High Frequency Meeting - 24 Feb 2011
From high frequency commissioning meeting on February 24, 2011:
The discussion this week was about strategies for finding high
frequency calibration sources.
Lorant gave a brief summary of the CALSUR program. Although progress
was made in taking data on sources at C and K band, the data in hand
needs to be re-reduced and checked. Although this could be done with
the AIPS pipeline, it would require some time and manpower. Joe suggested
that this could be done with RSRO people, but Frazer felt that this was
not a good use of those folks (i.e. Ph.D level not needed). Some further
discussion on manpower topics was continued; there were some people who
thought that perhaps the analysts could re-do the CALSUR data, or a part-time
student could be hired (though there are not funds for this). However, there
are no staff members who actually have the time for training or hand-holding
people at the level of analysts or student help.
Can we (easily) find the 50 mJy/beam sources needed ? What is the density
of sources ? Based on Condon's work, extrapolating to high frequencies
there are 0.3 50 mJy/beam sources per square degree at 45 GHz. This is
a direct quote from Condon.
Of course there are other surveys that can help address the source density
question directly (as well as give more possibilities for new calibrators),
for example, the AT20G
survey; a blind survey of the sky from the AT with
5890 sources above a flux limit of 40 mJy. This would be a good start, but
of course is limited to the southern sky (dec .le. 0 degrees). WMAP has
a flux limit of about 1 Jy.
We also had a brief discussion about Vivek's method of obtaining good
X band, A config calibrators using the NVSS (unfortunately Vivek was not
at the meeting):
1. Vivek used the NVSS catalog extractor, filter for size (unresolved)
and flux (50 - 100 mJy, for example) - this will produce a list of
sources (in Vivek's example it produced some 3000 sources in the Galactic
plane in region about 4 (square ?) degrees.
2. He then observed these ~300 sources with the VLA at X band in the A
configuration --- one snapshot per source, and a "known" calibrator every
10 - 15 minutes. He found that ~50% of the 300 sources were acceptable A
configuration calibrators at X band, i.e. they were either unresolved or
had simple structure. The X band fluxes were at least 10 mJy / beam on
the longest baselines.
Frazer voiced some skepticism over this method, but Michael and Lorant
have discussed this method with Vivek and it seems to work quite well
for finding A configuration calibrators at X band. Note that it hasn't
been demonstrated for higher frequencies. At some point, we'd like to
have Vivek write up his findings (and he has agreed to do so).
Near the end of the meeting, we discussed that we'd perhaps like to step
back and do a bit more research in the area of strategies for calibration
sources. Some questions that we perhaps need answered are:
- What density of small or unresolved sources that could be used for high frequency calibrators might be found on the sky as a function of flux density ?
- Should we survey sources selected from existing lists to see if they are useful for our needs (but see NVSS discussion above) ?
- Should a small blind survey near targets be done in order to find calibrators before observations are made ? (Note in writing minutes --- we discussed this before when Brigitte was working on CALSUR; if this were to be something we did for each project, we would have to find the manpower for it.)
- Are there other, better approaches ?
Joe and Frazer felt that this research might be usefully given to a RSRO
observer (i.e. an experienced observational astronomer), and they both have
approached Ken Kellermann to see if he would be interested in pursuing this
In the long term (i.e. a research project, not commissioning), we
might want to try what ALMA has planned to do: find the best gain/phase
calibrator in real time, by using the data from TELCAL and modifying
observation scripts in real time.
Finally, we felt that perhaps we should contact ALMA people who were
working on this problem. Joe agreed to contact Baltasar Vila-Vilaro.
After the meeting Adam Leroy and Todd Hunter sent Claussen an email
message essentially recommending the same thing; Todd suggested that
Bill Dent might be an active person to contact. We'll leave that up
Oh yes, just so we keep this in mind: known IDV sources should be