Transcript (HOWFS_type_ _#_of_subaps.ppt)

LGS wavefront sensor : Type and number
of sub-apertures
NGAO Team Meeting #4
V. Velur
Caltech Optical Observatories
01/22/2007
Presentation Outline
• Introduction - WBS dictionary definition
• LGS WFS type –
– Assumptions
– Handling extended LGS spots
– Pulse tracking techniques and comparison
• Number of sub-apertures - procedure
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Narrow field case SHWFS
Narrow field case PWFS
Wide field case SHWFS
Wide field case PWFS
Bad seeing, cirrus conditions on SHWFS and PWFS
• Conclusions
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WBS dictionary definition
• 3.1.2.2.6: Consider alternative WFS designs (e.g. Shack-Hartmann
vs. pyramid) for different laser pulse formats. Evaluate and compare
the advantages of e.g. short pulse tracking using radial geometry
CCDs and mechanical pulse trackers. Complete when LGS WFS
requirements have been documented.
• 3.1.2.2.7: Consider the cost/benefit of supporting different format
LGS wavefront sensors (e.g. 44 subaps across, vs. 32, vs 24.)
Consider the operational scenarios required to meet science
requirements in poor atmospheric seeing or cirrus conditions?
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LGS WFS type
• For this study I have only considered a Shack Hartmann and a
Pyramid WFSs.
• Assumptions:
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Handling an extended LGS spot
• The baseline design of NGAO uses a 150W from CW laser(s). The
LGS spots from these will produce elongated spots on the WFS
even if projected from the center.
• spot elongation (in seconds) is given by:
206264.81 (sec/rad)* (s * t * (cos(z))^2)/(h^2);
• Minimum spot size at the farthest sub-aperture when projected from
the center (in the radial direction):
(5 * 10 * 10^(3) * 1)/((90*10^3)^2)*206264.81 = 1.28 arcsec
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How a Shack Hartmann WFS handles
elongated spots:
1. Use sub-apertures that have a FoV greater than spot size in the
radial direction. This can be done by binning or by choosing a
suitable plate scale. But comes at the cost of extra noise in the
sensor
2. Use radial geometry CCDs as LGS WFS - Beletic et. al. are funded
by AODP and CARA to develop low noise CCD detectors with a
planar JFET based amplifier on a back-thined device (CCID-56b).
The noise estimates from these rectilinear CCDs is very
encouraging (1 e− RON at 1 MHz pixel rate). The radial versions of
these devices, as shown in figures 1 2, will be well suited to handle
a CW laser spots that are centrally projected. To reduce centroiding
error due to elongated spots either a noise optimal centroiding
algorithm or a matched filter scheme can be used.
3. Use a mechanical resonator to keep the spot in focus (will talk about
it in detail later in the presentation).
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How a pyramid WFS handles elongated
spots
1. Learn to work with an elongated spot. The only reference (Igelesias
et. al.) that I could find suggests that this is feasible. The paper
deals with performing WF sensing on the human eye and claims
that a point pyramid WFS becomes twice as sensitive (at the cost of
linearity) when working with an extended object. So, in principle one
could build a sensor that has the capture range (linearity) and
appropriate sensitivity. Word of caution : There is no SNR
calculations and the fact that there is no similar paper from the
astronomical community this approach must be further investigated,
if not ignored!
2. Use a mechanical pulse tracker or some other pulse tracking
scheme (see descriptions later). This will give the PWFS the
advantage of working with a AO corrected spot rather than a blurred
spot due to elongation (or a sub-aperture limited spot size as seen
by the SHWFS). The effect of this in terms of wavefront error is
shown in the second part of the presentation.
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Pulse tracking schemes for 1-3 microsec
lasers:
1. Radial geometry CCDs (only good for a SHWFS):
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Mechanical pulse tracker
1.
2.
3.
4.
5.
6.
7.
Built out of 6Al14V Titanium alloy or some similar aluminum alloy.
The geometry of the resonator may be step, exponential or a hybrid horn with active
cooling.
The input transducer stresses the input end and this is manifested as motion on the
other end. The design makes sure that the output end remains flat when the motion
is induced.
We may have to give the resonator manufacturer the mirror specification (size,
weight, density, geometry etc.), and we may have to figure out a bonding technique
that will withstand ultrasonic operation.
