By Michael Balogh, CATAC Chair
(Cassiopeia – Spring/printemps 2018)
The Wide Field Optical Spectrograph (WFOS) first-light instrument on TMT is expected to be a workhorse instrument that will appeal broadly to many in the Canadian community. This has proven to be a challenging instrument to design to requirements, and in the coming few weeks three different designs will be undergoing a technical and cost review process. CATAC has been working to understand these designs and gather input so we can present advice that reflects the needs of Canadian astronomers.
There are currently three compelling designs being considered:
- Fiber-WFOS: A fiber spectrograph that achieves a multiplicity of ~700 targets with R=5000 and full wavelength coverage, over a 79 square arcminute field of view. The challenge here is in achieving the necessary precision in sky subtraction and calibration, especially for faint objects at wavelengths redward of ~0.7 microns. There is no direct imaging mode, and the spectral resolution and angular sampling of each fiber is fixed. However, fibers can be bundled together to form multiple deployable IFUs. System throughput may be compromised in the near-UV and at wavelengths longward of ~0.9 microns.
- Slicer-WFOS: This design is an imaging spectrograph that uses image slicers to reduce the physical slit width to achieve R=5000, maintaining full spectral coverage from 310nm to 1000nm, but with a lower multiplicity of 33 over a 25 square arcminute field of view. The notional design for this option does not include imaging, although it likely could be added. The deployment of the slicer modules has been identified as a challenging operational issue.
- Xchange-WFOS: This is an imaging spectrograph with an object multiplicity of about 100 at resolution of R=5000, again over a 25 square arcminute field of view. Simultaneous coverage of the full wavelength range is not possible, but there is flexibility in using multiple gratings to achieve different resolutions. In terms of operational use, this design is most similar to multi-object spectrographs such as GMOS. Objects are placed on slits, using masks that are designed and cut for individual fields. Xchange-WFOS has an imaging capability. Like GMOS, there is also the option of adding an IFU.
All three designs have strong advantages and are exciting concepts; there are also technical and scientific trade-offs to be considered. The technical risks associated with all designs are being assessed by a committee that will advise the Scientific Advisory Committee (SAC) over the next couple of weeks. The SAC itself will meet April 9-10, 2018, and one of their tasks at that meeting will likely be to recommend proceeding with one or more of these three designs.
Scientifically, at a high level we might consider fiber-WFOS as best suited for “survey science”, where large samples of targets with a high density on the sky are gathered over large areas. On the other hand, the slicer- and Xchange-WFOS designs may be better suited for diagnostic spectroscopy: smaller samples of precision spectroscopy. But even this is an overly reductive description of capabilities.
CATAC needs to hear from you to provide appropriate advice. Some specific questions are:
- How do you imagine you (or your immediate descendants!) using WFOS in 2028? Are the sample sizes, target densities, and S/N or resolution requirements better achievable with one design or another? Are there science programs where you feel that MOS with slits would be advantageous compared to fibers?
- What spectral resolution or resolutions do you need for your science? If you could choose only one resolution, what would it be? Would it be acceptable to trade resolution against spectral coverage if the former were user-selectable?
- What wavelength coverage is most important for your science? Is there a need for coverage with maximum throughput between 0.3 – 0.4 microns, or at wavelengths longer than 0.9 microns? Is there an advantage of simultaneous coverage, or is it sufficient to use multiple settings?
- Is there a scientific need for seeing-limited (perhaps GLAO-assisted) imaging at visible – near-infrared wavelengths?
To help answer these questions we will be holding a public Webex meeting in advance of the April SAC meeting. Additional documentation describing the three instrument concepts will be made available, as possible, in the coming days via our website. We hope many of you will be able to attend, and/or send us your comments by email.
In other news, we await decision of the site selection. Disappointingly, the site permit for the ORM site in the Canary Islands has not yet been approved. There does not appear to be any fundamental obstacle to this permit, it is just taking longer than expected. In Hawai’i the political situation continues to change in a direction that is favourable to TMT construction. There are still two appeals in front of the Hawaii Supreme Court. One of them is being heard right now, and decisions on both are expected later this year. The good news is that the project continues to move forward in construction and design, with about 15 percent of the construction of components complete.