Multiple Object Fiber Optics Spectrograph Feed for the Hale Telescope

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Multiple Object Fiber Optics Spectrograph Feed For The Hale Telescope

Eldred F. Tubbs, Willis C. Goss, Judith G. Cohen

Eldred F. Tubbs, Willis C. Goss, Judith G. Cohen, "Multiple Object Fiber Optics Spectrograph Feed For The Hale Telescope," Proc. SPIE 0331, Instrumentation in Astronomy IV, (16 November 1982); doi: 10.1117/12.933468 Event: 1982 Astronomy Conferences, 1982, Tucson, United States

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 31 May 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use Multiple object fiber opticsoptics spectrographspectrograph feed for the HaleHale telescope

Eldred F. Tubbs,Tubbs, WillisWillis C.C. Goss,Goss, JudithJudith G. Cohen* Jet PropulsionPropulsion Laboratory,Laboratory, CaliforniaCalifornia InstituteInstitute ofof Technology, Technology, Pasadena,Pasadena, California 9191109 109

Abstract A preliminary designdesign ofof aa fiberfiber-optics -optics feedfeed for the prime-focusprime -focus spectrographspectrograph ofof thethe HaleHale telescope usingusing computer controlled movable fiberfiber hashas beenbeen completedcompleted andand aa testtest ofof aa proto-proto type configurationconfiguration carried out.out. The complete design will dividedivide aa 76mm76mm squaresquare fieldfield intointo 1010 strips andand willwill placeplace two movablemovable fibers in each strip.strip. The fiber pickups, which areare moved by stepperstepper-motor -motor drivendriven leadlead screws,screws, maymay bebe placedplaced anywhereanywhere inin thethe stripstrip subjectsubject to the limitation that they not pass eacheach other.other. The prototype consisted of a single strip withwith two fibers operated with manualmanual input to the stepperstepper motors.motors. In tests performed at the 5 meter Hale telescopetelescope inin AprilApril ofof 19811981 spec-spec tra of two brightbright 0 stars (B == 8.5 mag)mag) separated byby 5 arcarc minutesminutes werewere photographedphotographed withwith aa 3 minute exposure using aa 12001200 lineline/mm /mm gratinggrating and unbaked 103a0103aO plates. The performance of the prototype configuration waswas withinwithin aa factorfactor ofof twotwo ofof thethe unmodifiedunmodified prime prime-focus -focus spec-spec trograph indicating aa potentialpotential forfor aa tenten-fold -fold increase in the effective utilizationutilization ofof thethe telescope for spectrographicspectrographic surveysurvey workwork whenwhen fittedfitted withwith thethe 2020-fiber -fiber feed.feed. Introduction The traditional design ofof astronomicalastronomical spectrographs placesplaces the spectrograph entranceentrance slit onon the optical axis in the focal plane of thethe telescope.telescope. Depending upon thethe dispersiondispersion desired any of the various focifoci (Newtonian,(Newtonian, Cassegrain,Cassegrain, coude,coude, etc.)etc.) maymay bebe used,used, butbut allall locations suffer from the limitation that inin general onlyonly aa singlesingle objectobject cancan bebe observedobserved atat a time. It has recently been demonstrated thatthat itit isis feasiblefeasible toto connectconnect thethe focalfocal planeplane to the spectrograph slit with opticaloptical fibersfibers toto permitpermit thethe simultaneoussimultaneous observationobservation ofof manymany astronomical objects in a smallsmall angularangular field.field. The referenced demonstration was withwith fibersfibers fixed in position in the focal planeplane in order to intercept the light from a given set ofof astronomical objects, and it waswas necessarynecessary to prepareprepare a different fiber assembly for eacheach image field observed.observed. It did, however, show thatthat lightlight lossloss throughthrough thethe fibersfibers waswas reason-reason able and that a large increase inin observingobserving efficiencyefficiency waswas possible.possible. Objective The objective of this task waswas the demonstration of the feasibility of constructing a fiber-opticsfiber -optics spectrographspectrograph feedfeed suitablesuitable forfor useuse on the 200200-inch -inch HaleHale telescopetelescope ofof PalomarPalomar Observatory. The requirement on the proposed device was that it use optical fibers to couple the light from several objectsobjects in the image atat the primeprime focusfocus ofof the telescope to the existing prime-focusprime -focus spectrographspectrograph inin suchsuch aa wayway thatthat thethe fibersfibers couldcould bebe movedmoved underunder computer control toto taketake up thethe positionspositions requiredrequired forfor eacheach successivesuccessive observationobservation field.field. It waswas notnot the objectiveobjective to demonstrate computer controlcontrol perper se butbut only to demonstrate aa design adaptable toto computercomputer control.control. The specificspecific objectivesobjectives werewere toto do do a adesign- design-trade-off trade -off study to select a general approach, constructconstruct aa prototypeprototype instrumentinstrument usingusing atat leastleast twotwo fi-fi bers, and test it by observationsobservations atat thethe telescope.telescope.

