TOP LEVEL SCIENTIFIC AND OPERATIONAL REQUIREMENTS FOR NAOMI

• 1. Revised and extended requirements and goals
  • 1.1 Primary Scientific Requirements
  • 1.2 Instrumentation Interface Requirements
  • 1.3 Support Requirements
  • 1.4 Additional requirements
• 2. Relationship between the Science Clauses and Astronomical Projects

In the following specifications, Strehl ratio is used as a measure of image quality. This is the ratio of the central intensity of the delivered point-spread function (PSF) to that of an unaberrated image from the same telescope at the same wavelength. Where a high Strehl ratio is quoted (Clause 1) this means that a high proportion of the energy of the PSF will be present in a diffraction-limited core; the converse is also of great importance, that a small proportion of energy is in an extended halo. Image Full Width at Half Maximum (FWHM) is not a good indicator of adaptive optics system performance because very partial correction can produce a small diffraction-limited spike which, although it contains a very small proportion of the total image energy, has the best possible FWHM. The effect of the uncorrected seeing is expressed as the coherence length, r₀, at wavelength = 0.5 µm. r₀=10 cm corresponds to 1 arcsecond seeing and r₀=20 cm corresponds to half-arcsecond seeing. All specifications in terms of seeing apply to the seeing at the zenith distance of the object.

Upgrade routes available for extending the baseline NAOMI specifications are indicated in the same format as this paragraph. The maintenance of this upgrade potential is to be regarded as part of the baseline NAOMI specification and subject to the same change control.

1.1 Primary Scientific Requirements

1.1.1 Clause 1 (High Light Level Image Quality)

The AO system shall be capable of delivering an output Strehl ratio of at least 65% on-axis at a wavelength of 2.2 microns with guide stars of magnitude 8* or brighter when the visible (0.55 micron wavelength) atmospheric coherence length r_o is 20 cm or larger (optical (V) seeing =< 0".5). Performance at 1.25 microns shall be commensurate with the 2.2 micron performance after taking into account the additional Strehl ratio degradation introduced because of the wavelength dependence of the turbulence effects.

* This is the clause which defines the ultimate system performance, unlimited by the number of photons detected per sub-aperture. Please see the attached samples of current modelling which predict that this level of performance will be available with much fainter reference stars.

This clause drives the following specifications:

• system order (i.e. the number of correctable degrees of freedom of the AO system). The system order is expressed as the number of wavefront sensor subapertures across the diameter of an image of the WHT primary
• tip-tilt error budget
• uncorrectable mechanical flexure budget (i.e. those system aspects which the AO system will not self-correct)
• uncorrectable optical error budget (The assumption is made that uncorrectable telescope aberrations will not limit performance. Data on these aberrations have been requested by the programme.)
• the performance of the real-time control system and software and the system latency including the WFS image collection and pre-processing.

1.1.2 Clause 2 (Low Light Level Image Quality)

Minimum Requirement:

(a) At the same science wavelength, correction wavelength and r₀ as for Clause 1, the system should aim to achieve at least 25% Strehl ratio at the centre of the science field, with 25% sky coverage. Strehl ratio across the science field can vary according to separation from the guide star as would be predicted by standard tubulence models (being highest closest to the guide star).

(b) Any failure to meet these specifications, for either clauses 1 or 2, shall be due to atmospheric limitations alone. The feasibility of these clauses is predicated on models assuming atmospheric conditions equivalent to a single turbulent layer moving at 10 m/sec at 3 km above the telescope.

Goal:

(c) At the same science wavelength, correction wavelength and ro as for Clause 1, the system should aim to achieve at least 25% Strehl ratio at the centre of the science field, with 50% sky coverage. Strehl ratio across the science field can vary according to separation from the guide star as would be predicted by standard tubulence models (being highest closest to the guide star).

This clause drives sky-coverage issues:

• Wavefront Sensor (WFS) and (for the upgrade goal), Tip-Tilt Sensor (TTS) sensitivities
• the ability to change the spatial sampling on the WFS (i.e. to reduce the system-order)
• conjugation capability - the ability to perform correction at an image of atmospheric turbulence.
• guide star search field

1.1.3 Clause 3 (Seeing and Wavelength Operating Conditions)
(a) The system shall give a net gain in imaging signal-to-noise ratio (SNR) of a point source at wavelengths ranging from 1.0microns to 2.5 microns (goal: 0.82 [to include the Ca+ triplet] to 4.1 µm) in atmospheric turbulence conditions as poor as r₀ = 8 cm (1".2 optical seeing) and with guide stars at least as faint as visual magnitude 14.

