For wavelengths from 2.5 to 100 f/m SI RTF is the most sensitive telescope 

 now planned for Earth orbit. At very high spectral resolutions (X/AX » 10 3 ), 

 HST (with a possible second-generation infrared spectrometer) and LDR will 

 have an advantage at wavelengths less than 2.5 ^m and longer than 100 /am, 

 respectively, because of their larger collecting areas. At very high resolving 

 powers, this advantage overtakes the cooled-optics advantage of SI RTF. There is 

 an infrared spectrometer proposed for SI RTF that is planned to operate between 

 2.5 and 200 jum with two resolution modes of X/AX = 50 and 1000. The high- 

 resolution mode is adequate for extragalactic studies of the strongest infrared 

 fine structure lines; for example, it will be capable of measuring the very impor- 

 tant 157-^m CI I line out to a redshift of z - 0.3. The sensitivity of the SI RTF 

 infrared spectrometer is expected to be high enough that observations can easily 

 be made of other infrared lines required to trace the biogenic elements encased 

 within dense molecular clouds in either neutral or ionized states in normal 

 galaxies out to distances of 500 Mpc. This distance encompasses a very large 

 number of galaxies, enough that biogenic elemental abundance studies can be 

 correlated with other factors, such as galactic morphological type. 



There are two infrared spectrometers of high enough resolution for infrared 

 fine-structure line observations listed in the strawman instrument complement 

 of LDR. The large aperture of this telescope will allow good angular resolution 

 for abundance gradient observations of the biogenic elements and the large col- 

 lecting area will make it unequalled for submillimeter line measurements. 



Of all the planned Earth-orbiting telescopes, SI RTF is the best suited for 

 carrying out dust studies in other galaxies. The cold optics of this telescope will 

 allow for very high sensitivities. For example, the planned infrared photometer 

 will be capable of measuring the integrated mass of dust in normal spiral galaxies 

 out to red shifts of z = 2. The composition of the dust can be determined using 

 the planned SI RTF infrared spectrometer, which has a low-resolution mode that 

 was developed for detecting dust features in galaxies with large red shifts. In the 

 low resolving power mode of X/AX = 50, the SI RTF spectrometer sensitivity is 

 limited only by the background emission from the zodiacal dust. This high sen- 

 sitivity would allow observations of the 10-jum silicate dust feature in spiral 

 galaxies out to a red shift of z = 1 .5. While the warm optics of the LDR make it 

 less sensitive for observations of the most distant and faint extragalactic objects, 

 its larger aperture will allow studies of the distribution of dust in nearby galaxies 

 with 20 times the angular resolution of SI RTF. 



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