Scientific/Technical Abstract:

FIFI-LS will have two separate medium resolution (R ~ 1700) liquid helium cooled grating spectrometers with common fore-optics feeding two large Ge:Ga detector arrays (16 x 25 pixels each). With the two Littrow spectrometers we can simultaneously observe an object in two spectral lines in the wavelength ranges 42 - 110 µm, and 110 - 210 µm, respectively, in 1st and 2nd order.

Multiplexing takes place both spectrally and spatially. An image slicer redistributes 5 x 5 pixel spatial fields-of-view (nearly diffraction-limited in each wave band) along the 1 x 25 pixel entrance slits of the spectrometers. Anamorphic collimator mirrors help keep the spectrometer compact in the cross-dispersion direction. The spectrally dispersed images of the slits are anamorphicly projected onto the detector arrays, to independently match spectral and spatial resolution to detector size, thus enabling instantaneous coverage over a velocity range of ~1300 to 3000 km/s around selected FIR spectral lines, for each of the 25 spatial pixels.

For calibration and flatfielding, black body calibrators internal to the instrument are used. These have signal levels comparable to the thermal background of the telescope. An image rotator compensates for field rotation during long exposures.

FIFI LS Performance Summary:

The instrument sensitivity and resolution summaries are provided to permit estimating feasibility of scientific investigations. The FIFI LS performance summaries show the expected system performance for Full Operational Capability, which may differ from that during commissioning. An overview is provided in the table below.

FIFI LS Design

FIFI LS has two Ge:Ga detector arrays (16 x 25 pixels each). Each channel is 5 x 5 pixels in spatial coordinates and 16 channels deep in the λ direction. The projected pixel size for the Short channel is 6" x 6" resulting in an 30" x 30" instantaneous field of view (FOV). The Long "x 12" yielding a 60" x 60" FOV. A schematic of how FIFI LS arranges the incoming light onto the focal array is shown in the figure below.

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FIFI LS Angular Resolution

Shown below is the predicted FWHM image size for FIFI LS under nominal operating conditions, calculated as the root sum square (RSS) of the pixel size and the diffraction-limited telescope image size. Final images from FIFI-LS with angular resolution equal to the SOFIA telescope diffraction limit should be possible with use of appropriate observing techniques and post-flight analysis.

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FIFI LS Spectral Resolution

Wavelength range: 42 - 210 µm. The Short (S) channel coverage is from 42 to 110 µm and the Long (L) channel coverage is from 110 to 210 µm. Both bands have a free spectral range of 1300 - 3000 km/s. The central wavelength is set by manually adjusting the grating tilt. The gratings have a 40 degree tilt range. The wavelength setting accuracy corresponds to 20 km/s. The error in velocity determination is 20 km/s for unresolved lines.

The figure below shows the spectral resolution of both channels. The spectral resolution plotted corresponds to the FWHM of the instrument line spread function for a monochromatic line from a point source.

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FIFI LS Sensitivities

Below are shown plots of the continuum and emission line sensitivities for a monochromatic point source. The values are calculated for a S/N of 4 in 900 seconds. The Minimum Detectable Continuum Flux (MDCF) is in Jy and the Minimum Detectable Line Flux (MDLF) is in W m^-2. Both sensitivity values scale roughly as (S/N) / sqrt(t), where t = net integration time.

Line measurements in bright continuum sources may take longer to reach the same (S/N). Atmospheric transmission may preclude measurements at some wavelengths and reduce sensitivity at others. Further details for particular wavelengths of interest are available from the SI team; see contact information above.

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FIFI LS Observation Preparation and Data Handling

Once the observatory has been fully commissioned, additional information will be provided, including a full accounting of overheads associated with particular instrument set-ups and observing strategies; information on preparing observations using the SPT; and details regarding data formatting, calibration, and reduction.

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Disclaimer

All sensitivity and resolution data are preliminary, and based on anticipated performance of the observatory and the instrument.  Actual performance of the SOFIA telescope and instrument combination will be established after flight operations begin.  Telescope performance is expected to be upgraded during the first two years, and instrument performance may be upgraded, or additional modes or capabilities may be added.

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Additional References:

Klein et al., "FIFI LS getting ready to fly aboard SOFIA," Ground-based and Airborne Instrumentation for Astronomy III, Ian S. McLean, Suzanne K. Ramsay, & Hideki Takami, Editors, Proc. SPIE 7735, 77351T (2010), DOI: 10.1117/12.856530 [pdf]

Klein et al., "FIFI LS: the far-infrared integral field spectrometer for SOFIA," Ground-based and Airborne Instrumentation for Astronomy, Ian S. McLean & Masanori Iye, Editors, Proc. SPIE 6269, 62691F (2006), DOI: 10.1117/12.671505 [pdf]

Raab et al., "Characterizing the system performance of FIFI LS: the field-imaging far-infrared line spectrometer for SOFIA," Ground-based and Airborne Instrumentation for Astronomy, Ian S. McLean & Masanori Iye, Editors, Proc. SPIE 6269, 62691G (2006), DOI: 10.1117/12.671483 [pdf]

Looney et al., "FIFI LS: a far-infrared 3D spectral imager for SOFIA," Airborne Telescope Systems II, Ramsey K. Melugin & Hans-Peter Roeser, Editors, Proc. SPIE 4857, 47 (2003), DOI: 10.1117/12.458631

Raab et al., "FIFI LS: the optical design and diffraction analysis," Airborne Telescope Systems II, Ramsey K. Melugin & Hans-Peter Roeser, Editors, Proc. SPIE 4857, 166 (2003), DOI: 10.1117/12.458629 [pdf]