Observing Strategy
We intend to use the position-switching mode of observation for
SgrB2(N-LMH) and switch in azimuth 60 arcminutes off source every two
minutes. SgrB2(N-LMH) is extended on the scale of several arcminutes
and we have found that this procedure permits the detection of weak
signals. For example, we know of other observers who have wasted valuable
GBT time trying to nod just a few arcminutes between receiver beams,
only to find later that this "more efficient" observing method
actually switched into signal in the off position, thus precluding
the detection of weak species like prebiotic molecules.
In the 300 MHz to 50 GHz frequency range of the GBT there is 40.4 GHz
of useable bandwidth spread across 16 different receivers. What is
described below is an approximation based on an average of how much
time is needed to complete this survey. Having observed at almost
every band on the GBT, we are aware of observational difficulties
including RFI, the variation of system temperature across the bands
and the loss of sensitivity due to weather and what occurs at the
band extremes. Thus, some bands will require more observing time
than what is described below, and some will require less. With
this in mind, we assume an average value of the noise level
expected in a particular amount of time on source.
For SgrB2 observations, we intend to use four 200 MHz bandpasses, but
each successive bandpass will overlap the next one by 5 MHz so that no
molecular lines will be lost due to ambiguities at the bandpass edges;
this will result in 775 MHz of contiguous coverage for each
observation (i.e., one 6 hr track on SgrB2). On average, we assume
there will be 40.4/0.775 = 52 different tracks on SgrB2; to achieve a
noise level of ~2 mK, will require that approximately 2 tracks be
devoted to each tuning which results in a total of ~104 tracks for
SgrB2.
Thus, we could accomplish all SgrB2 observations in 2.5 yrs by
observing an average of 4 tracks per month. The foregoing time
estimates are reasonable, given that some bandpasses will be plagued
by RFI, and, hence, will not be entirely observable (e.g., RFI due
to communications satellites operating at 4 and 12 GHz). Moreover,
we acknowledge that for very low frequencies, we may have to observe
SgrB2 at 6.1 kHz channel spacing in order that good velocity
resolution is achieved on spectral lines. These decisions will
have to be made at the telescope since many of the receiver
systems have not been used for spectroscopy of the expected
weak lines that characterize prebiotic molecules.