Re: Aids for visual measurements

From: Greg Roberts (
Date: Tue Mar 10 2009 - 15:23:22 UTC

  • Next message: Alberto Rango: "4541 SATOBS 09 MAR 2009."

    Hi Peter
    Yes one could use an illuminated graticle if you set up on your stars before 
    hand and wait for the satellite to cross the line. One chap in the early 60's 
    used a photomultiplier tube to detect this. The MOONWATCH program also used 
    illuminated graticles in the M-8 and Apogee scopes- was quite sexy having the 
    red line in the field of view that was the graticle. I dont know how practical 
    it would be if you try following a leo and try to line up the graticle etc --  
    things happen so fast one runs out of time and hands thats why I would recommend 
    plotting the expected path beforehand - say in HEAVENSAT- and then chosing two 
    stars to form the "finishing line" and then wait - will work fine for most of 
    the leos provided they have been tracked relatively recently.Im not aware of 
    anyone nowadays using an illuminated graticle - video observers dont need such 
    and the usual binoculars dont have illuminated graticles- at least not the kind 
    I can afford :-))
    A "complication" could be you dont find a finishing line that is at right angles 
    to the orbit track - you then need a graticle that you can rotate - in double 
    stars this would be referred to as the position angle although you dont need to 
    know the value of this in getting a position. You obviously cant have too bright 
    a graticle ( red is the recommended color) as even too bright a red graticle can 
    decrease your "night vision" so you need to be able to adjust the brightness and 
    set it as faint as possible. The other thing to watch out for is to make sure 
    the graticle is at the focal point of the objective otherwise you could have a 
    parallax effect but I would imagine this will be relatively small even if you 
    are slightly off focus. You will also need what is known as a positive eyepiece 
    as the graticle is normally mounted in the focal plane of such an eyepiece.
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