FLAMINGOS-2 OIWFS, Leckie, 2003

FLAMINGOS-2 OIWFS
Brian Leckiea
, William Gardhouse, Murray Fletcher, Robert Wooff, Tim Hardy
National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road,
Victoria, BC, V9E 2E7
Keywords: OIWFS, FLAMINGOS, CCD, Shack-Hartmann, Gemini
ABSTRACT
An On-Instrument Wavefront Sensor (OIWFS) designed, built and tested by the National Research Council of Canada
(NRC) for the FLoridA Multi-object Imaging Near-IR Grism Observational Spectrometer (FLAMINGOS) is described.
The University of Florida is building the FLAMINGOS-2 IR spectrograph for the Gemini Observatory as a near copy of
the original multi-telescope FLAMINGOS instrument. NRC/HIA was subcontracted to build the OIWFS based on the
Gemini Multi-Object Spectrograph (GMOS) design.
The FLAMINGOS-2 OIWFS patrols the bulk of the FLAMINGOS-2 field-of-view and will accept the Gemini f/16
input beam as well as the f/30 beam from the Gemini Multi-Conjugate Adaptive Optics (MCAO) system. The portion of
the probe arm that enters the FLAMINGOS-2 field-of-view is cooled, to avoid contaminating the infrared images. The
OIWFS uses the same CCD and CCD controller as was used on GMOS (e2v CCD39 and ARC GENII). Mechanically,
the OIWFS is a modified version of the GMOS OIWFS. It comprises two stacked rotational stages, each operating on a
single bearing. The top stage supports an optics package, which includes a lenslet array, pickoff arm and CCD. The
optical design uses a four subaperture Shack-Hartmann lenslet array. The mechanism is controlled using EPICS based
software that includes GUI engineering screens.
Test results showing the OIWFS to be fully compliant with design specifications are presented.
1. Introduction
An OIWFS is used to monitor the position and focus of a guide star in the instrument field of view, in order to
compensate for image shift due to telescope and instrument support flexure. It can also compensate for slight tracking
errors in the pointing of the telescope, telescope windshake and atmospheric image motion. Experience with previous
instruments shows that an OIWFS can significantly improve the delivered image quality and aperture throughput,
especially for long exposures.
The basic requirements for the FLAMINGOS-2 OIWFS are summarized below.
• Accept f/16 and f/30 beam.
• Four sub-aperture Shack-Hartmann.
• Limiting guide star magnitude the same as (or better than) GMOS.
• Patrol half the instrument FOV (full FOV coverage provided by cass. rotation).
• Vignette < 5% of the instrument FOV.
• Operate in vacuum.
• Probe arm cooled to < 140K.
• Position repeatability < 24µm RMS for 10 arcsec offsets.
• Probe flexure < 300µm per hour.
a
For further information contact brian.leckie@nrc.gc.ca
Ground-based and Airborne Instrumentation for Astronomy, edited by Ian S. McLean, Masanori Iye,
Proc. of SPIE Vol. 6269, 626945, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.670834
Proc. of SPIE Vol. 6269 626945-1
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• Pixel scale = 0.15 arcsec/pixel (field stop > 4 arcsec) for f/16 and 0.075 arcsec/pixel (field stop > 2 arcsec) for
f/30.
• Probe controlled as per Gemini ICD 1.9.f/1.1.11 (Flamingos-2 OIWFS to TCS Interface).
• OIWFS CCD read out as per Gemini ICD 1.6/1.10 (A&G to On-Instrument Wavefront Sensor)
2. Optics
The OIWFS optics are arranged on a probe which can be positioned so as to select a guide star. The design uses a four
subaperture Shack-Hartmann lenslet array. Figure 1 shows the optical layout from the probe tip to the CCD.
When the probe tip has been centered on a guide star the light passes through a weak positive lens which slightly
modifies the f/ratio of the beam to be fed to the remaining optics. Following the lens is a mirror which folds the
converging beam through approximately 90 degrees towards a field stop. An image of the guide star is formed at the
field stop. From the field stop the now diverging beam passes to a collimator lens. The collimated beam proceeds
through an optional filter to the Shack-Hartmann 2x2 lenslet array which is located at an image of the telescope exit
pupil. The lenslet array ‘dices’ the pupil patch into four converging beams which are intercepted by a re-imaging lens
that forms four images of the guide star on the CCD.
The positions of the four images can be used to monitor the guide star position and any focus changes that occur. Image
motion is manifested as synchronized motions of the four guide star images. Changes in focus result in the images
moving along diagonal paths in synchronized radial motions.
If the probe tip were to travel in a plane, the direction to the pupil would change with probe position, since the exit pupil
of the telescope is at a finite distance. This would result in the image of the exit pupil on the lenslet array moving
laterally. This would affect the amount of star light intercepted by each of the four lenslets and consequently the
Figure 1 Optical Layout of the OIWFS Assembly

