This paper describes the as-built performance of MOSFIRE, the multi-object spectrometer and imager for the Cassegrain
focus of the 10-m Keck 1 telescope. MOSFIRE provides near-infrared (0.97 to 2.41 μm) multi-object spectroscopy over
a 6.1' x 6.1' field of view with a resolving power of R~3,500 for a 0.7" (0.508 mm) slit (2.9 pixels in the dispersion
direction), or imaging over a field of view of ~6.9' diameter with ~0.18" per pixel sampling. A single diffraction grating
can be set at two fixed angles, and order-sorting filters provide spectra that cover the K, H, J or Y bands by selecting 3rd,
4th, 5th or 6th order respectively. A folding flat following the field lens is equipped with piezo transducers to provide
tip/tilt control for flexure compensation at the <0.1 pixel level. Instead of fabricated focal plane masks requiring frequent
cryo-cycling of the instrument, MOSFIRE is equipped with a cryogenic Configurable Slit Unit (CSU) developed in
collaboration with the Swiss Center for Electronics and Microtechnology (CSEM). Under remote control the CSU can
form masks containing up to 46 slits with ~0.007-0.014" precision. Reconfiguration time is < 6 minutes. Slits are formed
by moving opposable bars from both sides of the focal plane. An individual slit has a length of 7.0" but bar positions can
be aligned to make longer slits in increments of 7.5". When masking bars are retracted from the field of view and the
grating is changed to a mirror, MOSFIRE becomes a wide-field imager. The detector is a 2K x 2K H2-RG HgCdTe
array from Teledyne Imaging Sensors with low dark current and low noise. Results from integration and commissioning
are presented.
MOSFIRE is a new multi-object near-infrared spectrometer for the Keck 1 telescope with a spectral resolving
power of R~3500 for a 0.7″ slit (2.9 pixels). The detector is a substrate-removed 2K × 2K HAWAII 2-RG HgCdTe
array from Teledyne Imaging Sensors with a cut-off wavelength of 2.5 μm and an operational temperature of
77K. Spectroscopy of faint objects sets the requirement for low dark current and low noise. MOSFIRE is also
an infrared camera with a 6.9′ field of view projected onto the detector with 0.18″ pixel sampling. Broad-band
imaging drives the requirement for 32-channel readout and MOSFIREs fast camera optics implies the need for
a very at detector. In this paper we report the final performance of the detector selected for MOSFIRE. The
array is operated using the SIDECAR ASIC chip inside the MOSFIRE dewar and v2.3 of the HxRG software.
Dark current plus instrument background is measured at <0.008 ē s−1 pixel−1 on average. Multiple Correlated
Double Sampling (MCDS) and Up-The-Ramp (UTR) sampling are both available. A read noise of <5ē rms is
achieved with MCDS 16 and the lowest noise of 3ē rms occurs for 64 samples. Charge persistence depends on
exposure level and shows a large gradient across this detector. However, the decay time constant is always ~660
seconds. Linearity and stability are also discussed.
MOSFIRE is a unique multi-object spectrometer and imager for the Cassegrain focus of the 10 m Keck 1 telescope. A
refractive optical design provides near-IR (0.97 to 2.45 μm) multi-object spectroscopy over a 6.14' x 6.14' field of view
with a resolving power of R~3,270 for a 0.7" slit width (2.9 pixels in the dispersion direction), or imaging over a field of
view of 6.8' diameter with 0.18" per pixel sampling. A single diffraction grating can be set at two fixed angles, and
order-sorting filters provide spectra that cover the K, H, J or Y bands by selecting 3rd, 4th, 5th or 6th order respectively. A
folding flat following the field lens is equipped with piezo transducers to provide tip/tilt control for flexure compensation
at the 0.1 pixel level. A special feature of MOSFIRE is that its multiplex advantage of up to 46 slits is achieved using a
cryogenic Configurable Slit Unit or CSU developed in collaboration with the Swiss Centre for Electronics and Micro
Technology (CSEM). The CSU is reconfigurable under remote control in less than 5 minutes without any thermal
cycling of the instrument. Slits are formed by moving opposable bars from both sides of the focal plane. An individual
slit has a length of 7.1" but bar positions can be aligned to make longer slits. When masking bars are removed to their
full extent and the grating is changed to a mirror, MOSFIRE becomes a wide-field imager. Using a single, ASIC-driven,
2K x 2K H2-RG HgCdTe array from Teledyne Imaging Sensors with exceptionally low dark current and low noise,
MOSFIRE will be extremely sensitive and ideal for a wide range of science applications. This paper describes the design
and testing of the instrument prior to delivery later in 2010.
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