Sunday, 29 August 2010

Observing at the GMRT (Part II)

Filed under: Astronomy,India — Tags: , , , — George Privon @ 11:00

Yesterday I finished another 8 hour observing session at the Giant Meterwave Radio Telescope. I wrote about observing here about a week ago, but I wanted to add a bit more information.

Naturally, the control room is the same as before. But I neglected to show what I actually look at when I am “monitoring” the observations. An important part of observing is making sure you are actually getting useful information, and not wasting the telescope time. When optical observing this usually means doing simple calibrations of the images (or spectra) you are taking and comparing it to your expectations based on the rough capabilities of the telescope. (It’s almost important to make sure your telescope is pointed at the right object!). When observing with a single-dish radio telescope you are combining the spectra to look at the amount of energy emitted by an object at a given wavelength. Again, you are checking the quality of the data to be sure the systems are functioning normally.

Arrays of radio dishes combined into a single array are monitored a bit differently. Instead of doing a quick calibration of the data and checking the result (as done frequently in many optical and single-dish radio observations), the raw data products are easier to monitor. This is due to the fact that making images from an array is computationally intensive and somewhat time consuming. The time consuming portion can be alleviated by running the data through a pipeline, but it can still take some time to arrive at a “raw” data product suitable for monitoring the observation. In other words, it can take so long to get the “first look” data that it is too late to fix your observation.

These arrays work by comparing the signal recorded from pairs of dishes. So, you can monitor the health of the antennas by looking at these pairs of antennas. Below is a screen capture of a plot I viewed to monitor the status of the observation. The plot shows the amplitude (strength) of the signal against time (IST is Indian Standard Time). This amplitude is measured by comparing the signals received between two telescopes.

The key to each plot tells you what data you are looking at. For example: “W03 – USB 130:C09″ denotes the amplitude measured by the antenna pair W03 and C09 (an antenna on the Western arm of the array and an antenna in the core of the array). For this plot, I am only showing amplitudes on baselines with C09. Can you tell which antennas aren’t working properly? (Hint: they are reporting very low amplitudes.)

Observation monitoring

Antennas S04 and C01 both appear to have issues, based on the low amplitude of the signals.

There is also a range of amplitudes in the signals from the other antennas paired with C09. The highest amplitude values (at the beginning) are observations of the flux calibrator (for descriptions of the calibrators, see this post). The next highest values are observations of the phase calibrator. Finally, the values that are nearly zero at this scale are observations of the science target. The amplitudes are lower because the science target is over 100x fainter than the flux calibrator.

This is of course only one way to monitor observations. You can also look at the phase, because light is a wave. The phase contains information about the structure of the source, but rapid variations of phase with time are signs of a problem.

These plots are updated as more data is taken, so monitoring these throughout the observing run can allow you to detect some potential problems with the observation. Naturally, you can’t catch everything. But, the big problems will usually be visible. Once the observation is concluded, the more subtle issues can usually be resolved.

Sunday, 22 August 2010

Live from the Giant Metrewave Radio Telescope

Filed under: Astronomy,India — Tags: , , , — George Privon @ 22:02


Big and little GMRT

Right now I am 5.5 hours into a 9 hour observing run at the Giant Metrewave Radio Telescope near Pune, India. I am observing neutral hydrogen in other galaxies as part of my PhD thesis. We had a bit of a hiccup at the start, getting some parameters set so we were observing the right frequency. But that got sorted out and we got back on sky with only a little time lost.

The basic flow of the interferometric observing is as follows:

  1. Observe a flux calibrator: This is a source of known brightness which remains stable with time. If you compare the known brightness of the source with what you measure, you can determine how bright the science target is.
  2. Observe a phase calibrator: Because we are observing radio waves, you can measure the amplitude and the phase of the wave. Measuring the phase is crucial for interferometry (more on that another time). But the atmosphere has an effect on the phase and can can change it. So, by measuring an object with stable (0) phase (a point source), you can correct for changes in the atmosphere which would corrupt your data.
  3. Take science data: Finally, after calibrations (note that you can add in other calibrations depending on what your science goals are: polarization for example), you can finally point at the source and begin taking data on the galaxy of interest.

Of course, the atmosphere is changing with time. There are clouds going by, wind, thermal effects, etc. These can all change the phase of the incoming radio waves. So periodically you need to stop taking data on your source and re-observe the phase calibrator so you can see how the phase changed. If you have measurements of the phase over the whole time of your observation, you can track the effect the atmosphere would have on the measured phases of the interesting galaxy. The whole time you’re observing you are moving between the galaxy of interest and a phase calibrator. How often you need to check the phases depends on what wavelength you are observing and what science you want to do.

I am in the middle of the observing run now, and the computer controlling the telescope is alternating looking at the target galaxy and observing the phase calibrator. I am sitting comfortably in the control room, monitoring the progress.

Control Room

Yesterday, before observing I took a minute to walk to one of the nearby dishes. Seeing a 45m diameter wire mesh dish is pretty impressive. There was a bit of a breeze as well, and you can hear it blowing across the mesh. There is apparently wildlife around, including snakes, scorpions, and even the occasional leopard! I’ve yet to run across any though. The facilities here are quite nice as well.

Three dishes It's like a radio halo

GMRT Building NCRA courtyard

This is the first of two sessions on the telescope. I have my final one this coming Friday to look at another galaxy. I’ll spend the intervening time analyzing the data taken tonight.

