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Critical to the performance of the HST is staying on target for extended periods of time. Electromagnetic waves emitted from distant objects are often faint or weak, so the HST must stay perfectly positioned while the photons are being collected in sufficient quantities to form an image. To accomplish this, engineers used the Schlumberger oilfield photomultiplier-tube technology to design the FGS system.1 0 An FGS is essentially a targeting camera capable of making celestial measurements, locking onto guide stars and providing data for maneuvering the telescope.1 1


Two FGSs are used


to point the telescope at an astronomical target and hold that target in the telescope’s field of view;


accuracy


the third FGS can then be used for astrometry measurements.1 2 The FGS system can maintain pointing to 0.007 arcseconds, allowing the


telescope’s pointing-control system (PCS) to keep the Hubble telescope on target during camera exposure times of 10 hours or more.1 3


to achieve this The


PCS combines a number of different sensor subsystems


milliarcsecond


pointing accuracy. This level of accuracy and precision is comparable to training a laser beam on a target the size of a thumbnail from a distance of 442 km [ 275 miles] .


Within the housing of each FGS instrument are two orthogonal white-light, shearing interferometers, their associated optical and mechanical elements and four Schlumberger S-20 photomultiplier tubes (PMTs) (above right).1 4 These PMTs are based on the same rugged construction as those used in well-logging instruments. The photocathode was manu- factured using the same technology as tubes used in oilfield service applications. For use on the HST, the PMTs were designed to be sensitive over a spectral range of 400 to 700 nanometers (nm), with an efficiency of approximately 18% at the blue end of the electromagnetic spectrum and diminishing linearly to about 2% at the red end. Each FGS interferometer consists


of a


polarizing beam splitter followed by two Koesters prisms. To measure the direction of the light emitted by a guide star, the pairs of Koesters prisms are oriented perpendicular to one another. The angle of the wavefront in the X and Y planes gives the precise angular orientation of the guide star relative to the HST’s optical path. These data, once fed into the PCS, are used to control the telescope orientation relative to a guide star.


5 . NASA— Hub b le’ s Conception: http: / / hub b le. nasa. gov/ overview / conception-part1. php ( accessed April 18, 2006) .


6. NASA, reference 5 .


7 . Adaptive optics is a technology used to im prove the perform ance of optical sy stem s b y reducing the effects of rapidly changing optical distortion ty pically


resulting from changes in atm ospheric conditions. Adaptive optics


w ork s b y m easuring the distortion and rapidly com pensating for it using either deform ab le m irrors or m aterial w ith variab le refractive properties.


8. Sm ith RW: The Space Telescope– A Study of NASA, Science, Technology and Politics. New Y ork City : Cam b ridge University Press, 19 89 .


9 . Sm ith, reference 8.


10. For m ore on photom ultiplier tub es: Adolph B, Stoller C, Brady J , Flaum C, Melcher C, Roscoe B, Vittachi A and Schnorr D: “ Saturation Monitoring With the RST Reservoir Saturation Tool, ” Oil eld Review 6, no. 1 ( J anuary 19 9 4 ) : 29 – 3 9 .


Spring 2006


11. Space Telescope Science Institute– FGS History : http: / / w w w . stsci. edu/ hst/ fgs/ design/ history ( accessed March 14 , 2006) .


A guide star is one of m any b right stars used for telescope positioning and triangulation.


12. Astrom etry is a b ranch of astronom y that deals w ith the positions of stars and other celestial b odies, their distances and m ovem ents.


13 . A second of arc, or arcsecond, is a unit of angular  3 , 600 of a degree of arc or 1 1, 29 6, 000 ≈ 7 . 7 x 10-7


m easurem ent that com prises one-six tieth of an arcm inute, or 1


of a circle. It is the angular diam eter of an ob j ect of 1 unit diam eter at a distance of 3 60x 60x 60/ ( 2π ) ≈ 206, 265 units, such as ( approx im ately ) 1 cm at 2. 1 k m .


14 . Interferom eters w ere  rst used b y Michaelson, w ho w on the Nob el Prize in 19 07 for his w ork using an optical interferom eter to accurately m easure the speed of light.


D


Dielectric beam splitter


K oesters prism


ACB


I ncident wavefront Alpha


> Guiding Hub b le. Light from the HST Optical Telescope Assem b ly ( OTA) is intercepted b y a pick off


m irror in front of the HST focal plane and directed into the  ne-guidance sy stem ( FGS) ( left) . The light ray s are collim ated, or m ade parallel, and then com pressed b y an aspheric collim ating m irror and guided to the optical elem ents of the star selector assem b ly . Sm all rotations of the star selector A and B assem b lies alter the direction of the target’ s collim ated b eam , and hence the tilt of the incident


w avefront w ith respect to the K oesters prism ( right) . As the w avefront rotates ab out Point B, the relative phase of the transm itted and reect ed beam s change as a function of angle alpha. When the


w avefront’ s propagation vector is parallel to the plane of the dielectric surface, the relative intensities of the tw o em ergent b eam s detected b y the photom ultiplier tub es w ill b e eq ual. When alpha is not zero, the intensities of the left and right output b eam s w ill b e uneq ual and the PMTs w ill record different photon counts, thus providing the telescope guidance control sy stem w ith data allow ing for pointing correction. [ Im ages courtesy of NASA and The J ohns Hopk ins University Applied Phy sics Lab oratory ( J HUAPL) . ]


Aspheric collimating mirror


P hotomultiplier tube with pinhole lens assembly ( 4 )


Star selector mirrors


Doublet lens ( 4 ) K oesters prism


B eam-splitter prism F ilters ( 5 in wheel)


P ickoff mirror P M T B F ield stop F ield lens


P ositive doublet


P M T A F ield stop F ield lens


P ositive doublet


Correction group Deviation prism


O ptical bench


47


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