The European Space Agency (ESA) is providing the Near Infrared Spectrograph (NIRSpec) developed by EADS Astrium GmbH to fly on the James Webb Space Telescope (JWST). NIRSpec covers the 0.6-5.0 µm domain. It will be primarily operated as a multi-object spectrograph, using a MEMS micro-shutter array (MSA) provided by NASA to select multiple objects from the field of view at an intermediate image plane formed by the NIRSpec fore-optics. The MSA apertures form multiple entrance slits of the spectrometer section.
The Dark Energy Survey Instrument (DESI) is a 5000-fibre optical multi object spectrograph for the 4m Mayall telecope at the Kitt Peak National Observatory. Ten identical three channel spectrographs will be equipped with 500-element fibre slits. Here we focus on the architecture of the science slits and the interchangeable auxiliary slits required for calibration.
The use of sodium laser guide star for Extremely Large Telescopes (ELT) adaptive optics systems is a key concern due to the perspective effect that produces elongated images in the Shack-Hartmann pattern. In order to assess the feasibility of using an elongated sodium beacon on an ELT, an on-sky experiment reproducing the extreme off-axis launch conditions of the European ELT is scheduled for summer and autumn 2016. The experiment will use the demonstrator CANARY installed on the William Herschel Telescope and the ESO transportable 20W CW fiber laser, embedded in the Wendelstein LGS unit. We will discuss here the challenges this experiment addresses as well as the details of its implementation and the derivation of the error budget.
CHOUGH is a small, fast project to provide an experimental on-sky high-order SCAO capability to the 4.2m WHT telescope. The basic goal has r0-sized sub- apertures with the aim of achieving high-Strehl ratios (> 0:5) in the visible (> 650 nm). It achieves this by including itself into the CANARY experiment: CHOUGH is mounted as a breadboard and intercepts the beam within CANARY via a periscope. In doing so, it takes advantage of the mature CANARY infrastructure, but add new AO capabilities. The key instruments that CHOUGH brings to CANARY are: an atmospheric dispersion compensator; a 32 × 32 (1000 actuator) MEMS deformable mirror; 31 × 31 wavefront sensor; and a complementary (narrow-field) imager. CANARY provides a 241-actuator DM, tip/tilt mirror, and comprehensive off-sky alignment facility together with a RTC. In this work, we describe the CHOUGH sub-systems: backbone, ADC, MEMS-DM, HOWFS, CAWS, and NFSI.
CANARY is an on-sky Laser Guide Star (LGS) tomographic AO demonstrator in operation at the 4.2m William Herschel Telescope (WHT) in La Palma. From the early demonstration of open-loop tomography on a single deformable mirror using natural guide stars in 2010, CANARY has been progressively upgraded each year to reach its final goal in July 2015. It is now a two-stage system that mimics the future E-ELT: a GLAO-driven woofer based on 4 laser guide stars delivers a ground-layer compensated field to a figure sensor locked tweeter DM, that achieves the final on-axis tomographic compensation. We present the overall system, the control strategy and an overview of its on-sky performance.
Wavefront (WF) sensing using Sodium (Na) Laser Guide Stars (LGS) is a key concern for the design of a number of first generation Extremely Large Telescope (ELT) Adaptive Optics (AO) modules. One of the main challenges is the mitigation of the effects induced by extreme LGS spot elongation on the WF measurements. Before the final design studies of the E-ELT instruments, a Na LGS WF sensing on-sky experiment at the E-ELT scale is mandatory to achieve the full validation of the proposed mitigation strategies and their performance. This experiment will provide unique spatial and temporal WF measurements on a true Na LGS, perturbed by the atmospheric turbulence and mesospheric variability. The fine comparative analysis of such data with synchronously acquired WF measurements on at least one natural guide star (NGS) will be fundamental to test a number of algorithms, configurations for spot sampling and truncation and WF reconstruction schemes including multi-LGS configurations. A global error budget for the whole experiment will be derived with time to feed into the numerical simulation and the design of subsequent E-ELT LGS-AO modules. The data produced will be made available to the E-ELT community. We propose to use CANARY, the Multi-Object AO demonstrator installed at the 4.2m WHT which is a modular AO platform, equipped with several NGS WF Sensor (WFS) and Rayleigh multi-LGS unit and WFS. The transportable 20W Sodium laser unit (WLGSU), developed at ESO, will be positioned at a varying distance from the WHT to provide off-axis launching (up to 40m), simulating the whole range of LGS spot elongations obtained on the E-ELT. In such a case, the WHT pupil will represent an off-axis sub-pupil of the main E-ELT pupil. In addition, this experiment will include a Na layer profiler and the capability for open and closed loop operations. The experiment is scheduled before the end of 2016.
