The Status of the HST

Fine Guidance Sensors

B. E. McArthur, McDonald Observatory, U. of Texas

E. P. Nelan

STScI

W. H. Jefferys and G.F. Benedict

Formerly Astronomy Dept., U. of Texas

Calibration

• Map and remove optical distortions whose effects on measured positions exceed 1.0" (linear component) and 0. 5" (non-linear components)

• How to calibrate? Use a star field in M35 that we have observed since 1992

• Distortions are greatly reduced over much of the FGS1r field of regard

• This model is called the Optical Field Angle Distortion (OFAD) calibration

M35 NGC 2158

We use M35, an open cluster in Gemini. This is a distant field that is along the ecliptic and observable much of the year

OFAD Calibration orbits

Analysis

• Use the GaussFit LS/Maximum Likelihood programming language to model the distortions and star motions)(Jefferys, Fitzpatrick, & McArthur

1987, Celestial Mechanics, 41, 39)

• Perform a simultaneous solution solving for relative star positions (the catalog), the pointing and roll of the telescope during each orbit (as quaternions), the magnification of the telescope (constrained), the OFAD polynomial coefficients, and lever arm parameters, rhoA, kA that track scale changes over time.

• Also must model the star motions (proper motion and parallax) of the field and intra-orbit drift of the observations

The Distortion Model

x' = a00 + a10x +a01y + a20x2 + a02y2 + a11xy + a30x(x2+y2) + a21x(x2-y2) + a12y(y2-x2)

+ a03y(y2+x2) + a50x(x2+y2)2 + a41y(y2+x2)2 + a32x(x4-y4) + a23y(y4-x4) + a14x(x2-y2)2

+ a05y(y2-x2)2

y' = b00 + b10x +b01y + b20x2 + b02y2 + b11xy + b30x(x2+y2) + b21x(x2-y2) + b12y((y2-x2) + b03y(y2+x2) +b50x(x2+y2)2 + b41y(y2+x2)2

+ b32x((x4-y4) + b23y(y4-x4) +b14x(x2-y2)2 + b05y(y2-x2)2

We used FGS 3 as the astrometer until the servicing mission which replaced FGS1 with a newer better instrument.

S-curves much improved in FGS1 over 3 and improvement in OFAD quality of at least 0.5 mas – even though some ground calibrations had been misplaced (asphere and pickoff).

FGS 1r Lateral Color CalibrationEach FGS contains refractive elements. The position

measured for a target star (relative to a differently colored set of reference stars) could depend on its intrinsic color.

FGS 1r Cross-filter CalibrationIts large dynamic range (3 ≤ V ≤ 17) requires a neutral-

density filter. There will be a small shift in position (due to filter wedge) when comparing the positions of a bright star to a faint reference frame.

Additional Instrument Calibrations

Two kinds of OFAD Calibration Observations

OFAD orbits – establish new calibration parameters in response to instrument changes in FGS3 and 1; many different pointings and offsets

LTSTAB orbits – check stability of instrument – monitor scale changes – use only two pointings a fall and spring orientation

OFAD & LTSTAB History

FGS3 – 1993 Full OFAD 20 orbits; 1 failed over 1 day 1994 Mini-OFAD 6 orbits over 1 day

1995 Mini-OFAD 11 orbits over 1 day 1997 Mini-OFAD 5 orbits over 1 day

FGS1 - 2000 Full OFAD 14 orbits over 5 days 2008 OFAD 18 orbits over 1 day (all astrometry/all time) 2010 Recovery OFAD 7 orbits over 1 day 2010 Recovery OFAD 9 orbits over 6 days

FGS 1 & 3 – Fall and Spring orientations – 105 orbits

Two modes of FGS Operation

Position Mode – and transfer mode – where a transfer scan is used to measure the distance between binary stars

In 2009, before SM4, only FGS and WFC2 were operational, so time was available to reset AMA to restore FGS1r S-curves.

