[Difx-users] integration time
Mark Kettenis
kettenis at jive.eu
Thu May 18 07:54:21 EDT 2017
> From: Hagatiana Andrianomena <handrianomena at ska.ac.za>
> Date: Thu, 18 May 2017 11:46:42 +0200
>
> Hi Mark,
>
> I have used the clock values from SFXC for example:
> # def BD;
> # clock_early= 2017y059d13h00m00s : 216.030 usec : 2017y059d14h30m00s
> : -2.20e-07 usec/sec;
> # enddef;
>
> My understanding is that DiFX expect the rates in usec/sec which is the
> case here, so I have used
> that value in the .v2d file, as for the clockEpoch I used
> 2017y059d14h30m00s here (converted to mjd).
> For sure I am having issues with some values I am using, reason I do not
> find fringes at some baselines.
It is my understanding that DiFX expects proper dimensionless rates
(i.e. sec/sec) so for the example you give above, you'd have to put
-2.20e-13 as the rate into the v2d file. Also, I'm not sure the
clocks in the .v2d file use the same sign convention as the VEX file.
Hopefully one of the DiFX expert can tune in and confirm/deny this.
Regarding the clock rates. It usually is "safe" to set the clock
rates simply to 0 if you suspect they might be wrong. For most VLBI
stations the clock rates are small enough that there is no significant
decorrelation within a couple of seconds. That might not apply to NK
though as I understand it uses a Rubidium clock.
Good luck,
Mark
> With regards to EOP, I used the values from NASA.
>
> Thanks for your reply Richard.
>
> Regards,
> Sam
>
>
> On Thu, May 18, 2017 at 1:08 AM, Richard Dodson <richard.dodson at uwa.edu.au>
> wrote:
>
> > Hi
> >
> > I agree with all the above that the most likely source of a high delay
> > offset is either incorrect position for the antenna in VEX, or a
> > mis-translated value for the initial delay offset. If there are no fringes
> > to any station it is unlikely to be antenna positions (as one being wrong
> > is possible, but some of the ones you are using are very well established),
> > so I put my money on Mark's suggestion. But another possible thing to
> > check, if the above are correct, are the Earth Orientation Parameters. If
> > these are wrong you essentially have introduced an offset in all the
> > antenna positions.
> >
> > happy hunting!
> > Richard
> >
> > On Thu, May 18, 2017 at 12:33 AM, Hagatiana Andrianomena <
> > handrianomena at ska.ac.za> wrote:
> >
> >> Hi Mark,
> >>
> >> Many thanks for that very useful information.
> >> I will double-check the $CLOCK values I was
> >> using.
> >>
> >> Cheers,
> >> Sam
> >>
> >> On Wed, May 17, 2017 at 5:02 PM, Mark Kettenis <kettenis at jive.eu> wrote:
> >>
> >>> > From: Hagatiana Andrianomena <handrianomena at ska.ac.za>
> >>> > Date: Mon, 15 May 2017 08:51:59 +0200
> >>>
> >>> Sam,
> >>>
> >>> Note that if you are using $CLOCK parameters from a correlation with
> >>> SFXC, the rates may not be specified in a form that DiFX expects. If
> >>> the "unit" isn't specified, SFXC will interpret the rate as usec/sec,
> >>> whereas DiFX will interpret the rate as sec/sec.
> >>>
> >>> Cheers,
> >>>
> >>> Mark
> >>>
> >>> > Hi Cormac,
> >>> >
> >>> > Please see attached the vex. I am interested in EF-YS-NK-HH-O8.
> >>> >
> >>> > cheers,
> >>> > Sam
> >>> >
> >>> > On Mon, May 15, 2017 at 7:45 AM, Cormac Reynolds
> >>> <cormac.reynolds at csiro.au>
> >>> > wrote:
> >>> >
> >>> > > hi Sam,
> >>> > >
> >>> > > just following up on Adam's point. The most common cause of a very
> >>> high
> >>> > > residual fringe rate (as yours appears to be) in an otherwise
> >>> functioning
> >>> > > array is an error in the station position(s). If you send the vex
> >>> file you
> >>> > > used, and the stations you are interested in, it should be
> >>> straightforward
> >>> > > to say if that is a likely cause or not.
> >>> > >
> >>> > > cheers,
> >>> > > Cormac.
> >>> > >
> >>> > > On 13/05/17 19:57, Hagatiana Andrianomena wrote:
> >>> > >
> >>> > >> Hi Adam,
> >>> > >>
> >>> > >> Many thanks for your explanations.
