# Presentation: 505509

## The Gemini MCAO System

**Andy Foster**
**Observatory Sciences Ltd**

## Adaptive Optics - The Movie

## Classical AO Comparison

## The plague of Classical AO...Limited Field of View

**Wavefront sensors measure perturbations along line of sight to guide star. **
**These will differ along line of sight to the science target. **
**See images below taken 20 arcseconds apart.**

## The plague of Classical AO...Sky Coverage

**Need bright stars close to science target for AO.**
**Only 5% of the sky is accessible.**
**Use Laser Guide Stars (LGS) to get round this. Fluorescence of sodium atoms at 90/100km. **
**But not without problems. Cannot determine Tip/Tilt information from an LGS. This is where most of the phase correction comes from.**
**Finite height of LGS leads to “The Cone Effect”.**

## The plague of Classical AO...The Cone Effect

## The Solution - MCAO

- Telescope

- DM1

- DM2

- Turb. Layers

- #2

- #1

- Atmosphere

- WFS

**UP**

**Notes:**
MCAO can solve all these problems. This figure presents a sketch of an MCAO system. In this figure, two WFS’s are looking at two off-axis guide stars. Turbulence in the atmosphere occurs in layers and the two deformable mirrors are optically conjugated to two layers in the atmosphere. That is, each one looks at one layer. Each WFS measures the combined effect of the turbulence from both layers. But this will be different for each guide star due to the spatial shift or phase shift between the sources. From these two signals it is possible to calculate the corrections to be applied for each layer by solving an algebraic system of two equations. These corrections are then used to drive the deformable mirrors. This loop would typically be run at about 1 kHz. 

Note that we still need natural guide stars as well as laser guide stars to be able to determine tip and tilt measurements. However, because we have now extended the field of view (with the LGS’s), there is more chance of finding an NGS close to the field.

## MCAO & the Cone Effect

- 90 km

**Notes:**
Allows uniform compensation over a 1 square arcminute field of view or 30 times the size in area of classical AO.

## The MCAO Subsystems

**MCAO contains the following subsystems:**
**The Laser System (LS)**
**The Real-Time Controller (RTC)**
**The Safe Aircraft Localization and Satellite Acquisition System (SALSA)**
**The Beam Transfer Optics and Laser Launch Telescope Systems (BTO/LLT)**
**The Beam Diagnostics Sensor System (BDSS)**
**The Adaptive Optics Module (AOM)**

**Notes:**
The laser system includes all the components needed to produce and maintain 5 laser beams at the sodium D2 wavelength.
The control system for this is non-EPICS, running on a Windows PC, with a socket based interface using ASCII strings.
Coherent Technologies Inc. - “That at 14 W/beam, this is the most powerful solid-state sodium laser ever produced for astronomical adaptive optics”
The RTC is really the heart of the system. It is dedicated to the adaptive optics control loop; that is the analysis of both natural guide star and laser guide star data. The control system for this is non-EPICS and runs on a Windows PC with a 64-bit PCIbus interface to the hardware.
(tOSC – The Optical Sciences Company)
The SALSA system is not really a dance! It’s responsible for aircraft detection and satellite avoidance. It essentially consists of a bore-sighted camera and two all-sky cameras to look for aircraft. The control system is responsible for reading out the camera images, analysing them and then shutting down the laser via a signal to the interlock system if necessary. For satellite avoidance we have to rely on the US Space Command laser clearing house program. We ask them if we can fire the laser in a certain direction on a given night and they say “yes” or “no”!
The control systems for the subsystems shown in red have been designed and developed by OSL. They are all EPICS based.

## The BTO/LLT Subsystem

**Control System designed and developed by OSL. Commissioning this week!**
**The BTO/LLT  subsystem is responsible for taking the laser beams from the centre section of the telescope and transferring them up the telescope truss, across the secondary vanes and then launching them from behind the secondary mirror.**
**VME based**
**EPICS 3.13.9 with the Gemini extensions**
**vxWorks 5.4.2 (Tornado 2.0.2)**
**Two MVME-2700 PPC’s**