Motivation to restore S-curves is for TRANS mode performance. We were losing angular resolution and sensitivity for binary star observations. Now all is well again (~12 mas resolution on both x and y axis.

2009 Mirror Move

A decision was made to move the mirror to sharpen the S-curves with the assumption that it would have no impact on the OFAD. 4 orbits were taken before and after mirror move.

If before and after data sets are considered separately.

If they are modeled together.

Mirror Move impact on astrometry

Initial 7 orbit OFAD was not adequate to recover; additional 9 orbits were necessary.

Current OFAD

In 2012, finished reclibration of OFAD from scratch – ~7000 observations in two FGS

Current OFAD• redetermined proper motions and parallaxes with 20 years of M35

observations (~200 orbits); some pms determined to 20 microseconds of arc

Current OFAD• expanded drift model to include more sophisticated motion that has occurred over time

• used independent instrument parameters on OFADs that lasted over several days; SCALE changes on a daily basis based upon conditions

• redetermined all instrumental parameters

Results of New OFAD

• Newest parallax results have errors in the range of 80 microseconds of arc (Chaboyer – The Ages of Globular Clusters and the Population II Distance Scale)

• Proper motions of M35 known to ~25 microseconds of arc - adequate for an internal proper motion study

• Highly accurate M35 catalog used for calibration of NGST

Results of New OFAD

• Potential to redo previous parallaxes and reduce errors – on fields like the Hyades –

1

2

4

6810

2

4

68100

2

4

681000

HS

T P

aral

lax

(mas

)

12 3 4 5 6

102 3 4 5 6

1002 3 4 5 6

1000HIPPARCOS Parallax (mas)

-4

0

4

HIP

Res

(m

as)

y = a + bxa = -0.37 ± 0.26 masb = 0.995 ± 0.002

ParallaxesHIPPARCOS

vs HST FGS 3

• Seven targets in common

• 3.95 < V < 12.41 (d Cep Feige 24)

• HIP <sp> = 1.7 mas

• HST <sp> = 0.3 mas

HST FGS results are mentioned in the abstracts of ~ 600 scientific papers.Hubble Space Telescope Fine Guidance Sensor Parallaxes for Four Classical NovaeDistance Scale Zero Points from Galactic RR Lyrae Star ParallaxesAstrometry with the Hubble Space Telescope: Trigonometric Parallaxes of Selected HyadsNew Observational Constraints on the υ Andromedae System with Data from the Hubble Space Telescope and Hobby-Eberly TelescopeThe Mass of HD 38529c from Hubble Space Telescope Astrometry and High-precision Radial VelocitieThe Mass of the Candidate Exoplanet Companion to HD 136118 from Hubble Space Telescope Astrometry and High-Precision Radial VelocitiesAstrometry with the Hubble Space Telescope: Trigonometric Parallaxes of Planetary Nebula Nuclei NGC 6853, NGC 7293, Abell 31, and DeHt 5A Hubble Space Telescope transit light curve for GJ 436bHubble Space Telescope Parallaxes of AM CVn Stars and Astrophysical ConsequenceThe Mass of the Candidate Exoplanet Companion to HD 33636 from Hubble Space Telescope Astrometry and High-Precision Radial VelocitiesHubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity RelationsThe Extrasolar Planet ɛ Eridani b: Orbit and MassConfirmation of Errors in Hipparcos Parallaxes from Hubble Space Telescope Fine Guidance Sensor Astrometry of the PleiadesDetection of a Neptune-Mass Planet in the ρ1 Cancri System Using the Hobby-Eberly TelescopeThe Solar Neighborhood. IX. Hubble Space Telescope Detections of Companions to Five M and L Dwarfs within 10 Parsecs of the SunHD 137510: An Oasis in the Brown Dwarf DesertAn HST parallax of the distant cataclysmic variable V1223 Sgr, its system parameters, and accretion rateAn Astrometric Calibration of the MV-Porb Relationship for Cataclysmic Variables based on Hubble Space Telescope Fine Guidance Sensor Parallaxes …..

Legacy of the FGS HST