Detector Upgrade of Subaru’s Multi-Object Infrared Camera
and Spectrograph (MOIRCS)
Maximilian Fabriciusa , Josh Walawendera,b , Nobuo Arimotoa , David Cooka , Brian Elmsa ,Yasuhito Hashibac , Takashi Hattoria , Yen-Sang Hud , Ikuru Iwataa , Tetsu Nishimuraa , Koji
Omataa , Philip Taita , Naruhisa Takatoa , Ichi Tanakaa , Shiang-Yu Wangd , Mark Webera , andMatthew Wungaa

Subaru Telescope (NAOJ), 650 North Aohoku Place, Hilo, HI 96720, USA
W. M. Keck Observatory, 65-1120 Mamalahoa Hwy, Waimea, HI 96743, USAc
Institute of Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japand
Academica Sinica, Institute for Astronomy and Astrophysics, 11F of AS/NTU
Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C.
b

ABSTRACTDuring the past year, the Multi-Object InfraRed Camera and Spectrograph at Subaru has undergone an upgradeof its science detectors, the housekeeping electronics and the instrument control software. This overhaul aims
at increasing MOIRCS’ sensitivity, observing efficiency and stability. Here we present the installation and thealignment procedure of the two Hawaii 2RG detectors and the design of a cryogenic focus mechanism. The newdetectors show significantly lower readnoise, increased quantum efficiency, and lower the readout time.
Keywords: MOIRCS, Subaru, near-infrared, imaging, spectroscopy, MOS, H2RG, Hawaii 2RG

1. INTRODUCTION
The Multi-Object InfraRed Camera and Spectrograph1, 2 is Subaru’s workhorse instrument for seeing limited
observations in the near-infrared. By dividing its field of view through a roof mirror system into two identicalchannels, the instrument offers imaging in Y,J,H,K[s] with a field of view (FOV) of 7 0 × 4 0 and Multi Object
Spectroscopy (MOS) through cryogenic slit masks (R = 500 — 3200 for a 0.5 00 slit). For a description of the
instrument please see the parallel presentation [3]. MOIRCS has been one of the earliest instruments of its
kind (first light 2004, open use since 2005), but more recent instruments such as MOSFIRE4 offer a higher
sensitivity. As part of the nuMOIRCS project, in 2015 MOIRCS has undergone an upgrade of its science
detectors, housekeeping electronics, and instrument control system. Here we describe the replacement of the twoolder HgCdTe Hawaii II detectors by Teledyne’s more recent Hawaii 2RG (H2RG) models.

2. NEW DETECTORS AND READOUT SYSTEM
We purchased the new H2RG arrays from Teledyne Scientific Imaging. These detectors have 2048 × 2048 pixels
with an edge size of 18 µm with is identical to the size of the older Hawaii II arrays such that no modificationof the image scale was necessary. The outer four rows of pixels around each of the four edges of the detector
are not sensitive to light but serve as reference pixels that allow to correct for the channel to channel reset levelvariation. The H2RG detectors have 32 readout channels which in principle results in a factor of 8 increase inreadout speed over the older 4 channel system (' 100kHz readout). Further improvements of the new detector
generation are a lower readnoise and an improved quantum efficiency towards lower wavelengths due to theremoval of the detector substrate on the H2RG model.
We obtained a total of three science grade detectors with the third one serving as spare. We employ the
SIDECAR ASIC5, 6 for the detector readout in combination with the current SAM readout electronics, both from
Teledyne Scientific.
Further author information: (Send correspondence to M.F.)
M.F.: E-mail: mxhf@naoj.org

Figure 1. A rendered image of the GLS detector module. The module provides the mounting interface for the detector,
a light tight enclosure, heaters, temperature sensors and electrical connections.

3. DETECTOR HOUSING
We purchased the detector modules from GL Scientific, Inc. (GLS). The modules provide a light tight enclosurearound the detector and a mount for the detector and the Sidecar ASIC. They include resistive heaters andtemperature sensors, one for the detector and a higher power one for the module. Finally, they include appropriate
feedthroughs for the detector signals, the temperature control and the focus drive limit switches.

4. INTERFACE TO THE OPTICAL BENCH
A single piece aluminium window frame (see Fig. 2) mounts the detector modules onto the optical bench. We
incorporated reference surfaces for the tip/tilt, piston and lateral alignment of the detectors. Two G10 brackets
between the window frame and the module insulate the module from the optical bench. The image plane of
MOIRCS is tilted by 0.39 degree. The surface of the frame that faces the optical bench is tilted correspondingly.

