[daip] RE: Pband issues

[Namir E. Kassim]

Larry -

I recall now we had our own flux bias problem at one point, though it was at
74 MHz and not 330 MHz. The problem related to how we did our absolute flux
density calibration ... how did you do your absolute flux density
calibration? The "regular" way, i.e. SETJY on something like 3C286 ... or
did you use a model of Cygnus or Cas to set your flux scale?

-Namir

> -----Original Message-----
> From: Lawrence Rudnick [mailto:larry at astro.umn.edu]
> Sent: Wednesday, April 16, 2003 12:17 PM
> To: daip; Rick Perley; Namir E. Kassim; Aaron Cohen; larry Rudnick
> Subject: Pband issues
>
>
> Gentlemen:
>
> I solicit your wisdom on 2.5 Pband problems.
> D configuration, 22 field mosaic (with each pointing
> mapped with 61 facets) infinite number of
> details available if that is important.
>
> Problem 1.  If I compare Pband fluxes with NVSS fluxes,
> there appears to be a significant *offset* at low
> fluxes;  i.e., the slope of P vs. NVSS does not
> extrapolate to zero.  A plot of the running medians
> of sources found in both surveys can be seen at
> http://www.astro.umn.edu/~larry/LSS/#bias2
>
> have you seen anything like this before?  I have in
> very diffuse sources like Cas A, where we get a bowl
> around the outside, but not where field is all
> small sources...  (maps available on that same web page)
>
> Problem 2.  If I take the difference image between maps made @
> different pointing centers, there are residuals that are
> much bigger than the noise (after correcting for primary
> beam).  So I picked three strong sources, and measured
> their observed fluxes in (almost) all fields, and tried
> to see how well things fit the expected primary beam
> attenuation.  Results are at:
> http://www.astro.umn.edu/~larry/LSS/#primary1
>
> After applying a pb correction and comparing all the fluxes,
> the result is that each field looks a little "off", with
> most scale factors varying between 2 and 10% of the average.
> More description in the figure caption.
> Inspiration welcome.
>
> Problem 2.5  Total field was reconstructed using FLATN.
> As noted to Eric earlier, LTESS does not appear to be
> working properly.
>
> Any wisdom you have would be most appreciated.
>
> larry
>
>