[daip] CLCAL extrapolation, or lack thereof, with SELN

[Craig Walker]

Alessandra Bertarini reported to me that, when she uses CLCAL with 
INTERPOL='SELN', using an SN table from APCAL, she does not get 
calibration values for scans that do not have Tsys data, and hence SN 
entries.  It turns out that only one scan on that source had Tsys data 
on source OQ208 so CLCAL would have to make a significant extrapolation 
from that value.  I have a copy of her data set (TASAV version) on 
/DATA/NOATAK_1 in user 450 in
AIPS 1:   48  450 X_1         .TASAV .    1 UV 17-OCT-2008 14:56:32
I tried running CLCAL with GAINVER 1, SNVER 1, with INTERPOL = '2PT', 
'SELN', and 'SELF'.  For '2PT' and 'SELF', the extrapolation is done (eg 
Westford (18) at 0 23 01) while it is not for 'SELN'.

I'm not sure if this is a bug or a feature as you might imagine that 
'SELN' doesn't want to cross scan boundaries.  If it is a feature, it 
should probably be made more clear in the explain file so as to not 
confuse users like Alessandra (and me).

For now, I recommend that Alessandra use 'SELF', which is probably what 
she wanted anyway.  Therefore this is not urgent.

Cheers,

Craig

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     R. Craig Walker            Array Operations Center
     cwalker at nrao.edu           National Radio Astronomy Observatory
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