IV. Scientific Requirements for the SOAR Instruments

1. Introduction

The SAC, through its individual partner representatives, reviewed approximately 120 observing projects formulated by the astronomers at the partner's institutions (and in the case of CTIO, culled from the observing proposals submitted to the CTIO TAC). Many of these projects involved observations using several different techniques in tandem. The types of instruments and their characteristics needed to accomplish these projects were summarized by the SAC. (In cases, where requested instrumental characteristics differed from those deemed feasible, feedback was solicited from the astronomers who authored them.) A general description of the different types of projects is given in Part III of the SAC Science Requirements Document. Approximately 20% of these projects either were deemed most suitable for execution on the CTIO Blanco Telescope or were of sufficiently specialized nature that they required a specially built, user instrument. The remainder had instrument needs which were distributed equally in observing time amongst the following 5 general categories whose general scientific niches are summarized below.

An Appendix to this section (IV) describes the more general instrument requirements associated with the telescope structure and its interfaces and serves as a preliminary "instrument builders manual."

2. Proposed Initial Instrument Complement for SOAR

  1. High-Efficiency, Optical (Stellar) Spectrograph: The goals are to maximize efficiency for work on faint compact sources, to access both the large number of spectral diagnostics in the rest UV and also the Ca triplet at 850nm at zero-redshift, to minimize field variations in the monochromatic PSF, and to have low flexure for accurate background subtraction. The "B" options aim to enable limited stellar work (while not pushing R beyond a photon starvation limit set by CCD read noise), to reach optical spectral features at higher redshifts, to provide an ADC needed to maximize S/N for exposures away from the zenith, and to provide multi-object spectroscopy over a limited FOV. The "C" options are designed to extent the FOV for multi-object spectroscopy to the size of the isokinetic field, to improve throughput and to preserve full spectral resolution in poor seeing.
  2. High-Spatial Resolution Optical Spectrograph: The goal is a spectrometer providing proper two-dimensional sampling over ~5" X 10" area, with more grating options than the High-Efficiency Optical Spectrograph and overlapping in wavelength with the IR spectrometer. This will permit stellar work on the (crowded) cores of bright star clusters, the study of stellar populations in the cores of low-redshift galaxies, etc. The intent is to bench-mount this instrument at Nasmyth to maximize stability. Lower throughput than the High-Efficiency Optical Spectrograph is acceptable. The goal of the "B" options is improved performance for stellar population work and provision of an easy upgrade path to additional fiber feeds from multiple IFUs or an AO system. The extended options in the "C" list are for actual implementation of a multiple IFU (which would complement the Blanco Hydra-S by providing spatial resolution at each point and sampling closer than 23"), for implementation of an AO feed, and/or to extend wavelength coverage into the near IR.
  3. Near-Infrared Stellar Spectrograph: The"A" requirements call for a spectrograph with the capacity to obtain 2-pixel sampling of the best quartile, center-field, tip/tilt stabilized images (~0.1"/pixel at 1500nm) and enable "work between the OH sky lines" with accurate sky subtraction. The "B" options are for higher spectral resolution to allow a wider range of stellar studies, an echellete mode to span a longer baseline for extinction measurements and for simultaneous measurement of spectral diagnostics in the ISM, and for an IFU to permit 2-dimensional sampling of extended objects and crowded star fields. The "C" options are for a longer slit to improve linear mapping of extended emission-line objects, and for wider wavelength coverage.
  4. Near IR Imager: The goal is to preserve the near-diffraction-limited performance of the telescope at 1400 nm while enabling imaging in both broad bands (JHKK') and narrow bands over as wide a field as possible.
  5. Optical Imager: the goal is to provide 3-pixel sampling of best quartile, tip/tilt stabilized images and to cover the isokinetic patch in best quartile seeing. while enabling imaging in both broad bands (UBVRI) and narrow bands ( including diagnostic emission lines in the UV) and to do high-fidelity on/off-band image subtraction.

It was decided that the two classes of optical spectrographs would be considered "one instrument" for construction purposes, even though they will probably be physically distinct.

3. Detailed Instrument Characteristics

Using the partner's projects as a guide (and, again, with feedback from the authors) the SAC assigned the following (rough) priorities to the different instrumental features which are needed to execute the proposed scientific observations (whose basic goals are outlined in the section above). Our highest priority was to try and avoid the trap of designing an instrument by committee. Thus the features listed below are neither meant to be exhaustive nor restrictive.

  1. "A" - Features are those necessary for almost all of the projects. These are features which any acceptable instrument "must have".
  2. "B" - Features are those which a majority of the projects requested and are "highly desirable" in any initial instrument but may be optional if financial or engineering constraints so dictate.
  3. "C" - Features are also highly desirable and may well be important for innovative work but were not perceived as absolutely necessary for an initial instrument.

