3. Observational Techniques
The observing techniques used at the 140 foot depend not only
on whether the mode is continuum or spectral line but also depend on
the type of object observed, the frequency, and the available
hardware. The following sections offer suggestions on how best to
conduct certain standard experiments. If you plan observations that
are unusual, or that press the hardware to its limits, then contact
the "Friend of the Telescope" for suggestions.
All observers who use the telescope at frequencies higher than
5 GHz or whose observations require critical calibration or pointing
of the telescope should familiarize themselves with the continuum
capabilities of the 140-ft since the necessary data are obtained with
continuum measurements.
The unit of observation for continuum and spectral line work
is called a scan, each of which is given a unique designation called a
scan number. Maps and surveys are built out of individual scans.
a. Continuum Observing, Pointing, and Calibration
Depending upon the receiver, continuum observations can be in
one of the following modes: total power, noise adding, load switching,
and beam or position switching; check with the receiver engineer as to
which are possible. Observations with the Cassegrain reciever can
take advantage of the quick beam switching provided by nutating the
subreflector; position switching with prime focus receivers requires
physically moving the telescope. The excellent description of
continuum techniques found in Chapter IV of the "Digital Continuum
Receivers User's Manual -- Electronics Division Internal Report
No. 243" should be consulted by those who wish to make, but are
unfamiliar with, continuum measurements.
A scan is built out of one or more samples of the incoming
signal; the sample times must be multiples of 0.1 seconds. While
taking data, the telescope can be either stationary, tracking at the
sidereal rate, slewing in any direction, or moving back and forth
between positions in the sky (ON-OFF observations).
If you only need continuum data in order to check the
pointing, then total power with prime focus receivers and beam
switching with the Cassegrain system are adequate. At high
frequencies (> 5 GHz) we highly recommnend that you check the pointing
occasionally. Table 2 suggests how often pointing should be done and
how close a continuum source must be to your region of interest in
order to ensure adequate pointing accuracy (better than 1/5 of the
beamwidth). You should also check the pointing every time a move is
made to a source at a new and widely different position in the sky.
Table 2
Suggested Pointing Strategy
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Frequency How Often How Far
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22 GHz 1h best, 2h at most 10-15 degrees
11 GHz 2-4h 30 degrees
5 GHz 4-6h 30 degrees
1.4 to 5 GHz 6-12h 45 degrees
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Note that bad weather while observing at high frequencies can
make pointing difficult on even the strongest sources. You can
schedule pointing in an observing deck with the PEAK observing
procedure or, to eliminate the possibility of erroneous pointing
corrections, you can tell the operator when and on what object to
point. The operator will set up the control panel, execute the
observation, and only update the pointing corrections if the
observations were good (CAO, pp 13-16). The operators have an
extensive list of sources suitable for pointing.
The observers are responsible for correcting their data for
telescope efficiency, atmospheric opacity, and errors in the value of
the noise tube temperature. You can obtain most of these corrections
from continuum measurements of sources with known fluxes. Some types
of observing require as much time checking the calibration as
observing. Some types require only a quick check. In order to correct
your data for the shape of the beam of the telescope, you can make
continuum maps toward strong point sources. Upon request, the "Friend
of the Telescope" will recommend ways to calibrate data and make beam
maps.
b. Spectral Line Observing
Spectral line observations with the Mark IV A/C can be either
frequency switched, position switched, or, with a Cassegrain receiver,
beam switched (see description above and CAO, p. 26 for details about
the A/C). The integration time for a scan must be multiples of the
A/C dump time which itself must be multiples of 20 seconds. We advise
that the duration time for a scan should not exceed 1 hour; you can
always average scans together during the analysis. In the presence of
spurious interference, as found at low frequencies, we suggest that
you start off with short (20 or 60 sec.) scans. By analyzing
individual scans, you can throw away scans plagued by interference and
keep the good scans.
During the integration time, the telescope can be stationary,
tracking an object at the sideral rate, or slewing in any direction.
The appropriate way for your observations to set the internal
attenuators of the A/C, called balancing the A/C, can be discussed
with the "Friend of the Telescope".
