Mirrors Finished For James Webb Space Telescope 115
eldavojohn writes "On August 15th, sendoff ceremonies were held at Ball Aerospace (subcontractor to Northrop Grumman) for the 18 gold-coated, ultrasmooth, 4.2-foot (1.3 meters) hexagonal beryllium primary mirror segments that will comprise the 21.3-foot (6.5 m) primary mirror of the James Webb Space Telescope (JWST). Over 90% of the back material was taken out of these mirrors to make them light enough so that 18 could be launched into space where they must operate at minus 400 degrees Fahrenheit (minus 240 degrees Celsius). The mirrors will be adjusted by computer controlled actuators that are vital to JWST producing high-quality sharp images. The tennis court sized JWST will reside at L2 and is hyped to allow us to see 'back to the beginning of time.' NASA has provided a video of the computer animated metamorphosis with many more videos at the JWST site."
One shot at getting it right. (Score:5, Informative)
During Hubble's deployment a solar panel failed to fully unfold. An astronaut needed to manually extend the panel or Hubble would not have the power to operate. Hubble famously needed a "set of glasses" to correct for a deformation in it's mirror. This was accomplished with a space shuttle mission. In the years that followed Hubble needed gyroscopes replaced and has received upgrade packages to extend it's capabilities.
Webb will be four times further away from Earth than the distance between the Earth and Moon. That will make any effort to repair it more risky than an Apollo moon mission. Webb was almost cancelled for budget reasons. It's unlikely a rescue mission would be conducted if something were to go wrong.
I can't wait to see what a telescope more powerful than Hubble can do. I hope everything goes according to plan.
Re:memories of Hubble (Score:5, Informative)
The Allen Commission found that the null corrector used to test the mirror had a lens installed 1.3mm out of position. Citation The Hubble Space Telescope Optical Systems Failure Report [berkeley.edu] chapter 7.
The mirror was wrong when it left the factory.
Re:memories of Hubble (Score:4, Informative)
hubble was polished fine. It warped when it got to zero g...
No it wasn't, it's well known that the mirror was ground incorrectly due to the measurement instrument being 1.3mm out [wikipedia.org]
Re:yeah (Score:5, Informative)
Here's The scientific case for the James Webb Space Telescope [springerlink.com]. The earliest galaxies are redshifted into wavelengths that the HST can't resolve.
Re:yeah (Score:5, Informative)
Hubble = HST.Look again.
For instance:
The deepest images of the universe include the Hubble Ultra-Deep Field (UDF) in the optical (Beckwith et al., 2003), which reaches AB = 29.0 mag in the I band, HST near-IR images of the UDF, which reach AB = 28.5 in the J and H bands (Bouwens et al., 2005a), and the Spitzer Great Observatories Origins Deep Survey (Dickinson, 2004), which reaches AB = 26.6 mag at 3.6 m. Galaxies are detected in these observations at 6
Or how about this:
Hierarchical Assembly: The dark matter mass function of bound objects at very high redshifts can be uniquely measured in two ways with JWST. First, the dynamics of groups of galaxies or sub-galactic fragments can be used to determine the typical masses of halos (Zaritsky and White, 1994).
These measurements require observations of emission lines in the rest-frame optical, such as [OII] 3727, [OIII] 5007, and H. These are very difficult to measure from the ground when redshifted into the near-IR.
Second, JWST will measure halo masses through the gravitational bending of light. Using this weak-lensing method, ground-based programs have measured the mass within 200–500 kpc of galaxies at redshifts of z 0.1 (McKay et al., 2002) and z 1 (Wilson et al., 2001). Using the superior resolution of HST, these measure- ments are likely to be extended into 30–50 kpc for galaxies at z 1 (e.g., Rhodes et al., 2004; Rhodes, 2004). While there are some hints of variable halo struc- tures for galaxies of different luminosity and total halo mass, the radial penetration of these surveys, and the ability to compare galaxies of different morphologies are
518 J. P. GARDNER ET AL.
limited by statistics. We expect that HST will establish the statistical mass functions for spiral and elliptical galaxies at z 1, but not much beyond that, because of its limited sensitivity and sampling at > 1.6 m.
JWST will extend the equivalent measurements of galaxies to z 2.5 and thus determine the development of the dark matter halos during the peak growth of galaxies and star formation. JWST will require near-IR imaging with high spatial resolution and sensitivity to achieve this greater depth. Background galaxies with a size comparable to the resolution of JWST will be measured at 20 .
The same near-IR sensitivity and resolution will also make JWST superior to those of ground-based facilities and HST for the study of dark matter structures on larger scales, e.g., 1–10 arcmin or 2–20 Mpc (co-moving) at z 3. These volumes measure the clustering of dark matter on cluster or even supercluster scales, and would extend the study of the mass function into the linear regime. The goal of these observations would be to verify the growth of structure between z 1000 (the CMB large-scale structure) and z 2.5, i.e., during the period that dark matter dominated the cosmological expansion of the universe prior to the beginning of dark energy dominance at z 1.
It's not all about the redshift, the paper also describes potentially useful observations in the IR-- planet formation, star formation, etc. And,of course, optical telescopes have a hard time resolving what's behind dust clouds-- ir telescopes can see beyond them
Re:Have they been properly collimated? (Score:5, Informative)
Collimation wasn't the issue, the mirror was incorrectly shaped due to a fault in the QA process where a tool used to measure the sphericity of the mirror called a null corrector was assumed to be set up to spec.
More details here for those who are interested: http://en.wikipedia.org/wiki/Hubble_Space_Telescope#Origin_of_the_problem [wikipedia.org]
Collimation refers to the arrangement, or alignment, of the optical surfaces and lenses in relation to each other.
Re:What about maintanance of Hubble? (Score:5, Informative)
While there have been other telescopes that observed in the infrared, JWST will have a mirror 6x larger than the prevous space-based telescopes that operate or have operated at the same wavelengths (0.8 to 24 microns). This means that JWST will have a factor of 6 better resolution than previous telescopes and be incredibly more sensitive due to the larger collecting area. Ground-based telescopes cannot compete with JWST because of the sky brightness in the infrared making sensitive observations very time consuming. The science drivers of JWST are primarily the high-redshift universe, that is galaxies that were formed shortly after the big bang. This is something Hubble cannot do since it is not infrared optimized (the telescope is quite warm compare to JWST's operating temperature) and has too small of an aperture for the resolution needed.
The lack of future Hubble servicing has a lot to do with the retirement of the Space Shuttles, the only platform that can be used to service HST. Hubble will be kept going as long as possible since it is still doing outstanding science. In the 2020s it is hoped to launch an 8m class optical-uv telescope to truly replace Hubble.
Re:Have they been properly collimated? (Score:5, Informative)