STEREO - Solar Terrestrial Relations Observatory
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Image artifacts - Internal reflections

Light reflecting inside the telescope optics, and diffracting off edges within the telescopes, can produce some interesting effects. Consider this series of images showing the planet Venus leaving the field of view of the HI1-B telescope between Janary 26-31, 2009. As Venus approaches the edge of the field-of-view, a ring shape is seen apparently coming out of the planet. This is caused by reflections of the bright planet off of the camera barrel. (If you look closely at the full-field version of the January 26, 2009 image above, you'll see a large, faint bubble on the left side of the image, which is also an internal reflection of Venus.) The ring grows progressively larger as time goes by. On January 31, a horizontal streak appears near the position where Venus disappeared. This latter effect is caused by diffraction off of the optical baffles. Click here for a more detailed explanation.

Mercury, Venus, Jan 26, 2009   Mercury, Venus, Jan 27, 2009
The planets Mercury (left) and Venus (right) as seen in the HI1-B telescope on January 26, 2009. Click on the images to see full-sized versions.   Venus leaving field-of-view January 27, 2009. Ring caused by reflection off of camera surfaces.
Mercury, Venus, Jan 28, 2009   Mercury, Venus, Jan 31, 2009
Venus out of field-of-view, but still reflecting off camera surfaces, January 28, 2009.   Diffraction of Venus off baffle edges, January 31, 2009

Here's an image from the HI2-B telescope which shows a number of interesting internal reflections. The brightest object in the field is our own planet Earth, which is responsible for all the reflection effects. Also visible is the Moon, and the planet Mars. A similar event is described on this page.

Reflections of Earth, May 5, 2007
Internal reflections of the planet Earth, as seen in the HI2-B telescope on May 5, 2007. Also visible are the Moon, and the planet Mars. Click on the image to see the full-sized version.

Below is another example of internal reflections in the STEREO telescopes, this time from the COR2 telescope, taken when the extremely bright planet Venus was passing through the field of view. Circled is a faint reflection of the planet. Clicking on the image will bring up a movie showing the reflection entering the field from the right, and then slowly fading away. Also visible in the movie are some debris events.

Reflection of Venus, May 2, 2010
Internal reflection of the planet Venus, as seen in the COR2-A telescope on May 2, 2010. Click on the image to see a movie of the reflection (3 MB).

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