GPI Astrometric Calibration Quinn Konopacky et al.

The astrometric requirements in the OCDD are defined by proper motion follow-up requirements and orbital monitoring. Common proper motion confirmation requirement 4 mas Orbital motion detection requirement 1.8 mas 10 year orbital coverage Circular, face on orbit with a=30 AU and distance=30 pc

Astrometric precision ~1 mas greatly improves the ability to constrain orbital parameters. Over 10 years of monitoring, astrometry of 1 mas vs. 1.8 mas provides: 30% better constraints on eccentricity 60% better constraints on period 20% better constraints on inclination HR 8799 system shows that improvement of uncertainties from ~2 to ~1 mas will exclude certain families of orbits HR 8799 astrometric orbit models

Astrometric precision ~1 mas greatly improves the ability to constrain orbital parameters. Over 10 years of monitoring, astrometry of 1 mas vs. 1.8 mas provides: 30% better constraints on eccentricity 60% better constraints on period 20% better constraints on inclination Experience with HR 8799 system shows that improvement of uncertainties from ~2 to ~1 mas will exclude certain families of orbits HR 8799 astrometric orbit models 4

There are several requirements to obtain ~1 mas precision astrometry. Accurate plate scale Accurate orientation of the camera with respect to true north/elevation Distortion solution for the system Knowledge of primary location Knowledge of biases introduced by reduction algorithm (i.e., ADI/LOCI) Good PSF fitting

There are several requirements to obtain ~1 mas precision astrometry. Accurate plate scale Accurate orientation of the camera with respect to true north/elevation Distortion solution for the system Knowledge of primary location Knowledge of biases introduced by reduction algorithm (i.e., ADI/LOCI) Good PSF fitting

The expected distortion on GPI detector is of order ~3%. Likely caused by first two mirrors in IFS before beam hits lenslet array Accurate astrometry requires determining and solving for this distortion GPI distortion model at lenslet array

NIRC2 Distortion Solution (M92) Good distortion solutions require well-populated fields to be imaged on the detector. Binaries do not map whole field of view Pinhole grid limited by manufacturing and placement knowledge Does not sample entire optical path Residuals provide a “distortion map” NIRC2 Distortion Solution (M92) Yelda et al. 2010 (Do not distribute)

Hubble has extensively observed globular clusters, several of which have bright guide stars. NGC 6121 (M4) Brightest star I = 9.1 Closest globular cluster Observed with WFPC2 and ACS 16 23 35.4 -26 31 31.9

Hubble has extensively observed globular clusters, several of which have bright guide stars. NGC 6397 Brightest star I = 8.4 Second closest globular cluster Observed with WFPC2 and ACS 17 40 41.4 -53 40 25.3

Hubble has extensively observed globular clusters, several of which have bright guide stars. NGC 6656 (M22) Brightest star I = 9.3 Third closest globular cluster Observed with WFPC2 and ACS 18 36 24.2 -23 54 12.2

Hubble has extensively observed globular clusters, several of which have bright guide stars. NGC 6752 Brightest star I = 9.3 Fourth closest globular cluster Observed with WFPC2 and ACS 19 10 51.8 -59 58 54.7

Hubble cluster observers tend to keep very bright stars off the field of view…. NGC 6121 (M4) I = 9.1 2.5’ X

Hubble cluster observers tend to keep very bright stars off the field of view…. X NGC 6397 I = 8.4 2.5’

Hubble cluster observers tend to keep very bright stars off the field of view…. NGC 6656 (M22) I = 9.3 X 2.5’

Hubble cluster observers tend to keep very bright stars off the field of view…. NGC 6752 I = 9.3 2.5’ X

There is still the potential to use the clusters to find a self-consistent distortion solution. Method used to solve for WFPC2 distortion solution by Anderson and King (2003) Requires looking at the same field of stars at many orientations Also uses slight offsets and dithers of the field (potential issue for GPI) WFPC2 field coverage of Omega Centauri cluster used for solving for distortion, Anderson & King 2003

There is still the potential to use the clusters to find a self-consistent distortion solution. The position of a star in a given exposure (frame 1) is found via PSF fitting All other frames are transformed into the coordinates of frame 1 The average difference between the frame 1 position and all other frames is the residual Residuals are fit with 3rd order polynomials that becomes distortion model Anderson & King 2003 distortion residual trend for WF2 chip

Using WFPC2 images of region near NGC 6397 guide star, best-guess simulated fields are generated. Rough density of stars determined Assume all stars have early M spectral type for color Find “best case scenario” for potential stellar density on relatively small GPI field of view 2.5’ F814W, WFPC2

Using WFPC2 images of region near NGC 6397 guide star, best-guess simulated fields are generated. Average density of stars determined Assume all stars have early M spectral type for color Find “best case scenario” for potential stellar density on relatively small GPI field of view

Rotation of the field should provide sufficient orientations for solving for distortion, provided high enough stellar density. Simulate ±2 hour angle Provides 120o of rotation for NGC 6397 Allowance for 180o flip can cover both sides of detector Small dithers may be required to cover center of detector Is this possible? Does it change distortion?

Self-consistent solution requires separate method for deriving plate scale and PA with respect to north. Pinhole grid will provide first plate scale estimate With good distortion calibration, binary stars can be used to derive plate scale Binaries observed with other well-calibrated cameras better than plate scale binaries with uncertain orbits Calibration binary 99 Her observed with NIRC2

Summary and Outstanding Issues 4 globular clusters observed with HST have bright enough stars to lock GPI, but none on the field of view Self-consistent solution may work provided sufficient stellar density around guide star Simulations will be performed to determine the required density to achieve astrometric precision goals (~1 mas) Need to decide: How to measure plate scale If dithering is possible Observing mode for globular cluster measurements And probably much much more! Suggestions welcome!