More pictures from the GMRT are online here: NCRA, GMRT, and Travel on flickr

Sunday, 8 August 2010

Adventure transition

Filed under: Astronomy,School — Tags: , , — George Privon @ 15:52

This weekend has been one of transition. On Friday my class took their final exam. By the evening the exams were graded and I was left feeling purposeless. No lectures to prepare, no homeworks to write, no office hours to have. Teaching had completely consumed me for the past four weeks, so reaching the end was an odd feeling.

I had an excellent experience with the class. Preparing 2 hours of lectures every day is more than a full-time job, never mind the homeworks, exams, and grading. It was very challenging deciding what to talk about and then implementing that in a lecture. But it was also a lot of fun.

I am transitioning adventures at the moment though, and am returning to full time research. This first step actually involves leaving! I am doing some last preparation for an observing run at the Giant Metrewave Radio Telescope in India. It should be a very exciting trip! I will post pictures through the trip (depending on how often I have internet access).

Saturday, 24 July 2010

And I thought taking a class was a lot of work…

Filed under: Astronomy,School — Tags: , — George Privon @ 11:00

Yesterday marked the midpoint of the summer class I am teaching: Introduction to Stars, Galaxies, and the Universe. I knew it was going to be a lot of work but I don’t think I could have possibly understood how much. The content is split more or less between background & stars (two weeks), and galaxies & the Universe (two weeks). We just wrapped up the stars portion with supernovae and the deaths of stars. The summer classes here are intense, two hour lectures five days a week for four weeks. Factoring in somewhere between four to five hours preparation per hour of lecturing and it quickly becomes a sizable investment of time. Oh yea, you also have to write assignments, quizzes, and tests.

Getting to bed by midnight is becoming a luxury, reserved for the weekends! However, having said that I am very much enjoying the experience. I am fortunate that my advisor is okay with me taking four weeks off from research so I can focus on the class (and get a few sweet hours of sleep each night :) ). Teaching a class has been very informative, not just in terms of teaching, but also in terms of appreciating the broad knowledge base one needs when talking about science. As I am preparing lectures, I frequently realize that my presentation relies on prior knowledge that I cannot assume everyone has. As people say.. if you really want to understand something, try teaching it to someone else.

Two more weeks to go in the class, and we will be getting into the topics where sizes and distances become more and more difficult to comprehend. Galaxies outside our own, ranging from the nearby Andromeda galaxy all the way to galaxies we see when the Universe was a fraction of its current age! But before I can talk about it, I have to write the lecture. :)

Friday, 18 June 2010

Becoming a black-belt (radio astonomer)

Filed under: Astronomy — Tags: , , — George Privon @ 10:00

I spent the last week down in the New Mexico desert, attending the 12th Synthesis Imaging Workshop (can you spot me in the picture?). The aim of the workshop is to teach astronomers about radio telescopes, particularly interferometers. These instruments are very unique in their methods of attaining the highest angular resolution, despite the long wavelength of light involved. The workshop covered roughly a week and served as a “boot camp” for radio astronomy. The majority of the time was spent listening to lectures which described the theoretical and practical side of using radio telescopes to observe the Universe. However, two days were spent working with real data, and two days were spent “playing” (hiking and getting a tour of the Expanded Very Large Array).

I had been wanting to attend this workshop (which is held every 2 years) since 2005, when I began working on data from radio telescopes. So I was very excited to be there with the other 150 students.

Of course, I suspect recounting the excellent lectures (which can be viewed online or purchased) might not keep you reading, so instead I’ll put up some pictures from the extra-curricular activities from the week..

The first of which was a trip to Santa Fe and Taos. I had never been to either place and it was a nice change from Socorro. I won’t say much, just put some pictures up.

Mary

Crosses in the fence Beam Shadows

In addition to the two towns we also drove across the Rio Grande and walked around on a large bridge over the gorge. It was a pretty windy day, so our ankles were ex-foliated by the blowing sand ;)

Bridge over the Rio Grande

On Sunday all the summer school attendees were treated to a tour of the EVLA. I have been to the VLA two times before, but I can’t turn down a chance to visit a telescope! The EVLA has actually changed somewhat substantially since the upgrades have started. Most of the 20-30 year old electronics have been removed and replaced with new digital equipment to upgrade the instrument by a factor of at least 10. This includes laying optical fiber to collect data from the individual dishes.

By far the coolest part about the EVLA visit was this:

EVLA Observing my target

Yes, it is a slightly grainy/blurry picture of the EVLA from a distance. What makes this so cool? When I took the picture, it was taking data for my project! After the tours, I found out that my name and my EVLA project ID were on the screens in the control room. Just as we arrived, it finished taking data on a merging luminous infrared galaxy. I haven’t had a chance to analyze the data, but rest assured I will put something up here once I have.

Me on the dish LWA station

Another interesting part of the tour was the new Long Wavelength Array (LWA) station at the EVLA site. This is a low-frequency array and they have built the first demonstrator station (picture up and to the left). It’ll be interesting to track their progress over the next few years!

I managed to get a nice panorama of the EVLA as all the dished returned to their stowed position before resuming observations:

EVLA Panorama

There are more pictures online:
Santa Fe and Taos on flickr
EVLA tour on flickr

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