[mmaimcal] CBI Experience
Al Wootten
awootten at nrao.edu
Wed Feb 21 01:27:55 EST 2001
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NATIONAL RADIO ASTRONOMY OBSERVATORY
Charlottesville, Virginia
November 27, 2000
M E M O R A N D U M
To: P. Vanden Bout
From: R. Brown, B. Butler, S. Radford, T. Readhead, A. Wootten
Subject: CBI Experience on the Chajnantor Site and its Relevance to ALMA Planning
On November 6, 2000, Tony Readhead (Cal Tech), Bob Brown, Al Wootten, Simon Radford, and
Bryan Butler met in Brown's office to discuss observing and operating conditions at the Chajnantor
site. The Cosmic Background Imager (CBI) group has learned a lot about Chajnantor while operating
the CBI this past year, it is important to us that ALMA benefit from this experience.
1. Overview of the CBI Project on Chajnantor
The CBI group brought their equipment to the Chajnantor altiplano in August 1999 and began
operations in November 1999. The project is funded through next austral fall; additional funding
has been sought to extend this to August 2001. The CBI is a 13-element interferometer mounted
on a 6 meter platform operating in ten 1 GHz frequency bands from 26 GHz to 36 GHz. It sits in
a clamshell dome about a kilometer west of the ALMA site characterization containers.
Mostly, the experience on the site has been that the weather can be spectacularly good most
recently, a continuous run of 46 successive nights has been limited only by the receiver noise. On
1-meter baselines the CBI was thermal noise limited about 50% of the time during its first
six months of operation, whereas at the same frequency and baseline length at the OVRO
this occurs only a few percent of the time. This is a strong function of baseline length,
thus, for example, the OVRO Millimeter Array achieves the thermal noise limit even in
the summer daytime at CBI frequencies on all baselines.
1.1 CBI Experience
About a dozen nights in the past year have been problematic owing to weather at Chajnantor
but apparently these nights would not have been lost had the CBI been sited at Pampa la
Bola. Tony emphasized that 10 percent extra observing time would be important for
any astronomical facility
These lost nights can be characterized as follows: clouds form in the early evening over
Chajnantor, often with snow following. CBI could not open up for observation.
The 'far field' for that instrument is only a few 100 m above the site, so they actually image
the atmospheric clouds as well as the distant Universe. Formation of the cloud layer is seen
in the relative humidity, which rises through 90 percent when the cloud layer descends to the
altitude of the site. Condensation occurs on the dewar windows during these events, a
further impediment to observing. Wind has not caused much time loss, nor has
inaccessibility of the site. As the team heads back to San Pedro from the CBI site, they pass
through Pampa la Bola, where the sky is completely clear, and there has been no
precipitation. Presumably, these microclimatic events occur as moist air arrives at the top
of the plateau and undergoes a phase change.
Tony estimated the number of nights when it actually snowed to be around 12-15 nights of
bad snow which occurred in 4 or 5 bad storms each lasting 2-3 days; but after each bad
snowstorm the wind continues to redistribute the snow for a week or so, until it becomes
compacted. Over the course of four months in the period from mid-December to early-June
episodes of sporadic snow were common. Wind did not often pose a problem though there
was a very bad wind storm in July, where there were gusts up to 100 mph, and the mean
wind was above 40 mph over a 30-hour period. They were quite worried about the integrity
of the CBI dome (and were glad that they had reinforced it in Pasadena). The dome
suffered only minor damage, which was repaired in a couple of hours. One of the
lessons learned from experience is that the predictions being made by Prof. Erasmus for the
wind on Paranal were also accurate indicators of future high winds on Chajnantor. Tony
thinks that the survival wind spec for the ALMA antennas should include a time period over
which the antennas will survive an ongoing, gusty, wind.
Tony's experience suggests that the location of the LMSA/NRO container is also windy. In
fact, it may be as bad or worse than at Chajnantor because the wind appears to be funneled
between Cerros Chajnantor and Toco. If wind is an issue for ALMA one should search for
shielded areas of stagnant air.
Other conclusions from the CBI experience of relevance to ALMA are:
Ice is not a problem. Ice buildup has not been a problem, although there are
occasionally ~12" icicles on the external dome structure
Dust is not a problem. No effects due to blowing dust have presented problems,
however, the telescope itself is shielded by the dome.
1.2 Actions Recommended from the CBI Experience
The primary recommendation from the CBI experience is to understand the micro climate
episodes, where clouds form over Chajnantor but not over Pampa la Bola. By correlating the
CBI logs with the data from the other instruments (tippers, etc.) can we recognize these
episodes?
Steve Myers plans to try to identify the episodes for study from entries in the CBI logbooks.
At other times, clouds or invisible water vapor cause other effects. Sometimes increased
power on shorter baselines/water in virtual clouds can be seen in the CBI data and might be
detectable with the ALMA equipment. Under these conditions, the CBI receivers
individually detect the water vapor fluctuations, which are easily recognisable since
the individual antenna total power fluctuations are strongly correlated.