We describe the fiber system of the Dark Energy Spectroscopic Instrument (DESI). Its primary science goal is to provide
a survey of 14,000 square degrees of the extragalactic sky using the Mayall 4m telescope in five years. The fibre system
will provide a multiplex gain of 5000 so that more than 20 million galaxies can surveyed. Applying a number of tests to
the survey dataset should allow the evolution of the equation of state of the universe to be determined to greater accuracy
than before. The fibre system will provide a multiplex gain of 5000 with very high levels of performance.
CANARY is an on-sky Laser Guide Star (LGS) tomographic AO demonstrator that has been in operation at the 4.2m William Herschel Telescope (WHT) in La Palma since 2010. In 2013, CANARY was upgraded from its initial configuration that used three off-axis Natural Guide Stars (NGS) through the inclusion of four off-axis Rayleigh LGS and associated wavefront sensing system. Here we present the system and analysis of the on-sky results obtained at the WHT between May and September 2014. Finally we present results from the final ‘Phase C’ CANARY system that aims to recreate the tomographic configuration to emulate the expected tomographic AO configuration of both the AOF at the VLT and E-ELT.
The Centre for Advanced Instrumentation (CfAI) of Durham University (UK) has developed a conceptual design for the
Integral Field Unit (IFU) for EAGLE based on diamond-machined monolithic multi-faceted metal-mirror arrays as an
alternative to the glass IFU which is currently baselined. The CfAI has built up substantial expertise with the design,
manufacture, integration, alignment and acceptance testing of such systems, through the successful development of IFUs
for the Gemini Near-InfraRed Spectrograph (GNIRS) and JWST NIRSpec and 24 IFUs for ESO’s K-band Multi-Object
Spectrometer (KMOS). The unprecedented performance of the KMOS IFUs (Strehl < 0.8 across the field, throughput
rising from 86% at a wavelength of 1 micron to 93% at 2.5 micron) demonstrates that the current state-of-the-art
technology is sufficiently mature to meet the demanding requirements for EAGLE. In addition, the use of monolithic
multi-faceted metal mirror arrays will greatly simplify the manufacture, integration and alignment of such systems thus
potentially reducing technical and programmatic risks and cost. Through the timely completion of the KMOS IFUs,
which required the fabrication of an unprecedented 1152 optical surfaces, the CfAI have demonstrated that they have the
capacity to produce the required volume within reasonable schedule constraints. All the facilities (design, fabrication e.g.
diamond machining, metrology, integration and test) required for the successful realisation of such systems are available
in-house, thus minimising programmatic risks. This paper presents the opto-mechanical design and predicted
performance (based on the actual measured performance of the KMOS IFUs) of the proposed metal IFU.
We developed the technology of microslice integral field units some years ago as the next step in SAURON type
microlens IFU design with typically 5 times more spatial elements (spaxels) for the same spectrograph and spectral
length aiming at 1,000,000 spaxels IFUs. A full instrument for laboratory demonstration composed of the fore-optics, the
IFU, the spectrograph and the detector has now been built and tested. It has about 10,000 spatial elements and spectra
150 pixel long. Our IFU has 5 cylindrical microlens arrays along the optical axis as opposed to one hexagonal array in
the previous design. Instead of imaging pupils on the spectrograph input focal plane, our IFU images short slitlets 17
pixel long that keep the spatial information along the spatial direction then giving 17 spaxels per slitlet instead of one in
pupil imaging. This removes most of the lost space between spectra leaving place for more and keeps the spatial
information over the element size while pupil images lose it. The fore-optics re-images the field on the input of the IFU.
They are made of cylindrical optics to get the desired different magnifications in both directions. All the optics and
detector fit in a cylinder 35 mm in diameter and 280 mm long. With a different set of fore-optics on a 4-m telescope, a
field of 43" x 6.7" with spatial elements of 0.14" x 0.22" could be observed so 12 of these mini-spectrographs would
cover a field surface area of about 1 arcmin2 and 120,000 spaxels.
