TOP LEVEL SCIENTIFIC AND
OPERATIONAL REQUIREMENTS FOR NAOMI
wht-naomi-102
Internal document number AOW/SYS/RMM/6.10/07/96/NAOMI S & O Requirements
Version date: 2618
July 1996
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 A) 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 l = 0.5 mm. 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.
The requirement clauses have been grouped by category. The original denotations (A, B, etc.) have been retained for compatibility with previous documents. The current ordering is therefore not monotonically alphabetical.
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.
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
ro 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 effect of system order is illustrated in Fig. 1 (H-band) and Fig. 2 (K-band). The figures show the effect of the seeing coherence length r0 on Strehl ratio for various systems. 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.
Minimum
Requirement:
At the same science wavelength, correction
wavelength and ro
as for clause A, 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).
Any failure to meet these specifications,
for either clauses A or B, 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:
At the same science wavelength, correction wavelength and ro as for
clause A, 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
The system shall give a net gain in imaging signal-to-noise ratio (SNR) of a point source at wavelengths ranging from 1.0microns (the Ca+ triplet) to 2.5 microns (goal: 0.82 to 4.1 mm) in atmospheric turbulence conditions as poor as ro = 8 cm (1”.2 optical seeing) and with guide stars at least as faint as visual magnitude 14.
The NAOMI system alone shall have a throughput >70% to the Infrared Science Port at wavelengths > 1mm and a throughput to the wavefront sensor > 25% at wavelengths 0.9mm < l < 1.0mm
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.
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 methods
· pupil imaging/conjugation optics
· the overall control software and its interface to the telescope
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 ro > 15cm with an on-axis guide star brighter than R=10m.
This clause drives or affects the following specifications:
· electronics (camera, RTCS hardware) stability
· mechanical stability
· RTCS software robustness
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.
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 F drives the degree of automation of mode switching and optimization while Clause B 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
· links between the TCS and the Instrument Control System
· level of software and tools written for visualization, modal control and optmisation, target acquisition
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
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 330mm/arcsec
and its unvignetted field of view shall be at least 58 arcseconds in diameter.
The NAOMI system shall have an optical
wavelength port with field of view 2.9
arcmin in diameter and with throughput > 5865% between 0.54 and 0.87mm, which may be
used for acquisition. The focal ratio of this optical beam
shall be f/16.8.
The optical beam specified in Clause H shall also be available for optical science provided that this does not in any way compromise infrared science.
Any NAOMI
interface to the telescope or any instrument control system shall be via
DRAMA and shall conform to ING networking standards.
The interface to the IR science instrument
shall, as a minimum, permit the AO system to inititiate a windowed or
non-windowed exposure, to confirm the completion of the exposure and to obtain
the data. This entire sequence should complete in no longer than ?? seconds for
a 128x128?? pixel window.
The science instrument shall as a minimum be
able to command the AO system to open or close the control loops and to perform
a specified closed-loop offset.
It should be possible to carry out pre-use
alignment, calibration and testing off the telescope. A suitable off-telescope
mounting base shall be supplied with NAOMI.
It shall be possible for on-island staff to
install and align the equipment within 8 hours and to remove it to a WHT
storage point in 4 hours.
All these operations should be such that
they may be carried out safely by a maximum of two people.
Documentation shall include, as a minimum, a User’s Manual, full system (optics, mechanical, electronics, software architecture) engineering diagrams as built, maintenance procedures and trouble-shooting guidelines. The documentation approach shall be to recognize that the system must be supported by staff who have good appropriate mechanical, electronics and software engineering skill but who did not build the system.
The User Interface to NAOMI shall allow
operation by a trained telescope operator.
The NAOMI system shall provide optional
automatic adjustment of the number of corrected modes and of the operational
bandwidth. This adjustment process shall adapt to changing atmospheric
conditions. A manual override of these operating parameters shall be provided.
Information to support the manual choice shall be provided to the operator.
This information shall include as a minimum the values of r0 and the
Greenwood frequency.
The operational lifetime of any tip-tilt mirror and WFS camera shall be > 10000 hours. The deformable mirror shall have an operational lifetime of >3000 hours subject to one actuator failure and replacement.
The NAOMI system shall operate fully over a
temperature range from -10oC to 25oC, in relative
humidity from 10% to 90%, at 8000ft. The system shall be able to survive
relative humidity of 100%.
NAOMI
should be designed to conform to good
EMC practices. The NAOMI project shall consult with the ING in mutually
establishing these practices..