Continuum emission from the sky, ground, and astronomical
sources generates standing waves between telescope components above
the dish of the telescope. The standing wave, which will appear as a
8 MHz ripple in the data, can be partly removed by including MODFOCUS
on an unlabelled card in a "Setup" or Source" deck. (MODFOCUS
alternates the position of the subreflector or receiver by +1/8 and
-1/8 of a wavelength during an observation - see CAO, p. 34.) The
problems is most severe at high frequencies, because of atmospheric
variablity, and with wide A/C bandwidths (5-80 MHz).
In the discussion below, I am considering the case of an
emission line; the extension to absorption lines is straightforward.
i. Frequency Switching
During frequency switched or SPOWER observations, the observed
frequency alternates between frequencies chosen by the observer. The
observer supplies on the "L" card one on-frequency offset and two
off-frequency offsets. The A/C cycles the ULO through the sequence:
on - off 1 - on - off 2 - on - off 1 ... with each step in the cycle
lasting 1 second. The A/C subtracts the off observations from the on
observations in order to remove the instrumental bandpass.
Most observers use identical off-frequency offsets that are <
1/4 the A/C bandwith and an on offset of minus the off offsets. For
example, an observation at 15 GHz with a 20 MHz bandwidth for the A/C
could switch between 15.005 GHZ and 14.095 GHz by specifying frequncy
offsets of 5, -5, and -5 MHz on the "L" card; the spectral line will
appear twice in the bandpass of the A/C. One of the lines will be the
normal emission line at the correct velocity while the second will
appear 10 MHz away but as an absorption feature. The observer can
average the two lines with the analysis software thereby doubling the
effective integration time (see CAO, Appendix E).
If the amount of frequency change is small (< few MHz), SPOWER
provides one of the best ways of removing the shape of the
instrumental bandpass and provides the best signal-to-noise. If the
expected spectral line is wide or could fall over a large range of
velocities, or if more than one line is expected along the
line-of-sight, than SPOWER may not be the best method. In some
applications, SPOWER, because each channel in the A/C is sensitive to
two frequencies, reduces the number of useable channels by the amount
of the frequency switch.
ii. Total Power or Position Switching
In position switched or TPOWER observations, the telescpe
collects data not only at the position in the sky the observer is
interested in (the ON position), but also a nearby OFF position that
is assumed or known to be free of emission. The instrumental bandpass
of the telescope, measured at the OFF position, is subtracted from the
ON positions using the analysis software. Unlike frequency switching,
both the on and off measurements are kept as individual scans. TPOWER
provides more useable channels than SPOWER but can produce exceedingly
bad baselines in some cases.
Problems with TPOWER are: the off position may not be free of
emission; the atmospheric contribution to the system can differ
between the ON and OFF position; the receiver characteristics can
change between the ON and OFF measurements; and standing waves can be
more troublesome.
iii. Beam Switching and Double Beam Switching
An alternate way of position switching with Cassegrain
receivers, called beam switching, uses the nutating subreflector. The
observer specifies ZEROS for the frequency offsets on the "L" card and
takes SPOWER observations. The A/C, controlling the nutation of the
subreflector, measures the bandpass of the system every 1 sec when the
subreflector is in its reference position.
Next to frequency switching, beam switching provides the best
way of removing the changes in the atmosphere and response of the
instrument; it does not provide twice the signal as frequency
switching sometimes does. Since the beam throw of the nutator cannot
exceed 18' (CAO, App. G), the reference position could lie on the
source if it is extended; you then can't beam switch.
In double beam switching, an improvement to standard beam
switching, the telescope takes pairs of scans with the source first in
the signal beam of the nutator and, after the telescope moves by a
specified amount, with the source in the reference beam. The
baselines usually are good, but you cannot use double beam switching
with extended sources. A memo attached to the CAO explains the
mechanics of double beam switching.
Observers must be aware of problems with double beam switching
at frequencies higher than 10 GHz. Because the reference beam is off
the axis of the telescope, the shape of the beam is distorted; the
distortion depends upon the telescope's position and is unpredictable.
The efficiency of the reference beam is variable and can be 1/2 the
efficiency of the signal beam. The software must know the location of
the reference beam, but the center of the distorted beam changes
position so the observer must occasionally measure the location of the
beam.