A recent image of Comet ISON, recorded by the Hubble telescope, has surfaced online and caused quite a stir. Far from looking like a comet, we see an almost geometric pattern of streaks of light that look far, far different to the image released by the Hubble team.
Understandably, this raises more than a few eyebrows, and in the absence of qualified explanation, lends itself to all manner of interpretation - some quite elaborate! So this week we are going to take a look at the somewhat controversial image you see opposite, and explain exactly what's happening.
In the image above we see something that doesn't look a lot like a comet, and very much not like the beautiful Hubble Heritage image of ISON. We see three distinct nuclei that appear as short streaks oriented in different directions. Why is this?
The Hubble image of Comet ISON is not a single exposure but instead a series exposures taken at different times. Hubble is in orbit around the Earth, so is actually moving through space at a very high velocity. When compiling this image, the Hubble team aligned all of the exposures such that the stars remained fixed in space. This greatly enhances the detail of the stars and galaxies in the field of view and makes the image far prettier to look at. Unfortunately this also means that the comet (which is very much closer to Hubble than it is to the stars) appears in a slightly different location in each of the exposures that are taken, because the Space Telescope never sits still!
Furthermore, the images taken were long exposures -- up to 490-seconds. Any photographer will tell you that a long exposure of a moving bright light will lead to a streak in the image, and that is exactly what we see with the comet. Finally, the streaks are oriented in different directions due to Hubble's elliptical orbit around Earth. Depending on where the telescope is at a given time on this ellipse, it leads to a "streak" that trails in a different direction.
Read the full story at: http://www.isoncampaign.org/potw-sep02