Matching Plots with the LAB Survey Results

One of the nice things about the above activity is that plots obtained by observation can be verified against the results from an extensive professional radio astronomy survey of neutral hydrogen line emissions. There is an online form which can be filled in with target coordinates, antenna beamwidth and plot velocity limits.

For example, here is a composite plot which superimposes the result (in blue) from the 7-day integration of data (as described in the previous section) with the LAB survey results (in green) using a 6° beamwidth...

...which shows a fairly good correlation between the two.

It can be seen that the match is not perfect, with some small deviations from the LAB Survey result.  This is to be expected as even small changes in the position coordinates (say 1°) entered into the form results in large changes in the returned profile.  This is because the LMC is an extended object (actually larger in angular size than the 6° beamwidth of the antenna) and the resultant profile is the summation of small, separate regions with different velocities.  Moving the 6° beamwidth antenna by just 1° causes many regions to drop out and other regions to appear in the beamwidth.  This would not be the case for a more distant object where the angular spread of the object does not fill the beamwidth of the antenna.

The plot variation with a small (1°) change in declination is shown below...

...which indicates that attempting to get a perfect match for observations of the LMC using a 6° beamwidth antenna with the LAB Survey results is not a sensible endeavour.  To further illustrate the point, the right-most plot shows what the LAB survey returns for a 0.6° beamwidth antenna - meaning that if the 3 m TVRO dish doesn't have a 6° beamwidth as per theory, further deviations from the LAB Survey could be expected.

All in all, I am more than satisfied with the correlation between my observed results and the LAB survey results.