Corner Rafts
LSST Camera Review
SLAC
Oct. 14, 2008
Scot Olivier
LLNL
1
Corner Raft Assemblies: Reference Design
________________________________________________
•
Four corner rafts are located in the
corners of the focal plane
– Corner rafts contain wavefront
sensors and guide sensors
– Wavefront sensors are located in
the single inner position, nearest
the center of the focal plane, with
an area equivalent to one science
detector
– Guide sensors are located in the
two outer positions, farthest from
the center of the focal plane, each
with an area equivalent to one
science detector
Wavefront Sensors
(4 locations)
Guide Sensors
(8 locations)
3.5 degree Field of View
(634 mm diameter)
Corner raft
positions
Corner Raft Tower concept (Kirk Arndt, Purdue)
________________________________________________
• Mechanical and thermal
design of the corner rafts
is as similar as possible
to the science rafts
V-block kinematic mounts
based on Science Raft
design
Clearance
volumes for
Guider FEEs
Single Rail
Double Board
for WFS FEE
Cooling plate /
thermal strap
to Corner Raft
Triangular Tower fits
inside GRID corner
bays and mounts to
Cryoplate
• Electronics for operating
the wavefront sensors
and guide sensors are
packaged within the
corner raft volume
behind the detectors,
similar to the science raft
configuration
• Data acquisition and
control for the wavefront
and guide sensors are
managed using the same
infrastructure as for the
science detectors
Corner Raft/Towers in GRID/Cryoplate
________________________________________________
Guide sensors
WFS module
A single Corner Raft-Tower design in all corners of GRID/Cryoplate
 Corner Raft-Towers are interchangeable
 Sensor, FEE and raft-hold-down orientation w.r.t.
GRID/Cryoplate/Camera are rotated 4x 90 degrees
Simplified Thermal Models
________________________________________________
1-D model (electrical analog) for first-order calculation of thermal gradients
 Ignore lateral heat flows
 Rn = bulk + interface thermal impedance of component
 Tower cold mass = cooling plates + housing sidewalls
 Strategy: make R4 dominant, keep R5 minimum
 Makeup heater may be necessary to compensate for time variation of L3 and
cryoplate temperatures, and for controlled warm-up
6.4
1.1 W
0.36 K/W
6.0
0.27 K/W
5.7
0.12 K/W
5.5
+ CMOS
dissipation
4.36 K/W
0.34 K/W
3.3 W
Science Raft Tower
0.7
3
0.54
0
Corner Raft Tower
Wavefront Sensors
Scot Olivier
6
Wavefront Sensors Assemblies: Reference Design
________________________________________________
•
•
Four wavefront sensors are located in the
corners of the focal plane
– Tomographic wavefront reconstruction
algorithm developed for LSST was used
to evaluate the placement of wavefront
sensors
– Four wavefront sensors in a square
arrangement were found to be adequate
to meet requirements
Wavefront sensors are curvature sensors
– Measure the spatial intensity
distribution equal distances on either
side of focus
– The phase of the wavefront is related to
the change in spatial intensity via the
transport of intensity equation
– The phase is then recovered by solving
this equation
Wavefront Sensors
(4 locations)
Guide Sensors
(8 locations)
3.5 degree Field of View
(634 mm diameter)
2d
Focal plane
Sci CCD
40 mm
Curvature Sensor Side View Configuration
Tomographic wavefront reconstruction
________________________________________________
•
•
Collecting wave-front data from stars located at different field angles
enables a tomographic reconstruction of the mirror aberrations.
The tomographic problem can be reduced to a matrix problem by assuming
an annular Zernike expansion of aberrations at each of the mirror surfaces.
Tomography geometry
Ref: George N. Lawrence and Weng W. Chow, Opt. Lett. 9, 267 (1984).
D.W. Phillion, S.S. Olivier, K.L. Baker, L. Seppala, S. Hvisc, SPIE 6272 627213 (2006).
K.L. Baker, Opt. Lett. 31, 730 (2006).
