The CalUnit is intended to provide flat-field and wavelength calibrations for the Gemini instrument complement. An overview of the Phase I instrumentation programme has been given by Simons, McGonegal and Gillett [2]; Table 1 gives brief details of each instrument. The CalUnit must work over a range of spatial and spectral scales and at wavelengths from 0.3um-5.0um. The largest instrument field is 7' with GMOS [3]; this is the maximum science field delivered by the telescope. The majority of instruments have fields smaller than the unvignetted 3' field. Each of the Phase I instruments takes advantage of the high spatial and spectral resolution which can be achieved with the image quality expected from the Gemini telescopes. The highest spectral resolution currently anticipated is 120,000 with HROS [4]. Efficient optical coupling of even bright calibration lamps is required to illuminate the narrow slits and small pixel fields of view.
An effective flat-field source will illuminate the telescope focal plane in the same way as the astronomical object, and with sufficient flux to obtain a high signal/noise calibration frame in a short timescale. A typical goal for the Gemini CalUnit is that a flat-field for the NIR spectrometer with signal/noise ratio of 10 should be measured in 10s. The purpose of a flat-field is two-fold. The relative gains of individual pixels may be obtained by illuminating with a source with a slowly varying spatial function. The second purpose, to relate the flux in a source measured at one spatial position on an array to that from a source at another position, requires that the illumination have the same vignetting function as the telescope to the required photometric accuracy. The ideal calibration source is a perfectly flat, uniform source in the sky. The sky is used for this purpose in some observing modes, but the wealth of spectral features make it unsuitable for flat-fielding spectroscopic observations. The CalUnit will contain three continuum sources, chosen to provide sufficient flux across the complete 0.3um to 5um range (Table 2). The output of the CalUnit will be matched to the telescope by optimising the uniformity of the illumination and simulating the telescope vignetting function and illumination of the pupil by the f/16 beam. Several different lamps will be made available for wavelength calibration. The high spectral resolution requires that the throughput of the CalUnit be optimised, for efficient measurement of the faintest spectral lines. This is discussed further below.