Keck Adaptive Optics Note 237
Keck Precision Adaptive Optics: Technical Requirements and Constraints
P. Wizinowich, R. Dekany, M. Troy, G. Chanan, C. Neyman
July 14, 2003
1. Introduction
The AOWG strategic plan report of 11/12/02 stated that: “Our primary recommendation is for the
development of the first next-generation high-performance general-use AO system, which would deliver
stable high Strehl ratio infrared images in moderate field-of-view areas throughout the sky.” Further
details from this report included: “Such a system is envisioned to be one which would deliver extremely
high infrared Strehl ratios, high stability, near-complete sky coverage, a good knowledge of the delivered
PSF, and the ability to observe at high resolution into the visible (albeit with lower Strehl ratios).” The
AOWG dubbed this system KPAO for Keck Precision Adaptive Optics.
The purpose of this document is to begin the process of investigating and documenting the top-level
requirements for KPAO. We begin by listing the top-level requirements from the above AOWG report
summary, discuss their implications and list some preliminary design goals. We then discuss a number of
design constraints (additional requirements) imposed by the implementation of a KPAO system at Keck
Observatory.
2. Performance Requirements and Implications for KPAO
The top-level requirements from the AOWG report are listed below along with their top-level implications:
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High infrared Strehl ratios.
The first implication is that this requires significantly lower wavefront errors than the
requirements for the existing Keck AO systems. Significantly lower wavefront errors will likely
require reducing all of the current largest error sources including those due to measurement
(brighter LGS, less spot elongation and/or lower noise detectors), fitting (more subapertures),
focal anisoplanatism (more laser spots and wavefront sensors), bandwidth (faster wavefront
controller – also to handle more subapertures), the telescopes and science instruments (better
telescope and instrument wavefront quality and reduced vibrations).
We have selected two rms wavefront error values to evaluate as the initial design requirement for
KPAO: 120 and 180 nm.
o This is for the on-axis (LGS & NGS) case under median seeing conditions, within 45
degrees of the zenith, and assuming an NGS mV < 17. This includes atmospheric,
telescope, AO system and science instrument contributions.
o For reference this represents an H-band Strehl ratio of 0.81 and 0.63, respectively.
o There is no requirement to deal with isoplanatic effects. KPAO will not introduce any
significant off-axis errors.
High Strehl stability.
Although short-exposure photometric stability is predicted to improve with increasing absolute
Strehl value, this effect must be quantified through simulation and experiment.
Does this only mean maintaining the Strehl ratio within some tolerance or does it also mean
maintaining the PSF stability?
A potential initial design requirement for KPAO could be to maintain the rms wavefront error to 
15 nm. For the 180 nm rms wavefront error case, this corresponds to an H-band Strehl of 0.63 
0.05.
Moderate field of view.
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The AOWG discussions indicated that the Strehl and Strehl stability requirements were more
important than a large compensated field of view. The concept was that in order to achieve a
certain sensitivity over a given field it was quicker to mosaic together smaller fields with high
Strehls than to integrate longer with lower Strehls over a larger field. The conclusion was that the
moderate fields provided by the existing AO systems could be adequate.
For our design purposes we have chosen the following initial requirements:
 Science field of view: 30 arcsec radius.
 Wavefront sensor field of regard: 30 arcsec radius.
 Tip/tilt sensor field of regard: 60 arcsec radius.
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Near complete sky coverage.
There is no other obvious solution than to use a laser(s) to achieve significant sky coverage.
We are still assuming that natural guide stars, perhaps multiple NGS, will be required to perform
the low order correction. This is an important statement because the availability of suitable NGS
will largely determine the sky coverage.
Sky coverage is calculable based on the minimum acceptable performance, the
availability of suitable NGS and the atmospheric and AO system parameters. The best
performance sky coverage numbers will be for the case when the isoplanatic errors are
less than or equal to the rms wavefront error for the on-axis case. Larger sky coverage
numbers can be quoted for the case when the NGS error terms dominate the performance.
We would therefore like to define sky coverage as a function of performance; we will do
so in a future design note.
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Good knowledge of the delivered PSF.
Requires well-calibrated real-time diagnostic data from the wavefront controller and the tools to
convert this into a PSF estimate.
How we quantify this requirement in a useful and practical manner needs investigation. Ideally
we would be able to produce a PSF image corresponding to each science image and the relative
energy (relative to the total energy) in each pixel would be accurate to x%.