Where appropriate, the same type of
electronics components should be used as are already in use at the ING, as
defined by the ING. Where other components are used a minimum of one spare for
each type shall be supplied. Any exceptions (for example the Deformable mirror)
shall be subject to a specific agreement with ING. (In practice 2 weeks written
or email notice of the adoption of a card-level component should be given to
the on-island project manager before freezing the specification)
VxWorks should be used as the operating
system for non-specialised (i.e. AO-specific) local control processors.
NAOMI software shall be written to standards agreed with the ING
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.
This clause drives the following specifications:
· the thermal control of electronics on bench and in attached racks.
IRAF should be assumed as the offline
astronomical data analysis environment (i.e. any application provided should
not require the existence of some other complete astronomical data reduction
environment).
The FINAL archival format should be
DISKFITS.
The GUI shall be Motif- or Tk-based. The
scripting language shall be Tcl. These are rapidly deveolping areas and these
requirements may be revised by mutual agreement with ING.
The deformable mirror and its electronics
shall be supplied with its own carrying and tranpsort case. It shall be
possible to remove and re-install the DM safely, including its electronics, in
less than an hour and without dismantling the rest of the NAOMI system. Only
minor further optical alignment should be required after re-installation.
An upper limit to the emissivity at 2.2 microns and longer wavelengths of the total optical path to the cryostat window excluding the telescope shall be 20%, with a goal of £16%..
These numbers
derive from assuming reasonable emissivities (e.g. optimum coatings but not
necessarily freshly cleaned) for the minimum practical number of surfaces
commensurate with an off-axis paraboloid design (two OAPs, a segmented DM and a
dichroic coating compromised to give good throughput to the WFS.
· 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%.
The system shall be designed to permit an upgrade to Na laser beacon
operation so that the sky coverage over which high Strehl ratios at K can be
obtained is limited only by the availability of tip-tilt guide stars when used
with wavefront and tip-tilt sensing detectors of at least the sensitivity
required by clause C.
Note: this does not mean FULLY EXPLOIT a Na laser beacon - (that is for a generation-2 WHT AO system).
This clause drives the leaving upgrade of an upgrade path to install a separate tip-tilt sensor
A summary table is given below which associates important high spatial resolution astronomical projects with particular clauses.
A number of detailed astronomical cases have been made for AO. A new Gemini document, “Science Drivers for Adaptive Optics on Gemini-North” (1996) by Simon Morris et al. (DAO), is a particularly helpful source of material which distinguishes between high resolution observations most appropriate for 4m AO, 8m AO and HST. The reader is also referred to the AO internal document AOP/SCI/GG/1.0/09/95 from which many of the examples in the summary table below are drawn. Readers are also referred to the Gemini Adaptive Optics Working Group Report (Ellerbroek et al, Racine Chair, October 1993) and to the article by James Beletic in “Future of Space Imaging” (1994), the initial case for the HST Advanced Camera.. The latter, in particular, points out the importance of corrected PSF contrast (or core/halo ratio) when doing imaging or high-spatial resolution spectroscopy in the presence of a diffuse background or a very bright nearby source. Partial adaptive optics (Clause 2) leaves a high proportion of PSF intensity in an extended halo which can seriously affect contrast in many projects. It is for this reason many of the projects in the summary table below have been ascribed to Clause1.
As a general point, the use of the WHT NGS system (in Clause 1) will permit the imaging of any fainter point sources. (1 magnitude deeper in K despite the higher emissivity and reduced throughput inevitably introduced by the AO system optics).
|
Clause |
Section |
Specification |
Astronomical
Examples (near-IR) |
|
1 |
2.1 |
Image quality |
AGNs and starbursts, Stellar populations (in crowded fields and when seen against extended sources such as external galaxies and galactic nebulae), Young and evolved stars. Extragalactic distance scale (surface brightness fluctuations). Planetary Nebulae. |
|
2 |
2.2 |
Sky coverage |
Galactic Cores, High-redshift Galaxies, unbiased surveys in general. The ability to get improved resn and/or higher snr obs, on a wide range of unique or one-off type sources (i.e. a reasonable probability of being able to observe a specific target.) |
|
3 |
2.3 |
l/seeing range |
Faint stellar spectroscopy. |
|
4 |
2.4 |
Dithering |
To obtain best flat-fielding on IR array, beam switching
along a slit. |
|
5 |
2.5 |
Instrumentation |
Imaging, spectroscopy, coronography. |