1. GPI Astrometric Calibration
Quinn Konopacky et al.

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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)

9. 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

10. 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

11. 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

12. 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

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

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

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

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

17. 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

18. 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

19. 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

20. 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

21. 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?

22. 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

23. 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!