CFHT Information Bulletin Number 37, Semester 97II

MEGACAM: A Status Report

Derrick Salmon, CFHT (salmon@cfht.hawaii.edu)

The scope of the MEGACAM project

The MEGACAM project's goal is to provide a wide field, high resolution CCD imaging camera at the prime focus of CFHT. Image quality must be superb. Compared to CCD cameras currently available at CFHT, or for that matter at any other observatory on the globe, the field coverage and quantity of raw data provided by each image will be huge.

To gain a feeling for the size of the step from the cameras CFHT currently owns to MEGACAM consider that right now the largest CCD owned by CFHT measures roughly 30 mm on a side. MEGACAM will have the equivalent of 64 times as many CCD's inside a single camera. The detectors will cover a square area measuring roughly 250 mm on a side. Each exposure will generate half a gigabyte of data. Two images will produce enough data to more than fill the larger hard disks on most of the PC's in use at CFHT, and each night of use promises to produce on the order of 30 images. The camera's field is so large that the entire prime focus upper end of our telescope will need to be rebuilt with completely new wide field corrector optics, and a new prime focus cage. A new software and computer observing environment will be needed to provide access to the enormous quantity of data from each MEGACAM image. New high-speed data communications systems will be needed to handle the increased data flow. In short, MEGACAM promises to be an exciting technical development for CFHT, which, together with the AOB, should give us a strong research base well into the next century. We will undertake much of the MEGACAM development work here at CFHT. A lot more will be distributed among labs in Canada and France.

Background

MEGACAM was first proposed to CFHT several years ago by Laurent Vigroux of the French Atomic Energy Commission - CEA. Since his initial proposals and discussions with CFHT both the SAC and the Board of Directors have endorsed the project and have encouraged us to expand the scope of the project to support the research needs of as wide a range of Canadian, French, and Hawaiian astronomers as possible.

The project will be expensive. The BoD has identified funding for the project but has also warned us that it must not go over budget. At the moment we have the funds needed to take the project through the preliminary design phase to the point where we can justify our current cost estimates, and also to purchase the CCDs. Once we can demonstrate to the Board that the project can be completed within budget, the remaining moneys needed to complete the project will be made available. Currently we are hoping to be able to make the final funding proposals for the project construction phase to the Board sometime in the spring of 1998.

The MEGACAM Kickoff Meeting - March, 1997

The Kickoff meeting - held in Waimea in mid-March - brought together several technical groups interested in the project. At the meeting we partitioned the development work among the interested institutions, and discussed a range of technical issues needed to get the project going.

The 25 participants at the meeting represented the following 7 institutions:
CEA

Centre de Energie Atomique (the French Atomic Energy Commission in Saclay)

CFHT

Canada-France-Hawaii Telescope in Waimea, HI, USA

DAO

the Dominion Astrophysical Observatory in Victoria ,BC, Canada

DT

the Division Technique de INSU - the technical services division of INSU located in Paris (INSU is the science agency which provides support for Astronomy in France)

Observatoire de Toulouse

the Toulouse Observatory in south-west France

OPM

the Paris Observatory in Meudon just west of Paris

IfA

the University of Hawaii's Institute for Astronomy, Honolulu

The project has been divided into a set of 16 Work Packages. Each package is listed below along with the name of the institution which has committed to undertake the work. In deciding on the division of labor we tried to balance the strengths of each institution against constraints on available manpower and the timing for completion. As you can see from the list, the project scope is really quite ambitious and provides a wide range of technical challenges.

The MEGACAM work packages

Package

Responsible Agency

CCD Procurement

CFHT

CCD Camera Development

CEA

Computer Facilities

CFHT

Cryogenic System

CEA

Filter Mechanism

CEA

Filter Procurement

CFHT

Guide and Focus Sensors

DAO

Detector Focus

DAO

Wide Field Corrector

DAO

Image Stabilizer

OPM

Prime Focus Controller

CFHT

Prime Focus Environment

CFHT

CFHT Camera Lab

CFHT

System Integration

CFHT

Programme Administration

CFHT

Stray Light Studies

DAO

You may notice that neither the Toulouse Observatory nor the IfA received a work package. Toulouse's interests were in the guider package which ultimately went to DAO. DAO and Toulouse may well work together in some sense on this package. They are working out the details. IfA has a real interest in the camera and may help in the procurement of CCDs but will otherwise not be directly involved in the camera development work.

Current schedule

Conceptual designs for the camera, the prime focus environment, and the computer system should take place this summer. Preliminary design reviews are scheduled for November; final design reviews for the spring and early summer of 1998.