Curvature wavefront sensor
________________________________________________
•
•
Recording images on each side of
focus enables reconstruction of
wavefront aberrations by solving
the transport of intensity equation
Wave optics modeling has been
performed to analyze images from
curvature sensors
– Includes effects of atmospheric
turbulence and noise
Intra-focal intensity image
Dz=-1 mm
CWFS
Extra-focal intensity image
Focal Plane
d
d
40 mm 40 mm
Curvature wavefront sensor geometry
Dz=+1 mm
Wavefront Sensors Assemblies: Design Details
________________________________________________
•
•
•
•
Curvature sensor design images two
different fields at two different focal
positions
Design uses the same detector
technology as the science focal plane
array, but with half the size in one
dimension to enable shifting focal
position between two halves of sensor
Pinout can be identical to normal
science sensors (multilayer AlN
substrate)
Looks identical to Timing/Control
Module & CCS
Wavefront Sensors: Required Accuracy
________________________________________________
•
Wavefront sensor errors are propagated through the tomographic wavefront
reconstruction resulting in errors in the controlled shapes of the telescope mirrors
An image FWHM error budget of < 0.10 arc second is achieved for this wavefront
sensor configuration with < 200 nm wavefront sensor errors and 200 nm residual
atmospheric aberration for 15 second exposure
•
Telescope Design + Wavefront Reconstruction
Wavefront Reconstruction
Telescope Design
0.16
0.15
FWHM (arc seconds)
0.14
0.13
0.12
0.11
0.10
0.09
0.08
0.07
0.06
0.000
50.000
100.000
150.000
200.000
250.000
300.000
350.000
400.000
Curvature Sensor Error (nm )
•
Fundamental Issue: Are there enough field stars of the magnitudes necessary to
provide the required wavefront accuracy in each of the 4 sensors for each pointing?
Curvature WFS images and phase vs. stellar magnitude
________________________________________________
•
Wavefront sensor images of dim
stars are sky background limited
Stellar Magnitude
(i band)
Reconstructed
phase
Intra focus
CCD Image
Extra focus
CCD Image
19
18
Applied phase
17
16
15
14
Histograms of sky brightness in LSST survey
________________________________________________
RMS CWFS error vs. Stellar Mag.
________________________________________________
R U Y Band
RMS CWFS error [nm]
250
200
150
U
R
Y
100
50
0
10
12
14
16
Stellar Magnitude
18
20
Probability of finding stars as a function of magnitude
________________________________________________
(Probability of star in 88 sq arcmin)^8
Four split detectors
90 degree Galactic Latitude
1.0
0.9
0.8
0.7
0.6
U
0.5
R
0.4
Y
0.3
0.2
0.1
0.0
10
12
14
16
Magnitude at wavelength
18
20
Probability of finding stars as a function of CWFS error
________________________________________________
•
•
There are enough field stars of the magnitudes necessary to provide the required
wavefront accuracy in each of the 4 sensors for each field in g, r, i, z
At high galactic latitudes ~5% of fields in y and ~15% of fields in u will have degraded
accuracy due to the absence of bright field stars
If wavefront sensor errors are too large, the control system can delay mirror
adjustments until the next pointing with little degradation in optical performance
90 degree
(Probability of star in 88 sq arcmin)^8
•
1.0
0.9
0.8
0.7
0.6
U
0.5
R
0.4
Y
0.3
0.2
0.1
0.0
0
50
100
150
200
250
RMS CWFS error [nm]
300
350
400
Wavefront Sensors Assembly: Issues
________________________________________________
•
Wavefront Sensor Baseline Validation
– Is there a curvature sensing algorithm that works for a split detector at the edge
of the LSST field ?
• Challenges include variable vignetting, registration between extra-focal images, variable
atmospheric dispersion
• Yes, the vignetting can be corrected, and the correct registration determined, atmospheric
dispersion to an accuracy related to the uncertainty in the stellar spectral energy distribution
• Pistoning the detector between exposures also investigated
– current analyses indicate this approach, which introduces mechanical complexity, not
necessary to achieve the required performance
– What is the required axial shift for the extra-focal images?
•
•
•
•
Atmosphere – bigger shifts
Detector noise, crowding – smaller shifts
1 mm meets requirements
Crowding analysis shows isolated stars can be found
– What are the requirements on flatness and positioning of the wavefront sensor
detectors?
• Flatness spec nominally similar to science arrays but
– additional analysis ongoing to investigate possible relaxation of spec
– detectors likely to have characteristics similar to science arrays
• Positioning of detectors relative to focal plane relatively insensitive
– offsets of up to ~100 microns (1/10 of nominal 1 mm offset) can be calibrated on the sky
Analysis of variable vignetting
________________________________________________
Q=1.78 degrees
Outside corner
Q=1.75 degrees
Max field angle
Q=1.62 degrees
CWFS area center
Q=1.45 degrees
Inside corner
Pupil vignetting
•
Vignetting is variable throughout wavefront sensor field
Wavefront sensor
images including
vignetting
Q=1.62 degrees
CWFS area center
•
Q=1.45 degrees
Inside corner
Analysis demonstrates that wavefront sensor images of stars with different
vignetting can be successfully combined to produce accurate measurements
Analysis of crowded fields
________________________________________________
•
•
•
Typical star field with Dz=1 mm defocus
in Y band at galactic equator
~10 stars with magnitude y<15 are
isolated from all stars with magnitude
y<18
Analysis demonstrates that even the
most crowded fields still have usable
stars for wavefront sensing
– No need for more complicated
deconvolution algorithms
Purdue test station
________________________________________________
Includes a Cryostat (LN2 cooling + vacuum system), CCD controller, X-ray (Fe55) source, optical flat-field
source, focusing optics + motion control, monochrometer, electronic and mechanical shutters, and
instrumentation (modeled on setup at BNL).
