I have a unique view of the Hubble First Servicing Mission because I was at Perkin-Elmer, the company that made the perfectly wrong primary mirror, during the period when they made the wrong mirror. As a result, I know first hand the conditions that team was working under. In this first writing, however, I am choosing to point out something that was nicely featured in the recent PBS program, “400 Years of the Telescope” (http://www.400years.org/). At the time the problem was discovered, the press chose, not wrongly, to feature an early image of M100, a spiral galaxy, showing clearly a very fuzzy image, seen below.
Image courtesy of Space Telescope Science Institute
What they did not point out (at least in my memory), was the much more significant point that because the images were not sharp points, they were spread over multiple pixels and as a result the distance into space from which the Hubble could collect data was, by accident, almost exactly matched to what was already seen from the ground. As a result, there were no new objects to study, fuzzy or not. This was shown briefly on the PBS show. I had earlier assembled the relevant images, which I have been featuring as my “trademark” on all of my presentation title pages for the last many years.
The first image, shown here, was strategically taken from a large ground-based telescope. It was identified as the darkest point in the sky (field with the smallest number of object with a magnitude as low as 23) over the field of view of the Hubble, as seen from the ground. As can be seen, this image shows only a few objects, none of which look particularly interesting. The intent for the Hubble was to move the lowest magnitude images from the 23 as the lowest visible from the ground to around 28. It was said during the design that if space was considered a 1,000 page book, from the ground we are only able to read page one.
The second image represents one of the longest exposures of the Hubble taken specifically of this darkest region from the ground. This was taken with WF 2 camera. This dramatic photo illustrates the success of the Hubble First Servicing Mission. Nearly every scene in this image has only been imaged by the Hubble and each represents an object that is new to science. And to think, this is the darkest region in space!
It is not well known actually that the terminology WFPC2 (“whif-pick” 2) actually refers to seven different cameras, in two groupings. Three of the cameras make up the Wide Field (WF) format and four of the cameras make up the Planetary Camera (PC). This is illustrated nicely in the following figure.
WFPC2 field-of-view (image courtesy of the Space Telescope Science Institute)
There are four cameras involved with the Planetary Camera, for example, to increase the number of pixels that can be applied to the image. A four-sided pyramidal prism is used to combine the focal planes optically. The pyramid physically rotates to switch between the WF and PC format. The “2” meant it was a second-generation camera.
It was very fortunate that it was always planned to replace the instruments as more advanced detectors became available. One of the biggest problems with these very substantial NASA programs is that the technology that the design is based on is, in this case, over 15 years old by the time it is used. A lot happens in 15 years. This was anticipated with the Hubble and interfaces were designed to allow upgrades over the years.