Such events may have occurred but gone unrecognized in ALMA tipper measurements.
Every four and a half hours, the tipper stares straight up and measures the variation of total
power on the sky. This should show the water containing pockets which cause (a) the excess
of power on short CBI baselines and (b) power level fluctuations on individual antennas.
These data will be investigated in detail.
On clear nights when the CBI registers an excess of power on the shorter baselines might be
nights when anomalous refraction occurs. This also correlates well with relative humidity.
Tony estimated that this had occurred on only a few nights. We have estimated the
magnitude of the effect by extrapolation of data from the NRAO 300 m Site Testing
Interferometer measurements to 12 m baselines. However, a direct estimate at short scales
would be of great use for ALMA. Examination of data from these nights might give us some
insight into the structure of the pockets causing the effect. We will try to identify these
events for further investigation.
Other action items:
Correlate CBI evidence for liquid water in invisible clouds with 225 GHz tipper stare
mode fluctuations. The 183 GHz radiometers don't tip, but stare all the time. It may be
possible to see something in those data. Simon will discuss this with Richard Hills next
week.
Snow and black ice on the tarmac are dangerous. Several accidents have occurred this
year alone. The Jama road may not be cleared while drifting continues, limiting access
for periods of typically ~3 days.
Correlate RH meters at ALMA/EU and LMSA sites with the CBI results.
Consider ice prophylactic measures on antennas--watch out for open conduits
Doors on tepees should be upwind.
Consider weather stations at sites planned for array centers.
2. ALMA Requirements for Site Meteorological Conditions
Simon Radford reviewed a draft ALMA memo that summarizes the data taken nearly
simultaneously from the NRAO and NAO tipping radiometers. The 225 GHz optical depth
measured by the NRAO tipper, and the 220 GHz optical depth from these two devices at
Chajnantor and Pampa la Bola respectively provides a quantitative comparison of the atmospheric
transparency that favors the Chajnantor site in the sense expected from the higher elevation of the
Chajnantor site (5000 m vs. 4800m). While the difference in the quartile transparency at 225/220
GHz is not large between the two sites, approximately 0.01 at the 25 percent quartile, it is
possible to use the FTS measurements to infer the difference at all ALMA frequencies. The
difference can then be turned into a difference in the observation time required by ALMA to
reach a given sensitivity, the time always being longer at the Pampa la Bola site. The increased
observation time varies from approximately 10 percent at 230 GHz to 100 percent at 809 GHz.
For the CBI the difference in atmospheric transparency is not a decisive factor because the
transparency at either site at 26-36 GHz is so exceptionally good.
Bryan Butler reviewed a draft ALMA memo that summarizes the data taken nearly
simultaneously from the NRAO and NAO site testing interferometers. These instruments view a
geostationary satellite and record the effect of a varying path length delay through the atmosphere
on a baseline of 300 m. Here too, the data favor the Chajnantor site in the sense that the sky is
more stable over Chajnantor than over Pampa la Bola. In terms of observing efficiency, this can
be interpreted either as more time during which radiometric phase correction techniques will not
be required by ALMA, or it can be interpreted as a lessened requirement for those techniques on
Chajnantor as opposed to on Pampa la Bola. Again, this specification for ALMA has no direct
analogous requirement for the CBI owing to the lower frequency (26-36 GHz) and shorter
baselines (1-5 m) exclusively employed in the latter.
2.1 Actions Recommended from the ALMA Testing Program Experience
An effort should be made to bring the NRAO and NAO tipping radiometers together at
the same place to verify that they give the same quantitative result for the zenith
transmission;
An effort should be made to characterize the site microclimate.
3. Meeting Summary
The CBI and ALMA are sensitive to different atmospheric conditions and effects which may
strongly affect the optimum location for each.
The CBI images directly water-bearing clouds because such clouds are in the far field of the
CBI; hence the CBI cannot operate in the presence of clouds. ALMA sees such water-
bearing clouds only as an increase in system temperature; there is no direct image distortion.
Snow is debilitating to the CBI because it blows into, and collects within, the enclosing
dome. CBI observations cease during snowstorms, and it takes a few hours to clear
the snow from the dome following a serious snowstorm. The ALMA antennas operate
in the open air where the wind will act to remove snow from the dish (although snow will
collect against the pedestal mount); ALMA observations can proceed in the presence of
episodic snow. The need to remove snow from the site access ways needs to be an integral
part of ALMA site development and operations.
When the relative humidity exceeds 95% condensation is likely on the CBI antenna
windows, and therefore the CBI dome is not opened. For ALMA, condensation on the
primary mirror may prove to be a concern but not for the receivers and optics which are
protected by a Gortex window that is continuously IR heated.
Both the CBI and ALMA require a site with the exceptional conditions characteristic of the
Chajnantor science preserve. However, owing to the different requirements that ALMA and the CBI
have for conditions of wind, snow, clouds, atmospheric turbulence and atmospheric transparency, the
optimum location for each on the site is best determined by the local micro-climate and the specific
requirements of each instrument independent of the differently weighted needs of the other.
/bmr
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