relative brightnesses of the four images at the CCD. Excessive differences could affect the quality of the guiding and
focusing and also decrease the limiting magnitude for guide stars.
If left uncompensated, the patch position at the edge of the six-arcminute field of view would be displaced by 11% of
the patch diameter resulting in a ratio of 56% for the minimum to maximum image brightness. A goal ratio of 95%
requires that the optical axis of the OIWFS point to the center of the secondary mirror to within 10mm (1% of the
secondary diameter) for any position of the probe.
The method of compensation is modeled on that of the GMOS OIWFS, where the mechanism for moving the probe
about the field has tilted stages such that the optical axis of the probe always points to the center of the telescope exit
pupil.
3. Mechanics
As with the GMOS OIWFS1
, the Flamingos-2 OIWFS comprises two stacked rotational stages, each supported by a
single, preloaded, X-contact bearing (Figure 2). The top stage supports an optics package which includes a lenslet array,
pickoff arm and CCD.
Figure 2 OIWFS Mechanical Assembly
Both stages are driven by stepper motors through a worm gear system. The position of each stage is held by a power-off
brake on each motor. This allows the motor to be switched off between moves, thereby minimizing the heat dissipation
of the OIWFS.
The CCD and probe arm are cooled by a pair of cold straps. Electrical connections to the CCD are through a self
supporting Kapton cable. All other cabling is routed through the base stage’s center of rotation to the appropriate
bulkhead connections.

The guide star is directed to the CCD by a converter lens and a fold mirror positioned on the end of the pickoff arm. As
in GMOS, the arm has a double taper to ensure maximum rigidity with minimum weight.
The Flamingos 2 OIWFS uses a different strategy for limiting motion than does the GMOS OIWFS. Micro switches on
each stage provide the usual home and end-of-travel indications. There is also an interlock switch on each stage that
prevents the combinations of pickoff and base positions that would bring the arm into contact with the dewar wall. As a
final precaution, a set of mechanical end-stops are provided for each stage.
The axes of rotation between the two bearings are 77.5 mm apart while the distance between the pickoff stage bearing
and the fold mirror is 189.3 mm. The stepper motor’s motion is 0.18 °/micro-step. With the base stage gearing of 360:1
and the pickoff stage gearing of 180:1, minimum movements are 5.0 x 10-4
°/micro-step for the base stage and 1.0 x 10-3
°/micro-step for the pickoff stage. When summed in quadrature, a conservative estimate of the incremental movement
of the pickoff mirror is 3.4 µm.
The OIWFS sweeps as much of the 6.1’ instrument field of view as possible using a 180° rotation of the Cassegrain
mount. This leaves two small wedge shaped areas that cannot be swept, amounting to 3.5% of the FOV (Figure 3). The
spectrograph slits run horizontally in Figure 3, therefore, the impact on OIWFS coverage is minimized since the wedge
shaped areas are undesirable probe positions. The probe arm and fold mirror, vignette a maximum of 3.7% of the FOV.
Figure 3 Patrol Area and Vignetting
4. Electronics
The FLAMINGOS-2 OIWFS electronics consist basically of a control computer, motor support electronics to control
the probe and a CCD controller to readout the detector. The system includes a remote control power bar that permits any
component to be power cycled remotely. Probe motion commands and responses take place over the OIWFS Cntrl-