## Laser path

**A**

**Laser path**

**Five 14-W beams **
**Laser System location: **
**On center section (A)**
**On-axis LLT**

## BTO/LLT: Main Engineering Screen

**BTO/LLT: Main Engineering Screen**

**Notes:**
This is the top-level EDM display for the BTO. It shows the light path through the system. The laser is directed up the side of the telescope via the truss pointing and cerntering arrays. Each array contains 5 mirrors which can be controlled in tip and tilt. These would normally be controlled in a closed-loop with 1 second updates. Once the beam reaches the top-end it is directed to the FSA, a further array of 5 mirrors. The FSA is used to take out:
 pointing jitter in the laser
 motion of the LLT caused by windshake
 movement of the laser stars due to the turbulence not being the same on the upward path from the 45cm LLT to the sodium layer as it is on the downward path to the 8m telescope
Mainly because of this last point, the FSA has to have it’s loop closed at 800 Hz. One of the MVM2700’s is dedicated to the control of the FSA. After the FSA, the laser light passes through a beamsplitter where part of the beam is directed to the BDSS cameras. This is a separate control system which we’ll talk about later. The remainder of the beam is directed to the optical bench pointing and centering mirrors before going to the laser launch telescope itself. One extra thing of note on the EDM screen is this home developed widget (so we have used the “extensible” quality of EDM).  This indicates the direction of polarization of the laser.

## BTO/LLT VME architecture

**MVME 2700 – CPU board**

**OMS58-8S motor controller**

**Control LAN**

**Time bus**

**To servo motor **
**electronics**

**To switches, **
**relays, DC **
**motors**

**To TT **
**mirrors**

**Bancom 635 time board**

**MVME 2700 – CPU board**

**XYCOM 9660 + 4 XIP **

**VMIVME5588  RM board**

**Synchro bus**

**From **
**sensors**

**OMS58-8S motor controller**

**OMS58-8S motor controller**

**OMS58-4S motor controller**

**MCAO**
**BTO/LLT**
** ****VME**

**BDSS**

**Notes:**
Here we see the VME crate used to control the BTO/LLT. We have 2 CPU’s, one runs EPICS and is used to control all components of the system apart from the FSA. The second CPU controls the FSA by receiving interrupts from the Synchro Bus at approximately 800Hz. On interrupt the data is read from the bus and applied to the piezo-electric actuators on the back of the FSA. These data are also smoothed and integrated to 1 second and then applied to the optical bench pointing and centering mirrors to take out gross effects. Thus the FSA is kept within it’s dynamic range of movement. The Synchro bus data originate in the Real-Time Contoller subsystem as a result of the analysis of the natural and laser guide star signals.

The two CPU’s communicate using VxMP, Wind River’s shared memory product. This allows you to set up shared message queues and semaphores etc. Commands need to be passed from the EPICS CPU to the one controlling the FSA and we need to be able to read back status from the FSA CPU. The EPICS CPU is the master in the system.

We use the OMS58 motion controller servo card to control movement in Tip and Tilt for the truss pointing and centering array and for the optical bench pointing and centering mirrors.

The Xycom-9660 carrier board is populated with 4 XIP modules which I’ll discuss on the next slide.

## BTO/LLT EPICS Additions

**EPICS driver for Xycom-9660 carrier board (following Andrew Johnson’s “drvIpac” approach)**
**Device & driver support for XIP-2440 digital input module**
**bi, mbbi, mbbiDirect records, interrupt support**
**Device & driver support for XIP-2445 digital output module**
**bo, mbbo, mbboDirect records**
**Device & driver support for XIP-5320 analog input module**
**ai, waveform records**
**supports on-board calibration through a vxWorks task every 30 minutes**
**Device & driver support for XIP-5230 analog output module**
**ao record**
**Drivers can be compiled without EPICS**
**ao driver is used to drive piezo-electric actuators at up to 1 kHz for FSA control**

**Notes:**
One of the things we had to do especially for this system was write the driver support for the Xycom-9660 carrier board. This was done following Andrew Johnson’s “drvIpac” approach.
We also wrote device and driver support for 4 XIP modules. 
The XIP-2440 DI module. This is used to read limit switches and therefore the location of shutters in the system. 
The XIP-2445 DO module is used to send commands to move shutters, switch on amplifiers and move the Beam Dump Mirror in and out of the beam.
The XIP-5320 AI module is used to read various temperature sensors in the system.
The XIP-5230 AO module driver is used from the second CPU to drive the piezo-electric actuators for the FSA mirrors.  

