Yesterday, in the first space walk of the Hubble Servicing Mission 4, astronauts John Grunsfeld and Andrew Feustel replaced Hubble's Wide Field and Planetary Camera 2 (WFPC2) with the Wide Field Camera 3 (WFC3), thus aiding in the succession of a nearly royal line of cameras. These cameras are some of Hubble's workhorse instruments, and are responsible for many of the pretty pictures you've seen over the past 19 years.
The first Wide Field Planetary Camera went up with Hubble 19 years ago. It's reign was not all that glorious, due to Hubble's mis-shapen mirror. It was replaced in during the first servicing mission in December 1993 with WFPC2, which has special optics to correct Hubble's blurred vision. The difference is shown in the picture of the galaxy Messier 100 below. The image on the left is from WFPC 1, and is roughly as good as ground-based telescopes can do. The picture on the right is from WFPC2, right after installation.
Image Credit: NASA / STSci / JPL
If the cameras are royalty, WFPC2 is Queen Victoria and Queen Elizabeth II rolled into 1. It has worked admirably for over 15 years, taking over 135,000 images. WFPC2 was all but retired in 2002, when the Advanced Camera for Surveys (ACS) was installed. But, in January 2007 (just a few hours after I put in a proposal to use the ACS), the ACS suffered an electronics failure, and its optical camera was shut down. WFPC2 stepped up to the plate, taking much of Hubble's workload that had been destined for ACS (including my own images). WFPC2 images often can be recognized by their Stealth Bomber shape; this shape was due to the optics needed to correct Hubble's vision and the desire to put in a very high resolution detector in one corner of the camera. An example of this unique shape is the famous Hubble Deep Field image.
WFPC2 has been showing signs of age. It is not as sensitive as it used to be due to radiation damage, and certain parts of the image -- if I live as long relative to my life expectancy as WFPC2 has, I'll live close to 240 years. It's deserving of its retirement, and will be on display in the Smithsonian Air & Space Museum this fall.
WFPC2's replacement is WFC3. The "P", standing for "Planetary", was that high-resolution detector in WFPC2. It was designed for taking pictures of planets, but has been used for lots of high-res imaging. The entire WFC3 camera has the same resolution as the planetary camera, so it was decided not to include the "P" in the name. This seems silly to me, as a large fraction of astronomers, myself included, often wrongly refer to the new camera as WFPC3. Whatever.
WFC3 will really shine in the near infrared, or wavelengths of light just beyond what the eye can see. Currently, a camera on Hubble called NICMOS takes those images, but WFC3's sensitivity far outpaces that of NICMOS. An ever-increasing amount of astronomy is done in the near-infrared regime, so WFC3 will be used a lot in this mode.
In a few days, the shuttle astronauts will try and repair the ACS camera. This is a tough repair, because it involves replacing a circuit board. If the repair is successful, ACS will probably keep taking most of the optical images from Hubble, because it has a slightly bigger field than WFC3 and it is more sensitive in the optical. But if the repair doesn't work, WFC3 is more than capable of filling the void. Either way, WFC3 is new, and could operate for a decade or more! Early tests yesterday show that WFC3 is alive; we'll have to wait for the first pictures after the shuttle leaves to see just how good this camera is. I suspect it will be fantastic. Welcome to the Hubble family, WFC3!
In other repair news, the astronauts replaced the science data formatter that failed last fall, leading to the 9-month delay in this mission. The replacement also appears to be alive and ready to work!
As I type this, astronauts Mike Good and Mike Massimo (who you can follow on Twitter) are on the second spacewalk, replacing all of Hubble's six gyroscopes. Hubble needs at least two to point, and NASA prefers to use three. Hubble was down to two gyroscopes, so the fresh set was desperately needed and should keep Hubble going for a long time to come. They will also replace one of Hubble's two batteries. These batteries are as old as Hubble itself (imaging your car battery lasting 19 years!), and were threatening to fail. Again, the new batteries could keep Hubble going for another two decades, if time is good to the rest of the telescope!
For a well-written story on the life of WFPC2, check out this JPL press release.