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FGS Instrument Handbook
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Fine Guidance Sensor Instrument Handbook for Cycle 22 > Glossary of Terms

Glossary of Terms
The following terms and acronyms are used in this Handbook.
Astronomer’s Proposal Tool (APT): A graphical software system for designing HST observing programs.
articulating mirror assembly (AMA): Commandable adjustable mirror assembly (replaces FF3 in FGS1r) capable of tip/tilt motion; allows for in-flight re-alignment of the beam at the Koesters prism to mitigate deleterious effects of HST’s spherical aberration.
back-off distance: Distance (+dx,+dy) from target photocenter to which the FGS IFOV is moved before starting the Walkdown to FineLock phase of target acquisition.
beam walk: Movement of the axis of tilt of the collimated beam on the face of the Koesters prisms; results from clocking errors and misalignments of the SSA and SSB assemblies with respect to the Koesters prisms.
breathing (thermal breathing): Change in telescope focus possibly resulting from temperature changes incurred as HST crossed the day/night terminator during each orbit.
check star: Targets observed more than once per POS mode visit; used as positional references to detect and allow for the removal of the apparent motion of the FOV during the visit.
CoarseTrack: Search algorithm used by the Fine Guidance Electronics which compares the counts from all four PMTs to continuously update the estimate of the target’s photocenter.
COSTAR: Corrective Optics Space Telescope Axial Replacement; set of corrective optics installed in the 1993 HST servicing mission to correct the FOC, FOS, and GHRS for the spherically aberrated wavefront due to the misformed HST primary mirror. COSTAR does not correct the wavefront as seen by the FGS.
de-jittering: The process of correcting a POS mode observation for the effects of spacecraft jitter and oscillations by establishing a fixed, though arbitrary reference frame determined by the x,y centroids of guide stars observed with the guiding FGSs and compensating for motions of those guide stars during post-observation processing.
dominant guider: The FGS used to control the HST’s translational attitude during an observation.
drift: Relative motions between FGS 3 and FGSs 1 and 2; thought to be a result of thermal effects.
F583W: FGS “Clear” filter, effective central wavelength of 5830 , with a bandpass of 1500 .
F5ND: FGS neutral-density filter; attenuates by 5 magnitudes.
FGE: Fine Guidance Electronics, on-board microprocessors which control the FGSs during the search for, acquisition of, and Position mode tracking of the target.
FGS: Fine Guidance Sensors, term used to describe one of the three white light Koesters prism (shearing) interferometers aboard HST.
FES: Fine Error Signal.
FESTIME: Fine error averaging time; time over which PMT data are averaged for computation of the Fine Error Signal.
FGS X,Y detector coordinates: Coordinate system defined for each FGS originating from the telescope’s optical axis (the V1 bore sight); easily maps to the HST V1,V2,V3 coordinate system.
field dependency: Variations of measured quantities of the Transfer Function (i.e., morphology and modulation) as a function of location in the FOV.
FineLock (FL): Stage of target acquisition in which the star selectors are moved to continuously zero the FES via a closed loop feedback between the position of the IFOV and PMT counts.
FITS: Flexible Image Transport System; the standard astronomical data interchange and archival format.
fold flat mirror (FF3): Non-adjustable mirror assembly employed by FGS2 and FGS3 to redirect the light emergent from the star selector and filter wheel assemblies onto the faces of the Koesters prisms.
FOV: Field of view, for an individual FGS is approximately a quarter annulus with inner and outer radii of 10.2 and 14.0 arcminutes respectively. The total field of view is sometimes referred to as the FGS pickle.
GEIS: Generic Edited Information Set; HST file format, similar in many ways to the FITS format standard; data consists of a header file and a binary data file with multiple data groups.
GO: General Observer.
GTO: Guaranteed Time Observer.
guide star: A star acquired in FineLock by the FGS and used by the HST pointing control system to make continuous corrections to the telescope’s pointing; used to eliminate translational and rotational drift of the telescope during observations.
HIPPARCOS: High-Precision Parallax Collecting Satellite; European Space Agency satellite launched in 1989 to determine astrometric position and parallax measurements of bright stars to an accuracy of 1 milliarcsecond and 2 milliarcseconds/year respectively.
HST: Hubble Space Telescope.
IFOV: Instantaneous Field of View, the roughly 5˝ x 5˝ aperture located by the positions of the two star selectors for FGS observations.
IRAF: Image Reduction and Analysis Facility, the suite of analysis tools developed by NOAO to process general astronomical data.
Koesters prism: Prism at the heart of FGS interferometer; constructed of two halves of fused silica and joined along a surface coated to act as a dielectric beam splitter.
LTSTAB: Long Term Stability Monitoring; program of short POS mode observations of an astrometric field made in the F583W filter to monitor the long term stability of the OFAD and plate scale calibration; usually repeated monthly or bi-monthly.
M35: Open cluster in Gemini (RA 06h 08.9m, Dec +24 20'; mV ~ 5.3) from which several stars are used in LTSTAB observations.
NEA: Noise Equivalent Angle.
NOAO: National Optical Astronomy Observatories.
null (interferometric null): Position in IFOV corresponding to the zero-point crossing of the transfer function for a given interferometric axis.