## BTO/LLT EPICS Additions

**Velocity and acceleration of tip and tilt axis motors must provide ****smooth, linear travel**** between mirror positions**
**Multi-axis linear “coordinated” moves**** are supported by OMS 58 motion controller**
**As far as we could tell, this feature is not supported by any current EPICS records, so...**
**Coordinated Motion Record developed by Gemini to handle multiple axis moves**

## The Beam Diagnostics Sensor System

**The role of the BDSS is to provide feedback to the truss pointing and centering arrays to maintain alignment of the laser beams and to keep the FSA within it’s dynamic range.**
**Uses two commercial (Pixelfly) CCD cameras to image the near and far fields of the laser**
**Measures centroids of spots and calculates offsets from pre-determined positions**
**Corrections applied to truss pointing and centering mirrors by BTO**
**Provides measurements of the beam quality of the laser at the top-end**
**“****times diffraction limit” (1 for a Gaussian beam)**

## The Beam Diagnostic Sensor System

**Runs on a Linux (Red Hat 9.0) target**
**Running EPICS 3.14.5 with Gemini extensions**
**Use ESO’s Skycat tool to display images**
**Interface to cameras is through PCIbus, came with Linux driver**
**Wrote a SCAM (Simulated Camera) driver which manipulates memory buffers to test without cameras**
**Using a 2.66 GHz Pentium 4 (512 MB RAM), can read out both cameras (1390 x 1024 pixels) at 8.6 Hz.**

## The Beam Diagnostics Sensor System – Software Design

**“****epicsThreadCreate” called to create tasks which analyse and manipulate the image data. This is done through a “genSub” INIT routine at iocInit.**
**Interface between these tasks and EPICS records which hold the calculated offset values is through a library which accesses a protected memory area.**
**Standard EPICS records used to control functions of the cameras such as:**
**Binning**
**Exposure time**
**Reading the temperature**
**Commands are sent to the BDSS through standard Gemini CAD records.**
**Being able to use EPICS under Linux has allowed us to fully integrate this system with the rest of the Gemini Control System. This is very positive and we have seen no major problems using a Linux target.**

## BDSS: Main Engineering Screen

**BDSS: Main Engineering Screen**

**Notes:**
This is the top-level BDSS EDM screen. The top-left of the screen indicates the current health of the system and the heartbeat. If the health changes to WARNING or BAD then a message explaining the reason for the problem is displayed underneath.

The picture of the laser spots allows the user to choose which of the beams will be centroided.

The controls in the middle section of the screen allow you to quickly alter the exposure time, frame count, binning mode of both cameras and the current mode of operation. 
You can start and end an observation and write FITS (Flexible Image Transport System) files of the current exposure from this screen.

The 3 modes of operation are:
Propagation –  Images the near and far fields and determines centroids, during normal operation.
Alignment    - This is used at the beginning of the night to align the footprints of the laser beams on the top-end ring mirror.
Calibration   - This is used to set the pre-defined reference positions for the laser spots in the near and far fields. In this mode imaging of natural stars can also be used to do the calibration.

## BDSS: Simulated Far Field and Phase Retrieval Plots

**BDSS: Simulated Far Field and Phase Retrieval Plots**

- Skycat Display of Simulated Far Field

- Phase Retrieval Plots

**Notes:**
This shows a Skycat image of the far field of the laser.

The picture on the right shows plots of the intensity in the near and far fields. From these intensity plots we can determine the phase map of the laser beam (Gerchberg-Saxton algorithm) and from this we can determine a quantity called the “times-diffraction-limit” which is a measure of the quality of the laser beam – that is how close it is to a Gaussian Beam.

I’ll be giving a demo of the BDSS software running on my laptop in simulation mode. I’ll show these screens and describe the operation of the system in more detail then if anyone is interested.

## Adaptive Optics Module

**Notes:**
This is a schematic showing the components of the AOM and the light path through them.

The light from the telescope enters the system at the top RHS and is directed onto the 3 deformable mirrors. These are the mirrors which are conjugated to different heights in the atmosphere. The beam then reflects off a tip-tilt mirror and onto a beamsplitter. At this point, part of the beam is directed down to another beamsplitter where the laser light is split from the natural star light, probably by use of a filter at the sodium wavelength. The laser light is directed onto the LGS WFS’s while the natural star light is directed to an array of Avalanche Photo Diodes. The APD’s are used to obtain Tip/Tilt information while the LGS WFS’s are used to obtain higher order corrections for abberations such as coma, astigmatism, trefoil, etc.

The remaining star light, not picked off by the first beamsplitter carries on into the telescope instrument being used for the observation.

## AOM: Main Engineering Screen