Both the "B" and "C" features should be considered as goals for later upgrade if they cannot be accomplished in the initial instrument complement (whether for financial or engineering reasons.) These priorities are listed below for each general instrument class:

3a. Very High-efficiency Optical (Stellar) Spectrograph

"A" list:

  1. A simple short-slit instrument
  2. Optimized for highest through put with R (aprox) = 5000 at 850nm.
  3. Spectral response: UV atmospheric cutoff (320 nm required, 305 nm desirable) to 850 nm.
  4. spectral PSF consistency appropriate to better than 1% sky subtraction over the useful field of view. (measured as D'(80)/D(80) <2% (where D' is the deviation in D). >
  5. flexure <0.1 pix / hr.

"B" List:

  1. R up to 20,000 at lower throughput.
  2. Extended spectral range to 1000 nm.
  3. ADC Correction down to 360 nm, (must be removable to preserve U).
  4. multi-slit mask with ~3' FOV. (with ADC for accurate spectrophotometry).
  5. Motorized slit mask exchange.

"C" List:

  1. multi-slit mask with >3' FOV.
  2. Image slicer/dissector for spatial resolution (over a few arcsec FOV).

3b. High-Spatial Resolution (IFU) Optical Spectrograph

"A" List:

  1. 2D-coverage with 2-pixel sampling matched to best quartile, center field, tip/tilt stabilized images (0.15"/pixel at ~1000nm)
  2. using an integrated IFU or image slicer with minimum of 1500 contiguous spatial samples, arranged over ~5" x 10" field.
  3. <5% cross-talk introduced by seeing.
  4. R up to 30,000.
  5. wavelength coverage: 360-1000nm with one octave (factor of 2) interval on the detector at once.
  6. throughput: 15% at 350 nm (including CCD + telescope).
  7. flexure: < 0.04 pix/hr.
  8. sky subtraction 1% residuals over 180 degree field rotation
  9. multiple fibers in fixed sky pattern (or applicable sky suppression strategy)

"B" List:

  1. >15% throughput at <350 nm (including CCD + telescope).
  2. sky subtraction <1% residuals over 180 degree field rotation
  3. provision for slit translation or other means of accomodating interchangable fiber feeds.

"C" List

  1. Multiple IFU -- 1000 spatial samples divided between several deployable heads distributed across the isokinetic field.
  2. Operation to 1400nm with necessary thermal suppression.
  3. AO feed with spatial scale 0.08"/pixel to ensure 2-pixel sampling of top-quartile, center field, AO corrected images.

3c. Near-Infrared Stellar Spectrograph

"A" - List:

  1. spectroscopy of point sources over 1000-2500nm
  2. sufficient sky coverage for simultaneous sky determination
  3. Configured so that 2Ksq array will fill a single atmospheric window (J,H,K) with R=4000.
  4. 2-pixel sampling of best quartile, center-field, tip/tilt stabilized images (0.1"/pixel at ~ 1500nm).
  5. R ~ 18000 (2 pixels).
  6. Detector 1Ksq pixels
  7. < 0.1e-/s dark, <30e- ron
  8. Slit 20".
  9. Throughput 30% w/detector
  10. Flexure < 0.1 pixel/hr worst case.

"B"-List

  1. 0.3"/pixel for median seeing
  2. R > 20,000.
  3. Cross-dispersion 900 - 2500 nm coverage at low R.
  4. IFU
  5. Detector 2Ksq pixels
  6. < 0.01e-/s dark

"C" - List:

  1. 900 - 5500 nm range.
  2. Slit 60".
  3. 900 - 2500 nm echellete mode at higher R (~2000) than is contemplated in B priority list.

3d. Near-IR Imager

"A" - List:

  1. 0.08"/pixel.
  2. 80" FOV
  3. 6 filter positions
  4. Cryo pupil stop, D(80)<1% of pupil size
  5. Throughput 30% with detectors and filters
  6. spectral range: 1000 - 2500 nm
  7. 2500nm dark current < 1e-/s
  8. 1.6"sq subarray ( 20 X 20 pixels) readout at 100Hz

"B" List:

  1. 20 filter positions
  2. FOV 200"
  3. 16"sq subarray (200 X 200 pixels) readout at 20Hz

"C" List:

  1. "tunable" filter (R=100 to 800).
  2. R<2000 grisms + aperture masks
  3. Coronograph
  4. Extended spectral range to 4000nm.
  5. Field of view >200" diameter.

3e. Optical Imager

"A" List:

  1. 0.08"/ pixel (with provision to "bin up" in poor seeing).
  2. FOV matched to isokinetic patch (5' ?)
  3. ADC
  4. operation down to 320 nm
  5. Provision for a minimum of 6 parfocal filters

"B" List:

  1. More filters.
  2. Filter that gives spectral R tunable over 100-800.
  3. Dual ccds (one "red" sensitive, one optimized in the UV).