Hi All, Chris responded to the question: > > Does Heavens-above take atmospheric extinction into account? > with: > We also (for the time being) ignore atmospheric refraction, at least as > far as satellites are concerned, but not when calculating Sun/Moon rise > and set. This is a reasonable approach; neither IRIDFLAR nor SkyMap computes atmospheric attenuation when computing visual magnitudes. The reason is that stars at low elevation angles are just as attenuated as satellites, so magnitude comparisons only remain valid if extinction is applied to both (or neglected for both). When you attempt to estimate an ordinary satellite's visual magnitude without stellar references, you'll probably be lucky to get it within 1 or 2 visual magnitudes. But put a known set of bounding stellar references nearby, and you can improve your photometric accuracy signficantly -- perhaps 0.3 magnitudes, sometimes better. Thus it's the *relative* brightness that's important -- not the absolute brightness. So if you see an Iridium glint low on the horizon (just to the left or right of Vega let's say), and it looks twice as bright as Vega, then you'd know that the glint would have reached almost mag -1 if it had been high in the sky. Your observation would have been photometrically dimmer than this, but there is little practical value in knowing the apparent magnitude unless your goal is to measure atmospheric extinction. Cheers, Rob ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Mon Aug 27 2001 - 11:02:21 PDT