[Difx-users] integration time
Hagatiana Andrianomena
handrianomena at ska.ac.za
Thu May 18 08:27:53 EDT 2017
Hi Walter,
Thanks for that answer.
Cheers,
Sam
On Thu, May 18, 2017 at 2:04 PM, Walter Brisken <wbrisken at lbo.us> wrote:
>
> See the section on "Conventions" in the DiFX manual:
> http://www.atnf.csiro.au/vlbi/dokuwiki/lib/exe/fetch.php/dif
> x/difxuserguide.pdf
>
> Quick answer: everything in DiFX uses the opposite convention as the vex
> file, so the sign in the .v2d file should be opposite.
>
> -Walter
>
>
> On Thu, 18 May 2017, Mark Kettenis wrote:
>
> 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
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> Difx-users mailing list
>>>>>>>>> Difx-users at listmgr.nrao.edu <mailto:Difx-users at listmgr.nra
>>>>>>>>>
>>>>>>>> o.edu>
>>>>>>
>>>>>>> https://listmgr.nrao.edu/mailman/listinfo/difx-users
>>>>>>>>> <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
>>>>>>>>> Difx-users at listmgr.nrao.edu
>>>>>>>>> https://listmgr.nrao.edu/mailman/listinfo/difx-users
>>>>>>>>>
>>>>>>>>>
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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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>>
>>
> --
> -------------------------
> Walter Brisken
> Director
> Long Baseline Observatory
> (505)-234-5912
>
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