Initially configured for tests using single sensors for wavefront and guider studies. In the future, the test
station will be expanded to accommodate tests of a full Corner Raft/Tower.
Light Shield
Power Supply
Lamp Housing
RS-232 / USB
and GPIB
CCD Controller
Lens
Holder
Shutter
Controller
Dewar
Monochromator
Shutter
Adjustable
Slit
Long pass filter
Adjustable
Slit
Liquid Light
Guide
Light-Tight
Box
Integrating
Sphere
Calibrated
Photodiode
Windows
PC
Linux PC
Temp.
Controller
Stage
Controller
Power
Meter
Equipment setup in Purdue cleanroom
________________________________________________
Turbo vacuum
pump
Dewar from
Universal Cryogenics
site for CCD
controller
LN2 storage
cylinder
Dark box
monochrometer
integrating
sphere
All items purchased exclusively
for LSST using University funds
Guiders
Scot Olivier
22
Guide Sensor Assemblies: Reference Design
________________________________________________
•
•
Eight guide sensors are located in the
corners of the focal plane
– Guide sensors in each corner raft
occupy an area equivalent to 2 science
detectors.
Wavefront Sensors
(4 locations)
Guide Sensors
(8 locations)
Baseline guide sensors are CMOS detectors
– The Hybrid Visible Silicon H4RG is a
4K×4K optical imager produced by
Teledyne Scientific and Imaging which
has recently been tested on the sky at
Kitt Peak
– CCD detectors are still an option to be
evaluated
CCS +Y
CCS +X
3.5 degree Field of View
(634 mm diameter)
CCS +Z
Guider Requirements
________________________________________________
Requirement
Centroid noise
Update frequency
Wavelength range
Sky coverage
Latency
Definition
error between computed centroid
location and true location of star
frequency of centroid updates
range of wavelengths for guider
requirements
range of points for guider
requirements
delay between average time of
photon arrival in guide image and
when centroid is delivered
Number of guide groups
number of independent guide
locations
Acquisition delay
time between shutter open and
first centroid
Value
Units
Rationale
23.5 mas FWHM allocation from higher level budget
10 Hz
loop must converge faster than LSST
exposure time
All
SRD
All
SRD
60 ms
allocation from higher level budget
4
100 ms
At least 2 stars needed to solve for rotation;
4 stars needed to reduce uncorrelated
atmospheric jitter to acceptable level
Guide Sensors Assemblies: Design Details
________________________________________________
•
The baseline guide sensor is a CMOS
detector
– The Teledyne HiViSi H4RG CMOS
detector has been tested extensively to
evaluate its performance
– Results from these tests are promising
but more development is needed to meet
requirements
– CCD alternatives are under consideration
•
A guider processor should receive the
signals from the guide sensors via optical
fibers
The command interface to the TCS could be
TCP/IP, as latencies are not too critical
The command interface from the Guider
processor to the Telescope Servo should be
direct, this is necessary to guarantee a
transport delay of less than 10 msec, with no
latencies
The guiders should receive power,
communication signals, and cooling via the
common Camera/Telescope interfaces
•
•
•
H4RG detector
Guider SIDECAR ASIC Package
from
Don Figer, RIT
________________________________________________
• SIDECAR ASIC interfaces directly to analog
readout circuits
• Low power, high performance
• Integrated circuit contains microprocessor,
bias generators, clocks, plus 36 input video
channels, 36 parallel ADCs (12 bit /10 MHz, 16
bit /500 kHz)
• SIDECAR is being used for three JWST
instruments (4 port readout, 11 mW)
SIDECAR
• HST
SIDECAR
is keycm)to Hubble Space JWST
Telescope
Package
Package (3.5×3.5
Does notACS
show hermetically
lid over ASIC CCDs
(does not show protective package lid)
Repairsealed
– operates
Estimated 37mm x 15mm x 100mm size of JWST package
is the size of the Guide sensor FEE clearance volumes in
the CAD model shown in following slides
Guider: Atmospheric Model
________________________________________________
•
•
The 4 guider groups, will be looking at a
different patch of atmosphere above 136m,
therefore the middle to upper atmospheric
layer disturbance signal will be mostly decorrelated between the guider groups, and
the ground and low atmospheric layers will be
mostly correlated
The degree of correlated and de-correlated
signal will vary as a function of time,
depending on the particular atmospheric
conditions
Guide Star1
Guide Star2
610m
3.5deg
8.4m
