diffraction-limited imaging in the visible on large ground-based telescopes · diffraction-limited...

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Diffraction-Limited Imaging in the Visible

On Large Ground-Based Telescopes

Craig Mackay,

Institute of Astronomy, University of Cambridge.

La Palma & The WHT

• The Hubble Space Telescope (HST) will not last forever.

• Astronomers will expect instrumentalists to deliver comparable

performance from the ground.

• Adaptive optics systems work reasonably well in the infrared but

poorly in the visible.

• Despite vast sums of money spent (> $1 billion by astronomers), no

one has demonstrated Hubble resolution (0.12 arcsec) on a Hubble

sized telescope (2.5m) in the visible from the ground.

• La Palma is a superb site for the best astronomical imaging.

• We will look at how we can achieve better than Hubble resolution

from the LPO over most of the sky.

21 June 2010: IAC, La Laguna, Tenerife


Why is AO so hard?

• Conventionally, the Shack-Hartmann wavefront sensor is used.

• This breaks up the pupil into a large number of small cells, ~20-50 cm diameter.

• Each cell forms an image of a bright star.

• The star images are tracked to deduce the wavefront errors.

• The starlight is divided amongst many cells so the reference star must be bright.

• The star images are tracked to deduce

the wavefront errors.

• The starlight is divided amongst many

cells so the reference star must be bright.

• Need to determine errors and correct

them before they all change.


Why is AO so hard?

• Conventionally, the Shack-Hartmann wavefront sensor is used.

• This breaks up the pupil into a large number of small cells, ~20-50 cm diameter.

• Each cell forms an image of a bright star.

• The star images are tracked to deduce the wavefront errors.

• The starlight is divided amongst many cells so the reference star must be bright.

• The detectors must read the sensor quickly as the atmosphere changes rapidly

(~wind crossing time for one cell, so less than 10 ms).

• An example of the

image sizes over a

run of 85 seconds

of fairly average-

to-good seeing.

• Changes by factor

of two in a few

frames (~ wind

crossing time of

telescope).

• AO systems would

struggle to follow

many of these

steps.

Why is AO so hard? Seeing is terribly variable.


Why is AO so hard?

• This requires a bright reference star, typically 12-13 magnitude (very

scarce, ~0.1% sky coverage).

• Below this threshold, as star gets fainter, all cells loose lock together, so

cannot do any correction below a specific threshold.

• Even at small angles away from the reference star, the turbulence correction

becomes uncorrelated.

• This gives a tiny isoplanatic patch size (~few arcsec in the visible on the

good site).

• Much easier in the near infrared because cells can be larger, read rate slower

and reference stars are brighter.


A Radically New Approach Is Needed.

• Need to build a system to work with the real sky, not just with easy

technology!

• Good sky coverage requires reference stars I > 18.0 to 18.5.

• On a 4.2 m telescope, I = 18.5 gives ~5000 detected photons per sec in

broadband.

• The wind crossing time on a 4.2 m telescope on La Palma is on typically

~0.5 secs.

• Surely it is possible to do something with this kind of photon flux!

• The 4.2m WHT (10.4 m GTC) has an intrinsic resolution of ~50 (20)

milliarcsec in I-band and 32 (13) milliarcsec in V-band.

• It is in the visible that we know most about the Universe so this is the

angular resolution we must aim for.


Lucky Imaging In the Visible.

• This is a technique originally suggested by Hufnagel (1966) and developed

by Fried (1978).

• Images are taken fast enough to freeze the motion due to turbulence.

• On a 2.5 m telescope (the NOT) in I band, on a good site (LPO), under

typical conditions 10-30% of images are ~ diffraction limited at 20 frames

per sec.

• The best images are selected and combined to give a near-diffraction limited

image.

• Many results and papers have been published by the Cambridge group and

by others including the group at the IAC (Rafael Robolo et al).

• The isoplanatic patch size is much larger than with AO, typically ~ 60 arcsec

rather than ~3-5 arcsec diameter.


• 100Her is a double star with 14 arc sec separation.

• Here the two components are shown side by side.

• The scale is about 4 arc sec vertically

• Images were taken with 10 millisec frame time, and stars are each 6.0 magnitude.

Results with Lucky Astronomy


The Einstein Cross

• The image on the left is from the Hubble Space Telescope Advanced Camera for Surveys

(ACS) while the image on the right is the lucky image taken on the NOT in July 2009 through

significant amounts of dust.

• The central slightly fuzzy object is the core of the nearby Zwicky galaxy, ZW 2237+030

that gives four gravitationally lensed images of a distant quasar at redshift of 1.7


New Results with Lucky Astronomy

• Techniques are also

very popular with

amateur astronomers.

• This shows a short

movie of the moon

taken under poor

conditions (roof of

skyscraper in Hong

Kong!).

• Wah! used Registax

Lucky software.(Images courtesy Wah!, Hong-Kong)


New Results with Lucky Astronomy

• Techniques are also very

popular with amateur

astronomers.

• This shows a short movie

of the moon taken under

poor conditions (roof of

skyscraper in Hong

Kong!).

• Wah! used Registax

Lucky software.

