calibration results by item dispersion AUTHORS : A.R. Martel, G. Hartig
PURPOSE :
The main goal is to measure the wavelength dispersion of the G800L grism at several locations on the field-of-view of the HRC#1 and WFC#4 flight detectors.
LOCATION AND DATE : GSFC/SSDIF, Apr 6, 2001
INSTRUMENT CONFIGURATION :
ACS is in the TBF in the diamond configuration with RAMP+RAS/Cal at the entrance aperture, as in the acoustics alignment campaign. A diagram of the light source assembly feeding RAS/Cal is shown in Fig. 1. The Quartz-Tungsten-Halogen (QTH) lamp is simply a fiber light source with a projector bulb. A multi-mode fiber is moved by one of the clean room operators (K. Redman or V. Roberts) such that one end is very close to (and centered on) the RAS/Cal 4 micron pinhole and the other end is alternately moved back and forth between the fiber light source or the Ar lamp. In the latter case, light from the Ar lamp was focused onto the fiber tip with a microscope objective. The move to the different field points with RAMP, and the associated metrology, is performed by the clean room operator.
METHOD :
To measure the grism dispersion across the chips, a number of well-separated field points were selected, ideally at the field centre and near the four corners. The standard, pre-defined field points for RAMP were described in the acoustics alignment campaign (Figs 3, 4 and Tables 1, 2) for HRC and WFC. But to insure that the first-order grism spectra fall within the HRC field-of-view, it was necessary to define three new field points : H10, H11, and H12. These are shown in Fig. 2 below. These field points were simply defined by taking the mean of the nominal RAMP (x,y,z) positions of the nearest field points as well as the mean of their TM2 tip and tilt values. In particular, H10 was defined with H1, H3, H4, and H6, H11 with H7 and H9, and H12 with H2 and H5. The focus, tip-tilt, and astigmatism of these new field points is therefore not entirely correct. For the WFC, the measurements were done at the standard field points W1, W3, W5, W7, W9, and at a new point W11, which was not corrected for focus and TM2 angles. The WFC field points are shown in Fig. 3.
At each field point, a dispersed spectrum of the continuum QTH lamp and of the Ar line lamp are taken, as well as a direct image of the PSF through the F775W broad-band filter, which covers a similar wavelength range as G800L (note that F814W was used for H1). For the HRC, the PSF was obtained with the QTH lamp and for the WFC, the weaker Ar lamp was used to avoid digital saturation. The dispersed spectra are taken in pairs to increase the S/N ratio and remove the cosmic ray hits. To save disk space and reduce read-out time, the WFC data are read out as subarrays.
The resultant spectra are shown in Figs 4-6 (HRC) and 7-9 (WFC). The WFC spectra are oriented along the V2 axis (A-B and C-D axes) and are offset by ~40 deg for the HRC. The first order spectra dominate. They extend for 140 pixels in WFC and ~210 pixels in HRC. The zero-th order image as well as the low-level negative orders are also visible. The ideal lamp would produce a few well-separated emission lines over the wavelength interval of the grism. But practically, the available line lamps, in particular the Ar lamp, produce strong blends at the dispersion of the grism (~40 Ang/pixel for WFC and ~29 Ang/pixel for HRC). For example, on the HRC, five groups of blended lines centered on the brightest can be identified (see Results below).
RESULTS :
The HRC and WFC grism spectra were analyzed by the ECF group. Their results are presented in a series of reports.
CEI SPECIFICATIONS :
None.
REFERENCES :
