Converting the Comets Back into Stars
Star or Comet?Yesterday was a very long day at work. I was stuck in meetings with our collaborators for over 6 hours! To make it worse, we spent the entire time discussing a single topic. I even wrote my last paper on it. What could possibly be so captivating, you ask?
Remember the solar wind I wrote about a few weeks ago? This stream of protons does more than create comet tails and aurora, it also destroys all of those fancy electronics we work so hard to put into orbit.
The protons streaming from the sun carry a lot of energy, and they leave a lot of this energy behind as they pass through satellites and astronauts that don’t have the Earth’s atmosphere to protect them. The energy released wrecks havoc on the system, throwing electrons and atoms around like a game of ping-pong. This is one form of radiation damage.
Definitely a comet!
This radiation damage is harmless over short periods of time, much like an occasional X-ray at the dentist. However the solar wind becomes a problem for something like the Hubble Space Telescope or our proposed satellite SNAP which are exposed for many years.
To understand how a telescope degrades from exposure to radiation, let me give an extremely quick explanation of how we gather astronomical images. A telescope is very similar to a camera you buy in the store. The large mirror is equivalent to the lens on your camera. The part that suffers the most radiation damage is the Charge Coupled Device, also known as a CCD.
The CCD is essentially the same as the 8-megapixel chip in your digital camera. This serves as an electronic version of film, recording the image through the photoelectric effect rather than through a chemical reaction. If you can still remember how photography was in the days of film, I'm sure you can appreciate the relief of going digital. Astronomers realized this early on and were pioneers in the use of CCDs.
The photons from the subject of the photograph collide with electrons in the silicon of a CCD, knocking them free from their parent atom. The free electrons are then collected in a well near the site of the collision. Once the exposure is complete, charge is moved one well (or pixel) at a time toward a transistor which then reports the number of electrons found. This process is usually described through the analogy of a bucket brigade passing buckets of water from a reservoir to a fire.
When the CCD is brand new, the bucket brigade performs almost perfectly. If I want to observe a star, the image comes out crystal clear. However, after enough time in space and in the solar wind, the CCD begins to show its wear. The bucket brigade gets sloppy at work and has to contend with an increasingly difficult obstacle course, spilling a little bit of water (or electrons) during each transfer. That same star now leaves a trail of charge behind and begins to look more like a comet.
Now, if I am observing a star, I want my image to look like a star, not like a comet. Is that really too much to ask? Unfortunately, the CCD will inevitably deteriorate in space and astronomers have to find ways to predict and correct for this deterioration. This is what we spent yesterday discussing. We passed around some pretty good ideas but still have a bit of work to do before we can prove a new method for correcting the images. I just hope we it figured out before our satellite launches in 2015!
Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.
latitude: 37.8768, longitude: -122.251
4 Comments
Kyle,
Is there any way to utilize Gore's theory of a lock box… to sequester the CCD from the offending protons?
Hope that a trip to Colorado in March is in your future!
btw, you're looking a little green in your blog photo… Is everything OK with you? (I mean physically speaking, not emotionally, etc.)
i was thinking the same thing as mike. we don't die from exposure to solar winds because the atmosphere protects us. can't you just put a shield around the telescope that always faces the sun? maybe that is what you guys are planning already, or maybe something even more clever? I'd be curious to hear the proposed solutions.
Kyle,
Is it possible to have a redundant CCD system, whereby multiple back-up CCDs are kept shielded, and can be rotated into position to gaze at the stars when the primary CCD has been damaged?Certainly multiple back-up CCDs preloaded into a telescope would be cheaper than space missions to replace them after the fact.
I think it would be difficult to shield the actual CCD you are using for imaging purposes because you would effectively decrease its sensitivity for the radiation you are trying to detect.
My two cents,
Dave Winger
Somehow I didn't ever see the first two comments, and now that I see the third it's clear that I missed one important point:
There is some level of shielding from any spacecraft. We at LBNL would make Al Gore proud, our satellite is designed to have an average of 5 cm of aluminum shielding the CCDs, even when they are taking an image. This shielding is made possible by a series of mirrors, the CCDs face the mirrors and not directly into space. However, this shielding is not enough to stop the offending protons. Even if we had a backup, or redundant CCD laying in wait, that device would still degrade.
You could make the shielding infinitely thick, that would stop the protons and introduce all of the obvious problems. We are constantly flirting with the weight and size limits in our design, right now blocking 99% of the solar protons with our shielding model. However, there is still a lot of energy left in the remaining 1%.
We've actually developed other ways to deal with that last 1%. Instead of building shielding past the point of diminishing returns, we work on the design of the CCDs. We now use a device with implants that are less likely to create the defects in the first place. There are several other tricks we can play, we can change the temperature, change the readout speed, change voltages-all different ways to minimize the effects of clumsy bucket brigade in the silicon.