[daip] Combining two sets of UVDATA increases noise?

[landon at inaoep.mx]

Hello helpful people at NRAO,

First off, thanks for all your help. It is very much appreciated.

I have come across some counterintuitive results using IMAGR.

I took one track of 13CO data from OVRO and flagged all the visibilities that
occured in the same positions in the UV plane as the visibilities in both of 2
other tracks. I then did the opposite and flagged all the visibilities that
weren't common to all four tracks. I made cleaned maps of both of these (with
the same adverb settings) and obtained noise levels of ~1.5mJy/beam and
~2.5mJy/beam for a certain channel of interest.

The strange thing is that although these two sets of flagged UV data are
completely complimentary, the noise level of the cleaned map of the original
data (ie: equivalent to these two sets combined) is ~4.5mJy/beam. From what I've
learned, more data (especially complimentary data) should lower the noise level,
not raise it by a factor of ~2.

The cleans were all done with the same adverb settings and the beam sizes are
all the same. 

If you could shed some light on what possibly causes this seemingly
contradictory result, that would be great. Or possibly, this type of result is
common?

Thanks again,
-Landon Halloran