I got my peak-up data and I want to double check that it was done right. What should I look for?
Posted by z-Vandana Desai on 01 December 2008 03:03 PM
An onboard peak-up algorithm measures the centroid of the brightest source in the peak-up field. It performs two measurements. First, the centroid of the brightest source in the peak-up array field-of-view is measured at an "acquisition" pointing (ACQ). Next, the brightest source is moved to the array "sweet spot" (SS) and its centroid is measured again. This centroid position is then used to move the science target into the slit. At each position (ACQ and SS), three images (DCEs) are taken and processed onboard to produce the image frame that is used by the peak-up algorithm. This fourth processed frame is always DCE number 0003, and the exposure number is 0000 for ACQ and 0001 for SS.|
Hence, to examine the peak-up data yourself, the first step is to look at the files with exposure.dce numbers of 0000.0003 and 0001.0003. The FITS headers will contain information about the operation of the telescope and the peak-up algorithm (see below). The data in each FITS file shows the combination of the three individual exposures. The processing includes cosmic ray rejection, flat fielding, and background subtraction. The data are in units of DN. To convert to electrons per second, multiply by 4.6 to get to electrons, and then divide by the exposure time given in the RAMPTIME header keyword.
In the processed images, any source that is bright enough for centroiding will be clearly visible. Assuming that the predicted flux of the peak-up target was correct, you will see the peak-up target located near the center of the image. In the ACQ frame, it will be at or near pixel (107, 30) for Blue or (105, 92) for Red; in the SS frame, it will be at pixel (108, 28) or (106, 94) for Blue or Red, respectively. 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 integers coordinates.
It is possible that you will see other sources in the field, as well. It is the responsibility of the observer to have vetted the peak-up field to ensure that the brightest source is the intended peak-up target (see above FAQ). The brightest source in each of the ACQ and SS frames will be selected as the peak-up target by the peak-up algorithm.
Starting with software version S10, the world coordinate system (WCS) for the peak-up image will be provided in the FITS header. The Red and Blue peak-up arrays share a common WCS. The WCS is described in the system CTYPE1 = 'RA---TAN-SIP', CTYPE2 = 'DEC--TAN-SIP', not 'RA--TAN' and 'DEC--TAN'.
Note that with the current onboard software processing, you may see column-by-column variation in the image. This is a known effect (informally referred to as "jailbarring"). The peak-up algorithm has been demonstrated to work as advertised in the presence of this effect. Future revisions of the onboard software will mitigate against this effect.