EUCLID, the ESA Dark Energy Mission, contains a NIR and a visible imagers (NIP & VIS), and an NIR spectrograph
(NIS). Different designs of the NIS have been studied especially a slitless design, a Digital Micromirror Device (DMD)
design using grisms and another using prisms, and more recently a combination of the NIP and NIS into one instrument.
We present the design of the prism DMD NIS. This design has the advantage over the slitless design of having a DMD
mask which reduces the background by a factor of more than 100 and all the advantages over the grism DMD NIS that a
prism gives over a grism as a higher and more uniform transmission, the absence of parasite orders, and a choice of the
slope of the spectral resolution with wavelength. The field per spectrograph was made sufficiently large to reduce the
number of spectrographs to two. The design was made so that the mapping of the sky of the NIS is easily compatible
with the mapping strategy of the NIP and VIS. Two designs were made. In one, the field is larger but the surface shapes
of the optics are complex which makes manufacturing more challenging. In the other, the design was made to be fully
compatible with the manufacturing criteria of SESO after extensive discussions to carefully understand the
manufacturing limitations especially the formula for highly aspheric surface shapes as biconics. This was done by
directly integrating the criteria into the optimization process of ZEMAX. A calibration system that uses the DMD with
the micromirrors in their OFF positions was also developed.
The Euclid Near-Infrared Spectrometer (E-NIS) Instrument was conceived as the spectroscopic probe on-board the ESA
Dark Energy Mission Euclid. Together with the Euclid Imaging Channel (EIC) in its Visible (VIS) and Near Infrared
(NIP) declinations, NIS formed part of the Euclid Mission Concept derived in assessment phase and submitted to the
Cosmic Vision Down-selection process from which emerged selected and with extremely high ranking. The Definition
phase, started a few months ago, is currently examining a substantial re-arrangement of the payload configuration due to
technical and programmatic aspects. This paper presents the general lines of the assessment phase payload concept on
which the positive down-selection judgments have been based.
The CANARY on-sky MOAO demonstrator is being integrated in the laboratory and a status update about its
various components is presented here. We also discuss the alignment and calibration procedures used to improve
system performance and overall stability. CANARY will be commissioned at the William Herschel Telescope at
the end of September 2010.
EAGLE is an instrument under consideration for the European Extremely Large Telescope (E-ELT). EAGLE will be
installed at the Gravity Invariant Focal Station of the E-ELT. The baseline design consists of 20 IFUs deployable over a
patrol field of ~40 arcmin2. Each IFU has an individual field of view of ~ 1.65" x 1.65". While EAGLE can operate with
the Adaptive Optics correction delivered by the telescope, its full and unrivaled scientific power will be reached with the
added value of its embedded Multi-Object Adaptive Optics System (MOAO). EAGLE will be a unique and efficient
facility for spatially-resolved, spectroscopic surveys of high-redshift galaxies and resolved stellar populations. We detail
the three main science drivers that have been used to specify the top level science requirements. We then present the
baseline design of the instrument at the end of Phase A, and in particular its Adaptive Optics System. We show that the
instrument has a readiness level that allows us to proceed directly into phase B, and we indicate how the instrument
development is planned.
The Near Infrared Spectrograph (NIRSpec) developed by EADS Astrium GmbH for the European Space Agency (ESA)
is a spectrograph covering the 0.6-5.0 μm waveband to fly on the James Webb Space Telescope (JWST). NIRSpec will
be primarily operated as a multi-object spectrograph but also includes an integral field unit (IFU) allowing a 3×3 arcsec
field of view to be sampled continuously with 0.1 arcsec spatial resolution. The IFU, based on an advanced image slicer
concept, is a very compact athermal unit made of aluminium. It contains three 30-element monolithic mirror arrays
forming slicer, pupil and slit mirrors, and single-surface image relay and plane fold mirrors, produced using 5-axis
diamond-machining techniques. Many of the mirrors have complex surfaces like toric sections with 3rd-order corrections
in order to achieve the required performance within a small allowed volume, and could only have been fabricated with
the most advanced free-form machining. The mechanical design accommodates the differential expansion between the
aluminium IFU and its titanium parent assembly across a 250K drop to operating temperature using an isostatic
mounting system. This paper presents the development of the IFU from the design and diamond-machining techniques
to the optical and cryogenic testing of the assembled flight model unit.