Guider Atmospheric Model
Guider: Signal Model
________________________________________________
Guide Star1
Un-Correlated
Atmosphere
Correlated
Atmosphere
UpperLayer1
+
Guider1
Centroid
UpperLayer2
+
Guider2
Centroid
Guide Star2
Lower
Layer
Guide Star3
Optimal
Filter
UpperLayer3
+
Guider3
Centroid
UpperLayer4
+
Guider4
Centroid
Guide Star4
Correlated
Telescope and
Optics Jitter
Guider Signal Block Diagram
Correlated
Signal
Estimate
LSST Nominal Guider Signal Budget
________________________________________________
Seeing = 0.62”FWHM at Zenith (50%) ,Outer Scale = 23.4m(50%)
Air Mass = 1.243 ( 50% From Cadence Simulator)
Telescope Servo = 0.02”RMS
+
Telescope Total = 0.024” RMS
Ground Layer
( 0.0295” RMS)
Telescope Periodic Error = 0.0125”RMS
Correlated Jitter
0.052”RMS
Servo Error
Rejection
+
Wind Induced Motion
0.036” RMS
Centroid Noise
0.020” RMS
Upper Layers
(0.032” RMS)
+
Variable Bandwidth
0.01 - 50Hz
Decorrelated Jitter
0.019” RMS
0.038” RMS
Correlated Error
X
+
Pointing
Jitter
Servo Closed
Loop
Decorrelated Error
K=1 / Sqrt(# of Guiders) = 0.5
# of Guiders = 4
Guide Sensor Assembly: requirements
________________________________________________
Centroid noise error budget allocation < 0.02 arc second FWHM is met for
stars ranging from magnitude 13 (y) to 16 (g) for a guide sensor with 20
electrons read noise using a 10x10 pixel window around the star
– better performance can be obtained using a matched filter or
correlation algorithm.
0.1
0.09
0.08
FWHM (arc seconds)
•
0.07
U
G
0.06
R
0.05
I
0.04
Z
Y
0.03
0.02
0.01
0
10
12
14
16
stellar mag
18
20
22
Guide Sensor Assembly: Analysis
________________________________________________
•
•
•
Fundamental Issue: Are there enough field stars of the magnitudes necessary to
provide the required centroid accuracy in each of the 4 corners for each pointing?
There are enough field stars of the magnitudes necessary to provide the required
centroid accuracy in each of the 4 sensors for each field in g, r, i, z
At high galactic latitudes ~4% of fields in y and ~20% of fields in u will have
degraded accuracy due to the absence of bright field stars
The effect of this degradation on overall image quality may be acceptable for these
fields
1 star in each of 4 Detectors areas of 248 @ 90 degree
1.0
0.9
(Probability of star in 124x2 sq
arcmin)^4
•
0.8
0.7
U
0.6
G
0.5
R
0.4
I
Z
0.3
Y
0.2
0.1
0.0
0
0.005
0.01
0.015
FWHM
0.02
0.025
0.03
Guide Sensor Assembly: Analysis
________________________________________________
•
•
Increasing the FOV by slightly enlarging L3 and the filters, provides improved
probabilities of finding a star in each corner position with the required brightness.
With this modification, there are enough field stars to provide the required centroid
accuracy in each of the 4 sensors for each field in g, r, i, z, y
At high galactic latitudes ~10% of fields in u will have degraded accuracy due to the
absence of bright field stars
1 star in each of 4 Detectors areas of 328 @ 90 degree
1.0
0.9
(Probability of star in 164x2 sq
arcmin)^4
•
0.8
0.7
U
0.6
G
0.5
R
0.4
I
Z
0.3
Y
0.2
0.1
0.0
0
0.005
0.01
0.015
FWHM
0.02
0.025
0.03
Guide Sensor Assembly: Issues
________________________________________________
•
Guide Sensor readout time is specified in the baseline design at <10 ms.
– If the position of the guide star(s) is known, a CMOS device can read out a subarea around the star(s), reducing the required pixel rate to a manageable level.
• But using 10 50x50 pixel windows read in 10 ms implies 2.5 Mpixels/sec, which is ~10x higher
than we’d like to read the H4RG.
• Need to evaluate effect of increasing latency on guider system performance
•
H2RG is a much more established device, so we would prefer to use this if we can
tolerate the larger pixel size.
• Initial evaluation looks promising, but needs more work to evaluate effect on guider system
performance
RIDL Test System
________________________________________________
ARC LEACH Controller
Bias Voltage Source
TIS SIDECAR ASIC
Integrating
sphere
Cooling
He compressor
Monochromator
Lakeshore
Temp Controller
Light Source Power
Two filter wheels
Detector Mount
________________________________________________
• Detector is H2RG-147-SIPIN (18um pixels)
Spot Projector
________________________________________________
Pinhole
Lenses
Diffuser
Fiber Optic
Three Stage
Motorized mount
Guider Experiment Window
________________________________________________
Spot scan within window
Window around spot
full frame