Components

Updated April 4, 2024, by iromero

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GeMS consist of three main subsystems:

the Laser,
the Beam Transfer Optics (BTO), and
the AO bench Canopus.

All of these subsystems are linked together by loops and offloads. GeMS can feed two dedicated instruments: GSAOI and Flamingos2.

Gemini South Laser

GS Laser History:

The Gemini South (GS) Observatory acquired a 50W sodium Guide Star Laser System back in March 2010, as one of the key components of the Gemini Multi-Conjugate Adaptive Optics System (GeMS) project. After a successful post-delivery acceptance in the laboratory, the system was installed on the elevation platform of the telescope.

March 2010: The Gemini South Telescope receives its new 50W sodium laser

Following an intensive period of optimization, the Gemini South Laser Guide Star Facility (LGSF) delivered its first light on the sky in January 2011. Over the few months following the event, and while the LGSF was being commissioned, the GS laser team was able to gather considerable data concerning its laser system, and therefore ways to work on improving both its overall performance and reliability.

Picture of the Gemini South Laser team

GS Laser Design:

Chart of the Gemini Shouth Laser Layout

The system architecture is based on single pass Sum Frequency Mixing (SFM) of high power outputs of two Nd:YAG MOPAs (master oscillator-power amplifier) operating at 1064 nm and 1319 nm.

The 589 nm output is generated via SFM of 1064 nm and 1319 nm Nd:YAG lasers in a Lithium Triborate (LBO) nonlinear crystal.

The absolute wavelength is controlled via the Wavelength Locker Module.

Power and beam quality are monitored in real-time with a set of diagnostic tools (Shack Hartmann Wavefront Sensor, photo-detectors).

Description: GS Laser Bench.jpg

GS Laser performance:

Gemini South’s (GS) 1319nm & 1064nm MOPAs nominally produce average powers of 75 W and 110 W, respectively, with 250-300 pico second pulse widths.

Table 1: MOPAs performance

MOPA System	Description	Performance
1064nm	Average power (W)	110
	Beam Quality (% PIB)	80
	Polarization Contrast	50:1
1319nm	Average power (W)	75
	Beam Quality (% PIB)	80
	Polarization Contrast	50:1

The table below contains the system requirements and demonstrated results. The system displays good day to day repeatability after extended 10 hour warm-up periods. However, it does require monthly alignment maintenance to compensate for beam pointing walk off. The mostly likely cause is thermal hysteresis of MO’s opto-mechanical mounts.

Table 2 : GS Laser - 589 nm requirements and demonstrated performance

Description	Units	Requirement	GS
Average Power	W	50	50
Power Stability - short term Peak to Peak	%	10	16.3
Power Stability - long term RMS	%	5	6.0
M² - X	-	1.4	1.4
M² - Y	-	1.4	1.2
Frequency Stability (+/-)	MHz	100	100
Spectral Bandwidth	GHz	2	2.1
Pointing Stability - Transverse - X	mm	0.5	0.006
Pointing Stability - Transverse - Y	mm	0.5	0.006
Point Stability - Angular -X	urad	400	30
Pointing Stability - Angular - Y	urad	400	14
Polarization Contrast Ratio	-	20:1	50:1

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:

Schematic view of the full Beam Transfer Optics on the telescope

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).

BTO's picture with a line showing the BTO Optical Path

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.

Pciture of the BTO Optical Bench labeling its components

Fast Steering Array (FSA)

The Fast Steering Array (FSA) is a set of 5 Tip/Tilt platforms 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.).

Picture of the Fast Steering Array (FSA)

X Shaping Array (XSA)

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

Picture of the X Shaping Array (XSA)

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 the 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.

Picture of the Pointing and Centering Mirrors (PM & CM)

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 of 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 it to optimize the Laser spot size on the sky easily.

Below it's an image of the LLT where it has been installed on the telescope.