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Wavelength coverage
KPAO will allow science operations from 0.45 to 14 microns, with optical transmission to be later
specified.
The science wavelength coverage requirement for the current AO system is 1-14 microns. The
0.45 to 1 micron requirement would necessitate additional wavelength pass-through to the science
instrument versus a short wavelength cutoff of 1 micron for the current Keck AO systems. Notch
dichroics and/or filters would also be required for the sodium wavelength.
New science instruments would be required to take advantage of the 0.45 to 1 micron wavelength
coverage.
We need to define an atmospheric dispersion compensation requirement.
Facility-class system.
Although not explicitly mentioned in the AOWG strategic plan, there is an obvious need to make
KPAO a facility-class instrument. There are numerous issues and requirements that need to be
included under this category including: level of automation, tools for optimization, user interface,
reliability, etc.
We will revisit the above requirements with the AOWG to ensure that they satisfy the science requirements
and also to determine their relative priorities. We also need to ensure that these requirements are complete.
For example, we should tune the performance requirements to the science instrument(s), especially a new
instrument built specifically for KPAO. The above requirements also do not specifically address sensitivity
(e.g., signal to noise ratio in science image per exposure time) or the required angular resolution.
3. Design Constraints
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3.1 KPAO Location
The Keck telescope foci and Nasmyth deck storage locations are already heavily utilized. The current AO
systems occupy the left Nasmyth platforms of both telescopes. HIRES occupies the right Nasmyth
platform of K1. DEIMOS, NIRSPEC and KIRMOS are all intended to share the right Nasmyth platform of
K2. Both telescopes have Cassegrain instruments (LRIS and ESI, respectively) and K1 has a forward
Cassegrain instrument (NIRC and LWS). Both telescopes have some free bent Cassegrain ports, but the
space and weight capacities of these ports are small. The prime focus could potentially be available but
there would be lots of constraints on an instrument at this location. On the KPAO timescale we should
consider whether space could be made available by decommissioning an instrument; however since KPAO
is likely to be a Nasmyth platform instrument the most likely options would be to replace the existing AO
systems or possibly HIRES.
3.2 Science Instruments
We currently operate the K2 AO system with NIRC2, NIRSPEC and the Interferometer, and OSIRIS is
under construction. K1 AO is only used with the Interferometer. NIRC2 and OSIRIS are fed with an f/15
beam. The AO bench includes some re-imaging optics to feed NIRSPEC at ~ f/150. The Interferometer
expects to be fed with a collimated or f/15 beam depending on the science mode.
Presumably a requirement would be for KPAO to feed OSIRIS and the Interferometer and likely NIRC2,
but not NIRSPEC.
From the interferometer perspective the two AO systems must look essentially identical in terms of field
and pupil rotation, and polarization. This largely requires identical optical paths through the AO systems
on both telescopes. A higher performance AO system on one Keck telescope would likely be acceptable
from the interferometer perspective, although some means of matching the Strehl ratio or flux from the two
AO systems might be required.
Due to the timescale of building KPAO (and the resultant advances in instrument technology) and the need
for a science instrument matched to the higher capabilities of KPAO we should be considering the design
of a KPAO optimized science instrument in parallel with the KPAO design. This new instrument and
KPAO would provide design constraints on each other. By not considering the science instrument in
parallel we lose the opportunity to optimize them together and reduce the flexibility in the instrument
design. Note that a new science instrument need not be commissioned at the same time as KPAO, since we
will have NIRC2 and OSIRIS, but we do need to plan for it.
The extreme Adaptive Optics Planet Imager (XAOPI) is currently being designed to go behind the K1 AO
system and is only intended to be commissioned a couple of years before KPAO. A requirement may be
for KPAO and XAOPI to work together. The allocation of the KPAO wavefront error budget may be
influenced by XAOPI specific requirements.
Keck is currently participating in the early proof of concept phases of the OHANA project, a project to
interferometrically combine all of the AO equipped telescopes on Mauna Kea with infrared fibers. If this
project is successful then we would want to accommodate a fiber injection module at the output of KPAO.
3.3 Natural Guide Stars
Is there a requirement to work on natural guide stars (NGS)? And if so, what is the faint magnitude
requirement?
