SAFIRE: Performance Parameters
SAFIRE's unique capabilities enable a wide variety of science goals. Some
high level initial science projects can be chosen even at this point,
several years before SAFIRE's first light. Since the results of the
ISO satellite, it has become very important to be able to detect redshifted
Cii (158μm) from ULIRGs
such as Arp220 or Mrk231, which SAFIRE can do out to z~1. At longer
wavelengths ( l >160μm)
where ISO was not as effective and where ground-based telescopes cannot
observe, SAFIRE will be an important instrument for characterizing the line
emission from a variety of galaxies. Finally, projects which require large
area imaging spectroscopy -- for instance, the study of the Galactic center,
where measuring the spatial structure of emission lines will be revealing Ð
are ideal for SAFIRE's large detector array.
1. SOFIA Instrument Complement
SAFIRE can be compared to other SOFIA first light instruments on the basis
of its wavelength coverage, spectral resolution, and imaging area. A
spectral resolution well-suited to galaxies atl >200μm is not covered by any
other instrument (below left). SAFIRE also has a very large instantaneous
field-of-view (below right), which is designed to enable it to map nearby
galaxies quickly and efficiently. Simultaneous imaging has the obvious
benefit of increasing the speed of observation, and additionally has the
benefit of reducing the systematic errors associated with raster and jiggle
mapping.
2. Sensitivity Limitations
The background power on SOFIA is largely determined by the emissivity of
the atmosphere, shown in at left below. This can be translated into an
atmospheric photon noise equivalent power (NEP), as shown at right below. The
SAFIRE detectors will need to achieve a phonon and Johnson noise sum of
<3x10-18 W/sqrt(Hz) in order to be background-limited, and must be
able to absorb up to ~0.3pW in order to avoid saturation. This puts a
stiff requirement on the dynamic range of the bolometers, in that the ratio
of maximum power detected to the minimum power detectable in unity
bandwidth is about 105. At a fixed spectral resolving
power of 1000, the noise equivalent flux density (NEFD) and line
sensitivity can be calculated (lower figures). A typical NEFD is of order
1Jy/sqrt(Hz), while the line sensitivities are typically better than
10-17 W/m2 at the 5s
level in one hour of observation. For comparison, emission lines from
many interesting galaxies -- including the fine-structure lines from
luminous galaxies out to z~0.1 -- are of brightnesses of ~10-16
W/m2.
3. Wavelength Coverage, Spectral Resolution, and Field of View
SAFIRE achieves its high spectral resolution using a double Fabry-Perot
design, shown in the Instrument Design
section. A double Fabry-Perot is a straightforward optical system, in which a
low-resolution etalon provides order-sorting for a high-resolution etalon.
Because of the broad tuning range required to cover 100-700μm, a
filter wheel with six settings provides order sorting for the
low-resolution etalon. The resultant spectral resolution is shown below. A
standard set of 6 filters is used to provide the resolutions of
150-300km/s, completely covering the 100μm-700μm range. For
higher spectral resolution, a set of filters for certain atomic
fine-structure lines can be installed in this filter wheel, for the Cii line at 158μm, the Oi line at 145μm, and the Nii lines at 122 and 205μm.
These filters would permit observations at resolutions of ~100km/s, more
suited to sources in our own galaxy.
Spectral resolution vs. wavelength. A standard set of 6 filters is used
to provide resolutions of 150-300km/s over a wide band, while a separate
set of 4 filters provide resolutions of ~100km/s for certain lines.
On SOFIA, a large field of view is available with diffraction-limited
performance in the far-infrared. The spatial resolution at 200μm is
20''. It is therefore reasonable to set the plate scale in the focal plane
to 10'' per pixel. We are developing a 16x32 detector array for this
purpose using multiplexed superconducting transition edge sensor
bolometers. The field of view is then 160''x320''.
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