[Difx-users] Question of the loss in Int. time and the phase shift obtained from the correlations

[Adam Deller]

Hi Lupin,

On 19 May 2017 at 13:22, Lupin Lin <lupin at asiaa.sinica.edu.tw> wrote:

> To the experienced user of DiFX or HOPS,
>
> Sorry to spend you some time on two questions.
> The first one is related to the integration time presented on the final
> plot after the auto- or cross- correlations.
> (The DiFX I used is trunk ver. and and the HOPS ver. I used is 3.13)
>
> 1.
> Here the GLT team recorded two signals with 3 noise sources (only 1 is the
> same with a potential delay less than 1 ns) from the same ROACH, and made a
> test in correlation.
> We simply assumed that the Sky (Ref.) frequency is 230 GHz and the data
> were obtained from the same site (both G1 and G2 were assigned to the same
> position of GLT).
> We correlated the data of 5-sec. supported with the zoom-band mode at
> 1,200-1,328 MHz.
> ————(Contents in .v2d file)———
> vex = ext.vex.obs
> mjdStart = 2017y124d05h42m00s
> mjdStop = 2017y124d05h42m05s
> antennas = G1,G2
> startSeries = 7000
> dataBufferFactor = 16
> visBufferLength = 80
> nDataSegments = 8
> minLength = 0.5
> ————————————————
> But according to the attached figure of  “Zoom128_5sec.pdf”, the int time
> is only 4.995 sec.
> What’s the reason to cause a loss of the obtained Int. time?
>
>
This may be HOPS-related: I'm not an expert on HOPS so I can't comment.
But another possibility is that the correlation runs from 054200 to 054205
at the geocentre, and if the recorded data duration was only 5 seconds long
but at the telescope, then the geometric delay will mean than a few
milliseconds of data will be chopped off.  Maybe one of the HOPS users can
chime in on how that integration time number is calculated in the HOPS plot
- is it based on the weight?  If so then my hypothesis is plausible.


>
> 2.
> Following the previous test, I used Jan Wagner’s code to do the band
> subtraction (the original full band is 2GHz) and checked the obtained phase
> of different cases.
> I summarized my tests and my questions in the attached file “phase.pdf”.
>
>
> The 1st case is the same as shown in the attached “Zoom128_5sec.pdf”.
> What’s the possible origin to cause the phase shift?
> I originally suspect that such a shift is originated from the change in
> Sky/Ref. freq, but it might be wrong….
>
>
I'm sorry, but I can't follow what your are trying to do here. I'm not sure
what Jan's code is that you're referring to (is it something that filters
e.g. your 2 GHz band down to 128 MHz in a chosen window?) If that is the
case, what is the input - already quantised 2 bit VDIF data?  That will
lead to nasty re-quantisation errors - you'll notice that the amplitude
goes down, too.

But the root of the problem is: what are you actually ultimately trying to
achieve?  A verification of some kind?

Cheers,
Adam


> Sincerely,
> --
> Lupin Chun-Che Lin
> Supporting Scientist of GLT (GreenLand Telescope) project
> in Institute of Astronomy and Astrophysics, Academia Sinica,
> 14F of Astronomy-Mathematics Building (Rm: 1405),
> National Taiwan University.
> No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan.
> Tel: +886-2-2366-5464 <+886%202%202366%205464>
> Fax: +886-2-2367-7849 <+886%202%202367%207849>
>
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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
fax: +61 3 9214 8797

office days (usually): Mon-Thu
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