To take two extremes, astronomers can use a given amount of observing time either a) to image a wide region of sky with shallow photographs, or b) to image a small region of sky with deep exposures in a so-called ‘deep field.’ The Hubble Space Telescope observed its first deep field in 1995. The region was so small, similar in angular extent to that of a quarter coin on the moon as seen from the Earth, that we had to choose carefully the region to train the telescope.
Importantly, we needed to make sure that there were no especially bright objects in the way to obstruct our view of the distant universe. It works like this: imagine that you are on a road trip and see a majestic mountain in the distance which is in need of a photograph. You take the picture through the car window, and then later on wonder why there is evidence of the unfortunate end of an insect’s life taking up a good portion of the picture.
Th mountain is far away, but the photograph is a 2d image which does not know about distances or what you want and registers the events on the windshield as well as the landscape. With this in mind, we needed to choose a region of sky for the Hubble Deep Field that had faint stars in it so we could verify the pointing of the telescope, but not too many stars, and certainly no bright stars or we would be stuck looking ‘at the windshield’ instead of at the magnificent distant scene.
Astronomers knew there would be many results to come out of this Hubble Deep Field. Even so, we were surprised by what we found. There were more galaxy images in the field that were expected, a result we would not have discovered with a deep field of similar exposure from the ground. This is because images from the ground all show only washed-out images of distant objects. This has the effect of the loss of detail, of course, and also in the loss of detection of faint galaxies altogether as they would spear out until they became indistinguishable with the background black sky color.
Fortunately the Hubble Space Telescope is in space, and the view is entirely different. We could see the nearer galaxy shapes in exquisite detail, and could detect faint galaxies in unexpected numbers. There were thousands of galaxies in the Hubble Deep Field, and with a rainbow of colors reminiscent of Christmas tree lights. We learned that there were many more galaxies in the past than there are now, and that these galaxies were all smaller than galaxies today as well. This supported the point of view that galaxies like the Milky Way all came from the merging together of many smaller galaxies in the past.
The Hubble Deep Field also caused an unexpected level of excitement for the public. We were all able to take a moment out from our busy lives and share in the wonder of the distant universe. The next great observatory, indeed the ‘great pyramid’ of telescopes, is yet to come. It is called the James Webb Space Telescope with a launch in 2018.