[Difx-users] vex2difx freqId list

Thu May 17 10:54:35 EDT 2018

Hi all,

Just to confirm what has been stated: freqId is a blunt tool which works 
most predictably in situations where the array is homogeneous and zoom 
bands are not used.  In general, whenever it is used, the output files 
should be checked carefully.

 	-Walter

On Wed, 16 May 2018, Jan Wagner wrote:

> Hi Cormac,
>
> thanks for your reply! Yes freqId is global, I think it refers to
> the frequency table in difx.
>
> In the end I avoided vex hacking by experimenting more with the
> freqId list. Started with 0 and added new items one by one until
> things broke.
>
> And it was a user mistake...
>
> The freqId takes frequency ids, but I was looking at the subbands
> reported by vexpeek and vex2difx. There were two bands per frequency
> so "freqIds = 0,1,2,3,4,5,6,7,10,11" (vexpeek/vex2difx subband IDs)
> had to be reduced to "freqIds = 0,1,2,3,5" (freq IDs).
>
> Found later that the list of acceptable frequency IDs is actually in
> the .input file FREQ TABLE. The indices <n> of the first few "FREQ
> (MHZ) <n>" entries, just before zoom freqs in the same table, are
> indices usable in 'freqIds'. I updated the vex2difx wiki with this info.
>
> cheers,
> Jan
>
> Am 16.05.2018 um 14:14 schrieb Cormac.Reynolds at csiro.au:
>> Hi Jan,
>>
>> My understanding of this is that freqId is a global parameter, not
>> station specific, while the frequency lists are station specific,
>> not global. This poses an obvious  ambiguity where different
>> stations have different frequency structures.
>>
>> This has been discussed at a previous difx meeting, but IIRC the
>> fix was not quite trivial and the demand for a generalisation was
>> insufficient to force a change.
>>
>> In your scenario, selecting just freqId=2,3 (or perhaps
>> freqId=11,12) might do what you want,  as I'm not quite sure how
>> the selection logic works in detail, but have often found that it
>> does the 'sensible' thing in poorly defined scenarios.
>>
>> If not, a cheat I have used in the past is to set the frequencies
>> I wish to discard to some nonsense value such that they do not
>> form a correlatable baseline. This vex hacking can be a bit messy,
>> but has a very predictable result.
>>
>> Regards,
>> Cormac
>>
>> Sent from Blue <http://www.bluemail.me/r?b=12976>
>> On 16 May 2018, at 19:27, Jan Wagner <jwagner105 at googlemail.com
>> <mailto:jwagner105 at googlemail.com>> wrote:
>>
>>     Hi again,
>>
>>     got another potential user or interpretation mistake related to v2d.
>>     This cropped up while trying to fix something in v2d+vex for
>>     RadioAstron correlation. Basically we have stations with 16 MHz
>>     recorded bands that overlap only a 1 x 64 MHz band out of VLBA 4 x
>>     64 MHz. The other 3 x 64 MHz should supposedly be thrown away.
>>
>>     We tried to create 16 MHz zooms into just that 64 MHz VLBA band, and
>>     discard the other 64 MHz bands via including only desired bands in
>>     the v2d SETUP "freqId" parameter ("select which bands to
>>     correlate"). However, vex2difx then complains about mismatched
>>     bandwidth correlation. Despite the correct (I think) freqId band
>>     selection.
>>
>>     With 'vexpeek' the original list of bands is:
