Hi Ed and List, My 2 cents on Tom's extinction question: >Two questions: >(1) Does H-A consider atmospheric extinction when making >their flare magnitude predictions? Chris has already replied that it doesn't; I would add that it shouldn't. My reasoning is that when you're determining the stellar magnitude of some celestial object (or in this case a manmade satellite), you ideally want to compare to other celestial objects in the same general part of the sky. Particularly when making visual observations, the ability to distinguish small differences in visual magnitude diminishes rapidly with distance between the two objects being compared -- irrespective of differences in extinction. If two stars differ by only half a visual magnitude, I think many people would have difficulty telling the difference if those stars were more than 20 degrees apart. But put them side by side and it's easy. The main point of predicting a visual magnitude is to give the viewer some expectation of how bright a pass will be -- relative to nearby objects. If an Iridium flare and Venus are both predicted to be at magnitude -4.5, and both are only 10 degrees above the horizon, then the two should appear the same brightness -- even though both would appear brighter if they were magnitude -4.5 at 40 degree elevation. Ed mentioned a remark in the Iridflar documentation which is actually unrelated to the extinction question: > However, in the Iridflar 2.21 documentation Rob Matson wrote > the following -- which I don't fully understand -- about > Iridflar (and SkyMap): >Improved calculation of satellite lighting condition. Now >calculates lighting the same way that SkyMap does -- 15 km >or 20 km tangent height extinction (depending on season), >with 0.2 or 0.1 degrees of refraction respectively. These remarks have to do with the determination of when a satellite transitions from being sunlit to going into the earth's shadow. To properly calculate the lighting conditions at a satellite, you must account for refraction of sunlight by the earth's atmosphere, and choose a reasonable extinction height -- here defined as the tangent height below which no appreciable sunlight will pass through the atmosphere and back out into space. For spring-summer (determined by sun's declination and satellite's latitude), I use a 20-km tangent height cutoff. At this altitude, visible light transmission is less than 5% for high volcanic extinction. Atmospheric refraction at that tangent height is approximately 0.1 degrees. For fall-winter, the atmosphere is less dense, so a lower 15-km tangent height cutoff is used. Atmospheric refraction at this altitude increases to roughly 0.2 degrees. For either mode, when the highest point of the refracted sun (or moon, if computing moon illumination) reaches the designated tangent height, I consider the satellite to be eclipsed. --Rob ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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