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Hubble Space Telescope (HST) observed Comet ISON on May 8th, 2013. The telescope tracked
on the comet and kept
it still in the frame, and allowed anything else
in the image to move. 



What are we looking at in this image?


This is a composite, or stacked, image of multiple observations of Comet ISON as seen by Hubble.  


Why are the stars made up of little dashed lines?



The telescope observed the comet a total of 8 times.
During each exposure, the shutter was open for 171 seconds (a little less than 3 minutes). Since the telescope was locked on the comet, the stars "trailed" slightly during each exposure.


Why is the pattern of the dashed lines curved into an "S" shape?



The telescope began its observations of Comet ISON at 11:13am UT (Universal Time). It took 8 consecutive exposures with the last exposure starting at 11:53am UT. During this time, HST was traveling roughly one-half of its 97-minute orbit around the Earth and moving towards, alongside, and away from the direction of the comet. The stars show this parallax effect of the telescope's motion around the Earth.


What is all the white "snow" and faint streaks in the image?



High-energy particles called cosmic rays travel through space and produce bright spots on long exposure photographs and digital images. By taking two or more identical images and combining them, image processing software can remove cosmic rays to leave only objects that are in both images.



What would the stacked image look like if the individual exposures were aligned on the stars rather than the comet?


Can one get the actual telescope data from the
HST archive?


Archival images of Comet ISON are available through the
MAST archive and through High-Level Science Products.