Basic Usage

This applet calculates the expected RMS noise for an observation with the CARMA array, given inputs about weather, frequency, array configuration, number of tracks, mosaic pattern, and visibility weighting. Changing a text-field entry, radio button, or slider on the left hand side will update the calculated values on the right. Informational and error messages appear in the yellow box in the lower right. Array configurations are described in CARMA memos 20 and 24.

Details of The Calculation

The RMS noise is determined using the equation 6.55 of Thomson, Moran, and Swenson (1st edition) and assuming correlator efficiency 0.87, 10-m antenna aperture efficiency of 0.6 and 6-m antenna aperture efficiency of 0.65 . Typical weather at Cedar Flat is 5 mm water vapor, very good weather is 2 mm water vapor. Atmospheric phase noise is taken to be 40 degrees for typical weather, and 30 degrees for very good weather. Spillover fractions for 10-m antennas are 0.08*T_ambient and 0.04*T_ambient for 6-m antennas.

The Total Time on-source is calculated using the S/N optimization algorithm outlined in BIMA Memo #47 (see also the Hour Angle calculation applet, GetHA) and assumes a 30% overhead for calibration and slewing. So your time request when proposing should be the on-source time plus 30%.

Visibility Weighting

The slider on the lower left allows you to modify the weighting of the visibilities which in turn changes the synthesized beam and RMS noise. The weighting scheme uses the robustness parameter devised by Briggs. The robustness parameter can be used to down-weight excessive weight being given to visibilities in relatively sparsely filled regions of the uv plane. A robustness of -2 corresponds to uniform weighting and +2 corresponds to natural weighting. See help on the MIRIAD task invert for more details.

How Shadowing Is Handled

For all arrays configurations the synthesized beam calculated for low declination sources has shadowed visibilties removed and the total time on-source is reduced by the fraction of visibilities shadowed. Shadowing has been calculated using the MIRIAD task csflag, which properly takes into account the differing diameters of the 6-m and 10-m antennas. The shadowing calculation assumes some shadowed visibilities are kept, using 80% of the 6-m antenna diameter and 100% of the 10-m antenna diameter. For those familiar with the csflag task this means using cfraction=1.0,0.8. It is similar but not identical to using select=-shadow(5) in the MIRIAD task uvflag. Future enhancement may allow this to be configurable.

Mosaics

For mosaics, the RMS is calculated over the central region of the mosaic that has unity gain. Only standard hexagonal mosaic patterns are supported.