The National Reconnaissance Office has given NASA two telescopes with mirrors the size of Hubble's. This is potentially a game changer.
The CANDELS survey, which has the largest time allocation ever granted on Hubble -- 902 orbits, or about 3-4 months -- could be accomplished in just a few hours on such a telescope if it were to be outfitted with a camera with a wide field of view.
|The Hubble Space Telescope, seen from the space shuttle|
Hubble's main advantages over ground-based telescopes are:
- It is unaffected by turbulence and clouds in the earth's atmosphere.
- It can detect ultraviolet radiation, which does not penetrate the earth's atmosphere.
- It can observe efficiently at near-infrared wavelengths because the sky is darker from orbit and the view is unaffected by water vapor and other interference from earth's atmosphere.
Hubble's big disadvantages are:
- It has a relatively small primary mirror by today's standards. That means it has less light gathering power. The Keck telescope in Hawaii, with its 10-meter mirror (compared to Hubble's 2.5 meter mirror) captures 16 times more photons per second.
- It is warm. About room temperature. This means that the mirror glows at infrared wavelengths, which limits its observations to wavelength shorter than about 2 microns.
- It has a small field of view. Hubble's Advanced Camera for Surveys has a field of view of about 11 square arc minutes. That's about the size of the period at the end of this sentence viewed from normal reading distance (unless you have enlarged your font). Pretty small. If Hubble exposed on each spot for an hour, it would take it 1500 years to cover the whole sky. -- Correction. About 3000 years, since the earth gets in the way half the time.
|Artist's conception of the James Webb Space Telescope|
slated for launch in 2018.
Okay, so how do we improve on Hubble? To address the first two disadvantages, we would like to build a bigger, colder telescope. That's the James Webb Space Telescope (JWST). It has a 6.5 meter mirror, giving it about 7 times Hubble's light gathering power. And it will be orbiting a million miles beyond the moon, with a giant sunshade that will let the telescope cool to -405 degrees Fahrenheit (30 degrees Kelvin). It will be able to observe infrared radiation out to a wavelength of 28 microns. It will be spectactular for observing very faint, distant galaxies, probing the inner depths of star-forming regions in our own galaxy, and studying planets around nearby bright stars.
But the James Webb Space Telescope's field of view is not much larger than Hubble's. It is designed for detailed observations of small areas of sky, not surveys of large swaths of sky.
Use the right camera for taking a panorama
There are important projects that require a large field of view. For example:
- Finding thousands of distant supernovae to better constrain the behavior of Dark Energy.
- Measuring the subtle effects of weak gravitational lensing on the shapes of millions of galaxies, which provides a powerful cross-check on the supernova measurement and can help determine whether Einstein's theory of relativity is correct on very large scales.
- Using Baryon Acoustic Oscillations as a measuring rod as a cross-check on (1) and (2).
- Searching millions of stars for signatures of earth-size planets via gravitational microlensing.
- Studying the "archeological record" of past star-formation in nearby galaxies by measuring the brightness and colors of individual stars.
- Using measurements of galaxy clustering to study the relationship between galaxy properties and dark-matter halos.
The messy history of Dark Energy telescopes
The problem has been money. Well, money and astro-politics. Well, that and the fact that science is always changing.
The push for a wide-field space telescope started around 1997, with a project called SNAP (the Supernova Acceleration Probe). It was conceived as an optical and near-infrared telescope with a mirror about 2 meters in diameter and a field of view about 0.7 square degrees - 230 times larger than Hubble's. It was geared at studying supernovae and foundered for a variety of reasons: the difficulty of getting NASA and the Department of Energy to work together; difficulty getting support from the rest of the astronomical community given the narrowly focused science goals and the lack of clarity about access to the telescope for astronomers not associated with the project; complexity of the instrumentation that drove up the projected cost of the mission; and finally (and perhaps most importantly) rapid progress in developing competing probes of dark energy (2) and (3) above.