5. FOCUS DRIVE
MOIRCS has been equipped with a focus z-drive. The internal focus of MOIRCS, i.e. the relative focus between
the MOS image plane and the detector image plane proved to be very stable over time. However, some minor
drifts in the order of 50 µm are observed after a warm up of the instrument such that a focus drive is needed toreadjust. In addition to counteracting the degradation of image quality through defocus, the existence of a focus
drive also greatly simplifies the procedure of the detector tip/tilt alignment (see the following section).
The previous focus drive was incompatible with the new detector modules such that we had to design a newmechanism that was mechanically compatible with the modules (see Fig. 3). Specifically, they had to fit into thelimited space that was available behind the detectors. In our design a triangular base stage slides back and forthon three rods. A nutblock is placed inside the base stage and a stepper motor directly drives a lead screw to push
the stage back and forth. An additional triangular plate that is connected to the main stage via three standoffs
provides the mounting base for the detector. Braided copper straps provide a thermal connection between thedetector housing and the mounting base. All metal parts are machined out of 6061 aluminum. The nutblock andthe bushings around the rods are machined out of Vespel. We used Phytron (model VSS 52.200.2.5-UHVC-4Lp),
cryogenic stepper motors and the Phytron phyMOTION motor controller. Forward and backward limit switches
provide an absolute position reference and additional hardstops protect the detector from damage by accidental
out-of-range motion.

Figure 2. This photograph shows the array module integrated into the window frame that mounts onto the optical benchof MOIRCS. The yellow piece of material on the top is a G10 bracket that provides thermal insulation between the moduleand the frame. There is a corresponding second bracket at the bottom of the module that is hidden in this view.

We tested the focus stages under room temperature for tip/tilt stability and position repeatability. The
tip/tilt error upon return to the same position is less than five arcminutes and the position repeatability lies five
microns which is sufficiently small to cause no measurable degradation in image quality across the detector. We
also tested the stages extensively under cryogenic conditions before the installation in the instrument. For shortcycle test we cooled the stage with liquid nitrogen and for full function test we employed a test dewar with acryogenic closed cycle cooler. This led to several modifications in the original design. We found that we had to
remove a collar from the bushings that led to “gripping” of the bushings onto the rods at 77 K. Also, we foundthat is was necessary to open up the thread of the nutblock to avoid to strong gripping of the nutblock onto theleadscrew. In fact, this was simply achieved by re-tapping the block repeatedly immediately after dipping it inliquid nitrogen.

6. DETECTOR ALIGNMENT
Each precool of MOIRCS requires about 1600 l of liquid nitrogen and takes 72 hours. While Subaru has developed
a automated precool system to eliminate manual tank changes during night time,7 the precool still requires day
and night monitoring by observatory personnel. Therefore is was desirable to implement a strategy to align thedetectors with very few iterations, as each iteration required another precool.
The existence of the focus drive allows us to collect a focus series where the detector is moved through its fullrange of motion. We utilize a special MOS mask that has 96 pinholes, evenly distributed across each detector
field of view. By measuring the spot size at a series of focus positions we obtain 96 separate focus curves. Their
minima give the absolute location of the actual image focal plane of MOIRCS. We implemented a numerical
procedure in python that derives the focal plane location mostly automatically from a set of images of a focus
series. Relatively minor manual interaction is required to reject curves of a few individual pinholes that are
affected by nearby hot pixels. The routine models each focus curve as a parabola where the minimum, thelocation of the minimum, and the steepness of the parabola are free parameters. The location of the minima

Figure 3. The new focus stage. a) shows a rendered image of the focus drive with the triangular mounting plate for thedetector and two braided copper straps. b) presents the internal structure of the focus stage. The base plate moves along
three cylindrical rods. Vespel bushings provide a low friction interface between the rods and the stage. The endcaps thatare seen here on the rods (drawn in red) are precise hardstops to protect the detector from mechanical damage. Two limit
switches serve as absolute position reference. The stage is driven by a lead screw that is coupled directly to the shaft ofthe cryogenic stepper. c) shows the focus drive installed into the GLS detector module. We mounted a mirror insteadof the detector to test tip/tilt stability and repeatability. d) shows again the module with the focus drive. But now thecooling straps are installed and the module is ready for the integration of the detectors.

