[daip] vtess comments

[Sara Beck]

Hello,

I am using VTESS for the first time, on an extra-galactic source, and ran
into some puzzling behaviour at the short wavelengths (U and K band).  at
these wavelengths the source (seen in conventional CLEAN maps) is small
and weak, like a few mJy.  the vtess program would act sensibly only if
it was given an estimate of the total flux (FLUX<0 in the inputs) and if
the field mapped was limited to 6 arcseconds. otherwise it would hang up
somewhere, get into a state after a few iterations where the step and
gradient were 0 and stay there, and the final flux would be huge (1.45 Jy
instead of 3 mJy) and the map total garbage, broad bright streaks all
over.

also, the final fluxes for VTESS maps are way off, like a factor 2-3
higher, than the CLEAN maps at U and K. (these are the maps that behaved,
not the weird ones).

i get the impression from the explain vtes material in aips that the
quantitative results are not to be trusted unless the S/N is really high,
which it isn't here. so can i just ignore the high fluxes in the vtess
results? or do i figure that they weren't really good fits even though
they converged?

Note that at L, C, an X on the same source Vtess worked very sensibly and
the results are very consistent with the regular maps except they look a
little less blotchy/mottled, which is what we wanted from Vtess.

so thanks for any insight you have into the workings of this new method.

sara beck