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astronomers will soon have access to unprecedented spatial resolution and sensitivity in the far - infrared with the stratospheric observatory for infrared astronomy ( sofia ) .
far - infrared astronomical observations , which are impossible from the ground due to water absorption , are necessary to understand fully a number of important astronomical problems and issues .
many astrophysical conditions require far - infrared probes because the areas of interest are mostly inaccessible at other wavelengths due to severe extinction from interstellar dust or the physics of interest is only manifest at far - infrared wavelengths
. in particular , far - infrared spectroscopy , pioneered and developed on sofia s predecessor the kuiper airborne observatory ( kao ) and greatly extended with the infrared space observatory ( iso ) , will provide an injection of important data into astrophysical issues .
building upon the success of our previous imaging fabry - perot far - infrared spectrometer ( fifi@xcite ) that was designed for the kao , we are developing a successor instrument for sofia : the field - imaging far - infrared line spectrometer ( fifi ls@xcite ) .
fifi ls will utilize integral field spectral imaging in two wavelength bands : 42 to 110 @xmath2 m and 110 to 210 @xmath2 m .
this will allow the instrument to simultaneously obtain dual wavelength band , spectral imaging of a 5 @xmath0 5 pixel field , without scanning a fabry - perot or multiple pointings with a long - slit spectrometer .
thus , fifi ls is designed as a `` major - step '' forward and will take advantage of the unique benefits that sofia offers .    with the increased sensitivity and resolution provided by sofia , the main scientific targets for fifi ls will include the detailed morphological studies of : ( 1 ) the heating and cooling of galaxies , ( 2 ) star formation and the interstellar matter under low - metalicity conditions , as found in dwarf galaxies , ( 3 ) active galactic nuclei and their environment , ( 4 ) merging and interacting galaxies , and ( 5 ) large surveys of nearby galaxies .
to reach our scientific goals , very high observing sensitivities and efficiencies are essential , requiring a compromise of spectral resolution ; however , for the science objectives listed above , a comparably low spectral resolution ( r @xmath12000 ) is more than sufficient .
overall , fifi ls on sofia will be more sensitive than the iso long - wavelength spectrometer and have much higher spatial resolution and mapping capabilities . as a future option
, an extension of the instrument to the 25 - 42 @xmath2 m range is planned upon availability of the si : sb detector arrays developed for sirtf .
fifi ls achieves 2-dimensional spatial mapping and simultaneous spectral multiplexing by optically slicing , or re - arranging , the 2d field of view onto a single slit , which is then dispersed via a standard long - slit spectrometer .
this type of `` optical slicer '' was originally devised for laboratory spectroscopy@xcite and later successfully implemented for near - infrared astronomy@xcite , but this is the first time that this technique has been applied in the far - infrared .
specifically , fifi ls has a 5 @xmath0 5 pixel field of view which is sliced into five individual slitlets , re - arranged into a continuous 25 @xmath0 1 pixel slit , and finally , fed into a grating spectrometer and dispersed onto a 2d 16 @xmath0 25 pixel detector array .
one of the other unique aspects of fifi ls is its dual - channel nature . by using a dichroic beamsplitter in the entrance optics and two optical slicers ,
fifi ls covers the diffraction - limited field of view simultaneously in two wavelength bands : a short wavelength band 42 to 110 @xmath2 m and a long wavelength band 110 to 210 @xmath2 m .
fig . 1 shows how the dual channel integral field system is realized in more detail as a projection of the focal plane onto the two detector arrays with diffraction - limited optics . on the right side of the figure
is the long wavelength channel of the spectrometer which has a pixel scale of 14@xmath5 per pixel . on the left side of the figure
is the short wavelength channel which has a pixel scale of 7@xmath5 per pixel .
this of course implies that the shorter wavelength channel has a smaller field of view , as demonstrated in fig .
overall , this scheme ensures that for all spatial elements in the field , spectra are observed simultaneously in the two bands , thereby increasing observing efficency .    [ cols="^ " , ]      in order to comply with federal aviation administration requirements , we are designing the cryostat to withstand 3 times the operational pressure about 3.5 bars .
8 shows finite element analysis results for the three large vessels at 3.5 bars of internal pressure overemphasizing the deformation for display purposes the maximum deformation in the three vessels are 0.91 mm for the vacuum vessel , 0.20 mm for the ln@xmath6 vessel , and 0.24 mm for the lhe vessel .
in addition , we used fea for determining the best design for stability of the internal optical work surfaces and to minimize the amount of stress on the welding joints of the vessels . in the latter case ,
for example , we find that the best approach is to create a nearby zone that can elastically deform , thereby relocating areas of high stress away from the welding seam .
fifi ls employs two fixed pixel sizes of 7@xmath5 ( short wavelength spectrometer ) and 14@xmath5 ( long wavelength spectrometer ) , respectively , determined by the image slicer .
the 5 @xmath0 5 pixel fields of view are observed simultaneously with two ge : ga photoconductor arrays . observing wavelength
are adjusted by tilting the littrow mounted grating in each channel .
spectral coverage of @xmath1 1500 km / s around a selected far - infrared line is obtained simultaneously for all 25 spatial pixels .
a summary of important instrument properties is shown in table 2 .
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spie 2475 , infrared detectors and instrumentation for astronomy , a. m. fowler , ed .
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we present a progress report on the design and construction of the field - imaging far - infrared line spectrometer ( fifi ls ) for the sofia airborne observatory . 
 the design of the instrument is driven by the goal of maximizing observing efficiency , especially for observations of faint , extragalactic objects . 
 thus , fifi ls utilizes an integral field technique that uses slicer mirrors to optically re - arrange the two - dimensional field into a single slit for a long slit spectrometer . effectively , a 5 @xmath0 5 pixel spatial field of view is imaged to a 25 @xmath0 1 pixel slit and dispersed to a 25 @xmath0 16 pixel , two - dimensional detector array , providing diffraction - limited spatial and spectral multiplexing . in this manner , 
 the instrument employs two parallel , medium resolution ( r @xmath12000 ) grating spectrometers for simultaneous observations in two bands : a short wavelength band ( 42 to 110 @xmath2 m ) and a long wavelength band ( 110 to 210 @xmath2 m ) . 
 overall , for each of the 25 spatial pixels , the instrument can cover a velocity range of @xmath11500 km / s around selected far - infrared lines with an estimated sensitivity of @xmath3 w hz@xmath4 per pixel . 
 this arrangement provides good spectral coverage with high responsivity . 
 * this paper does not include figures due to astro - ph size limitations . 
 please download entire file at http://fifi-ls.mpe-garching.mpg.de/fifils.ps.gz . *