(b) The NAOMI system alone shall have a throughput >70% to the Infrared Science Port at wavelengths > 1µm and a throughput to the wavefront sensor > 25% at wavelengths 0.9µm < wavelength < 1.0µm

This clause drives the following specifications:

• science path throughput/emissivity
• system "availability" (i.e. under what range of seeing conditions should the system be able to work).
• the required transmission/reflectance curve of the dichroic mirror, other system surface coatings and the number of system optical surfaces.

1.1.4 Clause 4 (IR Observational Dithering)

The system should additionally be capable of producing a "dithering" offset of 18 arcseconds without shifting the pupil with respect to the science instrument and without losing lock. It shall be capable of continuously tracking objects at non-sidereal rates limited only by the guiding rate of the WHT for a self-referenced object and of 4 arcsec/sec in each co-ordinate for objects needing an independent guide star.

This clause derives from IR observational sensitivity requirements, ensuring best possible options are available for flat fielding using the sky. The clause drives the following specifications:

• WFS and TTS pick-off method.
• pupil imaging/conjugation optics
• the overall control software and its interface to the telescope

1.1.5 Clause 5 (Correction Stability)

The system shall maintain loop closure between the WFS and deformable mirror for at least 60 minutes in stable conditions of single turbulent layer windspeed less than 5 m/sec and r_o > 15cm with an on-axis guide star brighter than R=10^m.

This clause drives or affects the following specifications:

• electronics (camera, RTCS hardware) stability
• mechanical stability
• RTCS software robustness

1.1.6 Clause 6 (Observational Efficiency)

(a)) The goal of the system performance shall be to allow the astronomer to spend at least 50% of the night-time hours integrating on science targets or astronomical standards as required to calibrate the science instrument , when sky conditions are stable and photometric.

(b) Once installed and aligned the system shall require no more than 30 minutes to optimise/confirm the alignment in any 24 hour period.

This clause drives or impinges on the following specifications:

(some specifications are not completely orthogonal to others; for example, Clause 6 drives the degree of automation of mode switching and optimization while Clause 2 drives its presence in the first place).

• automation of the calibration functions
• automation of the moving slides, lenslets and filters in the optical chassis and WFS which are likely to be used routinely and with reasonable frequency during calibration and observing.
• links between the TCS and the Instrument Control System
• level of software and tools written for visualization, modal control and optmisation, target acquisition

1.1.7 Clause 7 (IR science port)

The science field of view shall be at least sufficient to illuminate all of a 1024x1024 imaging array fully-sampled at the 1.65 micron diffraction limit with no vignetting. Also the system should have a well- defined and accessible infrared science port around which other instruments such as spectrographs and a coronagraph can be designed.

This clause drives the following specifications:

• minimum science field-of-view and image quality specifications
• the environment and space envelope of the corrected science focus
• the overall control software and its interface to science instruments

1.2 Instrumentation Interface Requirements

1.2.1 Clause 8 (IR Science Instrument Optical Interface)

The output beam to the IR science instrument shall be f/16.5. The exit pupil of NAOMI in the IR science path shall be 66.7mm in diameter at a distance of 1100mm from the focal plane. The final angular image scale of the IR science instrument shall be 330µm/arcsec and its unvignetted field of view shall be at least 58 arcseconds in diameter.

1.2.2 Clause 9 (Optical Port: Acquisition)

The NAOMI system shall have an optical wavelength port with field of view 2.9 arcmin in diameter and with throughput > 58% between 0.5 and 0.8µm, which may be used for acquisition. The focal ratio of this optical beam shall be f/16.8.

1.2.3 Clause 10 (Optical Port: Science)

The optical beam specified in Clause 9 shall also be available for optical science provided that this does not in any way compromise infrared science.

1.2.4 Clause 11 (Network Interface)

Any NAOMI interface to the telescope or any instrument control system shall be via DRAMA and shall conform to ING networking standards.

1.2.5 Clause 12 (IR Science Instrument Control Interface)

'Faint IR' in this context means with an integrated flux not detectable in the raw spectrum at above 6-sigma in a 0.5 second integration at the wavelengths, slit width and spectral resolution of the proposed observation. Optically faint means fainter than 14th magnitude at R (number to be confirmed).

1.3 Support Requirements

The NAOMI system shall not, by its own thermal power input, degrade the uncorrected local seeing conditions at GHRIL by more than 0".1. The goal shall be for no detectable seeing degradation due to NAOMI's presence.

• The thermal control of electronics on bench and in attached racks.

• This implies the use of a minimum number of surfaces which are, so far as possible, all reflective and drives the choice of coatings of these surfaces.
• Including a predicted telescope emissivity budget for the WHT at Nasmyth focus the above specification gives a total system emissivity of <45% with a goal of <35%.