HSC Design Review Report
Review Panel:
Dr. Takashi Ichikawa, chair (Tohoku University, SAC), ichikawa@astro.tohoku.ac.jp
Dr. Mamoru Doi (U. Tokyo, TAC), doi@ioa.s.u‐tokyo.ac.jp
Dr. James Gunn (Princeton), jeg@astro.princeton.edu
Dr. Joe Jensen (Gemini), jjensen@gemini.edu
The design review was held in Hilo on Mar. 2, 2009.

Executive Summary:HyperSuprime Cam is an exciting instrument that will provide Subaru astronomers with a world‐
leading capability for exploring the nature of the universe. We recommend proceeding with HSC tocomplete the design and begin construction as soon as feasible. The primary risk to HSC appears to bebudgetary, not technical, and we encourage NAOJ to seek closure with Mitsubishi on the design,interfaces, and budget for the top‐end modifications and prime focus unit for HSC as soon as possible.

Charge I. Science Goals
HyperSuprime Cam will be a unique instrument for producing high‐quality, legacy data and high‐impact science results. The combination of excellent image quality over a wide field of view and thelarge collecting aperture of the Subaru 8‐m telescope make HSC truly revolutionary. The panel
recognizes the importance of the key science cases (Dark Energy measured via weak lensing andphotometric redshifts), but we also appreciate the broad range of other science projects that the Subaru
community and their colleagues in Taiwan and at Princeton are anxious to begin. Some of the panel
members attended the Subaru Users’ meeting held in Mitaka in Jan. 2009 where these other science
projects were discussed. Other programs such as galaxy evolution, surveys of stars in the Galaxy and
M31, surveys of clusters of galaxies, QSOs, AGNs, and SNe can either be done through open “PI” accessto HSC, or as a “piggy‐back” part of a large DE campaign. The HSC science capability will be unique in theworld when it is installed on Subaru in 2011. There is no other proposed facility that will be capable ofthe sky coverage and depth that HSC will achieve.
Recommendations:
We recommend doing a full simulation of the HSC data including realistic aberrations, fieldrotation (with respect to the corrector), dynamic and static errors, and from that simulated data extract
the weak lensing signal to see what the uncertainty is on the shear measurement.
Photometric redshifts are very difficult to measure with the accuracy needed (1% in redshift).
Spectroscopic redshifts from WFMOS may provide a strong complementary data set if WFMOS is built.
We recommend that emphasis be placed on extending the HSC wavelength coverage as red as possible,
at least to Y‐band (since other IR surveys are not expected to go deep enough to help with the HSC dataset). In this regard the HSC survey is not at any disadvantage compared to the competition, as the otherproposed surveys also rely on photometric redshifts measured at optical wavelengths. More attention
will need to be paid to calibration issues (photometric, astrometric, throughput, photo‐z training sets,etc.) to achieve desired precision. It is our opinion that with careful calibration the HSC data will support
higher accuracy than the presentations indicated. The panel recommends that the HSC teamdemonstrate how photometric accuracy sufficient to meet the scientific requirements of a broader
range of possible future science cases may be achieved. The software needed to achieve the best
science results with HSC is discussed further in section III below.
We understand the sensitivity of the Japanese community to large amounts of observing time
being allocated to dedicated campaigns. We would like to emphasize that HSC will have its greatest
impact in the time frame prior to LSST being completed. Resources and observing time should beallocated to make sure that HSC and its primary science goals are not significantly delayed, and thatsoftware be in place to make the HSC data available to the community as soon as the camera begins
science observations.

Charge II. Technical Design
HyperSuprime Cam has been successfully designed to meet the top‐level science goals. Designs
for most subsystems are mature and ready for further development and/or prototype construction. The
HSC team has used early prototypes to minimize risk in several technically challenging areas: new
Hamamatsu CCDs were installed and tested in Suprime Cam; a shutter prototype was used to explore
design options for this critical mechanical component; glass for some of the most challenging wide fieldcorrector elements has been purchased and tested.
Recommendations:
It was not clear what top‐end modifications were needed, and what would be provided by
Mitsubishi. This appears to be a big uncertainty in the project. We did not see any design details for the
HSC prime focus unit, and it’s associated hexapod alignment system, instrument field rotator, cablewrap, etc. Given the large cost of the top‐end modifications and HSC PFU construction, it is difficult to
say if there are technical issues with these systems. The designs for the wide field corrector, the camera,
dewar, shutter, filter unit, etc. were much more mature. If the cost of the Mitsubishi work has to be
reduced, what requirements will be relaxed? What impact will that have on the science mission or on
the other subsystem requirements? We recommend that NAOJ work with Mitsubishi to finalize thedesign and interface requirements, the budget, and the schedule as soon as possible, so that thetechnical risk to the project can be understood and any adjustments in other areas made as quickly as
possible. The HSC team repeatedly told the panel that the Mitsubishi work is not technically demanding.
The panel concluded that the technical risk due to lack of definition in the top‐end modification workand PFU development is smaller than the budget risk resulting from uncertain costs for Mitsubishi to do
the work.

