Tmt night

Adaptive Optics

Overview

Diffraction limited in J, H and K bands: 187nm RMS on axis; 208nm RMS on 34" x 34" in median seeing

Sky coverage: 50% at the galactic pole

Astrometry: 50µarcsec over 30" in H band for a 100 sec exposure

Photometry: 2% over 30" at @1µm for a 10 minute exposure

Optical throughput: 85% over 0.8-2.5µm with goal of 90% over 0.6-2.5µm

Background: 15% of ambient sky and telescope

Output ports: 3 ports, f/15 with a 2' FoV
 

 
AO Design

Multi-conjugate AO with 2 deformable mirrors.

Natural guide stars in NIR to sense tip/tilt/focus.

6 Laser guide stars and tomographic reconstruction.

High spatial (60x60) and temporal (800Hz) sampling.

Cooled (-30C) system with minimal optical surfaces.

 
Documents and Tools

Documentation

 
Contacts

AO group leader: Corinne Boyer

NFIRAOS project manager: David Anderson

NFIRAOS

NFIRAOS container with IRMS and IRIS instruments attached

Narrow Field InfraRed Adaptive Optics System (NFIRAOS)

The first light Adaptive Optics (AO) architecture for the TMT has been defined to provide near-diffraction-limited wavefront quality and high sky coverage in the near infra-red (IR) for the first TMT science instruments IRIS  and IRMS. Design, fabrication and prototyping activities of the TMT first light AO systems and their components have significantly ramped up in Canada, China, France, and in the US. NFIRAOS is an order 60 x 60 laser guide star (LGS) multi-conjugate AO (MCAO) system, which provides uniform, diffraction-limited performance in the J, H, and K bands over 34 x 34 arc sec fields with 50 per cent sky coverage at the galactic pole, as required to support the TMT science cases.  

The first light AO architecture for TMT consists of the following major systems: 

  • NFIRAOS, which is located on the TMT Nasmyth platform and relays light from the telescope to 3 science instrument ports after sensing and correcting for wavefront aberrations introduced by atmospheric turbulence and the observatory itself. NFIRAOS includes two DMs conjugated at 0km (63x63) and at 11.8km (76x76) with the DM conjugated to the ground mounted on a tip/tilt stage to reduce the number of optical surfaces. It also includes six 60x60 LGS WFS (one on-axis, and five in a pentagon with a radius of 35 arcsec), a 60x60 NGS WFS for operation without laser and operates at 800Hz.
  • The LGSF generates multiple LGS in the mesospheric sodium layer with the brightness, beam quality, and asterism geometry required by both the first light AO system (NFIRAOS) and later the second generation of TMT AO systems. It includes: i) the lasers, which are attached to the inside of the –X elevation journal facing the TMT primary mirror, ii) the beam transfer optics optical path, which transports up to 9 laser beams in a square pattern along the telescope elevation structure to the telescope top end, iii) the LGSF top end, which formats and launches the laser asterisms (up to 4 different asterisms) to the sky from the laser launch telescope, and iv) the laser safety system.
  • The On-Instrument wavefront sensors (OIWFS) of the NFIRAOS instruments dedicated for tip/tilt/focus sensing in the near IR (IRIS provides three OIWFS and IRMS only one), and four On-Detector Guide Windows in IRIS serving as truth tip/tilt sensors.
  • The Adaptive Optics Sequencer of the AO Executive Software, which automatically coordinates the operations of NFIRAOS, the OIWFS/ODGW and the LGSF with the remainder of the observatory for safe and efficient observations.
NFIRAOS Layout

NFIRAOS optical layout

Observation modes

The TMT facility AO system, NFIRAOS, enables NGSAO, LGS MCAO and Seeing-Limited observing mode. Each of the NFIRAOS science instruments will provide up to three low order on‑instrument wavefront sensors (OIWFS) and (for IRIS only) multiple on-detector guide windows (ODGW) to provide high precision tip/tilt, and (only in the case of 2x2 mode) focus and astigmatism measurements. When running at high speed, the OIWFS or ODGW measurements are used for tip/tilt, focus and plate scale control. When running at low speed, they can compensate for flexure between NFIRAOS and the science instrument. The ODGW additional compensate for flexure of the OIWFS because they are directly located on the science focal plane.

