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GMOX Feasibility Study

Introduction

The Gemini Multi-Object eXtra-wide-band spectrograph (GMOX) study was led by Massimo Robberto and managed by Stephen Smee at Johns Hopkins University. Main collaborators included the Space Telescope Science Institute and the Rochester Institute of Technology. The study began in April 2015 and concluded in October 2015.

Executive Summary [Extracted from the GMOX final report]

We present the Feasibility Study of GMOX, the Gemini Multi-Object eXtra-wide-band spectrograph, as the 4th generation Gemini facility instrument. We envision GMOX as a spectrograph covering the entire optical/near-IR wavelength range accessible from the ground (from 3500 A° in the U-band up to 2.4 m in the K-band) with resolving power R 5; 000, adequate to mitigate the effect of telluric airglow lines. Using existing Micro Electro Mechanical Systems (MEMS) technology, GMOX can simultaneously acquire hundreds of spectra of faint sources in crowded fields with unparalleled spatial resolution. GMOX optimally adapts to both seeing-limited and diffraction-limited conditions provided by ALTAIR and GeMS at Gemini North and South, respectively. On a large fraction of nights, these systems deliver nearly diffraction-limited imaging in the near-IR ({D  50 mas at 2 m) and exquisite, seeing-limited images across the visible. Fed by GeMS (f/33), GMOX can synthesize slits as small as 40mas (corresponding to a single HST/WFC3 CCD pixel) over its entire 852  452 field of view, reaching the ultimate sensitivity to point sources while resolving structures smaller than 300 pc across the observable Universe. Both the slit and field size double at the native f/16 focal ratio of Gemini.

Resolving galaxies through cosmic time is the key science driver for GMOX. Space telescopes like the HST, JWST and WFIRST have the capability of resolving typical galaxies at any redshift. Thanks to HST, we have learned that imaging alone is not adequate to disentangle the physical processes leading to the formation, growth and evolution of galaxies over time, both intrinsic (bursts of star formation, activity of central black holes,...) and extrinsic (mergers, feedback,...). Ground based spectroscopy in the seeing-limited regime provides integrated information that is often misleading, as galaxies are not homogeneous: individual sub-regions are dominated by different processes. The problem is exacerbated by the need to analyze statistically significant samples of galaxies in different environments and at different redshifts, from the voids and supervoids, to the sheets and filaments, up to the richest and densest clusters. Adaptive optics on 8m class telescopes could play a crucial role if equipped with a spectrograph delivering pinpoint accuracy, comparable to the diffraction limit of the telescope, flexibility to adapt to the actual AO performance, extremely large wavelength coverage and extensive multi-object capability. The combination of these requirements is prohibitive for multi-object spectrographs or integral field units. GMOX is the first instrument with such a capability, complementing JWST/NIRSpec below 1m and with superior spatial and spectral resolution up to 2.5m. Its versatility will enable exciting discoveries in many fields of astrophysics, from studies of individual, isolated faint sources to the analysis of highly crowded fields such as globular clusters, the Galactic Bulge, the Magellanic Clouds, nearby galaxies and galaxy clusters. We expect GMOX to become the ideal workhorse instrument for Gemini, highly requested and extremely productive in all observing conditions. Future facilities like LSST will also dramatically increase the demand for spectroscopic followup; GMOX will allow Gemini to play a critical role in this field well over the next decade.

GMOX is comprised of three arms: Blue, Red and Near-IR observing the same field through dichroic mirrors. The split focal planes are reimaged onto three Digital Micromirror Devices (DMDs) of the latest generation, commercially available; each DMD covers the field with 2048 1080 mirrors of 13:7m size, sampling at 0:0832/mirror at f/16 and 0:0402/mirror at f/33 (GeMS). Each mirror can be individually tilted by 12; those tilted in the "ON" state reflect the light to the spectroscopic channel, acting as slits. The multitude of other mirrors left in the "OFF" state project an image of the field to an ancillary imager for slit acquisition, AO tip-tilt control, or parallel deep imaging. The Near-IR arm is split into three channels, dedicated to the YJ, H and K-bands. With a total of 2 large format CCDs and 3 H4RG IR detectors, the full spectra of all selected sources are simultaneously collected, each spanning about 25,000 pixels.

Our conservative estimates show that under normal seeing conditions GMOX approaches Signal-to-Noise=5 at JAB 22 in 1000s, per resolution element. With Adaptive Optics, the capability of using ultra-narrow slits with minimal sky background allows GMOX to reach unprecedented sensitivity across its entire spectral range. The possibility of defining slit widths by multiples of 83 mas (at f/16) or 40 mas (at f/33 of GeMS) is, in fact, critical. GMOX can instantaneously and optimally synthesize slits that with any other approach would be completely impractical to handle, especially given the sensitivity of an AO-fed system to the weather conditions. GMOX removes the need for pre-imaging weeks in advance and fully relaxes the requirements on long term stability of the focal plane, while allowing real-time monitoring of the perfect slit alignment during the longest integrations. With AO, it enables adapting the slit width (and therefore the spectral resolution) to the quality of the PSF across the field. It enables tip-tilt control on fainter natural stars observed in white light within the field to maximize Strehl ratio and extend AO correction well into the visible range. It enables wide-field Integral Field Spectroscopy through the use of Hadamard transforms. As accurate slit positioning is nearly instantaneous, GMOX is extremely efficient even for the simplest, single source observations: the target can be acquired with a mouse click, regardless of its position in the field. Pointing corrections, blind offsets, etc., are no longer needed.

GMOX leverages previous studies to bring this technology to fruition for future space missions. Our team has unique experience with DMD technology, having built two DMD-based instruments, and experience in general with the concept, construction and operations of major instrumentation for ground-based and space observatories. We have crafted a well defined instrument concept for GMOX, one that addresses all critical technical considerations. We believe that GMOX will deliver unique science and superior performance (field size, wide-band coverage and sensitivity) at a lower cost than any competing IFU or multi-slit spectrograph.

Deliverables

The GMOX Deliverable are available to download: 

Contact Information

For more information of the GMOX  study, please contact the Principal Investigator:
Massimo Robberto:         robberto@stsci.edu

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.