L, PR130L, and PR200L). Users who wish to specify offsets from the field center by means of the POS-TARG option should do so by explicitly specifying the same POS-TARG for the direct imaging and prism exposures.

Table 6.4 lists the V detection limits for the ACS grism/prism modes for unreddened O5 V, A0 V, and G2 V stars, generated using the Exposure Time Calculator. WFC and HRC values used the parameters CR-SPLIT=2 and GAIN=2. An average sky background was used in these examples. However, limiting magnitudes are sensitive to the background levels; for instance, the limit magnitude of an A0 V in the WFC using the F606W filter changes by ±0.4 magnitudes at the background extremes.


Table 6.4: V detection limits for the ACS grism/prism modes.
Mode
V limit
(S/N = 5, exposure time = 1 hour)
Wavelength of reference (Å)

O5 V (Kurucz model)
A0 V (Vega)
G2  V (Sun)

WFC/G800L
24.2
24.4
24.9
7000
HRC/G800L
23.4
23.6
24.2
7000
HRC/PR200L
25.6
22.7
18.8
2500
SBC/PR110L
24.9
20.9
9.3
1500
SBC/PR130L
25.6
21.5
9.9
1500

Chapter 9 provides details of the calculations. Depending on the wavelength region, the background must also be taken into account in computing the signal-to-noise ratio. The background at each pixel consists of the sum of all the dispersed light in all the orders from the background source. For complex fields, the background consists of the dispersed spectrum of the unresolved sources; for crowded fields, overlap in the spectral direction and confusion in the direction perpendicular to the dispersion may limit the utility of the spectra.

The ACS Exposure Time Calculator supports all the available spectroscopic modes of the ACS and is available for more extensive calculations at http://apt.stsci.edu/webetc/acs/acs_spec_etc.jsp. The current version employs the on-orbit determinations of the dispersion solution and sensitivity determination where available.

For more detailed simulations of ACS spectra, an image-spectral simulator, called SLIM, is available. This tool allows synthetic target fields to be constructed and dispersed images from spectrum templates to be formed. SLIM can simulate spectra for all the ACS spectral modes. The simulator runs under Python and an executable version is available at: http://www.stecf.org/software/SLIM/SLIM10/index.html. Version 1.0 uses a Gaussian PSF but this has been found to be an adequate representation to the Tiny Tim model of the ACS PSF. A detailed description of the tool and examples of its use are given by Pirzkal et. al. (ACS ISR 01-03).


 6.3.5 SBC PR130L  6.3.7 Extraction and Calibration of Spectra
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