Most of the interferometer targets are bright enough to be the NGS. Potentially better performance could
be obtained in some scenarios with a bright NGS.
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It would be very useful to be able to use a bright NGS, prior to moving to the LGS system, during the
telescope integration and test phase. For this purpose the same number of actuators/subapertures as the
LGS system would be fine. The use of NGS would be very useful for system characterization.
Sky conditions, such as cirrus, could limit sodium laser operation. The absence of an NGS capability
would require backup programs with other instrument. This latter point could be handled via queue
scheduling; this would be most difficult on interferometer nights since both telescopes would need to be
rescheduled.
There are a number of implications to a NGS requirement, such as requiring the wavefront sensor to be
achromatic and dispersion compensated. Depending on the faint limit requirement this could imply the
need for control algorithms that interpolate between actuators and low hysteresis or monitored actuators, in
order to use larger subapertures.
3.4 Secondary Mirror
Both telescopes have f/15 secondary mirrors, as well as chopping secondary units that can accommodate
f/25 and f/40 secondary mirrors. The f/25 focus is at the forward Cassegrain location, while the f/15 and
f/40 foci are at the same Cassegrain, Nasmyth and bent-Cassegrain locations. K1’s chopper was primarily
tested for chopping while K2’s was primarily designed for fast tip/tilt correction, with more limited
chopping capability. The choice of f/15 secondary mirrors for the current AO systems was largely driven
by the resultant size of the AO systems (presumably 2.5 times smaller than for f/40) and by the availability
of PCS (Phasing Camera System) at f/15 via a rotation of the tertiary mirror. Note that the primary mirror
is the system stop for f/15 and the secondary is the stop for f/25 or f/40. Projecting at least one LGS from
behind the secondary mirror would reduce the total required laser power.
One option for reducing system emissivity and for a deformable mirror is an adaptive secondary mirror
(ASM) as was recently implemented at the MMT (0.6 m diameter with ~ 300 actuators). We could have
ASMs for just KPAO if appropriate. An ASM is most attractive from the perspective of significantly
reduced emissivity; but this is only really achieved if the ASM can provide enough correction (a 2 nd or
more DMs might be required in order to get enough degrees of correction or to have a DM conjugate to
another layer in the atmosphere). A Keck ASM would require an additional top end storage location or
decommissioning the chopping module (since it is currently only used on K1 for NIRC/LWS). In the case
of decommissioning the chopping module this top end module and storage location would become
available. Note that the existing f/40 and f/15 secondary mirrors are 0.5 m and 1.4 m in diameter,
respectively. It could also be possible to make an f/15 ASM that was used by all instruments so that no, or
very few, top-end changes were required. This would be very desirable if we had an ASM, especially with
laser hardware behind it, due to the potential damage to actuators, fittings, connectors and alignments
during top end exchanges.
The inability of the current Keck AO system to chop at the telescope pupil has been a limitation for thermal
IR observations. We should evaluate whether chopping at a pupil needs to be a requirement for KPAO.
This could potentially be accomplished with the secondary mirror (if it were the system stop) or another
mirror conjugate to the pupil.
3.5 Laser and Projector Location
The current laser is housed in a thermally insulated room on the K2 dome floor and on the elevation ring of
the telescope; and the projector is on the side of the telescope. These locations would also be available for
a next generation laser system. In addition, there may be adequate space behind the f/15 secondary mirror
at least for a projection telescope. There would be considerably less space behind the chopping secondary
unit, especially, as is likely, it was necessary to hide this in the mirrors shadow for emissivity reasons.
3.6 Schedule and Budget
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An additional design constraint is of course the timeline of when we want KPAO and when we can start to
seriously work on the design. In the Observatory’s current plan this design process will only seriously
begin in FY05. Similarly, the available budget will act as a design constraint.
4. Summary
This document represents a first step in the process of defining KPAO (the final version will result from
iteration with the AOWG and others). We have taken the statements from the AOWG strategic plan report
that first mentioned KPAO and the Observatory design constraints to develop the initial KPAO
requirements recorded here.
Our next step will be to produce an error budget and a point design(s) to support this error budget. This
will require understanding and documenting the error budget for the existing Keck AO systems and in the
process characterizing the atmospheric parameters. The requirements will need to be reworked as a result
of this error budget analysis. Reworking of the requirements, and the high level architectural choices, will
also necessarily follow from further development of the science case.
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