CHARA: Preparing for an observing Run

In just a few days I'll be departing for my last scheduled observing run for this season at Georgia State University's Center for High Angular Resolution Astronomy (CHARA) observatory located on Mount Wilson, CA (just to the North of L.A). In a similar spirit to my series of blog posts on observing at NASA's IRTF (preparing and conducting observations, what the data looks like, and what we hope to observe) I thought I would do the same for CHARA. Planning for the observing run consists of three stages: proposing, preparatory work for planning, and finally preparing the plan.

About CHARA

CHARA is an interferometer. Probably the best-known interferometer is NRAO's VLA which operates in radio frequencies/wavelengths. CHARA, on the other hand, operates in the optical and near-infrared (NIR) wavelengths which present many technical challenges. The basic principle behind the VLA and CHARA is to take the light from different sources and combine it such that the light arrives at the same time (that is, in phase) at the beam combiner. Consider the following figure:

Caption: A schematic view of a pair of telescopes forming a baseline. Taken from the CHARA Pictorial Overview.

In order to make the light arrive in phase the additional path length (B sin(theta)) must be added to Telescope 1. At radio wavelengths this process is a little easier as the light doesn't have to be combined immediately. Instead the signal from the source can be recorded along with timestamps from a high precision clock. This signal can be played back with all of the telescope information in-phase (by introducing an additional time delay on specific telescopes). With present technology, this cannot be done in opical/NIR wavelengths, instead the light must be combined at the detector in real time.

CHARA itself consists of six 1-meter telescopes located a maximum of 331 meters apart, a delay line and beam combiner lab, and a control facility (see image below). Construction was started in 1996 with first fringes three years later.

Caption: CHARA on Mount Wilson. Image from and additional details can be found in the CHARA Pictorial Overview.

Light from each telescope is sent through vacuum lines (the light pipes in the above graphic) to delay line and beam combining lab. After bouncing back and forth along the delay lines the light is directed to one of several detectors. We use the Michigan Infrared Combiner (MIRC) which is a four-telescope H- and K-band beam combiner with some spectral dispersion. The other beam combiners (CLASSIC, CLIMB, VEGA, PAVO) each have their own capabilities. VEGA for instance is a high resolution (R>30,000) spectrometer which functions as a beam combiner (so it does high resolution spectr-ointerferometery) that operates in optical wavelengths.

Proposing

Much like other major observatories, getting observing time at CHARA is done through proposals. A few years ago CHARA started to offer the greater scientific community access to the array through the NOAO program which provided about 50 hours of “community access.” Observing for these NOAO programs are done by CHARA staff and there is some assistance in reducing the data.

From what I understand Dr. Stencel started communicating with several persons at CHARA well in advance of the eclipse (and therefore well in advance my start of graduate school at the University of Denver) and before the community access time on NOAO. After the submission and acceptance of internal proposals, eps Aur was allowed to be observed.

Prep Work

Planning an observing run at CHARA can be quite complex. After you have identified your target objects, you need to find suitable calibration objects nearby. Our calibration stars (a.k.a. “calibrators”) need to appear as point sources to the interferometer in the wavelength of observation and be free of any circumstellar material that might be observed by the array. Fortunately 10 Aur and 45 Per were previously found to be good calibration objects in this area of the sky.

Next you need to find when your targets are observable by the array (presumably you've already selected the right time of year for your target). Aside from an horizon limit (or perhaps obstruction by a near-by array), there is also a limit between two telescopes because the delay lines are of finite length. A series of mirrors located in a tube below the delay lines (these are called Pipes of Pan, or POPs) which shorten/lengthen the delay lines, thereby changing the amount of time the light from two telescopes can interfere. Although changing POPs during the night is possible, it takes about 30 minutes as moving optics in/out of the beam require realignment of other components.

Several tools exist to help with planning. CHARA has an in-house program to predict the amount of time a particular object “has delay.” I've recently tried out Aspro2 which graphically shows the times (in either LST or UTC) during which the target is in delay on given baselines.

The above is a screenshot from Aspro for this Thursday's observing run. We'll be using the “outer-west” configuration (telescopes S1,E1,W1, and W2) during the beginning of the night (shown by the connecting lines). We'll switch to the inner-west (S2, E2, W1, and W2) during the night due to POP limitations. There's a lot to be discussed about the choice of array configuration including a few hours worth of work simulating the data I would get from the array and then reconstructing images to see which is “better,” but I won't discuss it in this post.

Next we need to choose a POP configuration, Aspro helps here too:

The above screenshot shows several different things. We'll start with the lower panel background colors. The shades of gray indicate the status of the Sun. Dark being set, medium being twilight, and light gray being above the horizon. The time eps Aur is above the horizon limits (set to 30 degrees above the horizon due to an oak tree in the way of W1) is shown by the blue line (second from the top) to be almost the entire night. All of the lines below the rise/set line show when individual delay lines are capable of observing eps Aur given the POP setting. The resulting four-telescope observing time is shown at the top in red along with the altitude of the target. The yellow triangle marks when eps Aur transits.

Preparing the plan

Now with all of the information at your fingertips, the last step is deciding the order in which the targets and calibrators will be observed. With eps Aur being the only target, this is really, really easy. I add one additional layer of complication and attempt to sample regions that my simulations predict will show interesting or rapid changes in the interferometric observables (more on this later). Typically I observe in a CTTC sequence (C=calibrator, T=target). If all goes well each time I observe a target or calibrator takes about 25 minutes. I like to see things graphically so I make a spreadsheet with each cell equal to five minutes. I then make 25-minute blocks of time labeled with the star's name along with any configuration changes or notable items in the schedule.

Well, that's it for now. In my next blog post I'll actually be at Mt. Wilson. I'll describe the actual observing process and show you some images of the data we get from the interferometer.