LAN. Probe motion is synchronized based on time stamps and the IRIG-B time bus. CCD control and data are
communicated over the ARC GenII fiber link.
The VME control computer subrack, motion control subrack and CCD power supply are installed in a glycol cooled
thermal enclosure. The mass of the thermal enclosure is 61 kg and the system power consumption is about 175W. The
amount of power dissipated in the MOS Dewar depends on the motion duty cycle; a 10% duty cycle results in about 4W
of dissipated power.
The seven-slot VME subrack contains:
1. An MVME2700 single board computer that runs the OIWFS component controller under VxWorks.
2. A Symmetricom bc635VME time bus card that decodes the IRIG-B time signal.
3. A Xycom XVME-240 TTL I/O card that reads the status of the limit and home switches and controls the
brakes on the two motion stages.
4. An Oregon Micro Systems VME58-4E motion control card that controls the steppers motors that rotate the two
motion stages.
The Motion subrack contains two active and one spare stepper motor driver circuit cards plus power supplies. Each
stepper motor driver card uses an IM481H micro-stepping drive module from Intelligent Motions Systems. It provides
micro-stepping resolutions from 2 to 256 (10 micro-steps/motor-step is used) and peak coil currents from 0.2 to 2.1 A
(0.8 A is used). All control inputs and outputs are optically isolated. A PLD permits flexible logic that disables the
driver based on the limit and interlock inputs. Driver disabling is direction dependent to allow backing out of a limit.
The ARC GENII CCD controller is mounted on the FLAMINGOS-2 instrument within a short cable run of the MOS
dewar CCD connector. The array controller is cooled by a glycol heat exchanger mounted on the top of the controller
housing. A Kapton flex circuit is used to route the signals from the MOS dewar wall connector to the CCD socket. This
flex cable is self-supporting and keeps the CCD signals away from the motor control wiring. The ends of the flex cable
are stiffened with FR4. One end of the cable supports the CCD socket and the preamps. The other end supports the
connector (41-pin hermetic jam nut), static protection and preamp power supply regulators.
The FLAMINGOS-2 OIWFS CCD array is the same type as used in GMOS, an e2v CCD39-01. This is a back
illuminated, split frame transfer device with four low noise output amplifiers. The image area is an 80 x 80 array of 24
µm square pixels. There are two differences between the F2 OIWFS CCD and the GMOS OIWFS CCD. First, the F2
device is in a standard windowless package, rather than in a TEC package. Second, the F2 device does not have an IMO
implant. Since the F2 OIWFS is operated in a cryogenically cooled instrument, it is fairly straightforward to operate the
CCD at a temperature where dark current and dark noise are negligible.
5. Software
The FLAMINGOS-2 OIWFS probe controller software was written in a VxWorks (Tornado 2.0.2) and EPICS
(3.13.4GEM8.4) development environment. The goal for the software portion of the control system was to reuse as
much of GMOS OIWFS software as possible, since it was a proven system design. However, a number of changes
were required.
The OMS VME44 motion controller used in GMOS was no longer available and was replaced by a VME58-4E. This
required a small amount of rework of the motor device record.
The region of allowed travel was more straightforward for the FLAMINGOS-2 OIWFS than for GMOS. This allowed
replacing the GMOS sin/cos potentiometer based limit circuit, with a set of limit and interlock switches. This change
simplified the software. It was now possible to make direct moves between any two valid positions. This was not the
case with GMOS, where it was necessary to make intermediate moves to safe positions. Also, indexing no longer
required an estimate of the current position and both stages could index independently of one another.

—d f2Opclop.dI
F2 OIWFS Probe Controller (OPC) — f2:wfs:
controller F2 OPC Cerro Pachon (V2.O)
ctn:RUNNING
OPC h&1th: GOOD
H&th&t: 33
IDLE
. I
I I
I I
I I
REBOOT ONC 1°C
NOTE: The OIWF S is contr oiled from the TCS
not from the F2 Instrument Sequencer - T
The original GMOS design was done on an MVME167 single board computer (SBC). The F-2 OIWFS controller is an
MVME2700 SBC.
Although the FLAMINGOS-2 OIWFS probe is generally controlled by the Telescope Control System (TCS), the probe
can be operated manually using a set of engineering screens. The top level engineering screen is shown in Figure 4.
Figure 4 Top Level Engineering Screen
The buttons at the right allow access to lower level screens. One of the most commonly used, is the probe assembly
control screen (Figure 5).

