Canadian Efforts Towards a Very Large Optical Telescope (VLOT)

Dennis Crabtree (NRC-HIA)


A very large optical telescope (VLOT) was identified as a high priority project in the Long Range Plan for Astronomy (LRP) with construction expected to begin in the middle part of the second decade of the 21st century. Since the LRP was developed in 1999, there has been rapid development of extremely large telescope (ELT) concepts world-wide, including Canada, and construction is now likely to begin within the next five years. Canadian efforts thus far have positioned us well for participation in an international ELT project. In this report I will summarize the current status of the VLOT project and the road ahead.

Project Structure

VLOT efforts began at a low level in early 2000. In FY 2001-2002, NRC provided a small amount of bridging money to NRC-HIA with the expectation of LRP funding to follow the next year. The NRC support for the LRP announced in early 2002 (see provided three years of development money to NRC-HIA for Canadian VLOT efforts. AMEC Dynamic Structures has also invested significant internal resources into enclosure and telescope structure studies and design during this time.

A VLOT Science Steering Committee (SSC) was established and an NRC-HIA project office was established in early 2001. The SSC has been providing advice to the Project Scientist (Carlberg) while the project office has been guiding the Canadian technical effort. The SSC was unanimous in its recommendation that Canada be a full and equal partner with a “second to none” share in any future project.

Science Drivers

What are the main science drivers for the next generation of ELTs? One of the major science goals will be to characterize the external planetary systems and protoplanetary disks that are currently being discovered at a rapid pace by existing telescopes. Planetary characterization could be a major driver of the telescope requirements as it will rely on highly precise optical elements feeding an extreme AO system with tens of thousands of actuators.

Another major science driver is the question of first sources of light in the Universe. Are the first light sources the hot high mass stars that theorists predict? Are they completely free of elements other than the Hydrogen and Helium that came out of the Big Bang? How did the rest of the elements build up through star formation in galaxies?

Other science drivers include the formation of evolution of galaxies, the evolution of large scale structure, the stellar IMF and the physics of fragmentation and the nature of dark matter and dark energy

VLOT Design

VLOT is a 20-m segmented mirror telescope with an aggressive optical design (f/1 primary) that could meet the Mauna Kea Master Plan requirements and fit on the CFHT site. The baseline design is to use 150 1.8-m hexagonal segments to form the 20-m aperture. The outermost segments are fairly aspherical (286 mm) in contrast to the CELT (California Extremely Large Telescope) approach of an f/1.5 primary with 1-m segments (20 mm asphericity). The fast primary means a shorter and stiffer telescope structure as well as a small and less expensive enclosure.

The design has two vertical Nasmyth platforms for instruments, one on each side of the telescope. This approach was chosen to eliminate the effects of a changing gravity vector on the instrumentation.

The telescope structure features two large hydrostatic bearing wheels 12-m in diameter, a monocoque mirror support structure and a quadrapod secondary support structure that utilizes both steel and carbon composite construction.

The enclosure design uses the Calotte concept, a rotating cap with a circular aperture. This configuration has several potential benefits in comparison to conventional slotted enclosures. The aperture in the Calotte configuration is the smallest possible enclosure opening size for a given telescope size. The Calotte configuration is expected to have larger and more uniform stiffness than a comparable conventional enclosure. Conventional enclosures typically have two large arch girders on either side of the slot. The arch girders create a relatively stiff zone near the slot and comparably softer areas away from the slot, leading to a highly variable weight distribution on the dome drive system. The power requirements for this type of enclosure and also much lower than for a conventional design.

Figure 1 . The VLOT telescope structure and enclosure concepts.

Canadian Efforts

Canadian efforts are focusing on the key challenges facing the next generation of optical telescopes. A large measure of the science to be achieved by ELTs will rely on adaptive optics (AO) to deliver diffraction limited images to achieve both higher spatial resolution and the D4 advantage in integration time for background limited point sources. The requirement of the telescope to deliver diffraction-limited image quality puts tight requirements on all aspects of the telescope design.

