<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div style="font-size: 14px;" class="">Hola,</div><div class=""><p class="MsoNormal" style="font-size: 14px;"><o:p class=""></o:p></p><p class="MsoNormal" style="font-size: 14px;"><o:p class=""> </o:p>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:</p><p class="MsoNormal" style="font-size: 14px;"><o:p class=""></o:p></p><p class="MsoNormal" style="font-size: 14px;"><o:p class=""> </o:p>“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”</p><p class="MsoNormal" style="font-size: 14px;"><o:p class=""></o:p></p><p class="MsoNormal" style="font-size: 14px;"><o:p class=""> </o:p><span style="font-family: Times;" class="">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</span><span style="font-family: Times;" class=""> </span><span style="font-family: Times;" class="">-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.</span></p><p class="MsoNormal" style="font-size: 14px;"><span style="font-family:Times" class="">I hope you can take a look
into this problem and correct the task. Thanks in advance.</span></p><p class="MsoNormal" style="font-size: 14px;"><span style="font-family:Times" class="">Cheers,</span></p><p class="MsoNormal" style="font-size: 14px;"><span style="font-family:Times" class="">Salvador Curiel<o:p class=""></o:p></span></p>
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