Science Considerations
Jack Baldwin and Marcos Diaz - Oct/97

The following is an attempt to say something about what kinds of science we should try to do on SOAR, on the basis of how SOAR can fit into a system including Blanco and Gemini. We would then take the charge from Gerald to be to take illustrative examples of the science in the areas where SOAR will do best, and use them as a means of working out what the basic instrument capabilities (FOV, resolution, etc) should be. We hope this is helpful.

I. Starting Point
The following simple-minded formulae were used to estimate the dependence of exposure time on other parameters in different observing regimes:
t = exposure time to reach a given signal to noise ratio (s/n).
F = object flux.
D = telescope diameter.
d = image diameter. ~ used here to imply proportionality rather than aprrox equal.
Regime. Dependence. Typical application.
1. Shot noise limited: t ~ 1/(D^2 F) Imaging or spectroscopy of brighter objects.
2. Background limited: t ~ (d^2)/(D^2 F^2) Imaging or spectroscopy of faint objects.
3. Detector-noise limited: t ~ 1/(D^4 F^2) High-res spectroscopy at intermediate mag.
4. Diffraction+BG-limited: t ~ 1/(D^4 F^2) Mid-IR, >5um for SOAR.

The detector-noise limited case assumes that multiple readouts are needed, with number proportional to total exposure time. If there were only one readout per observation, the result would change to t ~ 1/(D^2 F) .

The object flux is carried along explicitly just to remind everybody that the exposure times start to blow up rapidly as soon as you are fighting some other noise source than just the photon noise from the object itself.

II. OPTICAL SPECTROSCOPY (0.3-?? microns) Blanco SOAR Gemini
Wide field multi--object 1,2,3 x x
Narrow field multi-object x 1,2 2
2D over narrow field x 1,2 2
Single object low-res (1) 1,2 2
Single object high-res (1) x 2,3
Low-res 0.31-0.36um x 1,2 x

where 1=shotnoise limited, 2=b.g. noise limited, 3=detector noise limited, () signifies reasonable but not ideal performance.

The Blanco Hydra system will cover multi-object capabilities for looking at point-sources with fairly wide (> 23 arcsec separations) over a 45 arcmin field, over the range 0.36-1.0 microns. There will be a selection of FWHM resolving powers in the range R=1000-5000 plus in single echelle orders with R=16000 or 24000.

CCDs just plain don't work beyond ~1.1-1.2 microns (band-gap physics), but with an upgrade involving an additional camera with an IR detector ($300K???) these fiber-feed capabilities could be pushed out to ~1.7 microns.

CTIO plans to add an image-slicer fiber feed to the Hydra system, for looking at single objects. Predicted to be about as efficient as the existing RC spectrograph, which at ~15-20% total system throughput is reasonably good. Straightforward upgrade path to a cross-dispersed echelle system with R=30000 with simultaneous coverage over a wide wavelength range. The image-slicer feed could be a reasonable replacement for our existing cass RC and echelle spectrographs; we will have to wait and see how efficient it really turns out to be.

In any case, CTIO would retire the existing RC and echelle spectrographs.

Gemini South will offer GMOS with good multi-slit and IFU capabilities over a 5.5 arcmin field, with FWHM R up to 10000 and high image quality. This will presumably be the way to go for spectroscopy at the very faint limit (D^2 F^2 advantage for background-limited work). But GMOS will only work efficiently down to 0.36 microns. Like Hydra, it will not really get down to the atmospheric cutoff. And GMOS will probably only have 15-20% efficiency, so a really efficient spectrograph on a 4m could be fairly competitive.

