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Telescope Properties

This page describes some telescope properties relevant to ITC calculations. See the Gemini telescope pages for more details.

Mirror coating

The Gemini coating chamber is capable of depositing either alumini(u)m or silver. Currently the primary, secondary and tertiary (science fold) mirrors on both telescopes are silver coated. This graph shows the reflectivity of Al and Ag.

Instrument port

Gemini facility instruments may be mounted either on a side-looking or the up-looking ports of the instrument support structure (ISS). When mounted on a side port they are fed by reflection off the science fold mirror which incurs an additional small loss in transmission and an associated increase in emissivity. For signal-to-noise calculations it should be assumed that the instrument is side-looking. The default location for mid-IR instruments (e.g. T-ReCS, Michelle) is the up-looking port and is the side port for other instruments. 

Wavefront sensor

Use of a peripheral wavefront sensor (PWFS) or an on-instrument wavefront sensor (OIWFS) is required to provide tip-tilt error signals for the image motion compensation by the secondary mirror. Not all instruments have an OIWFS; see the specific instrument pages for details. When present, the OIWFS allows selection of a WFS star closer to the science field and therefore usually provides better image motion compensation and smaller images (see the image quality section of the observing condition constraints page for more details).



Last update June 20, 2005; Phil Puxley