EAGLE is an instrument for the European Extremely Large Telescope (E-ELT). EAGLE will be installed at the Gravity
Invariant Focal Station of the E-ELT, covering a field of view of 50 square arcminutes. Its main scientific drivers are the
physics and evolution of high-redshift galaxies, the detection and characterization of first-light objects and the physics of
galaxy evolution from stellar archaeology. These key science programs, generic to all ELT projects and highly
complementary to JWST, require 3D spectroscopy on a limited (~20) number of targets, full near IR coverage up to 2.4
micron and an image quality significantly sharper than the atmospheric seeing. The EAGLE design achieves these
requirements with innovative, yet simple, solutions and technologies already available or under the final stages of
development. EAGLE relies on Multi-Object Adaptive Optics (MOAO) which is being demonstrated in the laboratory
and on sky. This paper provides a summary of the phase A study instrument design.
KEYWORDS: Mirrors, Relays, James Webb Space Telescope, Spectroscopy, Spectrographs, Diamond machining, Space telescopes, Metrology, Molybdenum, Field spectroscopy
The James Webb Space Telescope (JWST) mission is a collaborative project between the National Aeronautics and
Space Administration (NASA), the European Space Agency (ESA) and the Canadian Space Agency (CSA). On board
JWST, the NIRSpec instrument developed by EADS Astrium for ESA is a near-infrared spectrograph covering the 0.6-5.0 μm domain at spectral resolutions of 100, 1000 and 2700.
NIRSpec will be primarily operated as a multi-object spectrograph (MOS) but it also includes an integral field unit (IFU)
allowing to continuously sample a 3"x3" field of view with 0.1". This IFU, based on the "advanced" image slicer
concept, is a very compact athermal unit made of aluminium. The slicer, pupil and slit mirror arrays are each machined
from monolithic blocks using diamond-turning techniques.
This paper presents the integral-field spectroscopy (IFS) mode of NIRSpec. After a brief presentation of its main
scientific objectives and expected performance, we will focus on its implementation in NIRSpec and the design of the
IFU to the diamond machining techniques applied for manufacturing. We will finish with a presentation of the status of
the development and of recent results from mirror machining and metrology.
EAGLE is an instrument under conceptual study for the European Extremely Large Telescope (E-ELT). EAGLE will be
installed at the Gravity Invariant Focal Station of the E-ELT, covering a field of view between 5 and 10 arcminutes. Its
main scientific drivers are the physics and evolution of high-redshift galaxies, the detection and characterization of first-light
objects and the physics of galaxy evolution from stellar archaeology. The top level requirements of the instrument
call for 20 spectroscopic channels in the near infrared, assisted by Adaptive Optics. Several concepts of the Target
Acquisition sub-system have been studied and are briefly presented. Multi-Conjugate Adaptive Optics (MCAO) over a
segmented 5' field has been evaluated and compared to Multi-Object Adaptive Optics (MOAO). The latter has higher
performance and is easier to implement, and is therefore chosen as the baseline for EAGLE. The paper provides a status
report of the conceptual study, and indicates how the future steps will address the instrument development plan due to be
completed within a year.
A European Laser Guide Star (LGS) test facility is proposed for the 4.2m William Herschel Telescope (WHT) on La
Palma. It will test the next-generation Adaptive Optics (AO) LGS technologies to aid risk mitigation of Extremely Large
Telescope (ELT) LGS AO systems. In particular, critical scaling of current LGS AO technologies to ELT dimensions
will be tested. For example, experiments addressing increased spot elongation, cone effect and the order of correction
required.
A pan-European consortium proposes to construct test facility infrastructure on the WHT for a number of risk mitigating
experiments. The infrastructure includes the construction of a Nasmyth platform based controlled environment 'Ground-based
Adaptive optics Innovative Laboratory' (GRAIL), an experimental test environment 'Testbed integration facility'
(TIF) and some common-experiment equipment such as the Common Re-Imaging AO System.
Experiments that are proposed for this facility cover the areas of laser technologies, spot elongation, LGS wavefront
sensing, parallel launch concepts, Multi-Object AO, atmospheric characterisation, co-phasing and real-time control
system risk mitigation.
Results of numerical simulations of the performance of GLAS (Ground-layer Laser Adaptive optics System) are
presented. GLAS uses a Rayleigh laser guide star (LGS) created at a nominal distance of 20km from the 4.2m William
Herschel Telescope primary aperture and a semi-analytical model has been used to determine the observed LGS
properties. GLAS is primarily intended for use with the OASIS spectrograph working at visible wavelengths although a
wider-field IR imaging camera can also use the AO corrected output. Image quality metrics relating to scientific
performance for each instrument are used showing that the energy inside every OASIS lenslet across the 10" instrument
FOV is approximately doubled, irrespective of atmospheric conditions or wavelength of observation.