General Design Considerations There are several possible designs for a system whichwhich provide for a number of fibers in the focal plane of a telescope arranged soso thatthat theythey cancan movemove underunder computercomputer control.control. The major questions that must be answered in arriving at an acceptable configuration are: (1) How manymany fibers mustmust bebe providedprovided in an operationally usefuluseful system? (2) WhatWhat constraints must be placed on theirtheir motion? (3) WillWill they bebe movedmoved simultaneously or sequentially?sequentially? (4) Will a fine adjustment be needed to locate the position of maximum signal for each object and if so how will thisthis bebe done?done? The first two questions are particularly closelyclosely related.related. If each fiber is free to movemove anywhere in the fieldfield subjectsubject toto thethe limitationlimitation thatthat itit isis ex-ex cluded from regions close to each of thethe otherother fibers,fibers, fewerfewer fibersfibers willwill bebe neededneeded thanthan ifif each fiber is restricted to a more limitedlimited regionregion ofof thethe focalfocal plane.plane. The last twotwo ques-ques tions are likewise closely related.related. The initialinitial setupsetup ofof thethe fibersfibers forfor aa givengiven fieldfield cancan certainly be done sequentially without occupyingoccupying anan excessiveexcessive amountamount ofof thethe observingobserving time.time. It can, forfor example,example, bebe done while the telescope isis being slewedslewed toto aa newnew field.field. If,

^California*California Institute of Technology

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Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 31 May 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use however, peakingpeaking-up -up isis required,required, thethe questionquestion becomesbecomes criticalcritical toto thethe designdesign becausebecause the peaking-uppeaking -up must must bebe donedone withwith thethe telescopetelescope tracking the measurement field.field. A possible articulation of the fibers so that each is free to movemove over a large portion of the field is shownshown inin Fig.Fig. 1.1. This has been describeddescribed asas thethe "fishing"fishing-pole -pole approach.approach. The mechanismsmechanisms are mounted around thethe edgeedge ofof thethe fieldfield atat equalequal intervals.intervals. Each mechanism is able to movemove its fiber inin polar coordinatescoordinates soso thatthat itit cancan bebe placedplaced anywhereanywhere inin aa sectorsector of radius at least equal to thatthat ofof thethe field.field. The fibers are kept from colliding with each other byby havinghaving the entire processprocess underunder computer controlcontrol andand byby havinghaving contact switches asas a backup. The figure illustrates such a system with 20 fibers applied to a hypothetical star field. The opposite extreme, that of subdividing the field into a numbernumber of independent areas each withwith a fiber movable only within thatthat area,area, isis shownshown inin Fig.Fig. 2.2. In this particular example the field has been divided intointo 3636 squaressquares andand thethe systemsystem appliedapplied toto thethe samesame hypo-hypo thetical star field used inin Fig.Fig. 1.1. The fibers intercepting objects are shown by the solid circles; whilewhile the unused fibers are indicatedindicated with openopen circles.circles. There are 44 objects,objects, shown byby 'anan "X" through a circle thatthat were interceptedintercepted byby fibersfibers inin thethe fishingfishing-pole -pole systemsystem but are not picked upup here.here.

