ACS Status

Linda Smith, lsmith@stsci.edu, & David Golimowski, golim@stsci.edu

During the third extravehicular activity of Servicing Mission 4 (SM4), on 16 May 2009, astronauts John Grunsfeld and Drew Feustel successfully repaired the Advanced Camera for Surveys (ACS), which had failed in January 2007. Through a breath-taking series of maneuvers, Grunsfeld deftly removed four circuit boards, one by one, from the Wide Field Channel (WFC) CCD (charge-coupled device) Electronics Box (CEB), and replaced them with a new module (CEB-R). He then attached a new low-voltage power supply for the CEB-R to a nearby handrail and connected the new power harnesses. The aliveness test, which followed immediately, showed that the first-ever on-orbit, board-level repair had been successful—the WFC is back in business! The WFC passed its functional test, and the first bias frames showed an even lower read noise than before the failure.

We had hoped that the High Resolution Channel (HRC) might also be restored by powering from the WFC CEB-R. This restoration was not successful, however, probably because the original failure caused a permanent short circuit in a location that could not be bypassed with the CEB-R.

The Solar Blind Channel (SBC), which was not affected by the failure of the CCD circuitry, has been used without interruption since January 2007.

First light for the WFC was achieved on 13 June, when the barred spiral galaxy NGC 6217 (see Figure 1) was observed as part of a test to measure the cross-talk between CCD amplifiers.

Figure 1. First-light image of the barred spiral galaxy NGC 6217.  The orbital verification program for ACS/WFC ended in mid-July and General Observer (GO) programs have been executing since then.

Figure 1. First-light image of the barred spiral galaxy NGC 6217. The orbital verification program for ACS/WFC ended in mid-July and General Observer (GO) programs have been executing since then.

After annealing (heating and cooling) the CCDs to repair hot pixels due to radiation damage, we began a campaign to optimize WFC performance by using the CEB-R’s Application-Specific Integrated Circuit (ASIC) to vary the bias, clock voltages, and data-transmission timing. The results show that the performance of the WFC CCDs under command of the new CEB-R—in terms of read noise, linearity, pixel full-well depth, and amplifier cross-talk—meets or exceeds the performance of the old CEB with the default settings used before January 2007. As expected, the dark current, hot-pixel fraction, and charge-transfer efficiency (CTE) have degraded after 28 months additional exposure to Hubble’s trapped-radiation environment.

The first set of bias and dark reference files, together with a table of the performance characteristics of each amplifier, have been delivered to the Calibration Database System for use in processing post-SM4 ACS data. More details on the performance of ACS are available at the ACS web site.

In summary, the ACS repair during SM4 has fully restored the imaging and grism capabilities of the WFC. Its performance is equal to or better than that before repair, after adjusting for 28 more months of exposure to radiation. It is clear that ACS/WFC is back and will once again provide spectacular new images and exciting science results.