The Data

The data are collected from a sequence of elevation scans, with 48 arcsec azimuth separation between scans; a boresight calibration preceding each scan.

This separation is the Nyquist interval (for observations at 11.7 GHz) of an aperture plane function which is band-limited to 110m.

The scan rate was 48 arcsec/second. We sample the data stream (at the Mk4 correlator) every 0.5 seconds.

This observing pattern has quite a high overhead at Effelsberg: there is a 30 second setup period prior to each step, which means that a full map consisting of 133 scans, each 133*48 arcsecs long, should take about 7 hours to complete (133 secs/scan drive time + 30 sec/scan setup + 30 sec/cal setup).

The VLB machinery adds a further penalty: data taking must stop every 44 minutes when the tapes reverse direction.

The circumstances of the two maps are summarised in table 1

Table 1: The observation log

date	start	end	raster size	weather	ID
29 May	9:50 UT	15:50 UT	110 scans	light cloud, T=20C	DAY
12 June	19:20 UT	3:40 UT	133 scans	very clear, T$\sim$10C	NIGHT