The Nasmyth Adaptive Optics for Multi-purpose Instrumentation (NAOMI) on the William Herschel Telescope (WHT) has been developed recently into a common user AO (Adaptive Optics) instrument to accompany OASIS (Optically Adaptive System for Imaging Spectroscopy), a multi-slit spectrograph and INGRID (Isaac Newton Group Red Imaging Device) an Infrared detector. The most recent changes are the addition of an Atmospheric Dispersion Corrector (ADC) to be used for the optical wavelengths and a Dichroic Changer mechanism to select either a pass band or IR light for the Universal Science Ports (UPS).
Future developments on NOAMI are planned as it is due to house the GLAS WFS (Ground Layer Adaptive optics System Wave Front Sensor), a wave front sensor for the future Laser Guide Star (LGS) system to be installed on the WHT in 2006.
This paper describes the changes made with respect to the science ports and the changes to be made for the GLAS WFS; focusing on the GLAS WFS and the optical path and interface to the NAOMI adaptive optics system.
The GLAS (Ground-layer Laser Adaptive-optics System) project is to construct a common-user Rayleigh laser beacon that will work in conjunction with the existing NAOMI adaptive optics system, instruments (near IR imager INGRID, optical integral field spectrograph OASIS, coronagraph OSCA) and infrastructure at the 4.2-m William Herschel Telescope (WHT) on La Palma. The laser guide star system will increase sky coverage available to high-order adaptive optics from ~1% to approaching 100% and will be optimized for scientific exploitation of the OASIS integral-field spectrograph at optical wavelengths. Additionally GLAS will be used in on-sky experiments for the application of laser beacons to ELTs. This paper describes the full range of engineering of the project ranging through the laser launch system, wavefront sensors, computer control, mechanisms, diagnostics, CCD detectors and the safety system. GLAS is a fully funded project, with final design completed and all equipment ordered, including the laser. Integration has started on the WHT and first light is expected summer 2006.
A Laser Traffic Control System (LTCS) for laser beam avoidance has been in use at the W. M. Keck observatory on Mauna Kea since 2002. Subsequent LTCS installations have occurred at Gemini North (2003), and at the William Herschel Telescope on La Palma, Canary Islands (2005). Gemini North laser tests in 2005 necessitated algorithm changes to provide support for multiple laser configurations. Operational differences for how laser-telescope priority resolutions occur on La Palma vs. Mauna Kea necessitated algorithm changes to address more generic specification of priority rules, collision event queries, and better display feedback. A joint collaboration between the W. M. Keck observatory and the Isaac Newton Group, to install the LTCS at La Palma and enhance its priority processing algorithm and display functions, occurred in 2005. The changes made should be sufficient to support LTCS software implementations at many different sites, current and future, where multiple laser/telescope configurations are planned. This paper will describe the algorithm changes, review outstanding issues, and describe planned development activities supporting a broader use potential to include sites with ELTs.
NAOMI is the AO system of the 4.2-m William Herschel Telescope on La Palma. It delivers near-diffraction-limited images in the IR, and a significantly improved PSF at optical wavelengths. The science cameras currently comprise an IR imager (INGRID), an optical integral-field spectrograph (OASIS) and a coronagraph which may be placed in the light path to either instrument. 19 science programmes were observed during 2002-3. Observing overheads are small, with as much as 60% of the night spent integrating on science targets. In late 2004 this year, the WFS will be equipped with a low-noise L3 CCD, giving a gain of a factor of 2 in S:N for faint guide stars. A Rayleigh laser guide star is under development, with first light expected summer 2006, providing a unique facility: AO-corrected optical integral-field spectroscopy anywhere on the northern sky.
Using a unique combination of empirical data collected simultaneously by the science camera (INGRID) and the wave front sensor in NAOMI plus the same night profiles of the turbulent layers measured by SLODAR, we discuss the accuracy of the analytic approach to modelling of AO performance. The WFS frames recorded for different atmospheric conditions allow us to make a detailed investigation of the influence of a restricted field of view and sampling of the WFS on the accuracy of the centre of gravity and its propagation to the residual variance. The predictions of Strehl, FWHM and FWHE derived for NAOMI+INGRID using our analytic approach are compared with on-sky performance demonstrated during the commissioning and science observations with NAOMI.
