Let’s consolidate this minor dispute between Yorzhik and I on the HDF
(And just for info – the term “HDF” in the astronomical community has come to mean the region of sky that the Hubble did it’s original long look into, as well as the Hubble results themselves from that exposure. So you often see mention of things like the Keck telescope in Hawaii looking at the HDF).
Originally Posted by ThePhy:
The area of sky chosen was not completely blank, and there were galaxies within it that had already been catalogued by telescopes that don’t come anywhere near seeing across the universe.
Yorzhik responded:
No, ThePhy, when that picture first came out it was touted as a picture from a blank spot in the sky and that the objects were from the outer reaches of the galaxy. And no, I don't get my information from only creationist sources.
Since Yorzhik made specific mention of getting his data from other than creationist sources, and he directly countered my claim that the HDF was known to not be completely devoid of astronomical objects before the Hubble was focused there, I asked:
Rather than supply the source for his claim, he said:
Are you denying that it was common knowledge at the time?
Yorzhik, are you saying that you don’t have a specific source, and that you are challenging my claim based on “common knowledge”? I wonder if you were even aware of the HDF prior to say, 5 years ago? Most non-scientists even now don’t know what it is, and even in the scientific community most people outside of astronomy are at best vaguely aware of it.
But our question is not what “common knowledge” is now, or even after the HDF exposure. Did NASA know before they selected that HDF region of sky that it had galaxies visible to ground-based telescopes?
I will leave it to you to dig into the technical reports detailing the rationale behind the HDF selection.
But some relevant background – Prior to the choice of the “blank region” of sky, there was some rather heated debate in the astronomical community over just where the Hubble should be pointed. Some were vying for the field of view to include one or another of the most distant QSOs (quasars) that were known at the time, so that at least something believed to be very very distant was in the field of view.
Since the advent of man looking at the heavens there have been projects to map the sky to the limits of the then-available viewing instruments. Examples – many objects in the sky are given “Messier” numbers, like M31 (The Andromeda Galaxy). These numbers were assigned by an 18th century astronomer, and they were intended to identify things that were visible in early telescopes, but had already been looked at and found to be not worth further investigation (a claim that later was shown to be enormously shortsighted). Messier was encouraging the astronomical community to not waste its time studying indistinct boring unchanging things as it mapped the heavens. Another example – in the 1930 Edwin Hubble himself exhaustively catalogued a limited region of the sky in which he added over 40,000 galaxies to the list.
The density and types of galaxies and objects observed in various parts of the sky has been crucial to understanding about the structure of the universe. Automated telescope surveys have created huge databases of every observable region of the sky.
Now it comes time to decide where to point the Hubble. Do you presume that the selection committee just happened to find the one spot in the sky that had never been deeply surveyed, and since they couldn’t see anything with their backyard binoculars, they said – hey this is it? Probably one of the most important, and expensive, telescope studies ever, and NASA didn’t think it worthwhile to preview that spot of sky to see if something anomalous was lurking just beyond human vision?
Since the object was to look into space as far as possible, it was expected that the hoped-for baby galaxies would be very dim. Rather like a conventional camera, having something bright in the foreground can obscure faint things more distant. So even though the “near” galaxies in the HDF are very distant by usual standards, they were not unknown. NASA was intensely interested in minimizing foreground obscuration, so they wanted to know as much as possible about the HDF field before focusing the Hubble there.
Look at
http://www.stsci.edu/ftp/science/hdf/project/field.html. Notice in that brief article when they talk about the field selection for the HDF, they say:
The field is devoid of bright nearby galaxies, stars, known nearby clusters, and bright radio sources.
Guess how they knew that. Just maybe they actually looked as part of the field selection process?
Now your source again, please.