Position Switched KFPA Pipeline Use Case
Figure 1: Image of the distribution of HC7N in the Taurus Molecular Cloud (TMC)
The KFPA imaging pipeline has two major modes of mapping calibration, 1) frequency switched and 2) position switched. These modes have many aspects in common concerning the control of the pipeline and parameters associated with producing the final images. Here we only address the differences in calibration between the two methods.
The sequence of observations and control of the pipeline is described in the frequency switched wiki page.
The goal of these GBT mapping observations is to determine the physical conditions of the regions of the Taurus Molecular Cloud (TMC) actively producing vast quantities of large organic molecules. In order to deduce the physical temperature, we need to estimates the effect of source structure on the apparent intensities measured at different frequencies. The primary effect, which must be modeled, is the varying angular resolution of the GBT as a function of frequency. Due to the un-blocked aperature of the GBT, the beam size may be accurately modeled, however the source structure must be observed, and the correction of source brightness temperature to be applied to each observation, based on the results of numerical integrations.
The mapping tests presented here use radio source 3C48 as the calibration standard. The vector calibration of the noise diode and system temperature is performed using these observations. The calibration is then applied to the line emitting region. We also include a vector estimate of the atmospheric opacity
The position switched calibration requires identification of signal free regions of the observation for use. Normally, the observer will schedule observations of reference locations before and after each mapping region. The pipeline will average the two sets of reference spectra and use these spectra to perform the (signal - reference/reference)*T_reference calibration.
Note the T_reference spectrum is a vector function of frequency and polarization and will be computed from the standard vector SCAL manner.
Observing and Data reduction log
The observations for project AGBT08B
_040 were carried out in three epochs, 2008 July 18, August 5 and 8.
The first observing session was short and was plagued by poor weather. These data are not useful. The second observing session went well. During this session, we observed the source in the frequency range 7 to 10 GHz. The third observing session also went well. These observations were made in the frequency range 12 to 14 GHz.
2008 August 8 (888)
These observations were performed with the Ku band receiver, using both beams for spectral line mapping. We observed TMC
and calibration source 3C48, performing ON/OFF, Nod and Mapping spectral line observations. We first present the
3C48 mapping data in the figure below. At the time of these observations, the SDFITS program used the GBT tracking
beam coordinates for the coordinate value of all beams in a single file. Note that in this case, the second beam
was pointed above 3C48, and that 3C48 is only partially observed, at the bottom of the second beam map.
Beam one map of 3c48 ||3C48 Beam 2 map