The selection of the CCD vendor is underway now. A list of potential providers and the details of their CCDs has been developed between CFHT, IfA, and CEA. CFHT and CEA staff have visited a few vendors to discuss a range of technical issues. One of the biggest problems here is to decide who will most likely be able to deliver the 36 to 40 thinned 2k x 4k CCDs the project needs on the time scale of a few years. We hope to have an order in place by the end of this summer - early fall at the latest.

We hope to complete the optical design review for the new corrector late this summer.

By the later part of 1999 the various subsystems for MEGACAM should be coming together for testing. We plan on having first light on the sky with the new prime focus environment in late summer of 2000. First use of the MEGACAM camera on the sky at prime focus should follow over the next months. Real science might start as early as the first semester of 2001.

The Toronto MEGACAM Science Meeting - May 1997

After the Kickoff Meeting there remained a number of contentious items which have direct bearing on the kinds and quality of science which will be produced by MEGACAM. These issues include the extent to which the camera optics need to transmit in the ultraviolet, whether the camera ought to be provided with an atmospheric dispersion compensator (ADC) to improve image quality when observing away from the zenith, and issues related to the availability of the MEGACAM images to the science communities. The Toronto Science Meeting was held to provide a venue for discussion and hopefully resolution of these issues and to improve the exposure of the project to the Canadian science community. Most participants agreed that the meeting was extremely useful. There certainly appears to be a wide range of science interest in MEGACAM and a whole range of science projects which will require the camera's unique capabilities of large field coverage combined with excellent image quality.

A number of important issues were resolved from the science point of view. In particular, the scientists present recommended that coverage of the ultraviolet wavelengths should extend to, but need not extend below 350 nm.

A second technical complication for the new wide field corrector was avoided by the decision that an atmospheric dispersion compensator (ADC) is not essential for the science which will be carried out with MEGACAM. An ADC is a series of large prisms mounted inside the wide field corrector. The prisms are free to rotate and must rotate in opposite directions in a such a manner as to introduce spectral dispersion into a star image to cancel a similar small dispersion naturally produced by the earth's atmosphere. By not including the ADC in the final design we not only avoid the expense of the added optics and their control, but we also avoid a number of potential technical pitfalls which are not readily modeled, but which nonetheless are serious concerns for the ADC's real-world performance. Little things like the uniformity of glue joints, the matching between `identical' prisms, and a list of similar manufacturing imponderables could rapidly degrade the expected image quality. The recommendation not to include an ADC has simplified the project considerably.

Another science issue with technical implications is the need for a grens or grism in the optical design. The science requirements clearly identify a need for one of these dispersing elements in order to provide a spectrum for each object in the field. Often the use of a grens or grism implies that the camera must rotate. The scientists however do not want to rotate the camera. Mechanical stability and the ability to precisely reproduce the orientation of the camera on the sky from night to night were the main issues here. Again, the recommendation not to provide a camera rotator has simplified the project. We still have to investigate the feasibility of providing the grens or grism.

The Toulouse SAC Meeting - May, 1997

At the last Scientific Advisory Council (SAC) meeting, SAC largely endorsed the recommendations of the science community outlined above regarding the ADC, UV coverage, the use of grisms and the rotator. However, SAC took issue with our current plans to cover only a 1.2 degree diameter field with a rectangular detector array. Since the field of view is circular, SAC would like to see the areas outside the currently-defined rectangular detector array to also be covered with CCDs. SAC similarly took issue with our choice of a 1.2 degree field. SAC prefers a 1.4 degree field, again to maximize sky coverage on each exposure. Technically, the bigger field and the added CCDs are possible. In the end the decision to cover more of the field or not will mostly be based on cost.

Where next?

One of the biggest efforts facing CFHT and the other participating institutions for the immediate future is the generation of the project Technical Specifications. These must be in place before contracts and memoranda of understanding can be signed.

As mentioned earlier, the procurement of CCDs is proceeding quickly. We need to start the design process for our new upper end structure, so clearly some brain storming will be needed fairly soon in this area. We have already started to explore the computing and observing environments needed for large imaging array detectors. Here, we have the opportunity to test many ideas using the CFH12k CCD camera which will go into operation early in 1998. The GenX project which aims to rethink, and to a large extent redefine or evolve, the computing architectures and observing environment at CFHT, has already extended its initial roots. There are many issues related to the new wide field corrector which need to be investigated. The list goes on. MEGACAM as a project has started.

Hopefully this note will have given you a feeling of what has been going on with MEGACAM over the past few months, and where CFHT is heading. If you have any questions about the project, please contact us. For CFHT, Dennis Crabtree is serving as project scientist; Derrick Salmon as the project manager.



CFHT Information Bulletin Number 37, Semester 97II

tmca@cfht.hawaii.edu
Copyright © 1997, Canada-France-Hawaii Telescope