[GMVAtech 5] Autocorrelation test using ALMA mode at Yebes

[Pablo de Vicente]

Hello

   Javier and me have been doing some tests to check if the ALMA mode 
was working properly at Yebes. Last Friday we tracked W3OH and recorded 
two scans with different LO setups to get an autocorrelation spectra of 
the 22 GHz maser. See the first attachment.

   We were using the ALMA mode with the 22 GHz receiver because the H2O 
maser is quite bright. Our first setup was setting the LO frequency to 
get the line close to 250 MHz in the base band. The result of this setup 
is in the second attachment. In order to check if the line that we 
detect was the real maser we moved the LO 100 MHz up and got the third 
figure that I attach. We expected the line to come up close to 150 MHz 
in the baseband.

   In order to have a good snapshot of the line profile from "m5spec" we 
increased the number of spectral points to 32000. The line profile is 
very similar to what we get from our FFTS except that the line is 
reversed, which is something that we expect since the signal is coming 
in the second Nyquist zone when entering the DBBC2 and the whole band is 
reversed. Therefore when increasing 100 MHz the LO, the line is shifted 
towards upper channels (~ 350 MHz) in the DBBC2 autocoreelation 
frequency base band whereas using the FFTS, our single dish spectral 
backend, the line is shifted to lower channels (~ 150 MHz) in the 
frequency base band. I hope I have not confused you with my text.

Commands used:

Immediately after recording two of these with a different LO setup:

   mk5=scan_set=::+20000000
   mk5=disk2file=/home/oper/data/systest.vdif:::w

At the Flexbuff:

   cp systest.vdif alma_h2o_centered.vdif
   m5spec systest_alma_h2o_centered.vdif VDIF_8000-4096-2-2 32000 4096 
spec_alma_h2o_centered.out

   cp systest.vdif systest_alma_h2o_shifted.vdif
   m5spec systest_alma_h2o_shifted.vdif VDIF_8000-4096-2-2 32000 4096 
spec_alma_h2o_shifted.out

  Cheers

  Pablo.

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