BATC : RAS/HOMS

Following the installation of the flight build detectors in late 2000, a series of calibration activities were conducted in clean room 5 of the Fisher testing facility at BATC. The Refractive Aberrated Simulator/Hubble Opto-Mechanical Simulator (RAS/HOMS) (Sullivan et al. 1993, SPIE Vol. 1996, 237), an HST simulator capable of delivering OTA-like external monochromatic point-source and broad-band full-field illumination above its refractive cutoff wavelength of ~3500 Ang, was used to obtain a final set of calibration data. The main RAS/HOMS calibration campaign was conducted on Feb 20-Mar 3, 2001 and the activities from this campaign are described in detail in the RAS/HOMS Calibration Plan. Also, BATC SER OAT-074 "ACS RAS/HOMS Calibration Test Plan with the Flight Detectors", based on these Web pages, includes some diagrams of RAS/HOMS in different test configurations. A reduced RAS/HOMS campaign was also conducted in Oct-Nov 2001, primarily to verify the alignment of the chief ray before shipping to the Kennedy Space Center (KSC). In RAS/HOMS, ACS was fully integrated in square configuration in ambient environment and the lab chillers were attached to the ASCS I/F plate. The clean room was kept light-tight by covering all the windows with black tarps.

The external light stimuli fed RAS in various configurations over the FOVs of the HRC and WFC detectors. Broad-band and monochromatic illuminations were provided by :

1. the standard HeNe point source illumination (6328 Ang)
2. a 250 Watts Oriel Quartz-Tungsten Halogen (QTH) lamp coupled to a tunable monochromator. Wavelengths from the near-UV to the near-IR (3500-10000 Ang) will be accessible and bandwidths of 20 or 40 Ang will be typically specified. Grating 2 isolates wavelengths below 5100 Ang and grating 3, longer wavelengths. A general monochromator calibration (slit, wavelength, resolution, gratings) was performed in Dec 2000. The wavelengths are selected remotely from a computer terminal in the control room.
3. a 150 W projector lamp for "white light" or broad-band illumination. We note that the QTH lamp could serve as the white light source but this would necessitate disassembling the monochromator/lamp configuration to couple the entrance end of the fiber cable to the QTH lamp. It is much more efficient to simply move the fiber cable between the 150 W and 250 W lamps, located next to each other on the bench.
4. a suite of emission-line lamps (Ar, Hg, Ne). For wavelength calibration of the monochromator, the lamps are placed immediately at the entrance slit of the monochromator.

Seven optical elements were placed interchangeably in the light path, either at the RAS focal plane or at the pupil plane, for specific calibration items :

1. the RAS source plate which consists of a metal plate with nine holes drilled into it and to which are coupled nine individual fibers, each of which is illuminated by an HeNe laser. A microscope objective focusses the laser beam onto the optical fiber tip. The light intensity through each fiber can be adjusted independentally. The location and label of the nine fibers on the WFC are shown in this figure. Only one fiber spot (from a separate fiber A10) falls in the HRC FOV. This configuration is used to image PSFs across the WFC and HRC fields at the standard 6328 Ang wavelength only. The plate, and hence the PSF spots, can be translated manually along the X and Y axes with a micrometer stage. The micrometers are read in inches and the plate correlations are about 1.15x10^-4 inch/pix on HRC and 2.14x10^-4 inch/pix on WFC. Only the HeNe lasers can be fed through the nine fibers, while the output from the monochromator and line lamps is fed through a single 1/4" fiber bundle which can be fit through a plug at the centre of the RAS plate.
2. a Mylar transmissive diffuser at the location of the pupil mask for flat-field illumination, either with the direct laser light (6328 Ang), the QTH + Monochromator + fiber bundle, or the 150 Watt lamp + bundle.
3. a Spectralon reflective diffuser placed slightly out of focus in front of the RAS source plate (on the HOMS side). It resides on a holder that rests on the source plate stage. The light source, such as the fiber bundle, is then placed in a holder or clamp, and installed off-axis in front of the RAS plate and directed towards the diffuser.
4. a Ronchi ruling with a transmissive diffuser (such as Mylar) in place of the RAS source plate for measurements of the geometric distortion. The micrometer stage is used to translate the Ronchi ruling.
5. an inverse field mask with a transmissive diffuser at the RAS focal plane for stray light evaluation.
6. a pinhole array with a collimating lens at the RAS focal plane for measuring the dispersion of grism G800L.
7. a set of neutral density filters (ND 1.5, 2, 3) which are inserted in front of the laser, before the microscope objective.

When the Ronchi ruling, the inverse field mask, or the pinhole array are put into place, the RAS plate is removed or flipped out of position to maximize the light throughput.