A thirdthird approach isis shownshown inin Fig.Fig. 3.3. Here thethe fieldfield isis divideddivided intointo 1010 stripsstrips eacheach con-con taining two fibers. The fibers are free to move anywhere withinwithin the strip subject to the

• Oo sr* • o • x l# • o o0 o0 o o o

*lK • _ o óÿ 0 • • o 4 o • #M i • 7 0o o • • • • x 0o 0o 0o o0 o

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Figure 1.1. Hypothetical starstar Figure 2.2. Same hypothetical Figure 3.3. Same hypothetical field with "fishing-"fishing-pole" pole" star field with fibers dis-dis star field with fibersfibers dis-dis actuation of the fibers.fibers. tributed in a grid pattern.pattern. tributed inin strips.strips. Note thatthat Solid circles indicatesindicates there are three unintercepted fibers intercepting objects.objects. objects andand twotwo unusedunused fibers.fibers. Open circles areare unused fibers. A circle with an X indicates objects not inter-inter cepted by fibers.fibers.

limitation that they may not pass eacheach other.other. They are kept from colliding by a combination of computer control, limit switches,switches, andand mechanical stops.stops. The system, when appliedapplied toto thethe hypothetical star field, intercepted 17 of the 20 objects and one additional one outside the original field.field.

In order to select a design for the fiber articulation, it is necessary to make an estiesti- mate of the number ofof fibersfibers requiredrequired forfor aa usefuluseful system.system. Since there isis somesome lightlight lostlost in going through the fibers, the numbernumber of objects observedobserved at one time mustmust bebe great enough to makemake a significant reduction inin totaltotal observingobserving time.time. If the factor by which thethe observ-observ ing time is to bebe reducedreduced is arbitrarily set at 5, andand if a factor of two is usedused to allow for transmission losseslosses and the inability to utilizeutilize everyevery fiber whenwhen observing a given field; wewe arrive at 10 as the minimum number ofof fibers.fibers. It is not as easyeasy toto setset anan upperupper bound, but the general complexity of any system probably limits the number of fibers to no more thanthan 40.40.

The first of the three designsdesigns offersoffers thethe greatestgreatest flexibilityflexibility inin fiberfiber utilization.utilization. The chief problem withwith it is the difficulty ofof protectingprotecting againstagainst fiberfiber collisions.collisions. The firstfirst line of defense is the computer program thatthat moves thethe fibers.fibers. The second levellevel ofof protec-protec tion, limit switches to shut off actuators if one fiber comescomes close to another, does notnot present unusual problems.problems. However, the third level, that of mechanicalmechanical stops, cannot be readily providedprovided for this design without aa considerableconsiderable increaseincrease inin complexity.complexity. The secondsecond

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Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 31 May 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use design provides good protection againstagainst fiberfiber collisions.collisions. Each fiber isis assignedassigned aa definitedefinite area and can be protected fromfrom incursionsincursions ofof allall otherother fibersfibers byby physicalphysical stops.stops. The diffi-diffi culties withwith this design are that fiber utilization isis not veryvery goodgood andand aa largerlarger numbernumber ofof fibers and fiber actuators are required thanthan inin otherother designs.designs. The third designdesign offersoffers aa compromise solution.solution. It provides-provides-fair-to-good fair -to -good fiber fiber utilization, utilization, relativelyrelatively simple mechan-mechan ical design and good protection against fiberfiber collisions.collisions. This was thethe designdesign finallyfinally se-se lected for the instrument. A possible addition toto thisthis designdesign isis toto provideprovide forfor thethe rota-rota tion of the entire instrument about thethe axisaxis ofof thethe telescope.telescope. This allowsallows thethe stripsstrips toto bebe placed in any orientation with respect to the field beingbeing observed to maximizemaximize fiber utilization.

