The Art and Science of PARALLAX Adric R. Riedel 2008 04 14
description
Transcript of The Art and Science of PARALLAX Adric R. Riedel 2008 04 14
The Art and Science of
PARALLAXAdric R. Riedel 2008 04 14
Parallax3
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
• Parallax is the apparent angular displacement of an object due to a change in the observing position (in space and/or time)
• There are seven common ways to get distance from parallax:
• Trigonometric
• Photometric (sort of)
• Spectroscopic (sort of)
• Orbital
• Secular
• Statistical
• Stereoscopic
El Yunque National Forest, Puerto Rico
El Yunque National Forest, Puerto Rico
Secular Parallax (sort of)5
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Statistical Parallax6
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Orbital Parallax7
• With a resolved binary, find the:
• angular diameter of the orbit
• period of the orbit• inclination of the orbit• v sin i of the orbit.
• With period and velocity you can find the linear size of the orbit
• Compare to angular size and get a distance estimate.
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
i
Spectroscopic Parallax8
• Get a spectral type• Find out what the Mv of
the star should be for that spectral type
• Compute a distance modulus
• Limited accuracy; spectral types are wide bins
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Photometric Parallax9
• Use magnitudes and colors to estimate absolute magnitudes
• No longer reliant on specific types but assumes all are main-sequenceGeneral Definition
Types of ParallaxThe ideal case
How stars moveHow the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
The Ideal Case10
• Use the baseline of the Earth; measure the angle through which a star moves.
2 A
U π (arcsec)
π (arcsec)
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
The Ideal Case11
But how do we know how the star moved from time to time?
Background stars!
2 A
U π (arcsec)
π (arcsec)
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
How Stars Move12
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
How Stars Move13
CENSORED
Proper Motion
Improper Motion
Proxima Centauri (CTIO 2002-2007)
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
14
• Around a barycenter (astrometric perturbation)• Definitely real and observable
• Secular acceleration• Second-order effect
• A side effect of projecting straight-line motion onto a line of sight
• Has been observed for a few stars (Barnard’s Star, Alpha Cen)
• Can be ignored for smaller motions
• Around the center of the galaxy• In principle this effect is there, but negligible over the
100+ years astrometric parallax has been done
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Other ways stars can move
Ways the Earth moves15
• Orbits the solar system barycenter
• Perturbations by the other planets (and the moon) directly
• RECONS uses the DE405 ephemerides produced by JPL
• Precession (360 degrees in 26,000 years is 49.8 arcsec per year)
• All coordinates are transformed to J2000.
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Atmospheric Effects16
• Atmospheric extinction• Only observe stars close to
the meridian (does nothing for stars at high declinations)
• Know the airmass; keep measuring (nearby) standards throughout the night
• Avoid clouds
Extra Airmass = Bad
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Atmospheric Effects17
• Atmospheric color distortion
• aka Differential Color Refraction (DCR)
• Only observe stars close to the meridian (not helpful for stars at high declinations)
• Observe standards throughout the night
• Choose reference stars of similar color (Where available)
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Telescope Effects18
• Tilted field of view• Easily corrected by
knowing the actual positions and separations of stars; find plate constants
• Warped/nonlinear field of view
• Inconsistent Plate Scale
• Requires careful correction for warping
Plate Tilt
Field of View
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
The Sad Tale of Peter van de Kamp19
• Director of the Sproul Observatory at Swarthmore University
• Conducted a long-running search for planets around nearby stars using astrometric perturbations
• Reported a 12 MJ companion to Barnard’s Star in 1963, revised to two planets in 1969. Peter van de Kamp, 1901-1995
The Guru of Astrometry
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
The Sad Tale of Peter van de Kamp20
• With collaborators, found many more planets around other stars.
• All perturbations were exactly the same.
• Gatewood & Eichhorn (1973) determined it was instrumental wobble in the Sproul reflector, coincident with telescope upgrades.
• Planets have since been found to exist around other stars
Peter van de Kamp, 1901-1995The Guru of Astrometry
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
The CTIO 0.9m21
• Underwent a Telescope Control System upgrade in April 2005
• Pixel scale is still 0.401”, on average and across each segment measured.
