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Canopus is the Adaptive Optics bench of GeMS . Below we described the characteristics of the main component of Canopus.
Canopus is equipped with a set of calibration sources. These sources are used to perform all the day-time calibrations, and to exercise the system in a control environment. The set of calibration sources:
GeMS uses 3 Deformable Mirrors (DMs) conjugated respectively at 0km, 4.5km and 9km. The characteristics of the DMs are summarized in the table below:
These DMs are based on a piezo-stack technology (developed by CILAS) the DM electronic has been made by XX
The Tip/Tilt Mirror Assembly is composed of a custom Tip/Tilt stage, a Silicon Carbide lightweighted mirror, a kinematic mounting interface for the Tip/Tilt Stage, and the assembly aluminum mount. Our TT mirror is located just downstream from DM0, and has a stroke of +/- 1.4 arcsec, with a bandwidth of 400Hz. It's a Physik Instrument device.
Science Beam Splitter Assembly
The science dichroic let the NIR through to the science instrument and reflects everything downward of 1 micron into the WFSs environment. The mechanism has 2 positions: the second dichroic has a cutoff at 850nm, to allow science at z band.
Science Atmospheric Dispersion Compensator
At large zenith angle, a compensation for atmospheric refraction within the science instrument bandpass (for broadband filters) will be needed. This can be done via the science Atmospheric Dispersion Compensator (ADC). The Science ADC can be IN or OUT the optical path. If it is not really required, it is recommended to leave it OUT, as it affects the transmission.
Canopus modifies the entrance f/16 beam to a f/32.5 beam
It is a dichroic that reflects the 589nm light into the LGSWFS and let the rest of the visible light through into the TTWFS.
Laser notch filter
Canopus is using five Laser Guide Star Wave-Front Sensors (LGSWFS) each one looking at one LGS. Characteristics of these LGSWFS are described in the table below:
Those are EEV39 80x80. They are controlled by a SDSU controller (San Diego State University). Each subapertures use quadcell, with 1.38arcsec/pixels (thus 2.76arcsec field of view).
LGS WFS CCDs
Because the LGS range varies from 90km (at zenith) to 180km (at zenith angle 60 degrees), plus the natural variation of the sodium layer altitude, we need the LGS WFS to accomodate this variations. This is taken care of by the LGS WFS zoom. There are many active elements in the LGS WFS which job is to:
LGS WFS Zoom
This device includes 3 TTWFS, each one a quadcell detector using Avalanche Photodiodes (APDs). Each one can be moved into position using X and Y linear stages. To facilitate dithering and insure the relative positioning of each probe w.r.t the other ones, probe #1 and #2 are actually located on top of probe #3. Thus, to dither, we need to move only probe #3 (#1 and #2 will move with it). Probe 3 can span the whole 2 arcmin field. Probe 1 is limited to the top-part, and probe 2 to the bottom part. See this page for all the details about probe positioning, and observing strategy.
Each of the TT WFS is in fact a quadcell. It is made of a 4 faces-pyramid, which apex is located in the infinity focal plane. Each face reflects the light into a small doublet, that focuses it onto a fiber. The light is transported through the fiber to the APD itself, located outside of the optical table. There are 3 TTWFS, each with 4 APDs, thus a total of 12 APDs. The APD does photon counting, and passes the counts to the RTC, at an adjustable rate (multiple of the LGS WFS sampling period, i.e. 800, 400, 200Hz if the LGS WFS works at 800Hz).
Because the Sodium layer altitude can vary slowly across a night, the science image would slowly drift out of focus, if only the LGS WFS focus information were used. To prevent such a drift, we need to sense the focus error on a nearby guide star. We have choosen to split the light from the TT probe#3 (about 70% for the TT WFS and 30% for the Slow Focus Sensor). The SFS provides a focus error every few seconds, which is sent to the LGS WFS zoom mechanism. Here is how the thing works:
Slow Focus Sensor (SFS)