Making Every Photon Count

Last week I went to a talk given by the leader of the Supernova Factory collaboration at LBNL. What is SN factory? This is an ambitious project to study supernovae like never before. I mentioned this project briefly in a previous post , now that they are so close to releasing their results I want to discuss it a bit more.

The main idea of this project is to study several hundred nearby supernovae using an instrument known as the Supernova Integral Field Spectrograph, or SNIFS. This type of instrument is essentially a blend between a traditional imaging camera and a spectrograph.

The resolution in an integral field spectrograph is defined in spaxels instead of the pixels that have become all too familiar with the advent of digital cameras. A spaxel is quite similar to a pixel, there aren’t nearly as many and each one carries at least a 1000 times as much information.

In your digital camera, the light passes through the lens and directly onto the CCD. Each pixel on the CCD counts the number of photons in the red, the blue, and the green. Typically, there are millions of pixels, each counting photons from a slightly different region of the subject of your photograph.

Now imagine that instead of just counting red, green, and blue, that each pixel counts the entire rainbow of light from your subject. Now you have a spaxel. In an intregral field unit, the light passes through an array of microlenses and prisms before landing on the detector. We would call each set of microlenses and prisms a spaxel. The resulting image carries information about every wavelength of light from every region of your target.

Spectrum of the first SN observed with SNIFSThe advantage to an integral field spectrograph like SNIFS is that you gain a lot more information than either an imager or spectrograph alone. With an integral field spectrograph you can basically identify and organize every photon that reaches the telescope.

Specifically designed to observe supernovae, SNIFS is being operated at the 88-inch telescope on Mauna Kea. Spaxels are quite expensive - this particular instrument has only 225. However, this is more than enough to observe the entirety of a galaxy, a supernova, and the background.

The members of the SN Factory have now observed over 100 SNe using this new camera. Last Thursday, I saw the data from the first 25 well-calibrated supernovae and was very impressed. The data showed the evolution of each supernova and the properties of the host galaxy in great detail. I’m sure the supernova community will be equally impressed when they first see these new results.

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.

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