Diffraction-Limited Imaging in the Visible On Large Ground-Based Telescopes. Craig Mackay, Institute of Astronomy, University of Cambridge.

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1 Diffraction-Limited Imaging in the Visible On Large Ground-Based Telescopes Craig Mackay, Institute of Astronomy, University of Cambridge.

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3 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.

4 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.

5 The detectors must read the sensor quickly as the atmosphere changes rapidly (~wind crossing time for one cell, so less than 10 ms). 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.

6 Why is AO so hard? Seeing is terribly variable. An example of the image sizes over a run of 85 seconds of fairly averageto-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.

7 Why is AO so hard? This requires a bright reference star, typically 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.

8 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 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.

9 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.

10 Results with Lucky Astronomy 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.

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

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

13 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)

14 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 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.

15 Large Telescope Lucky Imaging. 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. 14 December 2007: U3A, King s Lynn

16 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.

17 Large Telescope Lucky Imaging. 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.

18 Large Telescope Lucky Imaging. 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. 14 December 2007: U3A, King s Lynn

19 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 Imaging.

20 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 x fainter. Is substantially achromatic. The fainter the object, the fewer highorder modes may be corrected, but loworder modes are still manageable. (From Olivier Guyon, Subaru telescope, Hawaii). 14 December 2007: U3A, King s Lynn

21 Lucky/AO Imager for the WHT. 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.

22 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 ( bit FP processors, 3x10 9 transistors).

23 Lucky/AO Imager for the WHT. Optical Re-imager: uses WFPC-like pyramid to separate the contiguous field for 4 discrete thinned CCDs.

24 Lucky/AO Imager for the WHT. System specification: Reference star: mag (I band) faintest, within 60 arcsec of field centre. Field of view: 30 x 30 to 120 x 120 arcsec (adjustable). Pixel scale: 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.

25 Lucky/AO Imager for the WHT. 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.

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27 Instrumentation Group Institute of Astronomy University of Cambridge, UK