Re: Photographing Iridium Flares - Oops!

From: Bjoern Gimle (b_gimle@algonet.se)
Date: Sat Mar 24 2001 - 16:42:33 PST

  • Next message: JAY RESPLER: "Re: think global"

    It seems you mean UTC+3 with summer time:
    Ir#   Date   Local Time Azm El  Rng N SunEl M FlrAng Mag Max Flare Location
    12  01- 3-27 21:02:22.6 126 55  926 D -15.5 R  0.44 -5.2   8.5 km West
    16  01- 3-28 20:56:20.0 125 55  935 D -14.1 R  1.61 -1.0  29.2 km East
    29  01- 3-26  5:20:49.0 012 10 2338 A -22.9 R  1.90  2.1 Does not intersect
    33  01- 3-30 17:31:59.5 249 31 1327 D  25.3 R  0.01 -4.7   0.9 km West
    36  01- 3-26 19:23:17.9 194 61  883 D   3.1 R  0.16 -8.0   2.8 km West
    52  01- 3-29 20:50:15.0 125 54  941 D -12.8 R  3.50  1.6  63.5 km East
    55  01- 3-25  5:35:02.0 009 16 1924 A -20.6 R  3.84  3.9   281 km West
    60  01- 3-25  5:26:44.8 012 11 2282 A -22.1 R  0.67 -1.3  72.7 km West
    84  01- 3-26 21:08:26.0 126 56  918 D -16.8 R  2.55  0.5  47.9 km West
    
    I have seen about ten daytime flares, but missed at least that many!
    Looking at the right spot is essential - use a compass, a spirit level and
    some navigation/drawing tool to check the elevation. Beware of camera+tripod
    affecting the compass! A better method is to locate the correct RA/Dec
    (5:47, +9.1) at night, e.g. the +4 mag star above beta CMi (7:28 +8.95) at
    22:08 (summer time) on the 25th, if you can find a landmark and mark
    the tripod's place (or leave the tripod out all night+day!).
    
    (SkyMap plot of CMi + my photo attached to Anthony)
    
    I have photographed two daytime flares, one with a Maksutov type 250 mm
    telephoto lens. It was not focused correctly, so the satellite appears as a
    circle (the retro-reflector on the front corrector limits the aperture) This
    one I viewed at the same time. The other one was with a zoom, set to 85 mm.
    I pressed the shutter release at the second predicted, but didn't see it -
    but it appears as a dot on the picture! I took one as soon as possible
    after, and it doesn't show the dot. The exposure must be set to match the
    daylight sky (I used automatic exposure, but you may want to underexpose 1-2
    steps)
    
    You don't need any tracking, since no stars are visible, and the satellite
    moves only a few meters to a km during the exposure! At night you may want
    tracking to get a more attractive star background, but don't track on the
    satellite - I would think you would like to show the rise and fall of
    brightness, and possibly even some irregularities, so the 300 mm gives too
    small a field at night. Almost any camera could capture a daytime flare, but
    at night you need a camera that can keep the shutter open for many seconds
    without vibrations (cable release). If the sky is dark, you should use at
    least 30-60 seconds for good star images/tracks.
    
    For satellite tracks with a fixed camera, the efficiency of a lens is  focal
    length/(F stop squared), ie 250/5.6/5.6 = 8 for my reflector telephoto, 12.5
    for a 50 mm/2.0
    
    
    
    
    
    -- bjorn.gimle@tietotech.se (office)                         --
    -- b_gimle@algonet.se (home)  http://www.algonet.se/~b_gimle --
    -- COSPAR 5919, MALMA,    59.2576 N, 18.6172 E, 23 m         --
    -- COSPAR 5918, HAMMARBY, 59.2985 N, 18.1045 E, 44 m         --
    
    > >
    > >      As a final comment, can I assume that the length of exposure will
    > > be a function of magnitude? If so, I would love to see some feedback on
    > > exposure times (daylight and night time) as a function of magnitude.
    > >
    
    
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