Observing @ GBT: Signs of good & bad data

So far, all of the observations for the H₂CO densitometry project have been performed at the Green Bank Telescope. During a 10-day long observing trip here, I've learned a lot about diagnosing bad data.

This first image shows TSYS vs Airmass for good data. The high outliers are just sources with continuum in them - the continuum is the source of the extra signal, not atmosphere.  The receiver temperature is a nice 20.6 K, and you get about 5 K extra per airmass, suggesting a zenith optical depth of 0.018 assuming a round atmospheric temperature of 300K.

In the same style plot, there is a set of observations with low system temperatures: that stuff is good. There is also a set with clearly rising system temperatures, even at constant elevation. These data are bad. During this observation, the "blowers" that are meant to keep dew off of the receivers failed. Dew buildup on the receiver covers lead to higher optical depths and therefore system temperatures.

Finally, this data set was totally useless. Ku-band is not particularly sensitive to water in the atmosphere... but it's still not a good idea to observe during a rain storm.  Note that the fitted receiver temperature TREC is nonsensical.

Galactic Formaldehyde Densitometry Project

The Galactic Formaldehyde Densitometry Project is finally official and on its way. I am PI of a joint Arecibo / Green Bank Telescope project to survey formaldehyde, H2CO, in a sample of ~400 "dust clumps" in the Galactic Plane. The clumps represent condensations within larger molecular clouds that are likely to be forming stars, particularly massive stars and star clusters.

Formaldehyde is uniquely useful for measuring densities of these gas clumps. Because of a collisional selection effect, the low-lying K-doublet transitions of H2CO undergo "anti-inversion", which means their lower-energy states are overpopulated relative to the higher states, and it can therefore be seen in absorption against the cosmic microwave background. This unique property removes a key bias present in emission line studies in which variable excitation conditions can dominate the observability of a line.

For the next few posts, I will include calibration and data reduction information from the observations in this survey. The Green Bank observations are well underway, with nearly half of the sources observed (though many at poor sensitivity) and the Arecibo observations have been approved.

Neat new things....

1. sptool is a quick way to compare standards to stellar spectra. Nice, I'd been looking for a tool like that.
2. GNU screen captions are useful especially when working in a screen-within-a-screen environment (who does that, really?)
3. finally got SPLAT to work... turns out I just hadn't reduced my damned data
4. kill -STOP and kill -CONT are really useful ways to pause programs that are sucking up resources if you want to resume them later. Haven't tried this on "real" code yet.

Filled step plots in matplotlib

It's not possible to do a simple filled step plot in matplotlib using default
commands. Workaround:

def steppify(arr,isX=False,interval=0):
    """
    Converts an array to double-length for step plotting
    """
    if isX and interval==0:
        interval = abs(arr[1]-arr[0]) / 2.0
        newarr = array(zip(arr-interval,arr+interval)).ravel()
        return newarr

plot(xx,yy,linestyle='steps-mid',color='b',linewidth=1.5)
fill_between(steppify(xx[x1:x2],isX=True),
    steppify(yy[x1:x2])*0,
    steppify(yy[x1:x2]),
    facecolor='b',alpha=0.2)

My starred reader articles

Just posting up a few papers I found interesting.

Peng Wang and Tom Abel's paper on outflow feedback in clusters
A somewhat less interesting follow-up to the previous

HARP mapping of the Serpens cloud core

Neal Evans' review of low mass star formation observations

the MNRAS paper on supermassive stars in the LMC

Identification of molecular clouds from the FCRAO OGS

First!

First post by me on my own BLDG! I should transition to this address eventually...

Testing trackbacks?

I just want to see if "trackbacks" work at all. I recently posted about histograms on google spreadsheets .

While I'm at it, might as well throw up a link to my google code site .