Microlensing Studies in Crowded Fields Craig Mackay, Institute of Astronomy, University of Cambridge.
Introduction and Outline Will start by summarising the constraints we must work with in order to detect good numbers of Earth-size planets. We need to maximise the number of potential targets and develop the right technologies to observe them reliably and consistently. Will describe an instrument that may well be able to deliver the performance that we need relatively easily, relatively quickly and without being impossibly expensive! Finally will look at some advanced technologies that could give even better performance, and discuss the problems of operating such an instrument.
What is the problem we have? Gravitational microlensing is the only technique that appears to have the potential to detect good numbers of Earth-size planets. These events are rare so that large numbers of stars need to be observed. We need to use fields such as Baade s Window. Only main sequence stars will interest us: giant stars are too big to give the microlensing effect we seek. From Holtzman et al, ApJ, 115,1946 (1998)
What is the problem we have? This means looking at stars with I>18 mag. The brighter giant stars (~14-16 mag) make large parts difficult to observe. This is part of Baade s Window with HST. About 0.6% of the area shown! From Holtzman et al, ApJ, 115,1946 (1998)
In the range I~18-22 (M I =3-7), there are thousands of stars/sq arcmin, ~1/sq arcsec. Fainter stars also lens well. Ten times as many to I~24.5 Over a 30 arcminute field there are ~few million stars to survey. With better resolution we can see these stars reliably. Ground-based seeing is much poorer so large amounts of time are needed to observe a field. What is the problem we have? From Holtzman et al, ApJ, 115,1946 (1998)
These images are of the bulge, microlensing event MACHO-96-BLG-5. The difference in image quality is dramatic. The top images are ground-based with 1 arcsec seeing. Most current surveys use even poorer images. The lower images are from HST (left-hand is single frame, right-hand is multiframe drizzled). What is the problem we have? From Bennett, arxiv:astroph/0404075v1, 2004
The Photon Budget Is Also a Problem On a 4 m telescope, a star with I~22 will give 5-20 detected photons per second depending on band pass. A 2% (10%) planet lensing effect will require an SNR of 150(30) which will take 40 (1.5) minutes neglecting sky background. This is only possible if we have much better seeing than ~1 arcsec. Sharper PSF gives a much fainter background per star image.
And We Need Good Time Resolution! We cannot assume that high speed, high resolution follow-up will be available elsewhere. The microlensing signature from an earth-mass event may only last for a few hours or less. We need to be able to carry out accurate photometry every few minutes in order to get the most information from the event. That photometry must be done at the full resolution of the instrument. From Bennett, arxiv:0911.2703v2 (2010)
Achieving Higher Resolution How do we achieve this? HST time is far too valuable and the instruments have much too small a field of view. Space projects cost $zillions, and take for ever! Ground-based survey telescopes have relatively poor point spread functions so take much longer to survey a given number of targets. Adaptive optics can give excellent resolution but only over a tiny field of view (a few arcseconds in the visible). The only method that can routinely give Hubble resolution from the ground is Lucky Imaging.
Why is this so hard? Seeing is terribly variable. Image resolution over a run of 85 seconds in ~0.8 (good) seeing. Changes by factor of two in a few frames (~ wind crossing time of telescope). PSF therefore is changing very rapidly. Each image is different!
Lucky Imaging in the Visible. Technique originally suggested by Hufnagel (1966) and developed by Fried (1978). Images 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 neardiffraction limited image. The isoplanatic patch size is much larger than with AO, typically >60 arcsec rather than ~3-5 arcsec diameter. The key enabling technology are electron multiplying CCDs.
EMCCDs are standard CCDs plus an electron multiplication stage. One serial electrode runs at high voltage (~45 volts). An electron has a low probability (~1%) of a 1 electron avalanche. Gives 604 1.01 or 1.02 ~few x100 gain. EMCCDs: General Characteristics 22 March, 2012: Open University
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
Amateurs Are Enthusiastic about Lucky Imaging Image of the International Space Station, with Space Shuttle Atlantis & Soyuz, June 2007. Resolution ~20 cm at an altitude of 330 km altitude, or ~ 0.12 arcsec. Downward looking resolution is much better, ~20 milliarcsecs or ~ 2 cm.
GravityCam The EMCCDs used for Lucky Imaging are relatively small: 1024 x 1024 pixels typically although bigger ones could be made if funds allowed it. The CCDs cannot be butted but could be mosaiced to cover a large field of view. Working with astronomers in the UK and Switzerland we have been looking at an instrument that would use lucky imaging for weak gravitational shear studies. It turns out that the instrument is almost exactly what might be needed for gravitational microlensing detection of Earthlike planets.
GravityCam The two applications of the same instrument would be highly complimentary since microlensing is done towards the Galactic Centre and weak gravitational shear is done as far away from the Galactic The next problem is large telescope characteristics and their availability on a top site. Most telescopes > 2.5 m diameter have small fields of view. The NTT (3.6 m at La Silla in Chile) has excellent optical performance and a field of view of 30 arcminutes diameter. We have looked at how such an instrument might look and how it would perform.
The focal plane is covered with an array of independent EMCCDs each using lucky imaging. 100 of these EMCCDs would cover the focal plane with 70 milliarcsec pixels. An entire field of 30 arcminute diameter would be covered in six pointings. With 10 minutes on each pointing a 30`x30`field would be fully covered in ~ one hour. Other strategies are possible. GravityCam
The instrument would mount directly on the Naysmith focus of the NTT. The NTT has excellent optical performance and space needed for such an instrument. GravityCam
GravityCam The lucky images would not be diffraction limited, but deliver images of 0.2-0.25 arcseconds FWHM using at least 50% of the images and often up to 100%, seeing dependent. Would allow ~20 times as many stars to be observed. Detailed simulations of the performance have been carried out. Optically very simple: atmospheric dispersion corrector then directly onto focal plane array of CCDs. We have already used a simpler version of our LuckyCam very successfully on the NTT. Virtually all the technologies needed have already been demonstrated and the cost need not be excessive ($3-5 million).
GravityCam: Predicted Performance In a field such as Baade's Window, a single pointing of GravityCam will see approximately 0.5x10 6 stars with I<22. With six pointings, around 3x10 6 stars in one hour. The I=22 stars will have ~10,000 photons detected in 10 min, so with SNR ~ 80-100 after sky background correction. With an optical depth of ~2.4x10-6 (MACHO & OGLE estimates) we expect to see a few new events every hour. Can survey at least six fields (so ~2x10 7 stars) per night so should be able to detect ~20-50 new microlensing events/night. The planetary detection probabilities from Bennett & Rhie (ApJ,472,660,1996) suggest we should detect a good number of Earth size planets.
GravityCam: Advanced Technologies There are some advantages in using faster frame rate detectors if they were available. A new generation of solid-state detectors, scientific CMOS (scmos) are beginning to be used, with over 100 Hz frame rate. With massively parallel outputs and a buttable package, much higher focal plane coverage is possible. Could double DQE, and increase filling factor from 0.17 to about 0.75. Increase star imaging rates x6 or so. Upgrade would cost ~$2-3 million, but take 2-3 years to design detectors.
GravityCam: Advanced Technologies Data management will be interesting! Each EMCCD will generate approximately 50 MB per second (at 25 Hz frame rate), and each scmos could produce over 400 MB per second (at 100 Hz frame rate). Total 150 Tb (CMOS 1200 Tb) per night! This would need dedicated processing hardware such as the Nvidia GPUs with direct data transfer. The lucky imaging process could be significantly improved using techniques described by Bramich (MNRAS, 1210.2926, Difference Image Analysis). Nvidia Kepler core PCI-e card. ~4 Tflops. 250 GB/sec memory bandwidth. 2688 CPU cores.
Conclusions. Lucky Imaging offers a relatively inexpensive way of improving the seeing on ground-based telescopes by a factor of 3-5 (9-25 by area) under most conditions on a good site. It allows the observation of much higher density star fields, and to fainter limits. Main sequence stars towards the Galactic Centre may then be surveyed in very large numbers. GravityCam provides a model of a relatively simple instrument development programme. By observing ~2x10 7 (I<22) and seeing lensing at A>10 on ~2x10 8 (I<24.5) stars per night should allows new Earth size planet to be detected much more frequently.
Lucky Imaging Group Institute of Astronomy University of Cambridge, UK cdm@ast.cam.ac.uk