Following the installation of the flight build detectors in late 2000, a series of calibration activities were conducted at GSFC in the Space Systems Development and Integration Facility (SSDIF) in ambient environment from Mar-Aug 2001 (see the GSFC : Mar-Aug 2001 calibration plan for details). The light stimulus was delivered by the Calibration Delivery System (CDS) through the Reflective Aberrated Simulator for Calibration (RAS/Cal). ACS and RAS/Cal were covered with lumalloy and Kapton blankets to minimize stray light from the clean room ceiling lights. Some activities did not require RAS/Cal - the configuration of the external stimulus was then tailored specifically for these items.
A diagram of the CDS is shown in Fig. 1. It directly feeds RAS/Cal, located at the ACS aperture, through a series of five flat and concave mirrors. The focal length of the first concave mirror in the light path is ~1/4 m and of the second, ~1/8 m. ACS is in diamond configuration in the Thermal Balance Fixture (TBF). A choice of HeNe laser, Quartz-Tungsten-Halogen (QTH) lamp, or deuterium (D2) lamp (< 3500 Ang) feed the monochromator. The entrance and exit slit widths are typically set for a bandpass of 10-20 Ang. The monochromator includes two gratings for ``blue'' (VIS : < 5100 Ang) and red (IR) wavelengths. The wavelength and the grating can be changed remotely by computer commanding from the control room. The light intensity can be adjusted with a suite of neutral density (ND) and narrow-band filters (see Table 1).
Table 1 : CDS Filter Bank
|3 (C)||BLOCK||122 (Lyα)||OPEN|
|5 (E)||546 (Hg)||VISND0.2||OPEN|
A more detailed diagram of RAS/Cal is shown in Fig. 2. Light from the external source, such as the CDS, passes through one of five apertures : 2 micron, 4 micron and 100 micron pinholes, an OPEN hole (~5 mm), or a "More OPEN" hole (~8 mm). In general, the 100 micron pinhole is selected to set up the light source and the measurements are then made with the 4 micron pinhole to produce a point source. The light beam is then reflected off a fixed asphere followed by a pupil mask + conic. The tip/tilt of the conic can be toggled by means of two levers to produce the correct astigmatism for the WFC or HRC/SBC field center points. In the vertical direction, the turning mirrors TM1 and TM2 are separated by ~8". TM1 is fixed but the tip, tilt, and angle of TM2 need to be set to their correct values for each selected field point. For WFC, a +TIP moves the source down and left and a +TILT moves it up and left. A movement of 0.02 degrees corresponds to a translation of ~30 pixels. For the HRC, the directions are opposite for both TIP and TILT. The aperture wheel and tip/tilt of TM2 are selected remotely from the control room.
Images of ACS, RAS/Cal, and the CDS were obtained on 13-14 Aug 2001 with the SSDIF Webcam. The RAS/Cal and CDS covers (maroon and silver, respectively) and the CDS filter bank are visible. Lumalloy blankets cover ACS and in the last image, RAS/Cal is wrapped in Kapton sheets.
There are nine (HRC) and ten (WFC) pre-defined field points for ACS, shown in Figs 3 and 4 and defined in Tables 2 and 3. Only the centre field points are properly corrected for astigmatism. To move to a specified field point, RAS/Cal is mounted on the RAS/Cal Multi-Point Alignment Tool (RAMP), and by computer control from inside the clean room, is translated along the X, Y, and Z (focus) axes. The nominal tip and tilt of TM2 for W0 are TIP=-0.0665, TILT=+0.0150 and for H1, TIP=-0.1510, TILT=+0.0880. There are two photodiodes and two photomultiplier tubes (PMTs) on a stage, and each can be moved remotely from the control room into the light path to measure fluxes. One of the PMTs was removed for TV1/STUFF calibration (for the first thermal vacuum campaign in 1999) and would be the PMT of choice, since its absolute calibration has already been performed. One of the photodiodes was calibrated in Apr-May 2001 down to 2000 Ang (each diode has a lens which doesn't appear to affect their lower cutoff wavelength). Below ~3000 Ang, the PMT tube can also serve for the absolute flux calibration. We note that in the spring/summer calibration campaign of 2001, only the calibrated photodiode was used.
There are three methods to adjust the focus of the RAS/Cal point source : (1) Internally with the ACS corrector mechanisms. This is the standard focusing method when RAS/Cal is at its nominal position in front of the ACS aperture. But if RAS/Cal is in a non-standard position (as in the thermal vacuum chamber), then the range of the correctors may not be sufficient to bring the point source into focus. (2) Physically move RAS/Cal along the Z-axis. This is done by moving RAS/Cal either towards or away from the instrument's aperture while keeping the X-Y axes constant, as defined with theodolyte measurements. (3) Move the pinhole wheel along the Z-axis. Its nominal position for ACS is -2 mm and it can move up to a maximum of +1 mm.