When reducing data of weak sources (F_corr << 1 Jy, e.g. 200 mJy), there are a number of effects that need to be taken into account with respect to the standard EWS data reduction (midivispipe / midipipe) in order to not get biased results. Walter has written a routine weaksource.pro
that contains the procedures / functions meddelay, ph11 and procmed that takes care of this. It is not part of the standard MIA+EWS distribution (neither release nor snapshot as of June 2010), but can be requested from Walter Jaffe.
meddelay
): Another effect in weak sources is that the tracking precision of weak sources isn't as good as for stronger sources (e.g. a brighter calibrator). Since in weak tracks it is more likely that oirGroupDelay picks a wrong (noise) peak as true zero OPD (and not all of them will be flagged by oirAutoFlag due to excessive OPD or excessive differential OPD), you will at the end sum up some frames that are derotated by a wrong groupdelay (namely the one that corresponds to the noise peak in that respective frame and not to the true, maybe very weak, zero OPD peak). This will effectively decrease your signal, leading to a too low correlated flux. This bias can be partly removed by running a median filter over the delays found by oirGroupDelay. On the one hand, this improves the tracking for the weak sources by being less sensitive to individual noise peaks found by oirGroupDelay. On the other hand, by applying the same method to the calibrator, it will effectively deteriorate the tracking for the calibrator leading to the same effect as described above in the calibrator, thereby making the data better calibratable. The median filtering routine calls oirAutoFlag after it did the median filtering and produced a new groupdelay.fits file, see the Processings steps and Diagnosis of Fringe Tracking pages for more details. Taking care of this effect should effectively increase the reduced correlated flux.
ph11
does the following: after de-rotating with a crude groupdelay (found by standard data reduction) and med-delaying, runs groupdelay again with larger value of gsmooth.