OFAD: Optical Field Angle Distortion, distortion incurred as a result of the combined effects of the FGS/OTA optical train and errors in SSA and SSB encoder values; alters the measured relative angular separations of stars distributed across the FGS pickle from their true angular separations.
ORIENT angle: Angle measured from North (through East) to the HST U3 axis. This angle definition is used in proposals and should not be confused with roll angle. (See “roll angle”).
OTA: Optical Telescope Assembly.
parallax: Angular displacement in the apparent position of a celestial body when observed from separated points.
PI: Principle Investigator.
pickle: The total field of view (FOV) of a given FGS, roughly 69 square arcminutes in (angular) area.
pick-off mirror: Pickle-shaped plane mirror used to deflect light from the HST Optical Telescope Assembly (OTA) to the FGS; defines the field of view (FOV) of a given FGS.
pitch: Rotation of HST in the V1,V3 plane.
plate scale: Magnification of the combined OTA and FGS optical system.
PMT: Photo-Multiplier Tube.
Position (POS) mode: FGS mode in which a star is observed in FineLock; used to measure the parallax, proper motion, or reflex motion of a given target or targets.
POS TARG coordinate system: FGS coordinate system originating from the center of the FGS field of view, with the Y-axis positive in the radial direction from the FGS center and the X-axis rotated 90 degrees counter-clockwise to the A-axis.
Proper Motion: Apparent angular motion of a star on the celestial sphere over time.
PUPIL: A 2/3 pupil stop used to block the portion of the wavefront originating from the outer 1/3 of the HST primary mirror, reducing the deleterious effects of spherical aberration.
reflex motion: angular motion of an object about its local barycenter (e.g., as in binary stars).
roll: Rotation of HST along the V1 axis.
roll angle: Angle measured from North (through East) to the HST V3 axis. This angle definition is relative to the V3 axis as projected on the sky and should not be confused with the orient angle. (See “ORIENT angle”).
RPS2: A graphical software system for designing HST observing programs. It has been replaced by the Astronomer’s Proposal Tool (APT)
SAA: South Atlantic Anomaly; lower extension of the Van Allen radiation belts lying in a region above South America and the South Atlantic Ocean; no HST scientific or calibration observations are possible during spacecraft passages through the highest SAA contours.
S-Curve: The FGS’s interferometric fringe from a point source (also, see Transfer Function).
SM3: 1999 HST Servicing Mission.
SMOV: Servicing Mission Orbital Verification.
S/N: Signal-to-noise ratio.
Spiral Search: Search algorithm during which the star selectors command the IFOV in an outward spiral search around a target’s expected position.
Spoiler: A star nearby the intended target inadvertently acquired by the FGS.
SSA: Star Selector A Assembly; a two-mirror and five element optical corrector group used to correct for optical aberrations induced by the OTA and aspheric pickoff mirror; along with the SSB, defines and locates the IFOV aperture.
SSB: Star Selector B Assembly; a four-mirror optical group used to pilot the collimated beam through the filter assembly and onto the polarizing beamsplitter; along with the SSA, defines and locates the IFOV aperture.
STAT: Space Telescope Astrometry Science Team.
step-size: Angular distance covered by the IFOV during one photon integration period (FESTIME); default step-size is 1.0 mas.
strfits: STSDAS routine to convert FITS files to GEIS images, STSDAS tables or text files.
STScI: Space Telescope Science Institute.
STSDAS: Space telescope Science Data Analysis System, the set of data analysis and calibration routines used to process HST data; based on the IRAF set of analysis tools produced by National Optical Astronomy Observatories (NOAO).
sub-dominant (roll) guider: The FGS used to maintain control of HST’s rotation about the V1 axis during an observation.
SV: Science Verification, the process of taking observations used for HST instrument calibration and verification of science capabilities.
tilt: Angle of wavefront at face of Koesters prism.
Transfer Function (TF): The difference in counts of the two PMTs for a given interferometric axis, normalized by the sum of those counts, versus the tilt angle of the wavefront at the entrance face of the Koesters prism.
Transfer (TRANS) mode: FGS observing mode used primarily to measure the angular separations and relative magnitudes of multiple stars or the angular size of extended targets.
U1,U2,U3 Orient Reference Frame: Coordinate system fixed in the HST focal plane as projected onto the sky; U1 corresponds to the V1 axis, while U2 and U3 correspond to the negative directions of V2 and V3 respectively.
Upgren69: FGS standard reference star (RA 00h 37m 51.9s, Dec +84o 57’ 47.8"; mV ~ 9.58; B-V = 0.49) used to monitor the interferometric response of the FGS over time.
URL: Universal Resource Locator (or Link); the unique address used to define the “location’’ of a WWW-based document.
V1,V2,V3 reference frame: Fixed telescope coordinate system, with the V1 axis lying along the optical axis, V2 parallel to the solar-array rotation axis, and V3 perpendicular to the solar-array axis.
vehicular jitter: Variations in the HST pointing measured by the FGS’s tracking of guide stars; thought due to mechanical or thermal transients over the course of the orbit.
WalkDown (WalkDown to FineLock): Target acquisition mode in which the IFOV is moved toward target photocenter in equal steps along x,y until FineLock is achieved.
yaw: Rotation of HST within the V1,V2 plane.
zero-point crossing: Point at which the difference in PMT counts along a given interferometer axis is zero; ideally, this crossing should occur at interferometric null.

Fine Guidance Sensor Instrument Handbook for Cycle 22 > Glossary of Terms

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