Since there is no slit in the ACS, the Point Spread Function of the target modulates the spectral resolution. In the case of extended sources it is the extension of the target in the direction of dispersion which sets the achievable resolution. Simulations show that for elliptical sources, the spectral resolution depends on the orientation of the long axis of the target to the dispersion direction and is described in more detail in ACS ISR 2001-02
. The dispersion of the grisms and prisms is well characterized, but for the wavelength zero point it is important to know the position of the target in the direct image. For the grisms, the zeroth order will generally be too weak to reliably set the wavelength zero point. Given the typical spacecraft jitter, wavelength zero points to ħ0.4 pixels should be routinely achievable using the direct image taken just before or after the slitless spectrum image.
The jitter information can be used to obtain more accurate coordinates for the center of the FOV. These in turn allow one to determine better relative offsets between the direct and the spectroscopic images.
The wavelength extent of each pixel for the WFC and HRC G800L modes in the red is small enough that fringing modulates the spectra. For the HRC, the peak-to-peak fringe amplitude is about 30% at 9500 Ċ, and is about 25% for the WFC chips. In practice, for observations of point sources, and even more so for extended objects, the amount of detectable fringing is significantly reduced by the smoothing effect that the PSF, together with the intrinsic object size, have on the spectrum in the dispersion direction. Moreover, application of an optical model for the CCD fringing can allow the fringe amplitude per pixel to be reduced to below 4% peak-to-peak.
Because the STSCI pipeline does not provide an extracted spectral count rate vs. wavelength, an extraction software package, aXe
, is available to extract, wavelength calibrate, flat field, and flux calibrate ACS grism and prism spectra. Full details can be found at:
http://www.stecf.org/software/aXe/
.
The package is also available in STSDAS
.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |