Ph.D. - A New Infrared Camera for COAST

Note: this page is (hopefully!) supposed to be meaningful to non-astronomers. If you just want to know what on earth I am actually doing, please jump ahead to the applications section, or look at this introductory talk which I gave. Otherwise, please read on...

Since 2002, I have been pursuing a Ph.D. in Astrophysics at the Cavendish Laboratory in Cambridge University. I am working on "A New Infrared Camera for COAST", and my supervisor is Dr. John Young. The COAST group is led by Dr. Chris Haniff. COAST is the Cambridge Optical Aperture Synthesis Telescope. It is capable of extremely high resolution in the Optical and Near-Infra-Red wavelengths. There is an excellent introduction to COAST here, and some astronomical results and images may be found here.

What is COAST? Why Optical Interferometry?

Telescopes have essentially 4 problems to overcome:

• Sensitivity: we want to see faint, distant stars. This means we need the largest possible mirror area to collect light.
• Angular resolution: We want to see fine detail, on very small angular scales, to make out details of the surface of stars. We need to overcome diffraction, and, since θ = 1.22λ/d, we need a large-diameter mirror.
• Atmospheric Turbulence, otherwise known as "seeing". This is what makes an object appear distorted on the bottom of a swimming pool, and what makes the stars twinkle. However, it is a huge problem for astronomers. We can partly overcome it by using the closure phases on multiple baselines (COAST can have up to 6). Alternatively, we need to escape the atmosphere, by using a high mountain or a space telescope. Another method is Lucky Imaging developed at the Institute of Astronomy.
• Practicality: We need to make all this practical - and financially possible! But manufacturing a 100 m diameter mirror, with a precision of 0.2 μm, which must also be moveable to point at the target star, yet must not deform, or thermally expand/contract, is impossible.

COAST solves 3 of these problems:

• Angular Resolution: we are using the technique of Aperture Synthesis to build up the effective resolution of a single large mirror, by using 4 much smaller mirrors, widely spaced, and extremely precisely aligned [we use moveable mirrors on laser-guided trolleys to do path-compensation]. Together with the Earth's rotation, we can achieve the same angular resolution as a 100 m diameter mirror, and resolve details of stars.
• Atmospheric Turbulence: COAST has sufficient baselines to allow us to use closure phases to subtract away most of the effect of the atmosphere.
• Financial: COAST was a prototype, and built for a very low (well, compared to other projects!) cost.

MRO - the successor to COAST

MRO, the Magdalena Ridge Observatory, is an international collaboration between COAST in the UK, and our partners in the USA. The interferometer, which we are building, will be the successor to COAST, and will have extremely high resolution (due to long baselines up to 400 m), very high sensitivity to faint objects (due to large, 2.4 m diameter mirrors), and will avoid most of the atmospheric distortions (by being above most of the atmosphere on a high mountain, and by having 10 telescopes (i.e. 45 possible baselines), hence lots of closure phases).

My Camera - A New IR camera for COAST

OK...finally, we get to what I'm actually doing! Both COAST and MRO observe in the Infrared, out to about 2.5 μm. This means we need to have a detector which is sensitive to such long wavelengths. These detectors are similar to the CCDs which are found in digital cameras, but not quite sufficiently the same! Commercial CCDs are excellent, cheap, and only go slightly into the IR. Furthermore, we need to look at extremely faint objects: this means that noise (the visual equivalent of 'hiss' on a radio) must be almost entirely eliminated. Thus, we are using a very expensive HAWAII detector, made by Rockwell, and are cooling it in Liquid Nitrogen. My Ph.D. is essentially to design, build, test, and observe with the new camera.

The incoming Infra-Red light from COAST currently goes into our existing NICMOS-based camera. This has worked well, but it uses obsolete, 20-year-old technology, has a controller which cannot be interfaced to a modern system, and has a noise level of 16 electrons. My new design uses a modern HAWAII sensor (also made by Rockwell, 2 generations later than the NICMOS, and on loan to us from UKIRT), and a new control system which I have designed and constructed. We are currently testing it, and aim to achieve a read-noise of 3 electrons, using Correlated Double Sampling and multiple reads.

The system has a Linux computer hosting a Pulseblaster digital timing card, which controls the camera timing signals. The data is converted using 4 separate AD7677 ADCs (one per quadrant), and then buffered up to 1 full frame (1 Mpixel x 16 bit) by 4 IDT 72V2105 FIFOs. The data are then read back into the host computer via a QuickUSB USB 2 interface module. There is also hardware (dewar, filter wheel) and optics.

