by Sam » Thu Apr 21, 2011 4:14 am
oh_wellian wrote:But, Sam, I have wondered how exactly Hubble does it's Deep Field thing. I assumed that it was a matter of exposure time whilst pointed in any direction.
Indeed, galaxies so far away show up only because of the long exposure time. But there are limits on where one can expose. Since space is so big, and stars so small,
almost any direction given a small enough
field of view will not include a single star. The specific direction Hubble pointed for those images managed to
avoid bright stars and other sources which would have been overexposed over the course of the 10+ day-long exposure. Nonetheless, one can still identify single stars in the image by their diffraction spikes.
http://en.wikipedia.org/wiki/Hubble_Deep_Field wrote:
The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope.
It covers an area 2.5 arcminutes across, two parts in a million of the whole sky, which is equivalent in angular size to a 65 mm tennis ball at a distance of 100 metres. The image was assembled from 342 separate exposures taken with the Space Telescope's Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and December 28, 1995.
The field is so small that only a few foreground stars in the Milky Way lie within it; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known. By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe, with the associated scientific paper having received over 800 citations by the end of 2008.
...
Target selection
The field selected for the observations needed to fulfill several criteria. It had to be at a
high galactic latitude, because dust and obscuring matter in the plane of the Milky Way's disc prevents observations of distant galaxies at low galactic latitudes. The target field had to
avoid known bright sources of visible light (such as foreground stars), and infrared, ultraviolet and X-ray emissions, to facilitate later studies at many wavelengths of the objects in the deep field, and also needed to be in a region with a
low background infrared 'cirrus', the diffuse, wispy infrared emission believed to be caused by warm dust grains in cool clouds of hydrogen gas (H I regions).
What fascinates me here (and which hints at the answer to your pancake question) is learning how incredibly empty space is. Even toward the dense star-fields of Sagittarius, where Pluto currently is, occultations of stars are rare. So much bigger than our experience on Earth:
The typical ride for Owlice
Click to play embedded YouTube video.
---
Sam
[quote="oh_wellian"]But, Sam, I have wondered how exactly Hubble does it's Deep Field thing. I assumed that it was a matter of exposure time whilst pointed in any direction.[/quote]
Indeed, galaxies so far away show up only because of the long exposure time. But there are limits on where one can expose. Since space is so big, and stars so small, [i]almost [/i]any direction given a small enough [color=#0000BF]field of view[/color] will not include a single star. The specific direction Hubble pointed for those images managed to [color=#BF0000]avoid bright stars and other sources[/color] which would have been overexposed over the course of the 10+ day-long exposure. Nonetheless, one can still identify single stars in the image by their diffraction spikes.
[quote="[url]http://en.wikipedia.org/wiki/Hubble_Deep_Field[/url]"]
[b]The Hubble Deep Field (HDF)[/b] is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. [color=#0000BF]It covers an area 2.5 arcminutes across, two parts in a million of the whole sky, which is equivalent in angular size to a 65 mm tennis ball at a distance of 100 metres.[/color] The image was assembled from 342 separate exposures taken with the Space Telescope's Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and December 28, 1995.
[color=#0000BF]The field is so small that only a few foreground stars in the Milky Way lie within it[/color]; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known. By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe, with the associated scientific paper having received over 800 citations by the end of 2008.
...
[b]Target selection[/b]
The field selected for the observations needed to fulfill several criteria. It had to be at a [color=#BF0000]high galactic latitude[/color], because dust and obscuring matter in the plane of the Milky Way's disc prevents observations of distant galaxies at low galactic latitudes. The target field had to [color=#BF0000]avoid known bright sources of visible light (such as foreground stars), and infrared, ultraviolet and X-ray emissions[/color], to facilitate later studies at many wavelengths of the objects in the deep field, and also needed to be in a region with a [color=#BF0000]low background infrared 'cirrus'[/color], the diffuse, wispy infrared emission believed to be caused by warm dust grains in cool clouds of hydrogen gas (H I regions).[/quote]
What fascinates me here (and which hints at the answer to your pancake question) is learning how incredibly empty space is. Even toward the dense star-fields of Sagittarius, where Pluto currently is, occultations of stars are rare. So much bigger than our experience on Earth:
[i][size=85]The typical ride for Owlice[/size][/i]
[youtube]http://www.youtube.com/watch?v=au0Zjn3eB9k[/youtube]
:shock:
---
Sam