Gemini South will also have HROS, offering R~50000 echelle spectroscopy all the way from ~1 micron down to the atmospheric cutoff (assuming that the telescope optics have an aluminum coating). This will kill any 4m telescope for high-resolution optical spectroscopy of faint (V>17 or so) objects (D^4 F^2 advantage for detector-noise-limited observations declining to D^2 F^2 advantage for fainter, background-limited objects). Blanco can provide R=30000 spectroscopy of brighter (non-background-limited) objects using the image-slicer fiber feed, and in principle could do as well as SOAR because image FWHM will not be a parameter. Gemini will of course still have a D^2 advantage in this shot-noise limited regime, but the exposure times will be tractable with a 4m telescope (t \propto 1/F, rather than 1/F^2 as in the background-limited case). This argues for not putting high priority on high-resolution optical spectroscopy on SOAR. Blanco will be moderately good for this at the bright end, and Gemini, Keck et al will eat our lunch for the fainter objects and higher resolutions...

From the above, the areas where SOAR would most complement the combination of Blanco and Gemini South are:
a) Low-resolution spectroscopy which includes the 0.31-0.36 micron range.
b) A high-efficiency spectrograph with limited long-slit capability. GMOS throughput will probably be the usual 15-20%. It is possible to build a high-efficiency spectrograph that on paper would have 60% efficiency; an instrument like this on SOAR would be competitive with Gemini for point-source observations, and even if it didn't really hit 60% would certainly take a lot of pressure off Gemini at the brighter end of the background-limited range.
c) Real 2D spectroscopy working at the full angular resolution of the telescope. This will be good with tip-tilt, but way better with an AO system at which point we would be competitive with Gemini and HST (better image quality than Gemini, same image quality but bigger aperture than HST).

III. IR SPECTROSCOPY Blanco SOAR Gemini
Wide field multi--object to 1.7um x x
Narrow field multi-object x 1,2 ?
2D over narrow field x 1,2 ?
Single object low-res (to 1.7um) 1,2 ?
Single object high-res (to 1.7um) 1 2,3

where 1=shotnoise limited, 2=b.g. noise limited, 3=detector noise limited, () signifies reasonable but not ideal performance.

The entries for Blanco are for the case where the Hydra capabilities are extended to the transmission limits of the fibers, at 1.7 microns).

Single object high resolution assumes that at least Phoenix is available on both SOAR and Gemini.

The other Gemini entries are question marks because unless we wind up sharing GNIRS with them they have no plan for a first generation instrument, and both the timescale and capabilities of any second generation instrument are not known.
IV. OPTICAL IMAGING Blanco SOAR Gemini
Wide field, low angular res. 1,2 x x
Narrow field, high angular res. x 1,2 2
Narrow-band (FP, etc). x 1,2,3 x

where 1=shotnoise limited, 2=b.g. noise limited, 3=detector noise limited,

Wide field imaging will be provided by Mosaic imager at Blanco prime focus. Its unsurpassable 40 arcmin AD corrected field will deliver 0.5-0.9 (FWHM) arcsec images.

GMOS will provide imaging mode in a 6 arcmin field at Gemini, exploring the background limited targets with small image quality degradation.

In this scenario SOAR enters with a tip-tilt corrected field nominally 10-15 arcmin in diameter and ~f/10. Its image quality would be comparable with Gemini in the optical and at least the inner 2 arcmin may be correctable by low-order AO. SOAR may attend the high resolution imaging demand at intermediate magnitudes with the largest tip-tilt stabilized field.
V. IR IMAGING Blanco SOAR Gemini
Wide field, low angular res. 2?? x x
Narrow field, high angular res. x 2 2?,4
Narrow-band (FP, etc). x 3? 3?,4?

where 2=b.g. noise limited, 3=detector noise limited, 4=diffr. limited.

The Blanco entry for Wide field low angular resolution signifies that perhaps some instrument could be put on one of the sideports at cass, although this has not been discussed. In addition CTIO hopes to have a smaller (~2m) telescope that would be heavily used for IR imaging to a shallower limiting magnitude.

The Gemini entries with question marks signify that their present plans do not include such an instrument during the first several years of operation. The only definite IR imager for Gemini South is for the mid-IR (regime 4).