Applications

Here are some applications of this research:

• Really Ancient History: 6 billion years ago. Because MRO will be so sensitive, we can look at extremely faint light, which has been travelling for about half the age of the Universe (6 billion years or so; older than the Earth), and tells us a lot about galaxy formation and cosmology.
• Search for Other Earths. The search for exoplanets is difficult, because of size and contrast. Even a Jupiter-size planet is extremely faint (10,000 times fainter than an already-faint star), and is comparatively compact, and close-in to the central star. The angular resolution required to detect such a planet is very high, although about 200 have so far been found. It's also much easier to see them if we are lucky enough to be looking "down" on the orbital plane, rather than along it. Earth-like planets are probably quite rare, and, being smaller and closer to the star, will be very hard indeed to see. However, if we are really lucky, we might spot one. Such a discovery would be remarkable, because such a planet (being of the right mass, and distance from its sun) would have moderate gravity, an atmosphere, and the right temperature for liquid water. Life on such a planet is a distinct possibility.
• Surface Details of Stars. The angular-resolution provided by optical interferometry is so high that we can see details on the surface of stars, and the flow of hot gas from one binary-star to another.
• Looking at the Moon. Non astronomers are often surprised by this, but most powerful telescopes cannot "focus on" objects as close as the Moon. Technically speaking, the Moon is "resolved out", because it is not in the far-field (Fraunhofer) diffraction regime.

Publications

Here are some publications and talks which I have given:

• An introductory talk [PDF format, 530 KB], which I gave to the group in 2003, explaining the aims of my Ph.D. and the camera design. Please note that the technical details (especially on the slide "New Camera - Design Overview") were preliminary, and now have been significantly changed.
• A Poster which I gave at SPIE in 2004. This is the paper [PDF format, 137 KB], as it was printed in the proceedings of SPIE (A4, monochrome), and here is the poster [PDF format, 1.9 MB], which I actually presented (A0, colour). The reference is:

  R. J. Neill and J. S. Young. A new infrared camera for COAST. In J. W. Beletic and J. D. Garnett, editors, Optical and Infrared Detectors for Astronomy, volume 5499 of Proc. SPIE, page 423. 21-22 June 2004, Glasgow, SPIE Press, 2004.

• At the National Astronomy Meeting (NAM 2005) in Birmingham, I presented another paper [PDF format, 3.4 MB]. The reference is:

  R. J. Neill and J. S. Young. A new infra-red camera for COAST. UK National Astronomy Meeting, 4-8 April 2005, Birmingham, 2005.

• Just for fun, I gave this talk [PDF format, 4.1 MB] on "The Secret Life of Tux the Penguin": this was presented to the Astrophysics group, and concerns the real penguins I saw in Antarctica in December 2003, and conservation issues.

Software

I wrote rather a lot of software, for this, all of which is published here. Of particular note are the parser/utilities/driver for the PulseBlaster (here), and the data-capture and documentation of the NI 4462 (here).

Links

• My (brief) page at the Cavendish
• All COAST publications.
• My M.Phil - Silicon Nanostructures
• Stellarium - free software to view a simulation of the night sky

Supervising

For course details, please go to: this page.

I have also had the pleasure of supervising about 25 Natural Sciences students in IA Physics at Trinity over the last 3 years. Although hard work, it has been a great privilege, and extremely enjoyable. I look forward to hearing great things of you!

My advice to other Ph.D. students beginning to supervise, can be summed up in 3 concepts: Chocolate Cake, Experiments, and Digressions:

• Chocolate Cake: I have found that serving chocolate fudge cake (or pecan pie) is an excellent way to enhance supervisions. It isn't just delicious - but the sugar is useful to student and supervisor alike. I hope that I have managed to start some sort of Trinity tradition here! Now...if I could only claim it as an expense :-)
• Experiments: there are many small hands-on demonstrations which can be done in supervisions. These can really help to gain a physical intuition. Childrens' toys are surprisingly useful (Borders books have a good collection): E.g. slinky (waves, impedances); gyroscope (precession); silly putty (brittle fracture, when hit with a hammer), pocket interferometer (small pinhole(s) held next to eye + green LED torch for diffraction); electric field + candle flame; microwave + grape-halves (plasma), stepper-motor (short-circuit - Lenz's law)...
• Demonstrations: there are a lot of helpful demonstrations and animations on the web. Some of the dangerous experiments are best not attempted, but ought to be seen. For example: phase/group velocities, Tesla coils, Microwave experiments, and more. (For amusement, see Brainiac.)
• Digressions: if at all possible, try to find time to go off-topic, whether to bring in other interesting references, and especially to answer questions raised by supervisees which are not directly relevant to the material. This is especially the case when students have handed in complete (and correct) solutions to the examples questions! The supervisor's key rôle is not, as I see it, to mark examples papers, but to encourage and stimulate interest in the subject. Physics is about curiosity, and it is our most important task not to extinguish it!