Viewing dark objects against a bright background

From: Tony Beresford (
Date: Thu Jan 25 2001 - 03:37:22 PST

  • Next message: Bill T Bard: "Re: Viewing dark objects against a bright background"

    To all,
    The effect called irradiation limits the angular size 
    one can distinguish the silouete of an object against a bright background.
    In the case of the Sun this lower limit on the size is about 1 arcsecond.
    For Geostationary satellite ranges ( 40,000Km) this means about 200 meters
    or yards. So maybe just Milstar but not the ASTRA cluster.
    For the lower NEO's at ranges of 1000KM the lower size limit is 5meters.
    So I think detail would show on the lacrosses , the Keyholes
    and maybe HST. I also think that short exposures would be required
    ( or tracking) to reduce image motion blur to acceptable limits.
    This consideration comes up when considering the chances of seeing
    NEO asteroids against the solar disk. Brought up by Duncan Steel
    at the 1992 National Australian Convention of Amateur Astronomers,
    and shown to be not a useful technique because the asteroids angular
    diameter would be too small.
    Tony Beresford
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    This archive was generated by hypermail 2b29 : Thu Jan 25 2001 - 03:37:48 PST