KEYWORDS: Telescopes, Adaptive optics, Control systems, Electronics, Sensors, Servomechanisms, Space telescopes, Imaging systems, Temperature metrology, Domes
The William Herschel Telescope (WHT) has an adaptive optics (AO) suite consisting of the AO system NAOMI, near IR imager INGRID, optical field spectrograph OASIS and coronagraph OSCA. GRACE (GRound based Adaptive optics Controlled Environment) is a dedicated structure at a Nasmyth focus designed to facilitate routine AO use by providing a controlled environment for the instrument system. However, GRACE is not just a building; it is all of the systems associated with providing the controlled environment, especially the control of air quality, temperature and flow. A key concern was that adding the GRACE building to the Nasmyth platform would not adversely change the telescope performance. This paper gives the background to GRACE, its specification and design, the building construction and installation, the environmental controls installed and their performance, the services provided, the effect of the new structure on telescope performance, the results of the project, including the effect having a controlled environment on AO performance and its planned use for a Rayleigh laser guide star system.
We describe a coronagraph facility built for use with the 4.2 metre William Herschel Telescope (WHT) and its adaptive optics system (NAOMI). The use of the NAOMI adaptive optics system gives an improved image resolution of ~0.15 arcsec at a wavelength of 2.2 microns. This enables our Optimised Stellar Coronagraph for Adaptive optics (OSCA) to null stellar light with smaller occulting masks and thus allows regions closer to bright astronomical objects to be imaged. OSCA is a fully deployable instrument and when in use leaves the focus of the NAOMI beam unchanged. This enables OSCA to be used in conjunction with a number of instruments already commissioned at the WHT. The main imaging camera to be used with OSCA will be INGRID; a 1024×1024 HgCdTe cooled SWIR detector at the NAOMI focus. OSCA can also be used in conjunction with an integral field spectrograph for imaging at visible wavelengths. OSCA provides a selection of 10 different occulting mask sizes from 0.25 - 2.0 arcsec and some with a novel gaussian profile. There is also a choice of 2 different Lyot stops (pupil plane masks). A dichroic placed before the AO system can give us improved nulling when occulting masks larger than the seeing disk are used. We also present results from initial testing and commissioning at the William Herschel Telescope.
A rationale is presented for the use of a relatively low-altitude (15km) Rayleigh Laser Guide Star to provide partial adaptive optics correction across a large fraction of the sky on the 4.2m William Herschel Telescope. The scientific motivation in relation to the available instrumentation suite is discussed and supported by model performance calculations, based on observed atmospheric turbulence distributions at the site. The proposed implementation takes the form of a laser system, beam diagnostics, tip-tilt mirror and beacon launch telescope, together with a range-gated wavefront sensor and processing system. It is designed to operate in conjunction with the telescope’s existing facility-class natural guide star AO system, NAOMI. Aspects of the proposed implementation are described as well as the technical features related to the system model and the error budget. In a separate paper the NAOMI AO system itself is presented. Other papers describe a demonstrator system and preliminary Rayleigh beacon wavefront sensing measurements at the site.
Telescope performance can be characterised by two kinds of metric: those which reflect scientific productivity (e.g. citation impact) and those which monitor technical aspects of performance e.g. shutter open time and instrument throughput, assumed to impinge on eventual scientific productivity. These metrics can be used to guide an observatory’s investment of limited operational resources in such a way as to maximise long-term scientific productivity.
We review metrics used at the 4.2-m William Herschel Telescope (WHT) on La Palma, and identify key performance indicators.
Telescope performance can be characterized by a number of metrics e.g. mirror reflectivity, seeing, readout noise, observing overheads. In deciding where to invest limited operational resources to improve performance, one needs to predict the impact of given enhancements on scientific productivity. E.g. for the same cost, is it more important to reduce CCD readout noise by a factor of 2, or to improve instrument throughput by 30%? Knowing the mix of programs at a given telescope, and the dependence of signal-to-noise on the various parameters, the % gain in scientific productivity can be predicted for a given % improvement in any parameter, allowing optimal investment of the operational budget. We describe operational metrics used to monitor the performance of the 4.2-m William Herschel Telescope on La Palma, and give examples of current and planned enhancements which have been prioritized by comparing predicted gains and costs. These enhancements should deliver a total gain approximately 30% in productivity, equivalent to approximately 100 extra observing nights per year.
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