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BTO

Beam Transfer Optics

The Laser Guide Star Facility (LGSF) provides the telescope Adaptive Optics (AO) facility with a source of coherent light for the optical excitation of the mesospheric sodium layer to enable the production of an artificial beacon source or “guide star”. The LGSF subsystems are: the Laser System, Beam Transfer Optics (BTO), Laser Launch Telescope (LLT), and Safety Systems.
The Beam Transfer Optics (BTO) subsystem of the LGSF relays the laser beam(s) from the output of the Laser System to the input of the Laser Launch Telescope (LLT). The BTO include multiple mirrors, lenses, and beam splitters, as well as various sensors and diagnostic equipment. The part of the BTO that relays the laser beam from the Laser System to the top-end ring of the telescope is called the BTO Optical Path (BTOOP), while the part of the BTO that is located behind the LLT is called the BTO Optical Bench (BTOOB).
Below is a schematic view of the full BTO on the telescope, and a drawing of the optical elements of BTOOP and BTOOB:




Beside relaying the laser light from the laser to the LLT, other BTO functionalities include slow and fast compensation of telescope flexures and laser beam jitter, laser beam quality monitoring, beam shuttering, and laser polarization control. Beam alignment inside the BTO can be monitored at any time using a total of six video cameras imaging the EFM, TPM, TCM, TRM, FSA and LLT primary mirror assemblies. An example of such an image is shown below:

BTOOP

The laser exits the laser service enclosure as a single beam. It first travel through the “laser output box”, where the safety shutter and polarizing optics are located. Next, the beams travel through the BTO “torque tube” heading straight to the Elevation Fold Mirror (EFM) that redirects it to the Truss Pointing Mirror (TPM). TPM sends the beam up along the telescope truss to the Truss Centering Mirror (TCM) where it is redirected to the Truss Fold Mirror (TFM). After reflection on TFM the laser beam pass through three relay lenses used to image the output of the laser system onto the LLT entrance pupil. It then reaches the Top-end Ring Array (TRA), which is a combination of 4 beams splitter and a mirror, that actually divide the single 50W beam, into the 5 x 10W beams. At that points, the 5 beams are vertically aligned, and they are crossing the primary mirror of the telescope behind the spider, in a laser vane duct up to the BTO Optical Bench (BTOOB).

The combination of EFM, TPM, TCM and TRA provides open-loop compensation of the telescope flexures via an elevation-based look-up table (LUT) to maintain alignment of each laser beam relative to the narrow, 12mm-wide laser vane duct. The split of the single 50W beam into the 5 x 10W beams is done by TRA.


Image of the TRA

The 5 laser beams before entering in the laser vane duct



BTOOB

Inside the BTOOB, the five beams are received by the Fast Steering Array (FSA) which steers each of them independently to the X-Shaping Array (XSA) where the final five-star X-shaped laser constellation is formed. Finally the beams are reflected off the Centering Mirror (CM), pass through the BTOOB K-Mirror (KM), and are eventually reflected off the Pointing Mirror (PM) into the Laser Launch Telescope (LLT) for projection to the sky. A mirror can be inserted in the laser path between KM and PM so as to divert the laser light onto a power meter, thus enabling laser propagation through the entire BTO (minus PM) without projection to the sky. The entire BTO path is enclosed in tubes for safety reasons mainly, and the path is flushed with clean air to minimize dust deposition on BTO optics and prevent coating damage.

Fast Steering Array (FSA)

The Fast Steering Array (FSA) is a set of 5 Tip/Tilt plateforms that are used to keep each of the LGS well centered in-front of each LGSWFS. They are used in closed loop, based on the residual Tip/Tilt measured by each WFS, and at a rate of 100Hz. They compensate the uplink atmospheric Tip/Tilt and prevent the LGS to lie outside of the LGSWFS field stop, which is 3arcsec side. The averaged Tip/Tilt position of the FSA is offloaded to PM/CM (every 5s.) , and the averaged rotation is offloaded to the K-Mirror (every 10s.).

X Shaping Array (XSA)

X-Shaping Array (XSA), as its name says, is where the final five-star X-shaped laser constellation is formed.

Pointing and Centering Mirrors (PM & CM)

The Pointing Mirror (PM) and Centering Mirror (CM) are used to (respectively) aligned the constellation on the sky, and center the beams on the LLT. While telescope is tracking, we are using a LUT for PM/CM that keeps both the constellation and the location of the beams on the LLT well centered versus elevation and flexure. A second LUT controls the position of PM/CM vs. the K-Mirror position.

K Mirror (KM)

The K-mirror is located between the Pointing Mirror (PM) and Centering Mirror (CM) in the BTO - Optical Bench (BTOOB). Its role is to counter rotate the constellation while the Cassegrain Rotator of the telescope is tracking.

Laser Launch Telescope (LLT)

The GS Laser Launch Telescope (LLT) is a 450mm diameter aperture projector located in the shadow of the 1.0-meter diameter secondary mirror of the Gemini 8-meter telescope in order to provide on-axis launch of the five LGS constellation for MCAO operations. With an unvignettted field of view of +/-1.2arcmin and a 60:1 magnification ratio, the LLT enlarges the five, 5mm diameter gaussian laser beams overlapping on its entrance pupil, to a 300mm diameter footprint beam (these are diameters at the 1/e2 intensity points) on its 450mm diameter primary mirror. The LLT design consists in an unobstructed, afocal telescope using a diverging lens assembly followed by a fold mirror to expand the optical beam then direct it down towards an off-axis parabola (OAP) that provides collimated projection onto the sky. The opto-mechanical design includes a passive athermal focus mechanism that enables focusing. The LLT focus can be controlled remotely, which allows to optimize the Laser spot size on the sky easily.
Below it's an image of the LLT when it has been installed on the telescope.

Laser Constellation

The 5 beams are send in a constellation with 4 LGS on the corner of a 60 arcsec square, plus one in the center. This constellation rotates on the sky to compensate for field rotation using the BTO K mirror (see K-mirror). The position of each LGS can be adjusted by few arcseconds in the sky by using a combination of five tip-tilt platforms: the Fast Steering Array (FSA)