It wasn’t entirely clear which specifications have been demonstrated to be satisfied throughprototype testing, and which are met by design. The key question is what happens if some subsystemdoesn’t meet specification? How is the overall error budget affected, or how do the requirements forother subsystems change? A careful systems engineering approach to error budgets and requirements isneeded as the design is finalized.
One area of particular technical concern was the filter exchange mechanism. This is anextremely complex mechanism, with many motors, encoders, limit switches, precision interfaces, tightpositioning tolerances, and with complex optics and filters inside. It is also handled separately from therest of HSC, so handling procedures need to be carefully considered and followed. That it can only bechanged at zenith suggests lack of sufficient margin on the motor torques or other tolerances, which
was the case for the first filter exchange mechanism for Suprime‐Cam. Careful prototype testing should
be done before the design is finalized. The panel also suggests that the HSC team think about what it
would take to swap filters manually during the daytime if required due to an exchanger failure during anobserving run. The panel also recommends baffling the filter exchange mechanism to prevent scattered
light since it extends into the light path.
The review panel was concerned about the camera thermal performance with so manyelectronics in the dewar. The analog circuit boards are predicted to operate at about 20 degrees above
ambient, which will produce severe outgassing problems that may well overwhelm the proposed small
ion pump. We recommend looking for ways to ensure that heat is removed effectively from theelectronics in the dewar, in particular investigating liquid cooling and moving the high‐dissipation partson the boards nearer the thermal contacts. The panel noted the choice of brittle SiC for the dewar plate;there isn’t any viable alternative, so safety procedures should be carefully considered to minimize risk.
The panel strongly recommends testing the shutter prototype more thoroughly at typical Mauna
Kea operating temperatures (0 C) in a variety of orientations, and under typical humidity and dust
conditions.
The wide field corrector ceramic enclosure may be somewhat fragile. We appreciated the safety
discussion, but recommend taking the safety precautions one step further. Instead of protecting against
bumps, we recommend mechanical alignment guide rods, centering sensors or other guarantees that itnever contacts the top end ring unexpectedly. The WFC is a large and delicate optic, and key to both
HSC and possibly WFMOS. There should be no margin for error. We noted with some horror that thecontrol system for the TUE was proprietary and the observatory does not have documentation. It would
seem absolutely necessary for the safety of HSC that the horizontal motion of the TUE be disabled when
HSC is being lowered into place.
Optical design of the WFC seems to be quite mature. Current ghost analysis assumes 10%
reflectivity of most of glass surfaces, which may underestimate reflectivity of the interference filters. We
were generally satisfied with the careful treatment of possible ghost that has already been done.
Given how important image quality is to HSC, the reviewers felt it was important to understandhow residual flexure errors translate into errors in the weak lensing shear measurements. The targeterror budget is very demanding. Similarly, we could not judge how far off the optical axis is from theWFC or camera axis due to static flexure with elevation. We recommend that adding a way to adjust theposition of the camera dewar with respect to the WFC axis in addition to the hexapod (which aligns the
WFC to the telescope) would allow for better control of residual aberrations. For example, piezo flexureson the camera legs might give some additional flexibility in refining the camera‐to‐WFC alignment. The
current design requires that this flexure be controlled by passive flexure of the mounting cone, andtherefore any further adjustments would require modifying or replacing a key piece of hardware, andtherefore significant work disassembling and reassembling the instrument.
The review panel noted that one of the largest single sources of instrumental image quality
degradation is charge diffusion in the detectors (which is almost as large as all aberrations and guiding
errors put together). We suggest that the HSC team work with Hamamatsu and other CCD experts in theworld (e.g., LBNL) to find a way to reliably run the CCDs at higher bias voltage to minimize charge
diffusion.
The high image quality of HSC should not be compromised by poor tracking accuracy of thetelescope. Several problems with tracking have been recently reported, which might be caused by wear
after ten years of operation. Care should be taken to make sure that the added load of HSC not degrade
the tracking accuracy of the telescope.
Analysis Software and Calibration Recommendations:
The panel felt that the state of the planned software design was very far behind that of thedesign of the camera. The camera design was accompanied by detailed schedules, error budgets, andimplementation plans, none of which were seen in the software planning. Likewise, plans for theimportant task of survey calibration were essentially missing. NAOJ must realize that this instrument andthe surveys which will be accomplished with it require a very large investment in software and carefulplanning for photometric and astrometric calibration. Luckily, within the HSC collaboration there is agreat deal of talent and experience in both of these areas, but there is little evidence that even planningfor its use is in place. There is evidently some confusion about software for the HSC team weak lensing
science and general software for HSC surveys. While the specific requirements and goals may besomewhat different, the commonality is very large, and separate development of software for thevarious types of observations seems neither wise nor even possible, given the limited resources.
We recommend several steps toward this goal:1.

