[daip] Problems with CLCOR {External}

[Salvador Curiel]

Hola,
 I am having problems correcting the temporal drift of the sources that we are observing with the VLBA. We are observing sources that are close to the Sun (only a few parsecs away). A problem with these sources is that they have very large proper motions (several hundred mas/yr) and their parallaxes are also large (more than a hundred mas). This produces drifts of the sources of the order of 1 mas (or even more) during the observations. I am trying to get the best astrometric results I can get with the VLBA because I am searching for exoplanets using radio astrometry, and the astrometric signals due to the exoplanets are a fraction of milliarc second (less than 1 mas). I know there is a task in AIPS that can correct for this drift. I am using CLCOR with the options OPCODE = ANTP or ANTC, and I include the estimated movement of the source in mas/hr (e.g., 0.083 mas/hr in alpha and -0.013 mas/hr in delta) using the parameters CLCORPRM(8,9). The results I am getting indicate that the correction that this task does is incorrect. As the help of this task says:

 “The source position is shifted by CLCORPRM(8,9) times the time, where the time is relative to zero hours IAT on the start date of the observations”

 We observed the source for about 4 hours, starting close to 23:30 hrs and thus ending the next day at about 3:30 hrs. It turns out that the correction that I get using both options (ANTP and ANTC) is about  -2.108 mas in alpha and 0.252 mas in delta, which would be consistent with an observing track of about 24 hours instead of 4 hours. It seems to me that the correction starts at 0 hours and thus, when the source is observed during an amount of time (let’s say 4 hours), the correction depends on when the observation starts. The right answer will be when the observations start close to 0 hours.

I hope you can take a look into this problem and correct the task. Thanks in advance.

Cheers,

Salvador Curiel
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