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OCTOCAM Feasibility Study

Introduction

The OCTOCAM study was led by Antonio de Ugarte Postigo and managed by Pete Roming and Christina Thöne. The project was coordinated from the Instituto de Astrofisica de Andalucia (IAA-CSIC), with main collaborators at the Southwest Research Institute, Fractal SLNE, and George Washington University. The study began in April 2015 and concluded in October 2015.

Executive Summary [Extracted from the OCTOCAM final report]

The decade of the 2020s will be marked by the advent of large surveys and giant telescopes. Many interesting objects will be identified daily by high‐cadence surveys, such as the Large Synoptic Survey Telescope (LSST) and the Zwicky Transient Facility (ZTF) at optical and near-infrared wavelengths, and the Square Kilometer Array (SKA) in the radio regime. Only a very small fraction of these objects will be studied with the three planned extremely large telescopes (ELTs). It will be a great time for 8-meter class telescopes that are prepared to efficiently follow up and study exciting phenomena. To take advantage of this opportunity, we envision a highly efficient instrument that will stand apart in looking into the time domain window with simultaneous broad-wavelength coverage.

OCTOCAM has been envisioned as a workhorse instrument that optimizes the use of Gemini for broadband single-target observations, both for imaging and spectroscopy. The instrument design is science driven, and it enables answering fundamental questions for a very broad range of science topics. A large science team has identified the most exciting science cases within their fields, making sure that OCTOCAM delivers the most interesting scientific output. The science requirements to answer the big questions have resulted in technical requirements. In this process, most of the different science cases have been accommodated, but the focus has been on the cases that make optimal use of the combination of high temporal resolution, broad spectral coverage, good spectral resolution, the great sensitivity of Gemini, and synergies with the new facilities coming online in the 2020s. The technical requirements have resulted in the specifications shown in Table 1 and the design shown schematically in Figure 1. [See final report.]

OCTOCAM is based on the use of high-efficiency dichroics to divide the light into eight different arms, four optical, and four near-infrared.  It has the advantage of simultaneity, as it is capable of observing at the same time in the g, r, i, z, Y, J, H, and KS bands, with a field of view of 3'x3'. In its spectroscopic mode, it will obtain spectra covering from 370 to 2,350 nm in a single shot. It will use long slit observations to provide spectral resolutions of ~3,000‐4,000. Using the Integral Field Unit that we propose, it will perform spatially resolved spectroscopic studies in an area of 9.7" x 6.8", with the full spectral coverage and resolution. OCTOCAM will be capable of obtaining full‐Stokes spectropolarimetry simultaneously in the range 370 to 2,200 nm to study geometry and magnetic fields of astrophysical phenomena. Furthermore, thanks to the use of state of the art detectors, it will be able to reach high readout speeds, allowing science cases aimed at high time-resolution. Depending on the detector of each arm, we expect full frame rates reaching speeds of 10 to 76 Hz, which will be even higher for windowed modes. Its detector technology will virtually eliminate dead times in most observing modes, allowing duty cycles of roughly 100%.

OCTOCAM will efficiently cover a region of the (spectral-resolution)-(spectral-coverage)-(temporal-resolution) observational space that is not covered, and will not be covered, by any single instrument worldwide in the 2020s, giving the Gemini community a leading position in several fields of astrophysics. While the scientific scope of OCTOCAM is extremely broad, making it a true workhorse instrument, the science cases that make most optimal use of the characteristics of OCTOCAM within the observational landscape of the 2020s are transient phenomena, such as gamma‐ray bursts, supernovae, X‐ray binaries, neutron stars, active galactic nuclei, and tidal disruption events. Large survey instruments in various spectral regimes will provide an enormous amount of triggers, and OCTOCAM on Gemini will allow detailed studies of the most interesting sources that cannot be observed by smaller telescopes. OCTOCAM will also identify the most interesting sources for which follow‐up observations with one of the ELTs or the James Webb Space Telescope are warranted.

The scientific advantages of the OCTOCAM design are that Gemini can be used efficiently over its full spectral range simultaneously, and that consistent data can be obtained for the full range and at any time, which is particularly relevant for transient phenomena. The extension of the spectral coverage from the optical far into the near‐infrared, both in imaging and spectroscopy, is crucial for high-redshift sources in general and high-redshift transients in particular. With OCTOCAM we can use gamma-ray bursts to explore the Universe up to the very beginning of stellar formation, within the epoch of reionization. By studying active galactic nuclei at different redshifts we can acquire fundamental clues on the role of supermassive black holes in galaxy evolution. We can also study active galactic nuclei and X-ray binaries to obtain new insights into accretion physics near the black hole event horizon and properties of jets. The tidal disruption of stars by supermassive black holes allows for the measurement of black hole mass and spin. Studies of different types of supernovae probe their progenitors, the explosion physics, the evolution of dust in the Universe, and push current redshift limits in the Hubble diagram to further constrain the cosmological parameters. OCTOCAM will also enable to elucidate the formation and evolution of neutron stars, and constrain their equation of state, emission mechanisms, and properties of their atmospheres and magnetospheres.

While these are the most exciting science cases that make use of all the OCTOCAM characteristics while at the same time being prime sources of interest for the large surveys of the 2020s, there are several other science cases that make use of the temporal and spectral characteristics of OCTOCAM as well, for instance the identification and characterization of transiting extrasolar planets, to determine their mass and density, and the composition of their atmospheres; the search for ices on the surface of Trans‐Neptunian objects, unveiling their surface composition, structure, and potential companions and atmospheres; and asteroseismology to unveil the internal structure and radii of stars.

The addition of spectropolarimetry provides complementary information besides imaging and spectroscopy. In some research areas, spectropolarimetry is the most important diagnostic technique, e.g., stellar magnetic fields, and other where spectropolarimetry is used frequently, e.g., studies of supernovae, solar system objects, and circumstellar environments. High‐impact science will come from exploiting the spectropolarimetric capabilities in the continuum, spanning the entire spectral range from the optical to the near‐infrared. This would be a unique capability in the world, also enabling lots of discovery science.

This document presents the science cases driving the design of this high‐performance workhorse instrument, and shows that it is feasible both technically and financially, as well as attractive and competitive for the Gemini scientific community. It discusses several trade‐offs related to the optical and mechanical design, detectors, spectral coverage, and resolution, evaluating their impact on the project. Several alternatives to the design are also presented.

Deliverables

The OCTOCAM Deliverable are available to download: 

Contact Information

For more information of the OCTOCAM study, please contact the Principal Investigator:
Antonio de Ugarte:          deugarte@iaa.es

For more information about GIFS or Gen4#3, please contact the Gemini Instrumentation Program Manager:

Stephen Goodsell:            sgoodsell@gemini.edu 

Please visit the Gen 4#3 home page for the latest information.