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December 10: PROBA2 Off Pointing

  December 10: PROBA2 Off Pointing  
  PROBA2 Off Pointing  


Click on the above image for a movie showing what happens when PROBA2 off-points during times of solar activity



One of the things that makes SWAP unique as an EUV imager is that, while it is a full-sun imager like EIT on SOHO and AIA on SDO, it is not constrained to point constantly at a single point on the sun. Instead, PROBA2 can off-point, shifting its field of view to track interesting solar features or other unusual events, as we did a few weeks ago during Comet ISON's perihelion passage. As was the case with ISON, these off-pointing campaigns are not always successful, but when they are, they offer a chance to do some really special and unique science that no other EUV imager can do.

In fact, science is not the only reason to do off-pointing campaigns, however. Off-points can also be used to gain new information about the SWAP telescope and camera and to help calibrate the instruments on PROBA2. In the case of the off-points in the movie linked above, the goal was to measure SWAP's flat field. The flat field of a telescope can be used to correct defects in the images due to optical or electronic imperfections or the presence of dirt or other contaminants in the optical system. In SWAP's case, the big problem that has to be corrected is shadows cast by the metal grid that supports its two EUV filters. These shadows are hard to see in normal SWAP images and movies, but can be very distracting in some specially processed images, so they have to be corrected.

The problem SWAP scientists face is that, while it is possible to measure the flat field of a normal, ground based optical telescope by taking an image of an illuminated screen or even the evening sky on a clear day, there are so such tools available for SWAP. So, instead, we conduct complicated off-point campaigns from time to time to make these measurements. By imaging the Sun repeatedly over a short time, with the image Sun in many positions on SWAP's camera, we measure how the Sun's appearance changes as its image falls on different parts of the camera. By making many of these measurements and combining them using computer software, we can measure — and therefore correct — SWAP's flat field.