The STIS team routinely monitors the readnoise of most GAIN, binning, and amplifier modes available with the STIS CCD by measuring the rms value of bias images. Monthly monitors are posted at the STIS monitoring website: http://www.stsci.edu/hst/stis/calibration/Monitors/Read-Noise. Recently, it was discovered that the readnoise suddenly increased for GAIN=1 from 5.7e- to 6.2 e- on the STIS CCD between 5/14/2014 02:57 UT and 5/15/2014 00:54 UT. There does not appear to be any correlation with known telemetry values or the CCD housing temperature between these times, but more detailed investigations are ongoing. Similarly, the jump does not appear to be due to a change in the CCD amplifier gain or an increase in any fixed-pattern noise.
Other sudden jumps in STIS/CCD readnoise for GAIN=1 have occurred in the past. After SM3A in 1999, the GAIN=1 read noise permanently jumped from 4.0 to 4.4e- (see STIS ISR 2009-02). After SM4, the read-noise had also increased by 0.2 – 0.3 e- from its 2004 levels. There was also a larger increase of about 1 DN when the switch from side-1 to side-2 for STIS was made in 2001, but that was due to semi-coherent electronic noise that can in principle be filtered out, and that case appears different from the other read-noise jumps.
The biggest potential impact is for pending STIS/CCD programs that use GAIN=1 where they have multiple reads of point sources (or spectral regions) with low S/N per individual read. Most users may see a degradation in their S/N of a few percent, within the quoted 10% accuracy of the STIS CCD ETC. If any PIs are concerned about this impact on their programs, they are encouraged to get in contact with their program point-of-contact or the STScI help desk as soon as possible. The STIS team will be updating the ETC in the next few weeks, as well as the relevant reference files for CalSTIS.