(Images courtesy Wah!, Hong-Kong)


New Results with Lucky Imaging

• This image of the

International Space Station,

with Space Shuttle Atlantis

and a Soyuz Spacecraft in

attendance was taken with a

ground-based telescope

using Lucky Imaging in

June 2007.

• Resolution was about 20 cm

at an altitude of 330 km

altitude, or ~ 0.12 arcsec.

• Downward looking

resolution is much better,

~20 marcsecs or ~ 2 cm.


14 December 2007: U3A, King’s Lynn

• Lucky imaging techniques on

larger telescopes will not work.

• What can we do to improve our

luck?

• We can remove much of the

turbulent power with a low order

AO system, leaving Lucky to

work with what is left.

• We used the Palomar 5 m

telescope low-order adaptive

optics system plus our Lucky

Imaging camera.

Large Telescope Lucky Imaging.

• The Palomar 5m telescope

is >60 years old, so optical

quality is poorer than the

4.2m WHT or the 8.2m

VLT, for example.

• The PALMAO system is a

relatively old design, with

only 12 actuators across

the diameter of the

telescope, so image

sidelobes are visible.

• Nevertheless, great images

were taken throughout the

6-night run.



• Globular cluster M13 on

the Palomar 5m.

• Natural seeing ~650 mas.

• Imaged via the

PALMAO system and

our EMCCD Lucky

Camera.

• Achieved 17% Strehl

ratio in I-band, giving

~35 mas resolution.

• This is the highest

resolution image ever

taken in the visible.




• The comparison of our system, both

without Lucky/AO and with Hubble

Advanced Camera (ACS) is quite dramatic.

• The Lucky/AO images have a resolution

~35 milliarcseconds or nearly 3 times that

of Hubble.

Large

Telescope

Lucky

Imaging.

• The Cat’s Eye Nebula (NGC6543) on the Palomar 5m.

• Natural seeing ~1.2 arcsec.

• Green is V-band (4959/5007), red is H-alpha, blue is I-band.

• Imaged via the PALMAO system and our EMCCD Lucky Camera.

• ~110 mas resolution, limited by detector sampling, not Lucky/AO.

• Works well in V-band as well!




Large Telescope Lucky.

• Lucky Imaging + AO usually needs a

bright reference star.

• We are building a new kind of wavefront

curvature sensor.

• Much more sensitive than Shack-

Hartmann sensors for low-order AO.

• We use 4 planes to make out-of-pupil

images, and fit a wavefront curvature.

• Can work with reference objects x100-

1000 fainter. Is substantially achromatic.

• The fainter the object, the fewer high-

order modes may be corrected, but low-

order modes are still manageable.

(From Olivier Guyon, Subaru

telescope, Hawaii).

• We are building a system now as a visitor instrument for the VLT (8.2m).

• A similar system can now be built for the WHT (and indeed for the GTC).

• Will allow a wide range of problems to be tackled that require >HST resolution in visible.

• Examples include globular cluster physics, quasar host galaxies, AGN studies, compact gravitational lenses, MACHO surveys in crowded regions and many others.

• Also works as high-time resolution instrument.

• Photon-counting CCDs allow limited fields at 1000Hz.

• May also be used with Integral Field Unit (IFU) based spectrographs.

Lucky/AO Imager for the WHT.


Key technologies are:

• Electron Multiplying CCDs. They can be operated at high speed (30 MHz pixel rate, 1024x1024 at ~30 Hz frame rate), but have full thinned CCD DQE and essentially zero read noise so can count photons.

• Used both for wavefront detectors and science detectors.

• Use optical re-imager to give 2000 x 2000 pixels contiguous field of view of 30 x 30 to 120 x 120 arcsec.

• Use MEMS wavefront corrector.

• Large RAID arrays for data storage (200 Mb/sec continuous), plus NVIDIA Fermi parallel processors for real-time processing (512 64-bit FP processors, 3x109 transistors).



• Optical Re-imager: uses WFPC-like pyramid to separate the contiguous field for 4 discrete thinned CCDs.



System specification:

• Reference star: 18.5-19.0 mag (I band) faintest, within 60 arcsec of field centre.

• Field of view: 30 x 30 to 120 x 120 arcsec (adjustable).

• Pixel scale: 15-60 milliarcseconds per pixel.

• Use 10-40% of images, seeing dependant, typically 25-30%.

• Selectable percentage selection for trading off resolution against sensitivity.

• Isoplanatic field size: >60 arcsec, resolution and target dependent.

• Ideally joint project involving Cambridge, IAC and ING.



• First light of basic system in 15 months.

• Complete in 24 months with a reasonable level of effort. Many components already developed and only need duplicating.

• Main development effort in software (user interface, TCS interface, reduction pipeline user interface).

• Unique capability that really exploits the exceptional quality of the La Palma site.

• Opportunity for the LPO to take a world lead in the only way known to deliver diffraction limited imaging in the visible.



Instrumentation Group

Institute of Astronomy

University of Cambridge, UK

[email protected]


diffraction-limited imaging in the visible on large ground-based telescopes · diffraction-limited...

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