Formaldehyde absorption experiments:

1. It should be possible to detect a nearby dark cloud in a molecular absorption against the Galactic background. For example, Fig 1 of 1987A&A...184..315M shows that in L1709 (in rho Oph) the 4.83GHz H2CO line temperature reaches a peak of -0.3 K at the 140ft. They had Tsys=70K and used 40 min integrations in their point-by-point mapping. With Tsys~18K at C-band at GBT, the equivalent integration time is 2.6 minutes, but the beam is >5 times smaller in area, so the peak will likely be larger and thus detectable in tens of seconds. This cloud is extended enough that it could be detectable for up to 3 minutes in a drift scan through the peak. Beamsize is ~150 arcsec, so sidereal rate would yield 10 sec per beam. You could get 18 independent spectra. The time before/after the transit could be used as the "off" position in theory (not sure how difficult this will be in post-processing!). Could be repeated several times at slightly different declinations, then the data could be compared to star counts and visual extinction maps. Other targets cited in the paper above may be better, which the students could try to quantify beforehand.
2. It should be possible to detect general Galactic formaldehyde in absorption against a background quasar. I know of at least two bright quasars (3C111 and 3C454.3) that show strong Galactic CO absorption lines in J=2-1 spectra recorded at SMA. 3C111 is the best (tau~5 in CO), but its declination is close to the latitude of Green Bank, so the GBT would have to be positioned near the zenith. 3C454.3 with tau~1 transits at a more moderate elevation of about 68 deg. Better targets may be present in a literature search (by the students).

Note: Either one of these projects could be combined with the Cosmic/Galactic background experiment suggested by Larry above.

JimCondon?

-- JimCondon - 27 Mar 2007

It is possible to use the subreflector to 'chop' at timescales of ~10s. This would enable position switching on some source. The source itself might be of little interest but getting the chopping set up and reducing the data would be an interesting exercise in itself, if technical more than astronomical.

A 1.4 GHz continuum drift scan at any azimuth and elevation will be strongly confusion limited, as could be demonstrated by repeating the scan on two or more nights. The students could measure the GBT confusion P(D) distribution and deduce the number density of faint (down to about 20 mJy at 1.4 GHz) radio sources. They could compare the faint sources found in the GBT scan with the NVSS images and catalogs to see how many GBT sources are just blends, and how confusion affects the GBT positions and flux densities of the "real" sources.

-- JimCondon - 27 Mar 2007

A relatively easy project that can be done with a few minutes telescope time is to determine the dispersion measure of a bright pulsar. Method: Obtain average pulse profiles at three widely spaced frequencies, say 327, 1400, 2200 MHz which are carefully time-tagged. When I have done this before with other students, I have had the data acquisition run continuously, doing period-synchronous averages. I'm not sure we have the gear for this here. My system will do it, but I won't be around to help, and it is a bit too arcane to open up to summer students.

The challenge is that there will be several periods of dispersion between the frequencies, so one has to think pretty carefully about all possible solutions.

Downside: the dispersion measures have been measured and tabulated, so the students can easily look up the answer. I think I did this by presenting them with plots and not identifying which pulsar it was.

Another that might be straightforward these days would be to obtain the distance to a pulsar using absorption by galactic hydrogen.� On-pulse measurements give the HI absorption between here and the pulsar, then the off-pulse measurements give the HI emission for the total Galactic path. Using a Galactic rotation model the pulsar distance can then be determined. There are some caveats: I don't know if it is easy to get pulse-phase resolved spectra out of the existing analysis programs. One needs a relatively nearby, bright pulsar. Again, this has been done for virtually all of the pulsars for which it is practical, but it combines a number of disciplines (pulsars, galactic structure, HI spectrometry).

No doubt Paolo will come up with several pulsar timing exercises. I feel that pulsar timing is pretty arcane, and doesn't provide exercises in general principles, hence is not so good for a quick exercise.

If the GBT can be steered in elevation, then setting it up for a drift scan across a known bright pulsar might be interesting. If the frequency were low enough (300-500 MHz?) then there should be plenty of pulses in the scan to obtain a period. One might also see sidelobes of the beam, too.

This gives me another simple idea. To ram home the concept that the squared modulus of the Fourier transform of the aperture distribution of the electric field on an antenna is the beam power pattern, have the students compute (predict) the beam width for a particular wavelength, then measure it by doing a drift scan on a bright source. Then to get quantitative info, do formal Gaussian and/or sinc-squared fits to the drift scan and correct it for declination. This is dead simple in concept, but to do it right is reasonably involved. I suspect that many attendees do NOT have a feel for Fourier Transforms.

Cosmic/Galactic background radiation (Steven) - observe a strip of sky with the GBT in "drift scan" mode and calibrate this with hot + cold loads; compare to expected continuum brightness, e.g., on + off the Galactic plane, in the vicinity of bright point sources, etc., in an attempt to characterize both the Galactic synchrotron brightness and the cosmic microwave background temperature. Some years ago, I had undergraduates do this in a lab assignment described here: http://www.naic.edu/~gibson/asph405/cmbr_lab.ps.gz, using a rooftop radio telescope and 4 GHz receiver described here: http://www.ras.ucalgary.ca/grad_project_1996/. One complication with the GBT is that, if we cannot point it, then calibrating the atmospheric contribution may be difficult. One might try to constrain the problem further by observing the same strip on successive nights at different frequencies, but I don't know if this is enough to help, since the synchrotron contribution will also change with frequency. On the other hand, we could simply use a model of the expected atmospheric emission rather than trying to measure it.

HI line emission (I forget who suggested this) - observe a strip of sky with the GBT and 40-foot telescopes and compare to each other as well as the Dwingeloo HI sky survey -- a nice illustration of the effects of different beam sizes (and perhaps also how different doppler shifts affect the line frequency, if those are not corrected automatically)

Pulsar drift scan (Paulo) - look for pulsars that pass through the GBT beam (sorry I don't remember other details)

Sky brightness vs. the moon (Chris) - use the moon's known temperature as a reference to determine the sky brightness; may be tricky if we can't point the telescope

%META:TOPICINFO{author="LarryMorgan" date="1169237404" format="1.0" version="1.1"}% %META:TOPICPARENT{name="HandsonProjects"}% Please input ideas and discussion here.

-- LarryMorgan - 19 Jan 2007