On-Sky Tests of Sparse-Field Astrometry with GEMS and a 1-meter Telescope

AO4ELT3 May 28, 2013

On-Sky Tests of Sparse-Field Astrometry with GEMS and a 1-meter Telescope

S. Mark AmmonsLawrence Livermore National Laboratory

Olivier Guyon (UofA, Subaru Telescope)Eduardo Bendek (UofA)Bruce Macintosh (LLNL)Dmitry Savransky (LLNL)

Benoit Neichel (everywhere)


Outline

1. Precise sparse-field astrometry enables a wide range of new science for ELTs, including planetary mass measurement

2. How the diffractive pupil works

3. A test of the diffractive pupil on a 1-meter telescope

4. GEMS sparse field astrometric performance


Precision Sparse-Field Astrometry: A Challenging, but Rewarding, Goal for ELTs (and LTs)

Much progress has been made in crowded-field astrometry to date, where both atmospheric tip/tilt anisoplanatism (DTTJ) and instrumental systematics are reduced by shorter astrometric baselines.

But with < 100 μas performance for bright stars in sparse fields, targeted, ground-based astrometric surveys can be valuable for:

- planet detection via parent star reflex- mass measurement of exoplanets discovered by direct

imaging

What is the role of AO in sparse-field astrometry?


Why Sparse Field Astrometry is Difficult:Error Terms Grow with the Astrometric Baseline

Many error terms have a dependence on the astrometric baseline (distance between stars)

These include:- Tip/Tilt

Anisoplanatism- Optical Distortion

Errors- Atmospheric

Refraction

Crowded field cases

Bright star, wide-field cases

Relative astrometric error between two stars due to DTTJ


Diffractive Pupil Mask Calibrates Changing Optical Distortion

All astrometric distortions (due to change in optics shapes and deformations of the focal plane array) are common to the spikes and the background stars. By referencing the background star positions to the spikes, the astrometric measurement is largely immune to large scale astrometric distortions.


Direct comparison of the spike images between 2 epochs is used to measure the distortion, which is then subtracted from the measurement to produce a calibrated astrometric measurement.

O. Guyon, E. Bendek

Diffractive Pupil Mask Calibrates Changing Optical Distortion


Cancellation of Differential Tilt Jitter (DTTJ) caused by high-altitude turbulence

Control of low-order plate scale variations

Sharp PSF (better S/N term)

Spatially uniform PSF (less PSF modeling error) over a wide field (~2’)

Combining MCAO with a Diffractive Approach Addresses the Weakness of Each

Pros

Maps epoch-to-epoch changes in distortion simultaneously with observations

Stiffness requirement on diffractive grid more forgiving

DAR can be calibrated from spike motion (?)

Spikes serve as PSF references (?)

DTTJ no longer cancelled by multiple DMs Potential for epoch-to-epoch systematic changes in distortion (non-common path or changes in WFS zeropoint)

Tomographic blind modes introduce uncontrollable distortion errors

Cons

MCAO Diffractive Grid


Questions about this Hybrid Diffractive-MCAO Concept

1. Does the diffractive pupil concept reduce systematic errors in reality?

2. How large are the systematic errors expected to be in MCAO systems?

3. What is the expected precision of this hybrid concept?


Questions about this Hybrid Diffractive-MCAO Concept

1. Does the diffractive pupil concept reduce systematic errors in reality?

2. How large are the systematic errors expected to be in MCAO systems?

3. What is the expected precision of this hybrid concept?

We address these questions now with two on-sky experiments:1. Diffractive Grid test on 1-meter telescope2. Sparse Field Astrometry with GEMS


What is the Expected Random Component of the Astrometric Precision?

- Precision hits noise floor for 30-meter telescope due to reference star saturation in short exposure times

- Without addressing saturation, practical limit on random error terms Is ~3-10 microarcseconds for 8-30 meter telescopes

Includes DAR, chromatic DAR, Differential T/T jitter, and SNR of stars

1 hour exposure

I = 4 central star

Reference field: Bahcall & Soneira (1980) star count model to V = 22

Seeing = 0.8” FWHM

Bandpass = K

Nyquist sampling, 40% Strehl


We Test the Diffractive Pupil Concept at Lick Observatory

• Stiff CFRP honeycomb mounted at secondary produces diffraction spikes that map changing optical distortion

• Experiment designed to average down random errors and reveal systematics• Final generation mask manufactured in San Jose and designed by Eduardo Bendek


Crowded Field Precision without Mask: ~1.4 mas over 4’

• Data taken in NGC 6791 (average star separation: 10”)• Precision improves with averaging time, but not brightness, suggesting the

atmosphere is limiting at high stellar densities for t < 1 hour


October Run – First run with aperture mask

• First generation mask (perforated aluminum) produces diffraction spikes with even intensity across azimuth (as opposed to blocking blades design)


Observations of Hipparcos Stars will Reveal Stellar Parallax

• Second generation carbon mask provides improved throughput over perforated aluminum mask

Carbon mask installed on Nickel Telescope (Credit: Eduardo Bendek)

51 Per


Diffractive Mask Reduces Systematic Component of Error by ~2x

- Blue lines: fitted sum of a random and systematic component- Use of closest diffraction spike (reducing astrometric baseline) lowers systematic component by 2x

Distant diffraction spike used(similar to no diffractive mask)

Closest diffraction spike used


GEMS experiment: Astrometric calibrator TYC 7122

TYC 7122 is a distant supergiant surrounded by ~20 stars with K < 17 in 90 arcsecond field


GEMS short-term precision reaches ~0.5 mas in sparse fields for 2 minute exposure

- For short exposures, GEMS sparse-field precision approaches 0.5 mas for K < 15- Short term noise floor less than 0.5 mas- For long-term observations – need to keep field on same pixel due to large static

distortion


Summary

1. High precision sparse-field astrometry is a challenging goal for large telescopes, but the scientific rewards are great

2. We consider the merging of two techniques: a diffractive pupil for stability on long time baselines and MCAO for high S/N detection

3. On-sky tests of the diffractive grid technique indicate a reduction in the systematic component of the astrometric error by 2x

4. GEMS short-term level of systematic error is less than 0.5 mas (but may be higher over months).

Documents

On-Sky Tests of Sparse-Field Astrometry with GEMS and a 1-meter Telescope