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Calibration Wavefront Sensor |
The Calibration Wavefront Sensor
(CWFS) is used to tune SOAR's Adaptive Primary and Secondary
mirrors in order to achieve the best possible images. It was
built for SOAR by Adaptive
Optics Associates of Cambridge, Massachusetts and is based
on their Wavescope
laboratory measurement device. The CWFS is mounted at the center
(90 degree) folded Cassegrain port on the elevation ring.
The CWFS is a Shack-Hartman type wavefront sensor in which a microlens array is used to divide the telescope pupil into approximately 530 sub apertures (thus over sampling the spacing of the primary mirror actuators by slightly more than a factor of two). Each microlens produces a spot, the position of which is recorded using an Apogee AP6E Thermo-electrically cooled CCD camera. By comparing the position of the spots from a star, with those from a calibration source that projects a perfect spherical wavefront, it is possible to measure the slope of the wavefront across each sub aperture, and thus reconstruct the shape of the wavefront. The CWFS is capable of obtaining accurate wavefront measurement using stars as faint as R=12 in a 60 second exposure.
The control software for the SOAR Active Optics System (AOS) uses the CWFS measurements to compute the force required on each of the 120 Primary mirror actuators, and the position of the Secondary mirror, needed to correct the wavefront to have the ideal form. In normal operation the AOS is controlled via a lookup table containing the optimum parameters for the Primary and Secondary as a function of Zenith distance, and Temperature. The CWFS is used to populate this table by taking measurements of stars at various positions over the sky during engineering time. However, when the very best possible images are required it is also possible to tweak the parameters of the AOS by using the CWFS to measure a star near the science target, at a cost of about 10 min overhead.
The user's manual for the CWFS, and the related AOS software is in preparation.
Acquisition Camera
The CWFS also includes an acquisition camera consisting of a focal reducer with a magnification of 0.62, a filter wheel with space for 10 filters, and a second Apogee AP6E CCD camera, giving a field of view of 2 x 2 arcminutes at a scale of 0.12 arcsec/pixel. This camera is primarily used to acquire targets for measurement with the CWFS. However, during commissioning and early operation of the telescope it may also be useful for target acquisition for the science instruments.
Note: there is no instrument rotator at the position of the CWFS, thus the orientation of the Acquisition Camera rotates relative to the equatorial coordinate system (and is fixed relative to Az-El) as the telescope moves around the sky.
Key parameters of the Acquisition Camera are:
| Camera & Detector | Apogee AP6E with Kodak KAF1001E 1024x1024 24 micron pixel CCD | |||||||||
| Scale | 0.12 arcsec/pixel (5.0 arcsec/mm) | |||||||||
| FOV | 2.05 x 2.05 arcmin | |||||||||
| Filter | Open | V | R | I | SDSS g' |
SDSS r' |
SDSS i' |
(7nm) |
ND1.0 (T=10%) |
ND10.0 (T=1%) |
| Count Rate (R=10) | TBC | TBC | TBC | TBC | TBC | TBC | TBC | TBC | TBC | TBC |
| Gain | 7.3 e-/ADU | |||||||||
| RON | 12.5 e- RMS | |||||||||
| Dark | 1.27 e-/pixel/second @ -25.6 C | |||||||||
| Full Well Capacity | ~116k e- (16384 ADU) | |||||||||
The user's manual for the Acquisition Camera is in preparation.