The STIS has been recovered on Tuesday July 10th using the redundant "side 2" electronics, and several test exposures have been taken since then. We'd like to take this opportunity to inform the user community about the great news that the recovery has been successful. STIS is in business again!
All significant components of STIS have now been exercised on side 2. All detectors and mechanisms have been operated successfully, and their performance is very good.
On Tuesday July 10th the STIS was brought up for operations on side 2, and the CCD turn-on procedure succeeded right the first time. This was a rather major achievement, since the side-2 electronics involves power management that is quite different from that previously used on side 1.
On Wednesday July 11th, both MAMA detectors were ramped up to full voltage without any problem. The global NUV-MAMA dark rate was high, up to 4800 counts per second, which is about 3-4 times higher than the nominal dark rate. However, this high background was expected: it is a result of accumulation of the phosphorescent excitation in the detector window during the past 8 weeks (the STIS safing period) in which the detector was very cold. That excess excitation (out of equilibrium at the warm tube temperature) will "boil off" over the next few weeks, and we have made appropriate arrangements to schedule NUV-MAMA GO observations only after the dark current will come down to its nominal level. The same NUV-MAMA dark current behavior was seen after the lengthy telescope safing period that preceded Servicing Mission 3A.
As to the CCD detector, the first set of data taken encompassed bias frames, darks and flats associated with an anneal procedure for the CCD. Analysis of these data show that the read noise of the CCD was slightly higher than that encountered before the safing: The last measured bias frames reveal a read noise of 5.1 electrons, whereas a typical value before the safing was 4.4. The "extra" amount of noise is highly correlated and it may be possible to correct it through some sort of frequency filtering. We are currently waiting for more such data to come down to assess whether or not this slight increase in read noise is of a permanent nature.
A small Orbital Verification (locally referred to as mini-OV) program has executed over the past couple days, including a checkout of the Mode Select Mechanism (MSM) pointing and of the relative aperture positions. Analysis of those images is currently underway. The first few MSM positions checked show only very small shifts from the side 1 positions at a level that will NOT require updates to the onboard table. The relative aperture positions, on the other hand, must be known to very high accuracy (small fractions of a pixel), as the target acquisition procedure for STIS spectroscopy includes a blind offset from a reference slit to the desired observing slit. Some small corrections to the side 2 aperture table may be required to achieve the needed accuracy. This was expected, and the scheduling of STIS GO observations in the nearby future has been planned with the necessity for such aperture position changes in mind. STIS will only obtain imaging observations for a few weeks starting Monday July 16th, and full restoration of the spectroscopic capabilities of STIS is nominally planned for the week of Monday August 6th.