It appears toto bebe certaincertain thatthat peakingpeaking-up -up will bebe required.required. The combination ofof errorserrors inin the measuredmeasured positionspositions ofof the objects underunder study coupledcoupled withwith theirtheir properproper motionsmotions willwill create sufficient uncertaintyuncertainty in their location thatthat anan operationoperation maximizingmaximizing the signals from the individual objects will be required. The mode in which this is done is determined by the properties of thethe particular telescope.telescope. If the telescope is able toto executeexecute aa pre-pre cise raster scan pattern, thisthis cancan bebe usedused forfor thethe peakingpeaking-up -up operation. The Hale telescopetelescope can execute such a pattern, and the sequencesequence ofof operationsoperations wouldwould bebe thethe following:following: During the slew to a newnew field the fibers would be drivendriven toto thethe nominalnominal locationslocations ofof thethe objectsobjects to bebe studied, and the grating in thethe spectrographspectrograph would bebe replacedreplaced byby aa planeplane mirror.mirror. This willwill concentrate the light from eacheach objectobject onon aa singlesingle pointpoint ofof thethe detectordetector andand con-con vert thethe systemsystem intointo aa multimulti-channel, -channel, broadbroad band photometer. Once thethe pointing ofof thethe tele-tele scope hashas beenbeen established in the nominal locationlocation inin thethe newnew field,field, thethe telescopetelescope willwill bebe programmed to movemove in aa small rasterraster patternpattern andand the outputoutput ofof eacheach fiberfiber atat each pointpoint inin the raster stored by thethe computer.computer. At the end of the raster the telescope returns to the nominal position for the field, and the computer determines from the stored photometric values the amount each fiberfiber must bebe movedmoved toto achieveachieve thethe maximummaximum output.output. The fibers areare moved the few steps required to bring them to the position for maximummaximum signal while the mirmir- ror is being replaced by thethe grating.grating. The system is then ready to begin thethe observation.observation. Since the time to replace the mirror with thethe gratinggrating isis longlong comparedcompared toto thethe timetime toto movemove the fibers, the fibers can be moved sequentially.sequentially. Detailed design

A schematic of thethe prototypeprototype systemsystem isis shownshown inin Fig.Fig. 4.4. The pickup fibersfibers areare mountedmounted onon carriages driven by stepperstepper motors. A viewing system, which is movable in the direction of carriage motion,motion, allows visual inspection of the field to confirm that the correct objects are beingbeing intercepted by thethe fibers.fibers. A second viewing system mounted outside the main field allows offset guiding to be used. The drive electronics moves the motors under manually commanded digital control.control. The spectrographspectrograph isis thethe existingexisting primeprime-focus -focus instrument modified with a mount for thethe fibersfibers atat thethe entranceentrance slitslit andand withwith thethe normallynormally-used -used Schmidt cameras replaced withwith a simpler one for thisthis experiment.experiment. The individual portions ofof thisthis systemsystem areare discussed in detail below. Actuators

The actuation scheme as itit couldcould bebe usedused inin thethe finalfinal instrumentinstrument isis shownshown inin Fig.Fig. 5.5. One quarter of the assembly is shownshown inin twotwo views. The basic form of thethe carriages thatthat movemove

END VIEW SIDE VIEW

LONGITUDINAL DRIVE MOTOR

TRANSVERSE DRIVE MOTOR

SPECTROGRAPH FIBERS i

FIXED OFFSET PRISM GUIDE GRATING GUIDER FOCAL FIELD PLANE

ENTRANCE a MOVABLE REFERENCE VIEWER SOURCE

Figure 4.4. Block diagram of prototype system.system. Figure 5.5. Actuation ofof thethe fibersfibers inin thethe design for the complete instrument.instrument.

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Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 31 May 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use the fibers isIs that of a rectangular blockblock withwith itsits longlong axisaxis parallelparallel toto thethe telescopetelescope axis,axis, its intermediate axis parallel toto thethe longlong dimensiondimension ofof thethe strips,strips, andand itsits shortestshortest axisaxis inin the transverse direction.direction. The ways on which thethe carriagescarriages move areare pairspairs ofof rods.rods. Eight ofof these support two carriages while thethe otherother fourfour supportsupport singlesingle carriages.carriages. This arrangement packs the articulation of 2020 fibersfibers intointo aa reasonablyreasonably compactcompact space.space. The design of the individualindividual carriagescarriages isis shownshown inin Fig.Fig. 6.6. Although onlyonly twotwo werewere con-con structed for testing in the prototype, thethe designdesign isis thethe oneone intendedintended forfor thethe finalfinal instru-instru ment. The interior of the prototype instrument with the carriages and fibers is shown in Fig. 7.7. The outer portion of the actuator carriage is a set of parallel links with flexure

