Saturday, March 05, 2011

It's geosat flare season! (1)

Around this time of the year, just before spring equinox, the sun is moving through the orbital plane of the geostationary belt. As a result, two things happen:

(1) directly opposite the sun, geostationary satellites "disappear"in the earth shadow for a while;

(2) just before that, they can flare brightly (sometimes to naked eye magnitudes).

Last two evenings I spent some time photographing the relevant part of the geostationary belt, using the EF 2.5/50mm (24 x 18 degrees FOV).

Normally, this lens has too small an aperture to capture geosats (with the exception of the very bright Mentor's). But in the geosat flare season, scores of brightening geosats turn up on the images!

Below animated GIF shows Eutelsat W4 (left) and W7 (right). Both are flaring (normally I need the Zeiss 180mm lens to capture them!), and especially W7 becomes very bright near 21:04 UTC (March 4, 2011). The animation has been made using a series of 12 images taken at approximately 2 min intervals (Canon EOS 450D + EF 2.5/50mm Macro @ F2.8, 800 ISO, 10s):

The "wobble" of W7 is not real, but an effect of small changes in the camera tilt over the series (sorry, tripod was on a bumpy field of grass).

Below image is a crop from a single photograph (one of the series that also contained both Eutelsats above) taken around 4 March 20:46 UTC showing Turksat 2A and 3A both flaring, with Turksat 3A being extremely bright (it was visible by the naked eye). Twenty minutes later, both had become invisible due to entry in the earth's shadow:

click image to enlarge

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