> >>> > >>
> >>> > >> Cheers,
> >>> > >> Sam
> >>> > >>
> >>> > >> On Sat, May 13, 2017 at 1:37 PM, Adam Deller <
> >>> adeller at astro.swin.edu.au
> >>> > >> <mailto:adeller at astro.swin.edu.au>> wrote:
> >>> > >>
> >>> > >> Hi Sam,
> >>> > >>
> >>> > >> Integration time will affect the maximum fringe rate you are
> >>> > >> sensitive to. Shorter integrations = higher fringe rate before
> >>> you
> >>> > >> lose signal due to decorrelation. But normally this is not a
> >>> > >> limitation. I would be fairly surprised if a 1 second
> >>> integration
> >>> > >> time was a problem (unless there is an issue with the observing
> >>> > >> setup).
> >>> > >>
> >>> > >> Spectral resolution will affect the maximum residual delay you
> >>> are
> >>> > >> sensitive too. Finer spectral resolution = larger delay before
> >>> you
> >>> > >> lose signal due to decorrelation.
> >>> > >>
> >>> > >> These are not DiFX-specific things, but general to radio
> >>> > >> interferometers / correlators. To know what values are
> >>> reasonable,
> >>> > >> you need to know your instrument and what the expected residual
> >>> > >> fringe rate and delay are.
> >>> > >>
> >>> > >> Cheers,
> >>> > >> Adam
> >>> > >>
> >>> > >> On 12 May 2017 at 23:59, Hagatiana Andrianomena
> >>> > >> <handrianomena at ska.ac.za <mailto:handrianomena at ska.ac.za>>
> >>> wrote:
> >>> > >>
> >>> > >> Hi DiFX users,
> >>> > >>
> >>> > >> I got a question regarding the integration time in the
> >>> setup:
> >>> > >> SETUP default
> >>> > >> {
> >>> > >> tInt = 1.000000
> >>> > >> fftSpecRes = 0.12500000
> >>> > >> specRes = 0.12500000
> >>> > >> doPolar = True
> >>> > >> maxNSBetweenACAvg = 2000000
> >>> > >> }
> >>> > >>
> >>> > >> I got this default setup by running oms2v2d using the .oms
> >>> file
> >>> > >> from the experiment. When running the difx I got no fringe
> >>> but
> >>> > >> from changing the tInt to
> >>> > >> 0.08 and both fftSpecRes and specRes the result seems to
> >>> suggest
> >>> > >> I got fringe (see plot attached).
> >>> > >>
> >>> > >> I am not quite sure whether it is a naive question but I
> >>> wonder
> >>> > >> if there is a simple rule of thumb to get the right values
> >>> of
> >>> > >> those parameters in order to get fringe. I do not have much
> >>> > >> experience with DiFX I must say though.
> >>> > >>
> >>> > >> Regards,
> >>> > >> Sam
> >>> > >>
> >>> > >> _______________________________________________
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> >>> o.edu>
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> >>> > >> <https://listmgr.nrao.edu/mailman/listinfo/difx-users>
> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >> --
> >>> > >> !=============================================================!
> >>> > >> Dr. Adam Deller
> >>> > >> ARC Future Fellow, Senior Lecturer
> >>> > >> Centre for Astrophysics & Supercomputing
> >>> > >> Swinburne University of Technology
> >>> > >> John St, Hawthorn VIC 3122 Australia
> >>> > >> phone: +61 3 9214 5307 <tel:+61%203%209214%205307>
> >>> > >> fax: +61 3 9214 8797 <tel:+61%203%209214%208797>
> >>> > >>
> >>> > >> office days (usually): Mon-Thu
> >>> > >> !=============================================================!
> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >>
> >>> > >> _______________________________________________
> >>> > >> Difx-users mailing list
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> >>> > >>
> >>> > >>
> >>> > > _______________________________________________
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> >>> > >
> >>> >
> >>> > [1/2:text/html Show]
> >>> >
> >>> >
> >>> > [2:application/octet-stream Show Save:n17c1.vex (66kB)]
> >>> >
> >>> >
> >>> > [3:text/plain Hide]
> >>> >
> >>> > _______________________________________________
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> >>>
> >>
> >>
> >
> >
> > --
> > -------------------------
> > Dr Richard Dodson,
> > International Centre for Radio Astronomy Research
> > University of Western Australia
> > P: +8 6488 7842 E: richard.dodson at icrar.org
> >
>
> [2:text/html Show]
>
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