NFIRAOS has three instrument output ports. An Instrument Selection Mirror (ISM) is used to feed light to any of these three ports. At first light, these three ports are expected to be occupied by 1) IRIS, an imager and integral field spectrograph, 2) IRMS, a multi-object slit spectrograph and wide field imager that can work with modest or no AO correction, and 3) NSEN, a non-science instrument that contains an acquisition camera (NSEN ACQ) and a diffraction-limited NIR camera.

NFIRAOS has three operating modes:

  1. Laser Guide Star Multi Conjugate AO mode (LGS MCAO): 
     

    In  LGS MCAO mode, NFIRAOS uses six LGS wavefront sensors and the pyramid wave front sensor (PWFS) located within NFIRAOS, as well as up to three On-Instrument wavefront sensors (OIWFS) and/or up to four  On-detector guide windows (ODGW) provided by client instruments.The PWFS is used as a Truth WFS (TWFS) running at low frame rate for correcting aberrations rising from changes in the sodium layer profile.The OIWFS are generally used to provide tip/tilt, focus, and plate scale control ODGW can be used as well if bright guide stars are available within the imager focal plane to provide tip/tilt measurements If faint guide stars are available within the imager focal plane, the ODGW can be used as tip/tilt truth WFS running at lower speed, to provide flexure compensation between OIWFS and instrument focal plane.There are also cases when a fast TTF OIWFS measurement cannot be used due to reasons like 1) vignetting science target, 2) extended guide object that is comparable to seeing or 3) lacking infrared guide stars, etc., the PWFS may be used instead to provide high speed but less accurate tip/tilt/focus control. The TTF OIWFS could then be used as tip/tilt/focus truth WFS with a faint guide star while other TT OIWFS and/or ODGW may be used as tip/tilt truth WFS.

  2. Natural Guide star mode (NGSAO):
     

    In NGSAO mode, NFIRAOS uses the PWFS with a bright visible natural guide star, and optionally a TTF OIWFS and/or an ODGW, to close the AO loops. The NGS is usually a star but can be a small extended object. The PWFS running in 96x96 mode has an effective sub-aperture size of 0.31 meters and a magnitude limit of R~13.5 (RD1) for an on axis H band Strehl ratio of 50%. With a bright NGS (R<11), the NGSAO mode will provide superior on axis performance to that of LGS mode. But the performance will degrade for guide stars that are dimmer or at distances greater than about 5 arcsec from the science object.The diameter of the PWFS field stop is about 2 arcsec. Therefore the probability of missing the spots on the PWFS is about 60% as shown in , which warrants the use of the acquisition camera.

  3. Seeing-limited mode:
     

    In the case of IRMS, there will be science cases where NFIRAOS will be used as an active Optics system Seeing-Limited mode. This mode is implemented as degraded NGSAO mode with PWFS binned down and operating at lower frame rate, and an optional TTF OIWFS in IRMS.

Observation Preparation

In order to make efficient use of TMT, NFIRAOS and the science instruments, the observer will need to prepare their observation in advance. To make these preparations, the observer will need access to guide star catalogues and the observation preparation software tools, including the integration time calculators and AO performance modeling tools.

The Strehl ratio delivered by NFIRAOS will depend most critically on the seeing at the time of the observation. However, other factors may also have a significant impact on the system performance. These factors include air-mass, sky transparency and background (moon phase), brightness and location of natural guide stars, etc. There may also be other trade-offs to be made; for example, is the system better with a mildly extended tip‑tilt source close to the target or using a known star that is located at the outermost limits of the tip-tilt guide field?

To properly identify the observing conditions that are required for a successful NFIRAOS observation, and make informed decisions regarding guide stars, the observer will use the OIWFS selection tool (as part of OSW), which includes the NFIRAOS AO performance modeling tool to select the optimal OIWFS guide asterism ahead of the observation using known star catalogue information, or available images. If the selected guide stars have not been used before, backup stars should also be selected in case the stars are not point source or have wrong magnitude. TMT will keep a database for guide stars that have been successfully used.

The OIWFS selection tool could also be used during the acquisition step with acquisition images taken by the NFIRAOS acquisition camera or science instruments. The OIWFS selection tool should be able to select a near optimal asterism in less than 5-10 seconds.

