[daip] AIPS Cookbook-related question

[Samuel Conner]

To Whom it may concern,

    I have a question relating to the initial CALIBration step in which Flux calibrators are CALIB'd with Flux models SETJY'd in the SU table and Phase calibrators are CALIB'd with unit flux models:

    My prior experience with CALIB has been in the reduction of data which was FILLM'd with a very old version of FILLM (a '94 era version). 

    I have recently started using the 31DEC2004 edition of AIPS, and I notice that FILLM now populates the version 1 CL table with gain amplitudes slightly different from unity in order to represent a model of atmospheric opacity.

    Previously, when the CL version 1 complex gains were all (1,0), it did not matter whether one set DOCALIB to 1 or -1 in the initial CALIB steps. BUt now it makes a difference since the gains in CL 1 differ slightly from unity. 

    It appears to me that I should set DOCALIB = 1, GAINUSE = 1 to apply these opacity corrections to the observations in the initial CALIB step, effectively standardizing the observations to "zero airmass" prior to CALIBration. Since I will be interpolating the resulting SN table to a CL version 2 by applying the Gains to CL version 1, it seems to me that, contrarily, if I were to set DOCALIB= -1 in the initial CALIB step , then I would be effectively applying an opacity correction twice, once in the SN table gains and again in the CL version 1 gains when CL version 2 is formed in the CLCAL step.

   I think that I understand this, but am concerned that I may be being boneheaded. I see no mention of this subtlety in the Cookbook; the calibration sections may not have been updated since the introduction of the new features in FILLM. I would appreciate a confirmation that I should set DOCALIB = 1, GAINUSE = 1, or else a correction of my mistaken understanding.  

  I thank you for your patience with me.

Sincerely yours,

Sam Conner