Figure 4. The left image shows a 10 min. dark exposure after the first installation of the new detectors in MOIRCS.
Radiation events are seen distributed mostly homogeneously across the detector. The inset shows a zoom-in on the central
region of the detector. The plot on the right shows a histogram of the signal of the radiation events. The main peak lies
at about 590 ke− . The secondary peak at about 1200 ke− is a result of double counts. Note that the events do saturate
the detector and that the detector becomes significantly non-linear at such high signal levels. Therefore a derivation of
the actual radiation energy is difficult.

across all pinholes is then modelled by a simple three dimensional plane which corresponds to the image focal
plane. The z position of the plane and it tip and tilt are fitted together with the other parameters through astandard least squares algorithm (scipy.optimize.leastsq). With the location of the focal plane at hand wereturned to the 3D design of the detector mount and introduced a virtual plane to mark the focal plane location.
We then adjusted the detector location to be aligned with that virtual plane. As the position of the detector
module and the window frame are tied to the detector position, this allowed us to “measure” in the 3D design therequired shim thickness between the reference surfaces on the window frame, the optical bench and the upstream
field lens mount. We then proceeded to warm and open MOIRCS to install those shims of computed thicknesses
and continued with another precool to verify the improved detector location.
Unfortunately an incorrect wiring of one of the focus stepper motors initially led to a shimming in the wrong
direction in one of the channels. But after three iterations (three precools) we achieved a detector tip/tilt offset
of under 5 0 which is at the limit of what we can measure reliably from a focus series. More importantly thisresidual tilt results in a well acceptable image quality across the detector. The FWHM of the internal PSF acrossthe detector is 1.2 pixel on average which corresponds to 0.14 00 .

7. H2RG DETECTORS ARE α-PARTICLE DETECTORS
After the installation of the detectors into MOIRCS we were surprised to see what looked like a large number ofcosmic ray hits. They were, however, fairly energetic events with about 590 ke of signal per event and appeared
at a very high rate of several dozens over a 10 s integration. Based on reports of a similar experience at theneighboring NASA Infrared Telescope Facility (IRTF) during the upgrade of their SPeX instrument (MichaelConnelley, priv. communication, see also [8]) we immediately suspected alpha particle radiation emanating fromthe ThF4 anti-reflective coating of the last lens surface in front of the detector.
One of the significant advances of the H2RG generation of Teledyne’s Hawaii detectors is the removal of thebackside substrate after the growth of the photosensitive HgCdTe material through molecular beam epitaxy.
This led to a significant increase in the detectors quantum efficiency towards shorter wavelengths. However theremoval of the substrate also led these new detectors to become very sensitive α particle detectors.
Initially we explored options to remove the ThF4 coating from the last lens and to replace it with a nonradioactive coating. Unfortunately, the sensitivity of the material of the last lens (BaF2) and its very steep
curvature made this prohibitively expensive. As however the interaction probability of α particles with justabout any material is very high its penetration depth is very low. It was therefore clear that a thin glass window

Figure 5. The left image shows the α blocking filter, integrated in a light baffle that is mounted directly in front of thedetector. In this image the filter and baffle are installed in one of Subaru’s test dewars for cryogenic testing. The plotson the right show the transmission of the filter in the blocking and the non-blocking wavelength regions.

would block the radiation very effectively. Only, in order to avoid a too large difference in optical path length,
the filter had to be relatively thin and, in order to prevent ghosting, a high quality anti-reflective coating had
to be applied. We purchased Infrasil filters from Asahi Spectra Japan. Asahi was able to apply a broad band
anti-reflective (BBAR) coating on both sides of the filter which results in an average transmission of 98.5% withvery little ripple (97.6% minimum 99.2% maximum) across the entire wavelength range of 0.9 µm to 2.4 µm.
The filters are 50 mm × 50 mm in size with a thickness of only 1 mm. The manufacturer achieved a transmitted
peak-to-valley wavefront error of 0.07 waves.
Incidentally BaF2 is used a scintillator material in α particle detectors. The BaF2 optics of MOIRCS should
constantly emit a low level of blue light, to which the new substrate removed H2RGs are actually sensitive tosome degree. We estimated that the signal level of the scintillation glow is just about at the limit of what we candetect, but given that we applied a broad band coating to the blocking filter in any case, we could just as wellalso make it blocking at visible wavelengths. The transmission of the Asahi coating is on average 0.9 % below
0.7 µm.
We opened MOIRCS one more time in March to install the filters which in subsequent tests indeed turned
out to be effective at blocking the α radiation.

