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Knowledgebase: IRS
I think my peak-up failed. How can I tell?
Posted by z-Vandana Desai on 01 December 2008 03:06 PM
There are two ways for the peak-up to "fail".

1. If the algorithm does not find a valid centroid, then it will report a failure. In the peak-up images, particularly the processed DCE (see above), there will be no visible source. You can also identify these failures from the FITS header by looking at the value of the "peak-up centroid quality code", which is 0 for a failure and 1 for a success.

2. The peak-up algorithm can also result in a false positive. In this case, the peak-up software centroids on something other than the intended target and reports a success. You can look at the peak-up images to see if your intended peak-up target is at the centroid reported in the header. The value of the centroid is given in the AXCNTRD1 and AYCNTRD1 keywords; note that these values are in centipixels, so need to be divided by 100.0 to compare with pixel positions in the image. The onboard software considers the lower left corner of the array to be (0,0) with the lower left corner of that pixel (-0.5,-0.5); that is, pixel centers are labeled with integer coordinates.

In addition, the value of the PTGDIFF keyword in the header gives the difference in arcseconds between the requested and reconstructed pointing. This value is almost always less than 1 arcsecond, which indicates that the intended coordinates were placed on either the acquisition or sweet spot.