[Halld-cal] [EXTERNAL] FCAL calibration follow-up

[Shepherd, Matthew]

Hi Elton,

> On May 1, 2020, at 7:27 AM, Elton Smith <elton at jlab.org> wrote:
> 
> Hi Matt,
> 
> The reason to use non-linear corrections during the gain matching would be to minimize incorporating non-linear effects into the determined gains, as you point out. However, using non-linear corrections builds in the existing non-linearity corrections, which may be wrong, into the gain factors. In either case, one will be sensitive to the non-linearities at some level. Iterating over applied corrections is somewhat tricky,  which argues in favor of Collin's approach to do the gain calibration with raw inputs, and then make the energy correction.

Colin should clarify, but I don't think Colin uses raw inputs.  At least that is not how the code is setup.  You can check line #135 here:

https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_JeffersonLab_halld-5Frecon_blob_master_src_plugins_Calibration_FCAL-5FPi0HFA_JEventProcessor-5FFCAL-5FPi0HFA.cc&d=DwIFAg&c=CJqEzB1piLOyyvZjb8YUQw&r=qOykDDW07dxNIz2gqzRW_0sRJeff1E30DnojDqga1Jo&m=ZSxvCFbmrwhwoEAuVlSkSYV-YsHgPOxW3OtXD1CV2GI&s=iKV9-Afb0D88NMero_q6cJU4EW5TLb5vEGcALtMouGY&e= 

Note that the pointers photon1 and photon2 are the corrected showers returned by the high-level analysis objects.  Unless Colin has rewritten the DB with a set of nonlinear correction constants to effectively disable the non-linear correction, it will be applied.

This procedure, as evidenced by years of experience now, seems to have worked and stabilized.  I'm just raising this as potential mechanism to generate an apparent position dependence when, in fact, it one just has a uniform energy dependent response.  

Maybe one could argue that the calibration mechanism now would take a true position dependence (e.g., like on that arises from Cherenkov angular distributions) and shoehorn it into an energy nonlinearity.  

My point is more that we should be aware of and look for such effects if we desire to devise a new strategy for calibration as Igal is proposing.

Matt