8. DETECTOR OPERATING PARAMETERS AND CHARACTERIZATION
We found that the differential readout mode with the non-enhanced clocking scheme and the row-per-row reset
resulted in the lowest readnoise values for our system. A critical discovery was that the removal of the grounding
pin to the readout computer was essential, not only to lower the readnoise by about 10 e− but also to avoid
otherwise relatively frequent readout glitches (see also [6]). As [9] we do not make use of the kTC noise removal
option as this actually worsened the noise performance.
We mostly use Teledyne’s standard operating voltages. We did however find that a lowered bias gate voltage(VBiasGate) results in a lower cross talk between channels (see [10]). We found that a value of 1.80 V led tonegligible crosstalk for signal levels around half full well.
Following the manual, we tuned the reference voltage (VRefMain) to adjust the bias level to about 12000
counts. There is also some optimization with respect to the readnoise that can be achieved by tuning VRefMain(see Fig. 6), also we found that a value of more that 6500 counts is desirable to lower vertical striping.
We use the gain 10 setting, which according to the manual should result in a gain of about 2 e− /ADU. Froma photon transfer curve we did indeed derive values of 2.07 e− /ADU and 1.99 e− /ADU for channel 1 and channel
2 respectively. This is the value after the application of the 6% correction for inter pixel capacitance.

Figure 6. These plots show the results the change of bias level and readnoise for the two detectors. Too high values of
VRefMain result in increased readnoise. We also find that this lead to a worsening of a vertical striping pattern. On theother hand, high bias values led to a lowered dynamic range.

With these operating parameters we obtain a single CDS noise of 13.9 e− in channel 1 and 15 e− using a pixel
wise noise computation and a non-linearity of 1.7 % in channel 1 and 1 % in channel 2 at a signal level of 60 ke−(about half full well).

9. NEW SYSTEM THROUGHPUT AND OBSERVING EFFICIENCY
A total of 7 engineering runs were assigned to MOIRCS from December 16, 2015 to June 17, 2016. The firstnights still suffered from the α radiation problem and MOIRCS did not yet have the blocking filters installed.
There we tested primarily the software functionality of first the imaging mode and then in the second run the
MOS capability.
In May we were then able to conduct standard star observations with the alpha blocking filters in place.
Table 1 lists the system throughput that we reach in the two channels. In imaging mode the throughput averages
between 33 % and 40 % from Y to K band including telescope and atmospheric extinction. This represents amarginal increase at K band with respect to the old detectors only and a 5 % – 7 % increase at Y band which isof course owed primarily to the removal of the backside substrate from the new H2RG detectors. The parallel
presentation [3] gives and update of the throughput in spectroscopy.
More importantly, the observing efficiency — the actual fractional time that MOIRCS is integrating signal— has been improved by 17 % to 30 % depending on the exact mode of observation. This is driven by the now
32 channel vs. the older 4 channel readout. Further improvements are expected by the future change to a newreadout software that is expected to further lower the overhead from the setting of configuration parameters andfrom an optimization of the observing modes.
Table 1. Caption for the table.

Band
Y
J
H
K

channel 1
[%]
35.7
34.3
40.5
38.4

channel 2
[%]
29.9
32.6
42.0
41.4

10. CONCLUSIONS
The detector upgrade of MORICS was concluded with the recent first open use observation on June 18, 2016,
which was successfully completed with only minor software glitches. With the removal of MOIRCS from thesummit in April 2014 the total instrument downtime was 14 months. Out of these, 4 months were the result of
the mitigation of the unexpected alpha radiation problem.
Overall, the improvement of sensitivity lags somewhat behind similar upgrades that were undertaken elsewhere, which shows that the prior detectors had a fairly high quantum efficiency to begin with. The improvement
in readnoise is significant (' 30 e−1 down to ' 15 e−1 ). This together with the increase in observing efficiency
will increase the appeal of MOIRCS to the science community in the coming years.

ACKNOWLEDGMENTS
This work was made possible by funding support though a KAKENHI (23224005) Grant-in-Aid for Scientific
Research (S) thorough the Japan Society for the Promotion of Science (JSPS) (PI: Nobuo Arimoto).

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