Current status of astrometry activities using TMT/IRIS
Ryuji Suzuki (TMT), Matthias Schoeck (TMT), David Crampton (NRC Canada), Brent Ellerbroek (TMT), Paul Hickson(Univ. BC), Andrew Phillips (UC CfAO), Luc Simard (NRC Canada), + TMT/IRIS astrometry team

1. We want to achieve ~10 uas relative astrometry with TMT/IRIS!
There are exciting science cases which ~ 10 uas astrometry will open up (GC, Local dwarf galaxies, exoplanets, etc.)
10 uas can only be achieved by ELTs (because of the small PSF)
Many error terms that are negligible for ~100 uas astrometry which is the limit of current 10 m class telescopes become
important for ~10 uas astrometry.
We tried to identify and quantify ALL effects that might influence astrometry to the best of our current knowledge.

Top : Image of Galactic Center
obtained with Keck + AO
system (left) and expected image
with TMT (right) (fromastro2010 Science White Paper)

2. What should we consider?
Category
Reference star / Catalog errors
Atmospheric refraction
Other atmospheric effectOpto-mechanical errors
Focal plane measurement errorsObserving condition
Observing strategy
Calibration strategy

ExamplesWhat if natural guide stars are at different positions than we expect?
What if the atmospheric dispersion correction is not perfect?
What if the atmospheric dispersion changes over night?
How much is differential tip/tilt jitter?
Is PSF uniform over the field? How does the uniformity affect?
How does the optical distortion change over short/long periods?
How accurately can we derive true PSF center?
How much is the effect of confusion?
What if the observing conditions (transparency, seeing) vary during exposure?
What is the best observing scenario to obtain the best astrometric accuracy?
How accurately can the optical distortion be calibrated?

3. What have we done?
3.1 Atmospheric dispersion correction errors
The best material combination for the
ADC is S-NPH2/Spinel.
The ADC errors are ~10 uas in the worst
case when the object SED is unknown.
We need to know temperature andpressure to ~ 2 K and ~ 4mB, and adjust theADC control accordingly.
Residuals of atmospheric dispersion after the correction by ADC

3.3 Distortion correction errors
Optical distortion of the IRIS imager is expressedby 7th order polynomials.
Distortion pattern varies < 10 uas due to stability/
repeatability of moving parts in IRIS.
We will use a grid pinhole placed on the telescope
focal plane to correct the distortion.
Correction accuracy using the grid pinhole is < 10uas.
Position of the grid pinhole must be known with
~20 nm accuracy.

Right : Multiple planet system
around HR 8799 (Marois et al.
2008)

Item
Astrometry
Field
Post analysis

Cases
Absolute? Relative?
Crowded? Sparse?
Correctable? Or not?

 35 items and more forsubcategories are listed in aspreadsheet.

3.2 Differential tip/tilt jitter
Tip/tilt being different between NGSs and target objects causes astrometric error.
Residual tip/tilt errors computed for 7 x 7 stars in a 30 arcsec FoV at 800 Hz for
20 second exposure.
Lower order terms can be removed using “field stars”
Random measurement errors (S) contribute astrometry errors approximately as
S*(nmodes/nfield)1/2
Order

Residual (uas)Notes

0123

16755
16
7Linear termLinear term

3.4 Transparency and Strehl variation
Transparency variations and Strehl ratio variations during exposure coupled withany image motion will cause astrometric errors.
Both variations will contribute astrometric errors on the order of 10 uas for theimage drift of ~ 1 mas for short exposure (< 10 s).
Astrometric errrors increase with exposure time.
Residuals of distortion correction using the grid
pinhole4. What do we do next?
Keep investigating remaining items in the list.
We had the first astrometry workshop which put the people with a variety ofbackgrounds (astronomers, AO experts, instrumentation experts) together to share theunderstandings and draw a realistic scenario to achieve the unprecedented astrometricaccuracy.

We welcome anyone who is interested in astrometry using TMT/IRIS. PLEASE JOIN US!!
Contact : Matthias Schoeck (mschoeck@tmt.org)