FIBER FIBER IN IN RIGID FLEXIBLE JACKET JACKET

RETURN SPRING

FLEXIBLE MEMBERS

LATERAL-LATERAL-MOTION MOTION DRIVEDRIViE SCREWSCREW

LOLONGITUDINAL-MOTION NGITU DINAL-MOTI ON DRIVE SCREWSCREW

Figure 6.6. End view of a fiberfiber carriage.carriage. Figure 7.7. Interior of prototype instrumentinstrument showing fibers andand fiberfiber carriagescarriages hinges. This linkage supports the fiber and allows motion across the width of thethe strip. Motion along the length of the strip is provided by moving the entire carriage on a pair of cylindrical waysways with linearlinear ball bearings. Two of the bearings are mounted rigidly in the carriage whilewhile the third isIs onon aa stiffstiff leafleaf spring.spring. The smallsmall amount ofof compliancecompliance provided by the spring accommodates variationsvariations inin thethe spacingspacing ofof thethe ways.ways. This particular designdesign was chosen for severalseveral reasons:reasons: It lends itself to the stacking required toto accommodateaccommodate 2020 fibers byby havinghaving its shortest dimension perpendicularperpendicular to the axisaxis of the telescope, a direcdirec- tion in whichwhich space is limited, and its longest dimensiondimension in the direction of the axis wherewhere space is available. The configuration allows the flexure members to have a substantial width, which makes forfor greatergreater mechanicalmechanical stability.stability. In addition, flexureflexure membersmembers eliminateeliminate some of the problems of backlash and lost motion sometimes encountered with sliding or rolling motions. Both motions, longitudinallongitudinal andand lateral,lateral, areare drivendriven throughthrough lead lead screws screws by by 90 90°, °, permanentpermanent- - magnet stepper motors. The leadlead screwscrew pitch isis 52 toto the inch,Inch, and thisthis combinedcombined with a 6 to 1 geargear ratioratio givesgives aa displacement of .0008 in oror approximatelyapproximately 0.25 arc second perper step corresponding toto 1/41/4 ofof thethe bestbest-seeing - seeing diameterdiameter fromfrom aa pointpoint source. PermanentPermanent-magnet -magnet motors werewere chosen because of the detent action that remains whenwhen the power is off. The re-re sulting system has proved to be very stable.stable. It waswas found that thethe fibersfibers heldheld theirtheir posi-posi tions from one nightnight to the next over a period that involved a full night's observing at Cassegrain and numerous attitudeattitude teststests duringduring thethe day.day. Fiber cable selection Fiber-cableFiber -cable selectionselection criteriacriteria werewere spectralspectral transmission, corecore andand claddingcladding diameters,diameters, numerical aperture, flexibility,flexibility, andand protectiveprotective jacketing.jacketing. A requirement forfor transmissiontransmission over a wavelength range from 0.3pm0.3ym to 0.7pm0.7ym indicated the useuse of glass core rather than plasticp1as11c waveguides. TypicalTypica1 imageimage diametersd1ameters of 100100 toto 200pm at thethe telescopetelescope focalfocal planep 1 ane due to seeing conditions ledled toto thethe selectionselection ofof aa corecore diameterdiameter ofof 250pm.250ym. The fiberfiber numer-numer ical aperture waswas required to be adequate to accept and conduct the cone of light from the primary mirror. This did not present a problem because the numerical aperture of thethe tele-tele scope is 0.14 and the majority ofof thethe multimode, glassglass-fiber -fiber waveguideswaveguides availableavailable have numerical apertures inin excessexcess ofof 0.25.0.25. A jacketing design which provided forfor strainstrain relief

