[daip] Re: SDA in VLA datasets

[douglas bock]

Bryan,

My request was made as part of an investigation into some errors in the
data from our observations of the Vela pulsar region in September (the
data you have). I would have liked to check that the data were observed
with the correct flags in the 3rd card of the array file (i.e., T in
columns 13, 15).

However, perhaps you or others can instead tell me:
- that the flags must have been set or we would have seem nothing (or all
the pulsar),
or alternatively
- that their state cannot have given rise to our errors

or perhaps you can shed other light on the situation. So let me describe
it.

We made phased-array mode observations of the Vela pulsar with the
correlator gating on Sep 22, 26, 28, 29 (we phased up on a nearby strong
source, not the pulsar). On Sep 22 we set the phase center on the pulsar.
There was a substantial artefact at the phase center. Our working
hypothesis was that this artifact had something to do with the phased
array mode (i.e. the zero fringe rate might have meant some DC
terms/errors were not being subtracted). Perhaps the pulsar gating mode
was related.

On Sep 26 and subsequently, we pointed 1 arcmin north of the pulsar (after
setting the gate phase on the pulsar). The artifacts moved to the new
phase center. However, the artefacts were not the same on each day.

Furthermore, the gating worked with varying effectiveness on different
days. The best data are on day 2b, when we don't see the pulsar at all
(limit 40 uJy, say). On other days the pulsar residual is several hundred
uJy (the integrated data from the whole pulsar period is about 300 mJy at
this frequency).

Note that the gating setup was different on the two halves of day 2, so
those data are divided into day2a and day2b. The gates were set as
follows (all ms, wrt pulsar peak intensity):

day1:   -5,  +9.8
day2a: -10, +14.9
day2b:  -6, +10.7
day3:   -6, +10.3
day4:   -8, +10.9

the pulsar is known to have emission from -2 to +8 ms (period 89 ms).
Given the above, it seems odd that the pulsar disappeared on day 2b, but
was present for the rest of the time.

see the images at http://astro.berkeley.edu/~dbock/local/velavla/

FITS versions the images are there also. All gif images are to the same
grayscale (-100,+200 uJy/bm).

Thanks for any insights any of you can provide!

cheers

Douglas


[PS: I think have I have now got to the bottom of our sensitivity issues.
We are about 13% higher than the theoretical sensitivity of the
observation (calculated using the measured system temperatures of 60K,
which seem reasonable at C band for elevation=8-10). I think the last 13%
could reasonably be due to scattered sidelobes from either the artefacts
above, the residual pulsar, and/or the strong FRII source subtracted from
the images.]


On Tue, 16 Dec 2003, Bryan Butler wrote:

>
> hi douglas,
>
> eric forwarded your question on to me regarding looking
> at information in the SDA ("Subarray Data Area") within
> uv datasets.
>
> most of this information is thrown out when the data is
> read in to AIPS (well, it is often used to make decisions
> on what to do with data, e.g., flagging or not, but then
> not recorded anywhere within the AIPS uv dataset).
>
> however, it is retained in the archive data file, and i
> can get that information for you if you wish.  is this
> the same vela dataset you had me looking at Tsys for?
>
> i gather you want the information in section D.9 of memo
> 186 ("Array Control Bits")?  you should know that the
> bit that indicates correlator gating is a bit suspect
> (pardon the pun).  i asked ken sowinski about it a few months
> ago (i was doing some debugging of a program to parse those
> bits), and he indicated that it doesn't always map one-to-one
> with what's actually going on in the correlator.  i've
> cc:'ed him onto this email in case he wants to chime in
> here (if i've misinterpreted what he told me then).  here
> is a bit of an email exchange between he and i on this
> topic:
>
> bryan wrote:
> >>  - in the "Array Control Bits" words in the archive, is bit 4
> >>    ever actually turned on?  this is the bit that is supposed
> >>    to be "correlator is gated for pulsar observations".  walter
> >>    tells me that there is no information in the archive that
> >>    indicates pulsars are being observed.  it might just be that
> >>    FILLM knows nothing about this bit (or doesn't use it anyway).
>
> to which ken replied:
> >What is really means is that "this is an observation which gates
> >the correlator".  I suppose that includes some pulsar observations
> >but not necessarily all.  It is needed to force AP software to
> >really use the V_s the correlator reports even though it is not
> >the theoretical V_s expected based on the integration time.  The
> >default is to use the theoretical in case of a difference.
>
> if you let me know the date + timerange that you want this
> information, i can extract it from the raw data archive
> files and send it on to you.
>
>
> 	-bryan
>
> p.s. vla computer memo 186 is superseded by vla computer memo 188...
> most of the information is the same, but some is different.
>