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Altair Introduction

Altair is the facility natural/laser guide star adaptive optics system of the Gemini North telescope. It can feed a corrected beam to other Gemini instruments. Deploying Altair is "transparent" for the science instrument, as it reproduces the telescope focal ratio (f/16), pupil size and pupil position.

Altair uses a 177 actuator deformable mirror (DM) and a separate tip-tilt mirror (TTM) to correct atmospheric turbulence. It is equipped with a Shack-Hartmann wave-front sensor (WFS) that uses the visible light to measure the turbulent incident wavefront. The correction is done at a rate of up to 1 kHz (one DM/TTM update every milli-second), and is optimized automatically. Altair was built by the Herzberg Institute of Astrophysics (HIA), Canada.

Altair Natural Guide Star (NGS) function has been offered since 2004A. In 2007A, the Altair Laser Guide Star (LGS) commissioning was completed. PIs proposing for LGS should fully familiarize themselves with the Altair LGS Component webpage.

Overview of Altair Capabilities

Guiding modes

High Strehl NGS: Natural Guide Star (R<11, within 10 arcsec of target)
High Strehl LGS: Laser Guide Star with Natural Tip/Tilt Star (R<15, within 15 arcsec of target)
Low Strehl NGS: Natural Guide Star (R<15, within 25 arcsec of target)
Low Strehl LGS: Laser Guide Star with Natural Tip/Tilt Star (R<18.5 within 25 arcsec of target)

Above is only an overview of the guiding modes available. Please see the Performance and Use page for more detailed information.

Science modes	NIRI f/32 imaging and spectroscopy, 1.0-2.5 micron (broad band and narrow band filters) NIRI f/14 + field lens imaging and spectroscopy, 1.0-2.5 micron (broad band and narrow band filters) NIRI f/32, 3.4-4.1 micron imaging and spectroscopy (L', point sources only) NIFS 0.95 - 2.40 micron IFU imaging spectroscopy NIFS 0.95 - 2.40 micron IFU spectral coronagraphy See the NIRI web pages and the NIFS web pages for details of the science instruments that Altair can feed.
Wavelength coverage	Altair includes a dichroic beamsplitter that transmits light to the science path/instrument at wavelengths longward of 835 nm. Light shortward of 835 nm is reflected into the WFS and used for wave-front analysis.
Field of view	The field of view of Altair has recently been increased dramatically by installation of the Field Lens. The useful field of view is limited either by the instrument (e.g. NIRI f/32, giving 22.4 arcsec) or the size of the isoplanatic patch (see the performance and use page for more details as well as the Field Lens page). In addition, the output unvignetted field of view is limited to 2 arcmin in diameter by a hard mechanical stop at the Altair/ISS interface.
Wavelength Coverage:	Altair delivers 0.835 - 4.1 microns. The shortest wavelengths (z and J) suffer from poor correction and are not recommended. The long wavelengths (L') suffer from thermal emission and are recommended only for point sources.
Emissivity:	Emissivity is expected to be 10% or less, or a total lower than 19% for the entire optical train. The field lens increases the emissivity by an as-yet uncharacterized amount.
Zero points:	With NIRI, K = 24.111 and H = 24.702 (for 1 ADU/s). The approximate gain for NIRI is 12.3 electrons/ADU (see NIRI pages).

Last update Feb 22, 2007; Chad Trujillo