I apologize if I unintentionally introduced the "cloaking" side-track. I was referring to "natural" shading effects of geometry, but I agree that just orbital motion is not sufficient to explain Lacrosse's rapid changes. (Maybe if it needs to rotate the spacecraft, or reposition a SAR panel) > ... > Briefly back to the original topic - my reason for raising it was to point > out that just about anyone can produce a light curve from an electronic > image. And that includes scans of what were originally hard copy > negatives, > transparencies or prints. Your subtraction of sky background, using a strip adjacent to the track, is good. But in a wide angle shot, there are also effects of vignetting, and simple light reduction because 1. the lens area, as seen from from objects near a corner, is reduced 2. the light is spread out because it hits the film/sensor plane at an angle 3. the distance from the lens to the sensor is larger, making the image larger/faster. For a simple lens, all three factors may be near cos(angle), so raise this to a power of three! So these contributions should not be subtracted, but divided! For a 135 film equivalent focal length of 35 mm I find 31.7 degrees to the corner, and cos^3 = 0.615 Also, satellite speed and distance can vary substantionally across the image. /Björn ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Sat Jan 12 2008 - 18:11:19 UTC