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Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 31 May 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use and protection against abrasion along with flexibilityflexibility adequateadequate forfor aa bendbend radiusradius ofof oneone inch was also required. The fiber waveguide selectedselected was ValtecValtec PC10,PC10, withwith aa corecore diameterdiameter ofof 250pm250ym andand aa clad-clad ding diameterdiameter ofof 280ym.284m. The core is fused silica, the cladding is silicone RTV, and the numerical apertureaperture isis 0.30.0.30. Jacketing is in the form of an inner tube, which fits loosely around the fiber, aa layer ofof KevlarKevlar fibers for tensile strengthstrength andand cushioning,cushioning, and anan outerouter jacket of polyurethane. The fibers are supported andand protectedprotected byby stainlessstainless-steel -steel tubestubes fromfrom thethe carriagecarriage to the vicinity ofof thethe focalfocal plane.plane. These tubes are bent through a 90° curve and terminate about 10mm from the focal plane.plane. All jacketing is removed from the immediate tip of thethe fiber to permit clear viewing ofof thethe end.end. The outer jacket is clamped to the carriage just beyond the end of the steel tubing and isis clampedclamped againagain atat thethe pointpoint wherewhere itit passespasses throughthrough thethe wall ofof thethe housinghousing aroundaround thethe focalfocal-plane -plane assembly.assembly. The jacket isis alsoalso clampedclamped atat thethe spectrograph, and the fibers areare terminated immediately beyondbeyond the clampsclamps withwith commercial ferrules cemented to the ends of thethe fibers.fibers. These are clamped in a block which aligns them parallel to the axis of the spectrographspectrograph andand directsdirects thethe lightlight intointo thethe entranceentrance slit.slit. At the spectrograph end the fibers were opticallyoptically polishedpolished afterafter beingbeing mountedmounted inin thethe ferrules.ferrules. At thethe focalfocal plane thethe endsends werewere cleaved.cleaved. Although thisthis slightlyslightly reducedreduced the the light light- - collection efficiency, it metmet the practicalpractical requirement ofof allowing easy renewal of the fifi- ber end inin thethe eventevent ofof breakage. Three fiberfiber cablescables were prepared.prepared. Two were used toto pick up stellarstellar imagesimages andand oneone was usedused forfor aa spectralspectral referencereference source.source. The end treatmenttreatment ofof each cable waswas the same and allowed thethe referencereference fiberfiber toto serveserve asas aa spare.spare. Viewer and guiderguider opticsoptics Two sets of viewing optics were provided in the instrument, one for viewing the field covered by the fibers and one forfor offsetoffset guiding.guiding. The latter waswas one which had been built for the primeprime-focus -focus spectrographspectrograph and was used unmodified.unmodified. It waswas mounted to view aa regionregion just outside thethe threethree-inch -inch square main field.field. The other utilized a pair of projection lenses as relays and waswas equipped withwith an illuminated reticle conjugate to the plane of the fibers. This viewer was mounted on a slide which permitted it to be moved in the direction of the long travel ofof thethe fibers.fibers. Each fiber and the surrounding field could be viewed at any point in its travel.travel. Spectrograph The spectrograph used in this work was the one designed specifically for use at the primeprime-focus -focus ofof thethe HaleHale telescope by BowenBowen inin 1950.1950. It is aa planeplane-grating -grating instrument with aa Newtonian collimator andand aa SchmidtSchmidt camera.camera. Since the Schmidt camera isis inconvenientinconvenient toto use,use, it waswas replacedreplaced for these tests withwith a KodakKodak AeroAero EktarEktar f/2.5, 7 inch focal length and a 4x5 film plane. With thethe 12001200 lineslines/mm /mm grating used thisthis gavegave aa plateplate factorfactor ofof 33A 33A/mm. /mm. The entrance slit waswas not used toto determinedetermine thethe spectralspectral resolution.resolution. Instead the ends of thethe fibers served as the entrance slit.slit. This gave a resolution ofof approximatelyapproximately 10A.10A. Drive electronics Since actual computer operation was not required for the study, the drive electronics could be kept quite simple.simple. The control unit was built by Anaheim Automation as a variation on one of their standardstandard units.units. It is arranged so that control may be switchedswitched toto eacheach ofof the four motorsmotors in turn and operate eacheach inin anyany ofof threethree modes:modes: run, jog, oror index.index. In the "run" modemode the motormotor operates asas longlong asas thethe controlcontrol buttonbutton isis heldheld down.down. The "jog""jog" mode operates the motormotor through aa singlesingle stepstep eacheach timetime thethe buttonbutton isis pressed.pressed. The "index" mode allows operation throughthrough aa predeterminedpredetermined numbernumber ofof stepssteps setset on on thumb thumb-wheel -wheel switches.switches. Auto-Auto matic ramping of thethe motor speedspeed isis availableavailable inin thethe "index""index" mode.mode. Each motion ofof thethe car-car riages waswas fitted withwith limit switches that are switched in when the motor that drives that motion isis operating.

