Large spikes and unrealistic flux levels have been found in a subset of recently calibrated GHRS data. The affected observations were all made using the ECH-A grating in orders 35 and 36. Fortunately, this problem only affects a handful of observations in a few proposals. The PI's of the affected programs have been notified of the problem. A request has been made to have the affected datasets recalibrated and rearchived.
The bug has been tentatively traced to low level IRAF I/O routine used to access multigroup images. The specific problem occurs only when a "magic" higher numbered group is accessed before a lower numbered group. The calibration software, CALHRS, runs into this bug when reading the ECH-A vignetting reference file. CALHRS, because of the bug, is unable to extract good values from the vignetting file when it first accesses group 33 in the vignetting reference file ebe1353pz.r2h. This particular group is used to calibrate orders 35 and 36 of the ECH-A grating. The problem appears to be not only related the order in which the groups are accessed but also to the size of the file's group parameter block.
This bug is in the process of being repaired. As a workaround, the ECH-A vignetting reference file is being reordered and redelivered. By sorting the table in increasing spectral order, we can avoid one of the two conditions that leads to the observed problem. We have verified that no other reference files satisfy the two conditions necessary for failure and thus expect no other datasets to be affected.
The latest version of the vignetting file for Ech-A is fc11607fz.r2h. Using the latest version for your vignetting correction should assure that your data will not suffer from this anomaly.
Any questions, please contact your Contact Scientist.