A set of scientific, functional, and performance requirements for the software be
generated on as short a timescale as feasible; these requirements should be guided by
the planned surveys with HSC as well as specifically the lensing science, and should
certainly involve the developers and scientists throughout the consortium.

2.

Plans should be put into place quickly for developing the framework for the software sothat individual tasks which can be separated and accomplished efficiently can be
assigned to developers.

3.

Progress must be measured by regular phonecons/videocons/meetings, and a series ofreviews and demonstrations (which might also be accomplished by videocons, butideally would be face‐to‐face) should be held.

Without vigorous development on this front, we feel that it is quite likely that the camera will beon the sky in October 2011 and there will be completely inadequate software support. This should in noway be construed as a criticism of the individuals in the current effort; the resources currently being
utilized are, we feel, simply inadequate to the task. The consortium was only established recently,
probably too recently to impact this review, but it needs to get into high gear very quickly.
A calibration plan also needs to be developed in parallel. At the minimum, this plan needs toencompass:1.

Site‐monitoring instrumentation to establish photometricity. There is evidently inexistence a 10‐micron cloud camera which is not functional. It needs to be repaired or
replaced with some way of determining if clouds are present during an observation.

2.

Instrumentation to measure the response function and linearity of the camera. This canprobably be accomplished with the HSC while in storage off the telescope with arelatively simple monochromater, but such issues as filter installation need to beconsidered very carefully.

3.

A plan and tests for photometric calibration using transformations from SDSS or other(e.g., PanSTARRS) sources. This may involve negotiations with other survey teams to
obtain early photometric data.

4.

A plan and tests for astrometric calibration using SDSS, UCAC, 2MASS, USNO‐B, andother sources.

Charge III. Uniqueness of HSC
As discussed in Section I above, the HSC science mission is unique and world‐leading. HSC
provides a unique capability in that it samples a larger area more deeply than competing instruments.
Projects with similar goals like PanSTARRS will not be able to compete with HSC’s sensitivity. LSST will
not be built for ~10 years, giving HSC a clear advantage for the next decade. The HSC surveys willachieve an exceptionally faint limiting magnitude both because of the large telescope aperture and theexcellent image quality.
Recommendation:
The current HSC error budget and tolerances appear sufficient to meet the top‐level science
goals, but care should be taken during the final design phase, when cost and performance trades are
considered, to retain very high efficiency (throughput) and excellent image quality.

Charge IV. Recommendation to Proceed with HSC
We recommend continuing with design and manufacture of HSC. Many details of the final
design depend on interfaces with the new HSC PFU, so these should get top priority. The technicallyrisky areas are getting adequate attention, and prototypes are being built and tested. Other technicalrisks should be retired quickly and the design finalized. As noted above, the budget risk is largest for thetechnically least challenging area (top‐end modifications and PFU), but it is still the largest uncertainty inthe project. We recommend continuing with the design work and technical risk mitigation while waiting
for the agreement with Mitsubishi to be finalized.

Charge V. Schedule and Risk
The review panel is concerned by the extremely aggressive schedule, with no schedule orbudget contingency explicitly included. We recognize that the HSC team has mitigated many of themost significant technical risks by seeking early funding for prototyping of key subsystems (detectors inSuprime Cam, the shutter mechanism, and filters, for example).
Many of the subsystems appear to have insufficient schedule contingency. Most subsystems are
expected to be complete at the same time, about 1 year from now. The human resources needed for
those parts were not clearly identified in the presentations. The review panel would have liked to haveseen who will be involved in each part, and how many hours can be allocated for each one. The schedule
is very tight, and it wasn’t clear if the HSC team members will have enough time to complete the work
they have been assigned.
Traditionally, astronomical instruments are built in Japan with significant contributions fromstudents. The level of student involvement in HSC design and construction should be carefully
considered. The panel recommends allowing involvement of students in the design, construction,assembly, and testing of the instrument, which greatly helps the education of students in
instrumentation and science.
We congratulate the HSC team leadership for allocating resources early‐on to mitigate thelargest technical risks. The panel feels that the largest risk to the project, and the largest potential forschedule slip, arises from the uncertainties in the Mitsubishi top‐end and PFU work. These risks can only
be mitigated by concluding negotiations with Mitsubishi as quickly as possible.
The current budget contains no contingency at present. Budget contingency will certainly beneeded, probably for all subsystems and tasks.