Data Reduction and Calibration

The survey contains 128 separate 12 hour observations. Each generated a data set (consisting usually of the responses from 384 fan beams sampled every 24 s) which was formed into an image using the method of back projection (Crawford 1984). Further procedures including CLEAN enhanced the quality of these images. The treatment was similar to that used for the First Epoch Molonglo Galactic Plane Survey (MGPS1) as described in detail in Green et al. 1999.

Calibration consisted of a series of steps. First a provisional adjustment to each data set was based on the contemporary short observations of standard calibration sources. Next, all the images contained unresolved sources which were also present in other overlapping images. Fits for the position and flux density of these sources were made on the final CLEANed images and the results were intercompared to produce mean gain, right ascension and declination corrections for each image. These corrections were then applied to bring the whole survey on to common intensity and position scales.

To improve the sensitivity and to get an overview of the SMC, all observations were then combined into a single image. All the 128 CLEANed images were mosaiced with a weight based on their quality and with the above corrections included.

The final step in the overall calibration involved two further sets of observations. Firstly, the calibration of position. 15 of the stronger background sources in the survey have been observed by us at 5 GHz using the Australia Telescope Compact Array (ATCA) with a beam (FWHM) of 4 arcsec. Seven of these sources were found to be still unresolved and their positions at 5 GHz were determined to better than 1 arcsec. The mean position of these sources in the 843 MHz mosaic agreed with the mean of the ATCA positions within 0.5 arcsec showing that the positional scale had systematic errors of less than 1 arcsec.

Secondly, the intensity calibration. The 843 MHz flux densities of the 4 strongest sources unresolved at this frequency were measured with the MOST in a special set of observations in 1992 September. Each source was observed in sequence for four minutes at each of several hour angles. Several standard calibration sources were included in the sequence and enabled the flux density of the 4 sources to be established on the standard MOST scale with an uncertainty of 2.0 percent. The absolute scale itself, on which the sources 0409-752 and 1934-638 are 19.8 and 13.65 Jy respectively, is believed to be correct to within 5 percent (Hunstead 1991). After applying a single empirically derived factor to the mosaic, the fitted flux densities of the four sources agreed with the results of the special observations to within 2 percent. Hence the overall flux density calibration should be correct to better than 6 percent.

For individual sources, the random errors due to noise and other effects are likely to exceed the systematic uncertainties in position and flux density.

© Copyright Astronomical Society of Australia 1997