Picture of the Laser Launch Telescope (LLT)

Laser Constellation

The 5 beams are sent 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 a few arcseconds in the sky by using a combination of five tip-tilt platforms: the Fast Steering Array (FSA)

Diagram of the Laser Constellation

CANOPUS

Canopus is the Adaptive Optics bench of GeMS . Below we described the characteristics of the main component of Canopus.

Calibration Sources

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 controlled environment. The set of calibration sources are:

5 Laser Guide Sources: These are 0.8" sources (500um in the f/16 entrance focal plane). They can be moved following the optical axis to simulate sources from 90km to 160km.
6 Natural Guide Sources: These are 0.6" sources(300um in the f/16 entrance focal plane). They are conjugated to infinite, and they are used to simulate NGS seen through typical seeing conditions, and also for the calibrations of the NGSWFS.
24 Natural Guide Sources: These are diffraction limited (10um in the f/16 entrance focal plane). They are conjugated to infinite and they are used for Non Common Path Abberation (NCPA) calibrations and performance evaluation.

Deformable Mirrors

GeMS uses three Deformable Mirrors (DMs) conjugated respectively at 0km, 4.5km and 9km. The characteristics of the DMs are summarized in the table below:

	DM0	DM4.5	DM9
Altitude (km)	0	4.5	9
Total actuators	293	416	208
Valid actuators	240	324	120
Slave actuators	53	92	88
Actuator coupling	33%	33%	33%
Pitch	5mm	5mm	10mm
Magnification	7.9m -> 80mm
Layout
Layout notes	In the layout plots above, black crosses, red numbers show valid actuators. Blue crosses, blue numbers show slave actuators. The dotted lines mark the various beams (for WFS 0 through 4). For the altitude DMs layout (DM4.5 and 9), the solid line marks the outer edge of the WFS beams, that is, it encloses the sensed part of the DM (dotted and solid lines computed for a LGS altitude of 90km, i.e. at zenith).

These DMs are based on a piezo-stack technology (developed by CILAS). The DM electronics was made by Cambridge Innovations.

Tip-Tilt Mirror

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 of the optical path. If it is not really required, it is recommended to leave it OUT, as it affects the transmission.

Output Focal

Canopus modifies the entrance f/16 beam to a f/32.5 beam.

Laser notch filter

It is a dichroic that reflects the 589nm light into the LGSWFS and let the rest of the visible light through into the TTWFS.

LGSWFS

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:

Chart showing the characteristics of the Laser Guide Star Wave-Front Sensors (LGSWFS)

LGS WFS CCDs

These are EEV39 80x80. They are controlled by a SDSU controller (San Diego State University). Each sub-aperture uses a quadcell, with 1.38arcsec/pixels (thus 2.76arcsec field of view).

LGS WFS Zoom

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 must:

Compensate for the range induced defocus; and while doing so
Keep the wavefront aberrations small; and
Keep the deformable mirror / lenslet registration error small.

This is done by a combination of Look Up Tables (LUT) and active loop (see the SFS loop).

NGSWFS

This device includes three 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.

NGS ADC

The Atmospheric refraction across the visible (400-1000nm) is quite large. Thus the star images formed on the apex of the TTWFS pyramid (quadcell) will be somewhat elongated when working at large zenith angles. The use of an ADC will compensate for this refraction-induced elongation and will increase the SNR on the TT WFS.

TTWFS (Quadcells)

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 three TTWFS, each with four 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).

Slow Focus Sensor (SFS)

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 is 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 a summary of its functions:

Let's assume we start with everything in focus.
The Sodium layer drifts by 200 meters up.
This is seen by the LGS WFS.
The system (DMs) compensate immediately for this defocus (normal close loop, using default control matrix).
The NGS seen by the SFS will go out of focus (because this only corresponds to a sodium layer characteristics, and not a true defocus in the system, the NGS did not undergo any real defocus. However, it sees the defocus induced by the DM).
The SFS will report a defocus.
From this defocus, the system will drive the Zoom corrector.
Because of the zoom corrector motion, the LGS WFS will see a defocus and will correct it.
Assuming a proper calibration and sign, this will eventually converge and both the SFS and LGS WFS will be happy and with zero average focus.

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