• Rotation has changed, but is consistent afterward
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Observations23
The best times to take parallax frames:
Sunrise
Sunset
At these times you get the maximum parallatic angles
Observable during the night
Behind the sun
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
The Reference Field24
• Stars (five to fifteen) are identified around the target object.
• Their positions are used to denote a reference frame
• All other images are rotated and scaled to match the reference frame
• Scaling can be avoided by using the same telescope setup for all operations
• Be wary of changes to rotation and plate scale due to telescope servicing (we recalculate anyway)
• Also, don’t let the telescope age
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
The Reference Field25
• These stars were chosen for being solitary and bright, but not saturated
• Note that I’ve already broken my own rule
• Not so sure about star #4 either
• One frame per night is identified by hand, the rest are done automatically
1
2
345
6
7
8
a
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Reduction to an absolute frame26
• This is done by “Dependencies” (D), which contain all the rotation, translation and scaling necessary to place each reference star in each frame where it's supposed to be.
• These look nasty.• The position of the target star is calculated
based on the average of these dependencies.
All of this is done with IRAF routines
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Centroiding27
• We centroid to ~1/50 of a pixel (0.00802 arcsec)
• With more images come smaller net position errors
• Centroiding requires high accuracy, and good PSFs
• Not good for barely-resolved binaries or tracking errors
• New centroiding algorithm introduced last year.
A good star for centroidingA bad star for centroiding (binary)
Note that second lobe
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Observational Corrections28
• Corrections for a flat surface (The Mapmaker’s problem)
• Full conversion from x,y to RA (α) & Dec (δ):
• For very small fields:
00 cossin
tansin
y
x
22
00
1
cossinsin
yx
y
y0sec x
00 , ,
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Removing other motion29
• Using the reference frame, the proper motion (and secular acceleration) are removed.
• Parallax accuracy is limited by these factors
• The two years of data now provide just the parallax motion
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Parallax!30
Palpha is the fraction of the parallax in RA, Pdelta in DEC.
• They give the direction of the parallax wobble at a given time (usually, J2000 since we already have the star's position then)
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
The Reference Field is also moving31
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Reduction to Absolute32
• The true parallax is obtained against a truly fixed background. Motions of the reference stars must be taken out.
• Each term must be corrected (the position and secular acceleration are ignorable though)
• RECONS calculates photometric distances to the background stars, and adds the parallax they produce (decreasing the distance)
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Photometric Parallax33
• Use magnitudes and colors to estimate absolute magnitudes
• No longer reliant on specific types but assumes all are main-sequenceGeneral Definition
Types of ParallaxThe ideal case
How stars moveHow the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
A partial list of sources of error34
• Centroiding• DCR• Bad seeing• Limits of proper motion
accuracy• Time coverage too
short/sparse• Too few reference stars• Variable reference stars• Poor photometric parallax
to reference stars (giants, binaries)
• Poor positioning of reference stars
• Bad aperture correction
• Plate tilt
• Defocus
• Cosmic Rays
• Plate scaling and rotation
• Deterioration of the telescope
• Modifications to the telescope
• Tracking errors
• Weather
• The Seasons
• Marvin
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Works Cited35
• Finch, C. “A Step by Step Guide to Parallax Reduction” (internal) 2007.
• Henry, T. et al. 2004, AJ, 128, 2460H
• Jao, W.C. “CTIOPI Photometry Reduction User Guide”
• Van de Kamp, P. “Stellar Paths”, D. Reidel Publishing, The Netherlands, 1981
• "Parallax." Dictionary.com Unabridged (v 1.1). Random House, Inc. 14 Apr. 2008. <Dictionary.com http://dictionary.reference.com/browse/Parallax>.
Special thanks to:
Todd Henry
Wei-Chun Jao
John Subasavage
Jennifer Winters
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks Cited
Project ISPI36
General DefinitionTypes of Parallax
The ideal caseHow stars move
How the Earth movesAtmospheric effectsTelescope effects
Van de KampObservations
The Reference FieldAbsolute Frame
Reduction to ParallaxReduction to Absolute
ErrorsWorks CitedFuture Work