[Difx-users] integration time
Walter Brisken
wbrisken at lbo.us
Thu May 18 08:04:53 EDT 2017
See the section on "Conventions" in the DiFX manual:
http://www.atnf.csiro.au/vlbi/dokuwiki/lib/exe/fetch.php/difx/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
>>>>>>>>
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>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>> !=============================================================!
>>>>>>>> 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
>>>>>>>> !=============================================================!
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
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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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--
-------------------------
Walter Brisken
Director
Long Baseline Observatory
(505)-234-5912
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