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The VLA Exposure Calculator is a new tool
to provide observers with a quick method of calculating the rms noise
in an image, given the necessary parameters. The calculator is intended
to be flexible but simple, so some comments on the inputs is useful, since
some confusion might exist. For a detailed discussion on sensitivity calculations,
please consult the
Observational Status Summary .
Warnings !
In particular, for the lowest frequencies
on the VLA: 74 MHz, and 327 MHz, confusion, radio frequency interference
(RFI), and some instrumental effects limit the sensitivity. We have turned
off the calculation for 74 and 327 MHz; one should consult
the Observational Status Summary or a low-frequency expert for observations
at these frequencies.
For the frequency band 1.2 -- 1.7 GHz, the system temperature goes up
significantly as the elevation goes down. The added noise due to
this effect keeps one from attaining the theoretical limit. Thus the rms
noise level reached for a given time on source depends dramatically on the
range of elevations. The calculator does not take this effect into account
at the moment, so rms noise levels reported by the calculator are likely
to be optimistic. For more information consult the
Low Frequency Observing Strategy Guide .
For the 22 GHz and 43 GHz frequency bands, the deterioration of the
antenna efficiency with low elevations causes a similar effect as described
above. Again, the calculator does not take this effect into account.
The calculator has been tested under Netscape 4.78 and Mozilla 1.0 running
under Red Hat Linux 7.2, as well as Internet Explorer 6.0 and Mozilla 1.0
under Windows 2000 Professional. It might be necessary to go to the
Sun
plug in site and follow the instructions to install and enable Java
on your browser. This may take you up to 10 minutes.
Shown below is a screen shot (not the actual calculator)
of the exposure calculator. The top
(approximately) third of the calculator shows the inputs and outputs
of Effective Bandwidth, Number of Antennas, Time on Source, RMS Noise,
and RMS Brightness Temperature, with associated units. Below that is
a Frequency Band selection on the left, and an Array Configuration
selection on the right. Below the Frequency Band and Array
Configuration selectors is a message line (text in red).
You can enter values for the Effective Bandwidth, number of
Antennas, and the Time on Source. You may select (by clicking on
the buttons), the Frequency Band and Array Configuration.
The Effective Bandwidth is entered in MHz. For continuum
observations at the VLA, the default is to use 2 "50 MHz" polarization
pairs; each polarization pair at a slightly different frequency.
If you combine the polarizations and the two frequencies,
the "official" bandwidth is 200 MHz. However, due to the rolloff of the
50 MHz bandpass filters, the effective bandwidth is more like 43 MHz per
filter, or 172 MHz for the two "50 MHz" polarization pairs. Therefore,
a value of 172 MHz should be used for the effective bandwidth in the
exposure-time calculator for this standard continuum case.
For spectroscopy, the Effective Bandwidth (in MHz) should be
that for one spectral channel, one polarization. Dual polarization
observations at the same spectral frequency could be entered as twice
the effective bandwidth of one spectral channel, or twice the
time on source necessary in order to account for the two independent
polarizations.
The Time on Source is entered in hours. Note that you will have
to account for absolute amplitude, phase, and perhaps bandpass calibration
in addition to the time on source to give an estimate of the total time
required for an effective observation.
Given the Time on Source, the Number of Antennas, the Effective Bandwidth,
and the Frequency Band, the exposure calculator gives the rms noise for
a naturally-weighted image. Given also the VLA Configuration, the rms
Brightness Temperature is calculated.
In addition, at the bottom of the calculator, one may enter an HI Profile
Width. Assuming a low opacity neutral hydrogen emission line, with
this entered profile width (in km/s), the calculator gives the RMS HI column
density, if 1.5 GHz is selected in the Frequency Band selector. This
calculation assumes the profile is flat-topped
(i.e. rectangular) and spectrally resolved over the total profile width.
This calculation only depends on the Profile Width, not the Effective
Bandwidth.
The Calculator is still somewhat experimental. If the results seem unreasonable
please check the calculation by using the
Observational Status Summary .
Comments and suggestions about the Calculator may be sent to
mclausse@nrao.edu
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