Observing programs will need to provide the following inputs for the successful execution of the observations:

  • Coordinates (RA/Dec, epoch) of science object/field
  • Rate of apparent motion for non-sidereal targets
  • AO mode and corresponding coordinates (RA/Dec, epoch), magnitudes and colors or spectrum energy density (SED) of natural guide stars
  • Instrument configuration
  • Sequence of science and nighttime calibration exposures (arc, flats, darks, off‑source)
  • Type and sequence of daytime calibrations
  • Spectrophotometric or photometric standard star observations
  • Radial velocity standard star (if needed)
  • Weather, minimum image quality (Strehl ratio, enclosed energy, PSF FWHM, etc.) under which observations should be conducted.
  • Pattern for dithering.
  • These inputs will need to be provided by observers in a very detailed format that can be stored in the observing database so that observing programs can be efficiently sequenced. Sequencing will be key in meeting the required acquisition time budgets.

    For AO observations, a backup plan should be always prepared in case:

  • LGS MCAO observations are not possible,
  • NGSAO not suitable for planned target,
  • Image quality or Strehl requirement cannot be met,
  • An AO instrument is not available,
  • NFIRAOS is not available, etc.
  • The backup plan should ideally include NGSAO mode and seeing limited mode operations. Backup science targets may also be prepared, e.g. standard stars. Detailed parameters should be provided for each AO mode in the backup plan.

Performance

Coming soon...

  • Image Quality
  • Sky Coverage
  • NGS Limiting Magnitude
LGSF Specifications

Power: 25W per LGS 

Wavelength: 589nm

Beam Quality: 85% of the laser energy in core 1.2 times the diffraction limit

Tip/Tilt Jitter: <50mas

Polarization: 98% circularly polarized

Throughput: 75%

Pointing: Blind pointing accuracy of 1"

Operation downtime: <0.5%

Laser Guide Star Facility

Laser Guide Star Facility

Mechanical layout and system Overview of the Laser Guide Star Facility

The LGSF is responsible for generating artificial guide stars in the mesospheric sodium layer with the brightness, beam quality and asterism geometries required by both the NFIRAOS early light AO system and later AO instruments. 

The baseline LGSF consists of 3 primary sub-systems:

  • Laser System, which includes up to nine 20-25W CW or pulsed sodium lasers mounted on inside of the –XECRS telescope elevation journal.
  • Beam Transfer Optics and the Laser Launch Telescope (BTO/LLT) system, which is responsible for taking the beams at the output of the laser system and transferring them up the telescope elevation structure and then launching them from the LLT located behind the TMT secondary mirror.
  • Laser Safety System, which provides interlocks to prevent laser damage to personnel, the TMT observatory or the LGSF itself. In addition, the LGSF provides safety systems to avoid accidental illumination of aircraft and satellites, and to avoid beam collision with neighboring telescopes.

Asterisms

LGSF asterisms

Geometric distribution of the LGSF guide stars

The LGSF will provide several asterism configuration available for different operation modes:

  1. The NFIRAOS asterism, which consists of 6 LGS, 5 of which are equally spaced on a circle of 35" radius and one additional on-axis.
  2. A Mid-Infrared MIRAO asterism, which consists of 3 LGS equally spaced on a circle of 70" radius.
  3. A MOAO asteriem which consists of 8 LGS, 2 equally spaced on a circle of 70" radius and 5 equally spaced on a circle of 150" radius. The first-light LGSF system with 6 laser units will be upgraded to include 2 more laser units for MOAO asterism.
  4. A GLAO asterism, which consists of 4 LGS located at the corner of a 240" x 588" rectangle offset by 288" along the short edge of the field.
Team Members

TMT

  • Corrine Boyer  (AO group Leader)
  • Brent Ellerbroek (retired consultant)
  • Luc Gilles (AO analyst)
  • Gelys Trancho (System engineer)
  • Melissa Trubey (AO engineer)
  • Lianqi Wang (AO analyst)
  • Angel Otárola (System scientist)
  • Hugh Thompson (System engineer)
  • Tony Travouillon (System scientist)
  • Sean adkins (consultant)

 

 

 

 

NRC-Herzberg

  • David Andersen (NFIRAOS project manager)
  •  Glen Herriot
  • Jenny Atwood
  • Peter Byrnes
  • Kris Caputa
  • Jeff Crane
  • Adam Densmore
  • Jennifer Dunn
  • Joeleff Fitzsimmons
  • Brian Hoff
  • Dan Kerley
  • Olivier Lardiere
  • Malcom Smith
  • Jonathan Stocks
  • Jean-Pierre Véran

 

Institute of Optics and Electronics

  • Kai Wei
  • Muwen Fan
  • Changchun Jiang
  • Minli
  • Xiqi Li
  • Daoman Riu
  • Jinlong Tang