This photograph, taken in November 2001 during our brief but pleasant stay at COAA in the Algarve, is a tracked time exposure of the constellation Orion and surrounds. Camera Olympus OM-1; 28-80mm Hanimex zoom lens at 28mm; Fuji Super G 1600 film; exposure time 15 minutes. Scanned from a print.
The camera was mounted on a tracking platform, which compensates for the Earth's rotation. Briefly: the main axis points -- very accurately -- at the sky's north pole. Not quite at the Pole Star, Polaris, because it's actually a small distance away from the true pole position. A battery-driven motor then rotates the mount around this axis, once every day.
This image has been heavily processed. Here's the original:
Orion (unprocessed) [Portugal, 20 Nov 2001; Olympus OM1]
Mainly what you're seeing are the lights of Portimao, a coastal resort a few miles from COAA. Sky fog isn't very bad at the observatory, but there is some in this particular direction (though it's pretty much invisible to the naked eye). The main fault was that we over-exposed the film; we should have gone for 5-10 minutes. It was also very fast film (1600), which didn't help. And the aperture was wrong, and the lens wasn't that good, and....
Judging exposure times for wide-field astrophotography is not easy, with many variables all interacting with each other.
You can see details of the processing applied here.
Image detailsPhotographer: Chris
Taken: COAA, Algarve, Portugal, 12pm 20 Nov 2001
Original format: Scanned from a print
Camera: Olympus OM1
Exposure: 15min * ISO1600 * f?
Focal length: 28mm
Photodesk processing: Sky fog removal; gamma; saturation; scaled; clipped
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