[daip] Question about the relation between DPFU and Poly in GC table

[Eric Greisen]

On 06/21/2019 09:28 AM, Cui Yuzhu wrote:
> Dear Eric,
> 
> Thank you very much for your explanation.
> 
> Changing the order of DPFU and Poly means: for example, I create two GC 
> tables, GC1 and GC2.
> in GC1, DPFU=0.01,Poly=1;
> In GC2, DPFU=1, Poly =0.01.
> I expect these two GC table will bring same result. But in reality, they 
> are different.
> 
> To test this, in page 2, I created 19 GC tables in this way.
> After calibration in AIPS, I output the data and load them to DIFMAP to 
> check the gain factor by GSCALE.
> The offset is the gain factor obtained for each data with different GC 
> table.
> 
> For GC 1 to GC 10, they are all with fixed poly =1. I changed DPFU. The 
> amplitude is inverse
> proportion to DPFU (G in the formulae in page 2). That is consistent 
> with the expectation.
> 
> For GC 11 to GC 19, they are all with fixed DPFU=1. I changed Poly. But 
> the results are same.
> So I guess when we have only one parameter in Poly which represent the 
> DPFU is constant with the elevation,
> AIPS will scale the poly to 1 no matter what value it originally is.
> 
> Is this right?
>

I have looked at the code that I recommended to you.  In it, there is no 
normalization of the polynomial.  It returns a gain that is the product 
of the sensitivity (DPFU in your nomenclature) and the polynomial and so 
should return different numbers for your 19 GC tables

You say "after calibration in AIPS" - this includes rather more than 
just APCAL (which is the only task to use the GC table).  To see what 
changing the GC table does, you should examine the amplitudes in the SN 
table produced by APCAL.  It might be instructive to examine the 
amplitudes in the final CL table as well - which will include all of the 
other calibration tasks.  ACCAL and ACSCL also modify the gain so the 
latter might compensate for a change in the GC table.

Going to DIFMAP for some parameter "GSCALE" is confusing to me.  Use 
SNPLT to look at the SN and CL table gain amplitudes.

Eric Greisen