Space-based telescopes have changed our view of the Universe in the last 25 years and the future looks promising. A number of new observatories and observing techniques, both on the ground and in orbit, promise to broaden our understanding of the cosmos even further.
But there is a problem. One of the fundamental processes in observation is calibrating the instruments involved. Astronomers can easily test their ground-based telescopes with a decent light bulb. But one thing they cannot account for is the amount of light absorbed by the atmosphere, which can be significant.
It’s easy to imagine that this problem disappears with space-based observatories. But these too have to be calibrated. The Hubble Space Telescope, for example, has on-board tungsten light bulbs for just this purpose.
But these also introduce various uncertainties because of things like small changes in the output of a bulb as its temperature changes as the observatory moves in and out of the Earth’s shadow. Neither is there any way of cross-checking Hubble’s measurements of the tungsten bulbs with observations from the ground.
These uncertainties are now placing important limits on some types of observation, says Justin Albert at the University of Victoria in Canada. Perhaps the most important example is the measurements of the expansion of the universe which astronomers make by looking at the brightness of type 1a supernovas in distant galaxies. Better measurements will require better calibration.
Albert says there’s an obvious solution: place a light bulb in orbit that telescopes on the ground can use to work out exactly how much light the atmosphere is absorbing at any frequency. “The addition of man-made calibrated light sources in space to the arsenal of techniques for photometric calibration will provide a powerful new tool for increasing precision in astrophysics,” he says. Today, he outlines the various factors involved in his thinking.
The required light source is surprisingly small. He points out that a standard 25 Watt bulb in a 700 km orbit would be as bright as a magnitude 12.5 star. A tunable laser is another option but this would have to be accurately pointed at any ground-based telescope, thereby increasing the complexity of the design.
No decent light bulb is currently visible in space but there is a spacecraft up there with a laser pointed at the Earth. CALIPSO is a Franco-American satellite designed to measure the vertical profile of clouds and aerosols. To this end, it beams a green laser at the surface and measures the reflection.
Albert’s idea is that measuring this beam on the ground is a way proving the concept of telescope calibration from orbit. And he’s certainly been busy chasing the satellite and photographing the light that it produces using an array of seven cameras spread out over a few hundred metres..
His biggest problem is that CALIPSO’s laser wasn’t designed for the purpose he’s using it for. The beam has a footprint of only a 100 metres or so across. And since the uncertainties in the spacecraft’s orbit are greater than this, it’s hard to put the cameras in the firing line.
Also, the laser fires at a rate of 20 Hz which means that scintillation in the atmosphere becomes a factor (whereas a longer pulse or continuous beam could be time-averaged). Nevertheless, he’s done an excellent job of characterising the problems associated with this kind of work.
But it highlights the difficulty that observatories would have with this techniques. These types of laser observations are only possible when the satellite is directly overhead and only valid for that point in the sky, from that place on Earth, at that point in time.
Clearly, astronomers are going to need something better. A light bulb that can be seen from a wide angle is a good option.
However, reflectors are not. Albert points out that while several satellites have reflectors for laser range-finding, these cannot be used to accurately measure light absorption in the atmosphere. That’s because the reflectivity changes with the angle of incidence but exactly how is not known. Moreover, the reflectivity of these devices changes in time as the mirrors become pitted by micrometeoroid damage.
To carry his research forward, Albert has plans to send more advanced lights up in balloons so that he can better study this problem.
But in the end, the only way to really help astronomers of the future will be to put a light bulb in orbit. Given that the expansion history of the Universe is one of the most important problems in cosmology, perhaps its time to start thinking harder about how this can be done.
Ref: arxiv.org/abs/1101.5214: Satellite-Mounted Light Sources as Photometric Calibration Standards for Ground-Based Telescopes
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