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3. THE LST SCIENTIFIC CONFIGU RATION The LST is planned as a longÂ·lived orbiling astronomical observatory containÂ· In& a 2.4-m Rltchey-Chretien telescope with a focal ratio of f/24. The on-axis im:lgo is virtually diffrlletion.Jimit<d at 6328 A., whore the full width at half maximum Intensity Is a little la'l!er than 0.05 arc sec. The guidance sensfn& system is housed in three of four radial bays louted close behind the primary mirror and off the optical axis. Two methods of compensating ror the off-axis astigmatism in the guide stars have been pr~ posed to provide a point ins stability of 0.007 arc sec rms. The fourth radial bay houses the Focal P1ane Camera, a_n instrument of the highest priority. An on-axis flat mirror directs a 3-arc-min-$Quare ponion of the f/24 image to this camera. The only detector expected to be available is the SO X S(}.mm SEC Vidicon, which has 2,000 X 2,000 picture elements (pixels), each 25 X 25 lUll in size, correspond in& to 0.09 arc sec square. The 5Â·20 photocathode of the SEC Vidicon will allow Imagery from 1200 A. to 6000 A.. The Focal Plane Camer1 is desianed to image the brighter parts of galaxies in one orbital night, I.e., in half an hour. A single exposure wUireveal 26thÂ· mâ¢8'1itude star>. Reaching the sky background will require the "stacking" of ten or more Individual !mages by ground processing. A key feature of the Focal Plane Camera is its ability to image a portion of the sky at the same time any other scientific instrument on the LST is operatin&. This Â·~serenÂ· dipity mode" will produce a collection of high-resolution Un~es that are expected to reveal very distant cluster> of galaxies, as well as unsuSpected objects in the ultraviolet. The other scientific lnstrumenta are contained in four modular axial bays on the optical and geome1ric axis or the telescope. Foremost amons these instruments are the Faint Object Spectrograph (Fos) and the Faint Object Can>era (Foe). The FOS i1 characterized by resolving powers of ~/A~ â¢ 101 and 10' over the wavelength range 1200 A. to 8000 A.. in addition, the design can incorporate a higher resolution mode ()./A>. â¢ I!f). When ooupled to a photon-counting detector,the FOS wUI be able to obtain the stigmatic >peeÂ· trum of a 25thÂ·magnltude stâ¢r (>./A>. â¢ 101 ) in a IOÂ·hour exposure with a s:ignalÂ·tc>noise ratio or aboul ) 0. Tile FOC will have two modes of operation at focal ratios of f/48 and f/96. 8
Operated at f(96, It Is des~tned to retain the full resolvlna power of the teleÂ· ocope. For a detector litld of SOO X SOO pilteb, the correspond in& field of new is small (II arc 10t square). A I().how cumulative txpowre with a photon-countlna detector wiU yield 10 perocnt photometry on a 28th m"'l'l- tude nar In a 1()()0,\ band pus. The reeommtnded pbococothode respoJUO isS-20. The othtr hilh-priority instlWIXIItl beinl eoruiclered for tht two remalnln& ulal baya are the Planetary Camera and tht ttt Photomettr. The latttr C0Â¥<11 tht spectrwn frotn I to IOOOjUD with a ruoiYin&powtr ofabout )Jll). â¢ 10, obtained by a series of filttrs. A Si:As or Si:P photocoadiJCtor oo~n the opÂ«trum from I to 30 jllll, while thtlorctr wavelenatha art ruched by the ... of â¢ bolomttcr. Much of the Itt Photomtttr Is cooled to liquid helium ttrnperaturts by a dewu duigned to last at ltut 1 year In orbit . The t k Photometer b diffroetionÂ·limited at all wavtltrctlls. The Plonttary Camero is a 400 X 400 pixel unintendfled onoloa camera with a allleon photomt1rie rcaponat. When operated at f(96, lu field of view b 9 arc 1ec square, which is adequate to obtain images of Uranus and Nep- tune with â¢I-to-noise ratios in exCC$S of 100. When operated at f/24, the Planetary Camera is capable of lma&lna hi&hJy rtd-chifted pi axles (z < I) In one otbltal nl&ht. F'mally, astrometric observations eon be readily performed ulin& tht fme 1111danoe S)'ll.cm. 9