>>
>>     # 16 MHz stations
>>     # 22236 MHz +16M = 22236 MHz to 22252 MHz
>>     Subband[0]=[2.2236e+10 Hz, 1.6e+07 Hz, sb=U, pol=R]
>>     Subband[1]=[2.2236e+10 Hz, 1.6e+07 Hz, sb=U, pol=L]
>>     # 22236 MHz -16M = 22220 MHz to 22236 MHz
>>     Subband[2]=[2.2236e+10 Hz, 1.6e+07 Hz, sb=L, pol=R]
>>     Subband[3]=[2.2236e+10 Hz, 1.6e+07 Hz, sb=L, pol=L]
>>     # 22268 MHz +16M = 22268 MHz to 22284 MHz
>>     Subband[4]=[2.2268e+10 Hz, 1.6e+07 Hz, sb=U, pol=R]
>>     Subband[5]=[2.2268e+10 Hz, 1.6e+07 Hz, sb=U, pol=L]
>>     # 22268 MHz -16M = 22252 MHz to 22268 MHz
>>     Subband[6]=[2.2268e+10 Hz, 1.6e+07 Hz, sb=L, pol=R]
>>     Subband[7]=[2.2268e+10 Hz, 1.6e+07 Hz, sb=L, pol=L]
>>
>>     # 64 MHz VLBA stations
>>     # 22156 MHz +64M = 22156 to 22220 MHz ; *discard*
>>     Subband[8]=[2.2156e+10 Hz, 6.4e+07 Hz, sb=U, pol=R]
>>     Subband[9]=[2.2156e+10 Hz, 6.4e+07 Hz, sb=U, pol=L]
>>     # 22220 MHz +64M = 22220 to 22284 MHz, keep
>>     Subband[10]=[2.222e+10 Hz, 6.4e+07 Hz, sb=U, pol=R]
>>     Subband[11]=[2.222e+10 Hz, 6.4e+07 Hz, sb=U, pol=L]
>>     # 22396 MHz +64M = 22396 to 22460 MHz ; *discard*
>>     Subband[12]=[2.2396e+10 Hz, 6.4e+07 Hz, sb=U, pol=R]
>>     Subband[13]=[2.2396e+10 Hz, 6.4e+07 Hz, sb=U, pol=L]
>>     # 22460 MHz +64M = 22460 to 22524 MHz ; *discard*
>>     Subband[14]=[2.246e+10 Hz, 6.4e+07 Hz, sb=U, pol=R]
>>     Subband[15]=[2.246e+10 Hz, 6.4e+07 Hz, sb=U, pol=L]
>>
>>     The ZOOM section has zooms for only the desired one band out of 4 x
>>     64 MHz VLBA:
>>
>>     addZoomFreq = freq at 22220.00/bw at 16.0/noparent at true
>>     addZoomFreq = freq at 22236.00/bw at 16.0/noparent at true
>>     addZoomFreq = freq at 22252.00/bw at 16.0/noparent at true
>>     addZoomFreq = freq at 22268.00/bw at 16.0/noparent at true
>>
>>     The SETUP section has selects only the desired 16 MHz bands and the
>>     one 64 MHz band:
>>
>>     freqIds = 0,1,2,3,4,5,6,7,10,11
>>
>>     However, vex2difx still complains about mismatched bandwidths. The
>>     complaint disappears only when I add three more 16 MHz zooms so that
>>     a piece of each three undesired 64 MHz VLBA bands is included.
>>
>>     Apparently the "freqIds" list is either ignored, or not parsed
>>     correctly, or is checked too late?
>>
>>     The wiki describes the SETUP parameter "freqId" as "a comma
>>     separated list of integers that are freq table indexes to select
>>     which bands to correlate; default is to correlate all. Note: this
>>     should be used to select parent bands for zoom frequencies if
>>     difx2fits is to be run."
>>
>>     From the actual behaviour though it seems that "freqId" does not
>>     actually "select which bands to correlate"?
>>
>>     cheers,
>>     Jan
>>
>>
>>
>>     --------------------------------------------------------------------
>>
>>     Difx-users mailing list
>>     Difx-users at listmgr.nrao.edu
>>     https://listmgr.nrao.edu/mailman/listinfo/difx-users
>>
>
> _______________________________________________
> Difx-users mailing list
> Difx-users at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/difx-users
>

-- 
-------------------------
Walter Brisken
Director
Long Baseline Observatory
(575)-835-7133 (office)
(505)-234-5912 (cell)