[daip] FITLD for different digitizers

[Amy Mioduszewski]

Hi Michael,

So reproduced your finding (ratio=(digicor=1/digicor=-1)):
BR-HN (both antennas recording 1 bit) ratio=0.3441
AR-EB (both antennas recording 2 bits) ratio=0.3441
BR-AR (one bit correlated with two bits) ratio=0.3441

We suspect the error is on the DiFX side, in labeling the baselines 1 or 2 bit. 
    I will be away for the next week, so Leonia was going to pursue this.

Thanks!

Amy

Dr Michael Bietenholz wrote:
> Hi Amy
> 
>   I've got the rest of this correspondence on my HartRAO email,
> so I'm replying using this one (and I am currently at Hart, too).
> 
> In answer to your questions:
> The data set where I stumbled upon the problem was GB072 I think
> Leonia might have been looking at a similar dataset, GB071 whose PI
> was Andreas Brunthaler.  I think GB070 (which is now public domain)
> may be similarly affected, but I haven't confirmed that).
> 
> You learn something everyday - I used UVAVG to do the merging, not
> being aware of VBMERGE.  However the problem with scaling seems to
> happen before then. Basically, the visibility scaling produced
> by FITLD:DIGICOR=1 is the same for 1-bit and 2-bit data in
> a mixed data set, which I think means it must be wrong in one
> of the two cases - see below for a copy of an email I sent Leonia
> detailing what I'd learned so far.
> 
> As far as data, each of the three pieces of GB072 has both 1-bit and
> 2-bit antennas, so you just need one piece to check.  Should you need it,
> you have my explicit permission to  get the GB072 data from the archive,
> which would be far more efficient  than me trying to ftp it to you
> from South Africa.  However, if you need I'm happy to ftp it.
> 
>> Leonia asked me to look into this last week but then I immediately got sick
>> and was out for the rest of the week.  I plan to download your data and
> take
>> a look at it.  I asked Leonia to forward some of your discussion to me, but
>> he seems to have lost it, although he has given me some hardcopies, so I
>> apologize if I ask you some of the same things that Leonia did.
>>
>> Since this experiment was correlated in 3 passes did you load them all and
>> then use VBMRG to merge them?  If not which of the 3 passes are you looking
> 
> Here's what I did to test this FITLD with DIGICORR=1 -> call this GB072_DC.
> (I just used the first correlator pass for this)
> Then also FITLD with DIGICORR = -1 (but nothing else changed)
> -> GB072_XX
> 
> Now I looked at the ratio of visibility amplitudes between
> GB072_DC and GB072_XX.  This ratio should give the "digital
> correction" which was applied by FITLD.
> 
> For this run,  BR and HN recorded with one bit
> AR and EB recorded with two bits.
> 
> I would expect that the "digital correction", ie. ratio of the
> visibility amplitudes GB072_DC/GB072_XX would be different for
> each of these three baselines:
> 
> BR-HN (both antennas recording 1 bit)
> AR-EB (both antennas recording 2 bits)
> BR-AR (one bit correlated with two bits)
> 
> What I found, however, was that for each of those three baselines
> (BR-HN, AR-EB and BR-AR), the ratio GB072_DC/GB072_XX was exactly  0.3441.
>  (For auto-correlations, this ratio was always 0.22,
> although for the autocorrelations it varied by 1% or so, but
> not with any pattern related to whether antennas had recorded
> 1-bit or 2-bit data.)
> 
> Now I only have a vague idea of what the "digital correction"
> is, so I could be wrong in thinking that it should
> depend on whether its one-bit or two-bit data....
> 
> 
>                   michael
> 
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