This is NICMOS focus update as of September 4, 1998. Analysis of the data shows that focus in camera 3 may have started to recover from a downward trend seen since the end of May. Focus in that camera may have been impacted by extended periods of low Sun-angle HST pointings we had during this summer. Low angles seem to result in lower NICMOS fore-optics temperature. The latter is known to correlate with focus position, so low temperatures may have caused more negative focus. However, it is not clear if this is the only or even main cause of the recently observed focus behavior.
The measurements for cameras 1 and 2 have yielded a focus position very close to nominal. With fewer focus measurements for these cameras, it is hard to compare recent focus systematics in these cameras with those we have seen in camera 3.