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

Sun May 21 22:12:35 EDT 2017

Dear Adam,

Thanks for your repsonse to give the indication.
For the 1st question, I will further check with the expert of HOPS developer.

For my 2nd question, the Jan’s code is “m5subband” provided in the latest DiFX version to extract the specific frequency range information from a wide/full band data.
The original input is also the 2-bit VDIF file. 
I just compare with "the cross-correlation with two 2-bit wide band VDIF files at a specific frequency range using the zoom-band mode" and "one data subtracted a the same frequency range to cross-correlate another wide band file supported with zoom-band mode.”
A simple expression in the .v2d file is the comparison between:
(XXXXX and OOOOO are data with 2 GHz, and SSSSS is a data with the band of 256 MHz subtracted from XXXXX)
————————————————————————
ANTENNA G1
{
   file = XXXXX.vdif
   format = VDIF/8224/2
   phaseCalInt = 0
   addZoomFreq= freq at 230000.00/bw at 256.0
}
ANTENNA G2
{
   file = OOOOO.vdif
   format = VDIF/8224/2
   phaseCalInt = 0
   addZoomFreq= freq at 230000.00/bw at 256.0
}
————————————————————————  And,
ANTENNA G1
{
   file = SSSSS.vdif
   format = VDIF/8224/2
   phaseCalInt = 0
}
ANTENNA G2
{
   file = OOOOO.vdif
   format = VDIF/8224/2
   phaseCalInt = 0
   addZoomFreq= freq at 230000.00/bw at 256.0
}
———————————————————————— 

The loss in SNR or the amplitude is expected due to the re-sampling (FFT and IFFT for the original XXXXX.vdif) of the data, but we do not clearly understand the origin to cause the "possible shift" in the obtained phase distribution.
(We suspect the shift in phase is due to the adoption of the Ref. Freq., but the series of my tests cannot confirm this point.)

The motivation to do it is that we have limited/very slow internet speed in GLT. 
In order to check the preliminary result via the obtained observation, we would like to send only the band reduced data with a much smaller file size.
So to investigate/handle the comparison between the aforementioned two cases is required for GLT team.

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
Fax: +886-2-2367-7849

> Adam Deller <adeller at astro.swin.edu.au> 於 2017年5月22日 上午8:15 寫道：
> 
> Hi Lupin,
> 
> On 19 May 2017 at 13:22, Lupin Lin <lupin at asiaa.sinica.edu.tw <mailto: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 <tel:+886%202%202366%205464>
> Fax: +886-2-2367-7849 <tel:+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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