Guide fiber

As part of this task a design and fabrication study of a system for the automatic guiding of the telescope was carriedcarried out.out. The objective of the study waswas a system which could allow the observer to place a speciallyspecially designateddesignated fiberfiber onon aa selectedselected guideguide objectobject andand havehave guid-guid ance signals sent to the computer controllingcontrolling thethe telescope.telescope. The plan investigatedinvestigated was thatthat of combining four fibers intointo aa singlesingle "guide"guide fiber"fiber" ofof fourfour closeclose-fitting -fitting quadrantsquadrants byby careful lapping of the adjoining edges of each fiber and leading the light intercepted byby each quadrant toto aa separateseparate detector.detector. The dividing lines between thethe quadrantsquadrants werewere toto bebe aligned inin thethe northnorth-south -south andand easteast-west -west directions.directions. The northnorth-south -south guidingguiding signal would be generated by takingtaking thethe sumsum ofof thethe signalssignals fromfrom thethe northnorth-west -west andand northnorth-east -east detectorsdetectors minus thethe sumsum ofof thethe signalssignals fromfrom thethe southsouth-west -west andand southsouth-east -east detectors. Similarly thethe

SPIE Vol. 331 331 Instrumentation Instrumentation inin AstronomyAstronomy IV /V (1982) (1982) / / 293293

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 31 May 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use easteast-west -west signalsignal wouldwould bebe generatedgenerated by thethe differencedifference betweenbetween thethe sumsum ofof thethe north north-east -east an,an^ southsouth-east -east signalssignals and the sumsum ofof thethe northnorth-west -west andand southsouth-west -west signals.signals.

A signalsignal-to-noise -to -noise analysis analysis was was performedperformed andand detectorsdetectors selected. These werewere EGEG&G &G modelmodel HAD 1000A,100QA, which areare siliconsilicon photodiodesphotodiodes followedfollowed byby anan integralintegral operationaloperational amplifier.amplifier. A fabrication procedure for thethe fiberfiber was designeddesigned andand shopshop teststests carriedcarried out.out. A prototype was successfully constructed, but difficulties were encountered in a final assembly. Since there werewere insufficient resources toto putput aa largelarge efforteffort onon thisthis aspectaspect ofof thethe studystudy nono fur-fur ther work was done.done. It waswas concluded from the work thatthat was accomplished,accomplished, thatthat withwith suffi-suffi cient effort aa fiberfiber-optics -optics quadrantquadrant guider could be constructed. Visual guiding was used for the prototype tests.tests.

Tests The assembled instrument isis shownshown inin Fig.Fig. 8.8. Three sets of laboratorylaboratory teststests werewere per-per formed. The first of thesethese were interferometricinterferometric teststests ofof thethe carriagecarriage motions.motions. The results of one of these tests isis shownshown inin Fig.Fig. 9.9. The indication is that the deviation fromfrom the average position isis within ±0.1±0.1 arc second across thethe fieldfield ofof view ofof thethe instrument.instrument. The second testtest was thethe measurement of the transmission through the fibers in thethe visible regionregion ofof the spectrum.spectrum. This was found to bebe 8585%. %. The testtest was per-per formed with thethe fibersfibers inin theirtheir installed configuration where there were severalseveral sharpsharp bends contributingcont ributing toto thethe loss.loss. These curves will be smoothed out somewhat inin thethe finalfinal de-de sign which will furtherfurther reducereduce the lightlight loss.loss. The third laboratory testtest was aa seriesseries ofof exposures ofof spectraspectra toto deter-deter mine thethe dispersiondispersion andand toto check thethe focus.focus. The fiberfiber-optics -optics spectro- ,; graph feed was testedtested atat thethe prime focus of thethe HaleHale tele-tele scope during February andand April of 1981.1981. ThrThree e e problemsprobiems were discovered and correctedcorrected asas aa result of thethe tests.tests. It was found necessary toto addadd generalgeneral illumination inin thethe vicinity ofof the fiber tipstips soso thatthat theythey could be seen with thethe viewing system. It was also necessary to add an illuminatedilluminated reticlereticle in the viewing systemsystem forfor fo-fo cusing. The third problem arose from a failurefailure toto recog-recog nize thatthat thethe entranceentrance slitslit assembly ofof thethe primeprime-focus -focus spectrograph isis not perpendicu-perpendicu lar to thethe opticaloptical axis.axis. This construction allowsallows viewing ofof the stellar imageimage onon thethe slitslit Figure 8.8. Exterior ofof prototype instrument.instrument. The fiber by thethe observerobserver inin normalnormal use.use. are in the configuration for transportation.transportation. The block The initialinitial mounting bracketbracket on the nearnear side ofof the instrument is mountedmounted on the holding thethe fibersfibers at thethe slit spectrograph slit forfor operation.operation. was made toto hold thethe fibersfibers perpendicular toto thethe plane of the slit.slit. The result was that a considerable mount of the light leaving thethe fibersfibers missed the collimators. This was corrected by reworking the bracket to thethe correct angle.angle. Figure 10 shows a simultaneous exposure of HD 46149 and HD 46150 in the Rosette nebula together withwith a helium comparison spectrum.spectrum. These two stars are more than 5 arc minutes

