You Better Talk Right About My HST

There are some big stories out about the HST (Hubble Space Telescope) and how one of the instruments, the ACS (Advanced Camera for Surveys) has shut itself down.  What chafes me about the reporting, is that the headlines are calling the ACS the "main" camera.  Grr.  Okay, a little background.  After college, I worked for a cosmology group at UC Berkeley, and my main task was working with HST's 3 primary instruments at the time, the WFPC2 (Wide Field Planetary Camera Generation 2, pronounced wifpik two), NICMOS (Near Infrared Camera and Multi Object Spectrometer), and STIS (Space Telescope Imaging Spectrograph).  If you recall, it was WF/PC-1 that had to be replaced in 1993 because the HST's primary mirror is 2 degrees more spherical than parabolic in its curvature, and consequently could not focus properly.  The WFPC2 has an imaging mirror that corrects for the improper curvature.  The ACS was installed in 2002, and it is most decidedly not the "main" camera in the HST.  It is a third generation camera, has a wider field than WFPC2, but its set of filters is mainly used for broadband imaging, i.e. it's useful for finding objects, but not for studying them, hence the "for Surveys" part.  Take a look at the diagram below:

 

The WFPC2 is in the center of the primary mirror, while the ACS is located behind the mirror, off to the side. Coordinates are polar coordinates relative the solar array panels' rotation axis.  The scale is in arcseconds. Please click on the link above for a full size view of the diagram.

 

So, please, call the ACS the newest camera, the sexiest camera, the widest field camera, but do not call it the main camera.  Even now, the UDF (ultra deep field), which will be a much longer exposure than the HDF (Hubble Deep Field), is being done with the "main" camera, the WFPC2.