To help understand the tradeoffs in different ways of studying dark energy, NASA and DOE set up a task force in 2005 to look at the competing techniques and try to recommend a path forward to get the most bang for the buck from both ground-based observatories and space telescopes. Their work showed that techniques 1-3 were all complementary and argued persuasively that at least two techniques should be pursued. They defined a "Figure of Merit" for determining how well a specific set of measurements could constrain Dark Energy. Unfortunately, the figure of merit they chose was based on a set of assumptions which may or may not be correct (given that we don't understand dark energy), so the idea that we could just stack up proposed experiments beside each other and choose the one with the best figure of merit was widely rejected. There was a lively meeting on this topic at STScI in 2008. I highly recommend the panel discussion as entertaining and enlightening.
So in the middle of the last decade, NASA and DOE started a competition to select the best ideas for a Dark Energy mission (called JDEM at the time), but then called off the competition when it became clear that the money wasn't going to be immediately available, and when the European Space Agency started getting serious about doing something similar.
In 2010 the Decadal Survey gave the wide-field telescope a new name WFIRST (Wide Field Infrared Space Telescope) and gave it the top priority for a space mission following construction of JWST. The problem is that limited budgets have been limiting astronomers ambitions. The telescope has been whittled down to a telescope about 1.3 meters, which at near-infrared wavelengths provides an image quality that is barely sufficient for studying weak gravitational lensing. In the meantime, the Euclid mission is becoming more and more real, and it is starting to look silly for the US to send up a mission just to come in second on the Dark Energy measurements.
Anyone want a couple telescopes? Not even driven around the block...
So now along come two already-built 2.4 meter lightweight space telescopes. It doesn't take more than a few seconds of thought for astronomers to come up with things for them to do.
|Image of a test-unit version of the new telescopes, taken from|
Allan Dressler's June 4, 2012 presentation to the
Committee on Astronomy and Astrophysics
Credit: National Academies Board on Physics and Astronomy
Ah, but two telescopes, now that opens possibilities. My favorite right now is to do everything possible to get one of them up there fast. Don't try to build an all-singing, all-dancing set of instruments to do lots of different measurements. Just put big camera on board with a good selection of filters and start collecting the wide-area data. Do this as fast as possible. This would be good for measurements 2,4,5, and 6. Not so good for (3), and missing the crucial spectroscopic component for (1) -- although putting in some specialized filters could help.
What about the second telescope? I happen to think measurement (3) -- Baryon Acoustic Oscillations (BAO) -- are extremely important. I also think that measurements of distant supernovae (1) will continue to be useful -- that's part of what we are doing on a small scale in CANDELS. Both require a spectrograph. Measuring BAO involves measuring millions of redshifts; supernovae experts are hoping for thousands of supernovae. Current thoughts on how to do this involve simply dispersing the galaxy light and letting all the little spectra overlap. This has been the plan for WFIRST and JDEM. Cheap but ugly. Micromirrors provide the opportunity block out some of the galaxies and the sky in between. Launching the second telescope later than the first would give the opportunity to try to equip the telescope with a micro-mirror spectrograph, which would really be doing the job right. It would open up huge possibilities for science in other areas, not just measuring BAO. A 2.4 meter telescope is probably overkill for BAO, but it could do the BAO measurements fast and have time left over for lots of other kinds of astronomy.
So make the first observatory a into simple, quick, cheap imager. Make the second telescope a new-technology wide-field spectrograph.
Alternatively, equip the second telescope for ultraviolet observations. JWST doesn't have UV capability. Or go the other direction. If these telescopes can be cooled, then using the second telescope to provide wide-field imaging out to longer infrared wavelengths to complement JWST would also be extremely useful. I've also heard people suggest that one of the telescopes be outfitted with a coronograph for studying extrasolar planets. I'm skeptical that these are the right telescopes for that, but then I don't know much about them.
Will we stumble again?
In my view our entire scientific enterprise (astronomers, NASA, DOE...) has been stumbling along for far too long on deciding what to do for a Dark Energy experiment and debating how much it is worth and how to fund it, and what else to do with it. SNAP wasn't the best conceived mission, but if we had just collectively decided to do SNAP or something like it circa 2000, it might have been in orbit by now gathering great data. Instead, we are still debating designs on powerpoint presentations.
So can we get on with it? Will NASA be able to find or free up enough money to get one of these telescopes up into orbit fast enough to remain competitive with Euclid? Will US astronomers put aside their differences quickly enough? Are there sufficient incentives to motivate the standard NASA contractors to look for quick low-cost ways to get one of the telescopes into orbit? Will the private sector help in any way?