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The Deepest Photo Ever Taken
Astroturtle writes "Astronomers using the Hubble Space Telescope's powerful new Advanced Camera for Surveys (ACS) have taken the deepest visible-light image ever made of the sky. The 3.5-day (84-hour) exposure captures stars as faint as 31st magnitude, according to Tom M. Brown (Space Telescope Science Institute), who headed the eight-person team that took the picture."
Direct link to the full-resolution JPEG. (~4.9MB)
/ a/formats/full_jpg.jpg
http://imgsrc.hubblesite.org/hu/db/2003/15/images
See also the press release with tons of photos. Enjoy your new wallpaper ! :)
Many spacecraft have small jets that push them into different positions in space. Hubble has no jets because the exhaust gas from jets could damage its delicate mirrors. Instead, Hubble uses momentum to move.
When Hubble needs to move to a new target, engineers on Earth radio a signal to the HST flight computer. The flight computer then activates the Reaction Wheels.
Reaction wheels are heavy fly wheels that spin. As they spin, the momentum from their motion causes the telescope to move. There are four Reaction Wheels. By spinning each one at a certain speed and in a certain direction, engineers can point the telescop e anywhere they want.
Here is a link to a higher resolution image.
Hubblesite.org
Astrophotos are commonly made by combining many shorter exposures. Each additional exposure improves the signal to noise ratio yielding progressively greater detail.
As far as color and reciprocity, Hubble color shots are not always as the eye sees them. The famous "pillars of creation" shot for example, presented the light from oxygen ionization in one color, the light from sulfur ionization in another color, the light from hydogren ionization in another color.
The image is not actually a single exposure of 3.5 days in duration, but is actually made from 250 separate exposures taken from Dec. 2 to Jan. 11, 2003. The total exposure time was 3.5 days.
For those who are interested, the original hubble press release is located here.
The site includes the image in a variety of different formats, including a 123 MB tiff file.
CCDs do not suffer from Reciprocity failure like film does. However there are other problems that will turn long exposures into junk (such as cosmic rays as HST is not sheltered by the earths atmosphere!). So many shorter exposures are taken and then coadded to make a 3.5 day exposure.
As they spin, the momentum from their motion causes the telescope to move.
Well, it's techincally a litter different than that. The wheels don't actually cause hubble to translate within a plane. Instead they rotate hubble. By turning the spinning wheels, a torque is exerted on hubble, causing it to rotate.
neurostarModern optical/IR/UV telescopes typically have a large primary mirror, which reflects light back to a smaller secondary, which reflects the light through a small hole in the primary to the detectors. The secondary is supported by little rods. It is diffraction of light by those supports which cause stars to have distorted shapes.
(Astronomers understand the diffraction issues very well... it's usually not a problem; it just looks weird.)
- A friendly neighborhood astrophysicist
3.4 days is the effective exposure time, from stacking many shorter exposures. If HST integrated for 3.4 days without reading out the CCD, the entire chip would be saturated with cosmic rays, not to mention the fact that the Earth is typically in the way for half the orbit(*), limiting individual exposure times to about an hour or so.
:) ]
(*) except for a small patch of sky called the CVZ: continuously visible zone
BTW, if you're keeping score at home, 30th magnitude is 1 trillion times fainter than the human eye can see!
[*shameless plug* Tom Brown is using my thesis code to analyze these data
Liberal (adj.): Free from bigotry; open to progress; tolerant of others.
The best way is to download the processed HST images and see what the count rate is for a faint star. Then multiply by the gain (in the header of the image) which will give you the number of photons detected. A way to guestimate the number of photons is to compare the flux of the faintest star with the Sun. At the Earth's distance the Sun has a flux of 1.36x10^6 erg s-1 cm-2 and the apparent mag of the sun is V=-26.8. If we assume that we have a star with V=31 mag (the 50% completeness level is V=30.7 mag) then the flux recieved from the star is given by: F2/F1 = 100^((m1-m2)/5) where F1 and m1 are the flux and magnitude of the sun and F2 and m1 refer to the star. This gives 1.03x10^-17 erg s-1 cm-2. Convert the ergs into photons by the de Broglie frequency (E=hv) where we assume that a V-band photon has a wavelength of 550nm or a frequency of 5*10^14 s-1. Thus, each photon carries 3.61x10^-12 ergs which gives a rate of 2.85x10^-6 photons s-1 cm-2. So a 3.5 day exposure is 302400 secs and HST has an aperature of 240 cm so we get about 50000 photons at the entrance of the telescope. Remember.. detection of these sources means having a low background so that these photons are not lost in noise! I should also point out that HST does not leave the shutter open continuously for 3.5Hs, instead it takes a series of short exposures that are co-added. I hope this helps (and doesn't freak people out!)
The "streaks" centering on stars are diffration spikes from the secondary mirror support. The colour alternates as different wavelenghts cause different diffration spacings.
The big bright cluster is actually a member of Andromedae (M31). Very impressive! The appearance of fuzziness is because the CCD oversamples the resolution of the telescope - which is necessary for good photometry - if you want it "sharp" then just bin the pixels by 2x2 or 3x3 or whatever looks best!
http://wuarchive.wustl.edu/users/tom/mirrors/hu
is a mirror of the full JPEG - about 5M. Enjoy.
Something I've wondered for a while... what's up with the points coming off the stars?
As was mentioned in another post, those are diffraction spikes from the supports for the secondary mirror.
Newtonian reflectors and classical Cassegrain telescopes support their secondary mirror with "spiders" that produce diffraction spikes. There have been various efforts over the years to eliminate these from that type of telescope. One method is to seal the tube with an optical flat (a flat piece of optical glass) which supports the mirror. The trade-offs include longer times for the scopes to reach temperature equilibrium, distortion from imperfections in the optical figure of the flat, and slight light loss. Other attempts have included the use of spiders with curved support arms, which reduce or eliminates spikes at the cost of slightly degraded overall image contrast.
Other telescope types, such as refractors, Maksutovs, Schmidt-Cassegrains, and Yolo reflectors have no diffraction spikes, but they are all more optically complex (Yolos, for instance, require toroidal mirrors) and are more difficult to produce as a result. Refractors have the added problem of chromatic abberation, which is the fringing of color on the edge of bright objects. Various complex, multi-element objectives have been developed to reduce, or even practically eliminate, this problem. The problems are optical complexity, cost, and light loss. Figuring a 3-element objective lens for a refractor means grinding six optical surfaces with precise curves. Compare that to a Newtonian which has a single parabolic primary mirror and a flat optical secondary.
There are many other telescope types than the few popular types I mentioned here and each have their proponents. Most designs that have survived the test of time can be made to perform well, but each has trade-offs.
You're right, if you take deepest image to mean "image of most distant objects" instead of "faintest objects". However, the Universe is 13.7 Gyr old, not 20 Gyr.
8 _ilc_64 0.jpg
Here's your deepest image then:
http://map.gsfc.nasa.gov/m_ig/020598/02059
That's from the recent WMAP mission, which mapped the cosmic microwave background in exquisite detail, pinpointing the age of the Universe (and many other cosmological parameters) to high precision. You're looking at an all-sky image of the Universe as it looked when it was 100,000 years old, and became transparent for the first time. IOW, you are literally seeing the fires of creation.
Liberal (adj.): Free from bigotry; open to progress; tolerant of others.