The VLOT mirror segments will have to be polished to much tighter requirements than the Gemini or VLT mirrors. We recently funded a study, jointly with CFHT and France, by Sagem (formerly REOSC) to look at the technical approaches, cost and schedule for producing the VLOT mirror segments. Sagem feels that using a combination of existing and newly developed techniques they can produce and test the required VLOT segments in a reasonable amount of time. We will likely be funding the production of a demonstration 1.8-m segment as the next step towards actual production of primary mirror segments.

The effects of wind, both in terms of loading on the telescope structure and mirror cell and in effective thermal flushing of the enclosure, are critical to VLOT’s performance. The enclosure plays a key role in both protecting the telescope as much as possible from the wind and providing adequate wind flushing of the interior of the enclosure. NRC-HIA held a wind modeling workshop in late February that included participants from NRC-HIA, NRC-IAR (Institute for Aerospace Research), UTIAS (University of Toronto Institute for Aerospace Studies), RWDI (a commercial firm specializing in wind loading on structures), AMEC and UBC. As a result of this workshop, it is clear that Canada can play a central role in understanding wind effects through both wind tunnel testing and CFD (computational fluid dynamics) studies. The VLOT project will be contracting wind studies over the next two years to optimize both enclosure and telescope design with respect to wind effects.

Figure 2 Wind velocity contours for a calotte enclosure design

Integrated Modeling

Integrated modeling of VLOT will be crucial to an understanding the trade-offs between different design approaches and in choosing a design that best meets the scientific requirements. The integrated model (IM) is designed to model the dynamical and optical performance of the complete telescope system to various disturbances such as gravity, wind and thermal. For example, wind blowing on the primary mirror cell will exert pressure on the mirror segments and slightly move the segments which will be detected by the edge sensors.  This information will be fed into the mirror control system which will move the actuators to correct the placement of all 150 segments. While this control loop is going on, an optical ray tracing package will be calculating the effect on the delivered image quality of the wind loading. This is a much simplified example as in reality one must include the wind loading on the telescope structure, the secondary and tertiary mirrors, the thermal effects (mirror and enclosure seeing), AO performance, etc.

NRC-HIA has invested significant resources in the IM and will continue this investment over the next two years. The wide-range of expertise at NRC-HIA will be invaluable for the development of the IM.

Figure 3 Schematic outline of the Integrated Model

The Road Ahead

In Europe, the two main ELT projects OWL and Euro 50 have more or less merged and will be pursuing study funds from the European Commission. France has committed a limited amount of resources to looking at a successor to the CFHT. At the moment this effort appears unlikely to play a major role in the French “prospective”, their 5-year planning exercise that is currently taking place.

In North America, the GSMT (Giant Segmented Mirror Telescope), as outlined in the US Decadal Review, was seen as a 30-m telescope that would be a joint public-private partnership. CELT and the TMT (Twenty Meter Telescope) group are the two US “private” projects that are to be taken seriously at the moment. The public side of the GSMT is likely to be realized through AURA’s New Initiative Office that is funded mostly by NOAO with some support from Gemini.

Canada has been interacting with CELT, TMT and the NIO on both scientific and technical levels. The NSF has formed a GSMT Science Working Group for input on the science drivers for a GSMT and Carlberg is on this group. Canada has kept in contact with both the CELT and TMT as they groups have furthered their development and funding plans. The Canadian project office has a good working relationship with the NIO in the sharing of technical knowledge and plans. This technical coordination has recently expanded in the area of wind modeling to include CELT.

In Canada, the development of ACURA (see article in this issue) is changing the traditional funding and management approach to large astronomical infrastructure projects. In terms of funding, ACURA will allow our community to access the significant funds available through the Canada Foundation for Innovation (CFI). To this effect, a sizeable CFI proposal will be submitted this May to allow for Canadian participation in the design work and initial construction of an ELT project. It is likely that Canada will soon signal its intent to join US efforts and form an international project. Canadian efforts thus far have positioned Canada to being significant scientific and technical capital to the table as a “full and equal” partner in a future ELT Project.