For example, a 20 KHz resonator is 5” in length and 3” at the transducer end and
0.25” at the output. The size for a 50 KHz resonator is approximately 2/5 th. The
lower the natural frequency the greater the amplitude obtained from the resonator.
Typical amplitudes obtained from a 20 KHz resonator is 300 microns. So a 50 KHz
resonator will yield a stroke of ~120 microns.
Georges et. al. describe a optical system that can convert a 15 mm focus shift
(native to telescope/AO) to a 120 micron motion. The design uses 14 extra optical
surfaces. We may come up with a more novel design.
The cost of the resonator is between $2500 - $3500 (word-of-mouth quote from Krell
Engineering depending on complexity). There are multiple vendors for drive
electronics (Branson Ultrasonics, Sonics and Materials etc.) and the cost for the
electronics is about $4000. These units consume about 2KW of power.
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Georges et. al.’s optical design
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Other schemes (B. Bauman’s ideas)
3. Use of 2 orthogonal cylindrical lenses with a continuously varying
RoC shaped to fit along the circumference of a rotating disc. Uses
only 4 extra surfaces!
4. Mechanical MEMS resonator
Comparison table:
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Number of Sub-apertures Trade Study
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In this trade study merit is indicated by WFE (based on Rich’s error budget
tool) and the laser power required is assumed to be cost driver (without
actual mention of $ value). It also assumes that the complexity in switching
plate scales and lenslets in not the cost driver!
The study looks at the performance of the NGAO system with SHWFS and
PWFS using 44, 32 and 24 sub-apertures all working with a 64 actuator DM
in wide field and narrow field conditions.
Narrow field case : on axis TT star w/ 10% of TT path light going to a slow
WFS, TT star mv = 19, Chris Neyman's atmospheric parameters, SCAO,
quincunx radius=5 arcsec. The H - band Strehl was optimized by varying
the TT and HOWFS loop rate.
Wide field case: sky coverage = 15% (19 mv star limit with a search radius
that is varied). I optimize HO integration time, TT int. time, TT guide star
brightness (at the cost of search radius) to optimize H band Strehl. The LGS
asterism radius is set to 33 arc-sec. as suggested by KAON429.
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Narrow field case SHWFS
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Narrow field PWFS
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Wide field SHWFS
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Wide field PWFS
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PWFS vs. SHWFS
• When modeling a PWFS the spot size advantage during centroiding
is used, also the charge diffusion is set to zero (which is assumed to
be .30 pixels based on Palomar #s, this is different from van Dam’s
#s).
• Will present plots and tables on the report.
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Bad seeing conditions
• No cirrus and r0=0.10 m, wind velocity = 15.0 m/s, laser power =
150W
• Bad seeing and bad laser (which I also assume is bad cirrus
condition, which is optimistic because I don’t account for the extra
scatter caused by the extinction) - r0 = 0.10 m, wind velocity = 15.0
m/s and laser power = 50W. The extra scatter will be accounted for
after the results from the Rayleigh scatter study are in.
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Bad seeing, Cirrus clouds cases:
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Conclusions
1. If we are going to be using a CW laser it is easiest to work with a SHWFS
and a radial geometry CCD. Both technologies are mature (as compared to
counterparts) and the advantage of PWFS is only a few nm in WFE as
presented by current models.
2. With a pulsed laser, still the option of SHWFS with a radial format CCD,
seems like the simplest and most efficient way to proceed w.r.t pulse
trackers.
3. It is useful to have the option of multiple sub-apertures only in case of low
laser power at the 50W level. Otherwise there is only a few nm of WFE
difference between the 32 sub-apertures and 44 sub-aperture case. The 24
sub-aperture case performs quite badly except for the 50W laser power
case. So the EC must make a choice of either 24 and 32 sub-apertures or
24 and 44 sub-apertures if the incremental cost or packaging constraint of
supporting 3 lenslet gets to be too much work.
4. In case of bad seeing, serious Cirrus conditions only narrow field science
must be performed and 32 sub-aperture case gives optimal performance for
both cases. There is no significant difference between the PWFS and
SHWFS case. For the wide field case the WFE in case of bad seeing and
cirrus is 610 nm and that in case of bad seeing and no cirrus is 567nm.
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