—ii
Jog Controller:
Jog: Target:
x:IQJQL—J 0.000
Y:Ics5cQJ 0.000
Targets (urn')
II
rovr. ____________________________
Error Message
Configuration ++
Full Controller
Device Control ++
Status ++ EXIT
£2: wE s: probeAssembly
_________ II I I
—.
Cnrrent X/Y:
X: 0.0024mm
Y: —0.0152mm
Probe Pmgle:
12 . 61 deg.
CAR:IDLE
Base STOPPED
Pickoff:STOPPED
Figure 5 Probe Assembly Control Screen
This screen allows the operator to initialize, index (datum), move and park the Probe in much the same way as the TCS
command interface does. Three move fields are provided to make motion repeatability testing more convenient. Also,
there is a means to easily move the probe a relative distance in X or Y to “jog” the probe into a desired location.
Additional screens are available to move each stage individually, to monitor configuration parameters and to check on
limits and interlocks.
6. Test Results
Flexure and motion repeatability were measured by using the motion of an artificial star image on the WFS CCD. Both
were better than the required performance and better than the results obtained with the GMOS OIWFS2
(see Table 1).
Motion accuracy was measured by using an alignment telescope to compare the OIWFS probe position to a precision
grid in the focal plane. The achieved accuracy was also much better than the required performance (see Table 1).
The probe temperature requirement for preshipment tests was estimated based on the final requirement. For preshipment
testing, it was necessary to use a different heat sink temperature and the OIWFS was in a different thermal radiation
environment than the MOS dewar. The preshipment test result was 2K warmer than the estimated requirement.
However, in the final acceptance test, the probe reached the required temperature of 140K when installed in the MOS
dewar.
In order to check that the pupil patch remained centered on the lenslet array for any probe position, a target was placed
on the optical axis at the distance of the Gemini secondary mirror. An auxiliary imaging lens was installed on the re-
imager lens cell. This lens imaged the lenslet array and thus the secondary mirror target onto the CCD. As the probe
was moved to various points in the FOV, a shift in the target image on the CCD was an indication of misalignment.

Refinements to this technique allowed confirmation that the positions of the projected axes of the base and pickoff stage
bearings were within 10mm of the center of the secondary mirror (see Table 1).
Difficulties were encountered when measuring the system throughput. Combining the measured optical throughput of
70% with the measured CCD QE, resulted in a system throughput of 63%. However, attempts to measure the system
throughput using a calibrated artificial star, gave results ranging from 47 to 50%. The reason for this discrepancy is not
yet understood.
Table 1 Test Results Summary
Test Required Performance Measured Performance
Flexure < 300 µm per hour 9 µm per hour
(GMOS: 14 µm per hour)
Motion Repeatability < 24 µm RMS 4.8 µm RMS at 22 °C
(GMOS: 12.4 µm RMS at 22 °C)
16 µm RMS at -5 °C
(GMOS: 28 µm RMS at -5 °C)
Motion Accuracy within the field of view or ± 1.9mm ± 190 µm
Probe Temperature ≤ 140K at final acceptance
≤ 114K with the heat sink at 80K
140K at final acceptance
116K with the heat sink at 80K
Plate Scale 0.15 arcseconds/pixel ± 5% 0.146 arcseconds/pixel
Pupil Patch Position Bearing axes point to the center of the
secondary to within 10 mm.
base bearing 4.3 mm from center
pickoff bearing 4.4 mm from center
CCD Read Noise < 10 e RMS at 500 kpixels/s 7.7 e RMS worst case at 500 kpixels/s
CCD Dark Current Negligible reduction in dynamic range
and negligible noise contribution in a 5ms
exposure.
2860 e/px/s at the expected CCD operating
temperature of 278K. 14 e of dark signal
and 4 e RMS of dark noise in a 5ms
exposure.
CCD Quantum
Efficiency
≥ 85% at 600 nm 90% at 600 nm
System Throughput ≥ 62% at 640 nm 63% at 640nm (separate tests)
47 to 50% at 640 nm (combined test)
7. Conclusion
An OIWFS has been built that meets or exceeds the requirements of the FLAMINGOS-2 instrument. In some cases the
performance exceeds that of its predecessor, the GMOS OIWFS. A novel technique has been used to test that the pupil
patch remains aligned with the lenslet array for various probe positions. This feature of the GMOS OIWFS was not
tested.
8. Acknowledgements
The authors thank our colleagues at HIA and at the University of Florida.
9. References
1. Roberts, S. et al., "Opto-Mechanical Design of the Gemini Multi-Object Spectrograph On-Instrument Wavefront
Sensor", SPIE, 3132, p. 184-195, 1997.
2. I. Hook, et al., "The Gemini North Multiobject Spectrograph Integration, Test and Commissioning", SPIE, 4841, p.
1645-1656, 2002

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