294 // SPIE SPIE Vol.Vol. 331 Instrumentation inin Astronomy IV (1982)

Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 31 May 2019 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use apart inin thethe sky.sky. The fiber positioning func-func 0.3 tioned wellwell and it was apparent thatthat setset upup forfor 0.2 bright objects isis relatively easy.easy. In addition, there waswas nono detectable flexure inin thethe system.system. The photographic speedspeed ofof thethe instrumentinstrument waswas found to be asas expected.expected. The best performance of the primeprime-focus -focus spectrographspectrograph withwith the originaloriginal f/1 camera as used until about 19701970 was quotedquoted byby Jesse Greenstein as:as: B = 16.7 magnitude inin 44 hours with aa 600600 lineline/mm /mm gratinggrating producedproduced a weak 0.5 1.0 1.5 20 2.5 3.0but but usable usable exposure exposure onon bakedbaked IlaOIIaO platesplates using aa THOUSANDS OFOF STEPS STEPS 1 by 12 arc secondsecond slit.slit. The substitutesubstitute lens,lens, asas has already been indicated, was an f/2.5, 7 inch Figure 9.9. Results of interferometric aerial-cameraaerial -camera lens. This lenslens had somewhatsomewhat poorpoor test of carriage motion. The dashed image quality towards the edge ofof thethe field.field. The lines indicate ±0.1±0.1 arcarc-second - second deviationdeviation best performance in a very limited amount of from the average position.position. The totaltotal telescope time was B = 8.5 magnitude inin 33 minutesminutes carriage travel represented here is with aa 12001200 lineline/mm /mm gratinggrating andand unbakedunbaked 103aO103a0 57 mm. plates. Allowing for the change in camera focal length 5016 length and the change in grating, thethe lacklack ofof 5016 trailing required for the fibers, thethe use ofof thethe Wynn corrector lenslens with thethe fiberfiber opticsoptics spec-spec 4922 trograph feed but not for thethe original spectro-spectro 4713 graph, the change in aperture of thethe cameracamera lens,lens, and the difference inin thethe exposureexposure time,time, thethe per-per formance comes within a factor of twotwo ofof thethe original spectrograph.spectrograph. A firmer statement cannot be made without evaluating more carefully the relative speedspeed ofof thethe photographicphotographic emulsionsemulsions andand the transmission ofof thethe WynnWynn corrector.corrector. It is,is, therefore, reasonable to conclude onon thethe basis ofof limited tests that the instrument performedperformed up toto expectations 44714471

-01 4388 Acknowledgements We wish to acknowledge the contribution of Harry T. Fearnehough, who diddid muchmuch ofof thethe detaildetail design and all of the mechanical fabrication.fabrication. 4144 This paper has presented thethe resultsresults ofof oneone phasephase / of research carried outout atat thethe JetJet PropulsionPropulsion 4121 Laboratory, California InstituteInstitute ofof Technology,Technology, under ContractContract NAS7NAS7-100, -100, sponsored by thethe 4026 National Aeronautics andand SpaceSpace Administration.Administration. --- 3964 References - 3889 1. E. N. Hubbard, J. R. P. Angel,Angel, andand M.M. S. Gresham, Astrophysical Figure 10.10. Simultaneous spectra of HD46149 Gresham, Astrophysical JournalJournal 229,229, 10741074 (1979).(1979). and HD46150 (5(5 arcarc-minutes - minutes apartapart in the sky)sky) taken with thethe prototypeprototype FOSF"POSF onon thethe 5m5m-Hale -Hale telescope during engineering tests.tests. These are bright 0 stars.stars. The helium comparisoncomparison spectrum waswas obtained with an optical fiber with one end viewing thethe helium lamplamp andand the other end located at the spectrograph entrance slit.slit.

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