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

[Lupin Lin]

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? 


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….

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
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