Re:Bright Sat Photometry

From: Greg Roberts (grr@telkomsa.net)
Date: Tue Sep 15 2009 - 09:24:17 UTC

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    Morning
    
    As already indicated by several people the estimation of satellite 
    magnitudes is a rather hit and miss affair. There is no accurate way in 
    which an amateur can estimate the exact magnitude of a satellite - Im 
    talking about accurate to say 0.1 magnitude or better. Experience enables 
    some observers to give a good estimate but I would rate this around 0.5 
    magnitude accuracy. The magnitude depends on a large number of factors- 
    phase angle being one of the more pronounced. Then there is the question of 
    are you wanting to work with "live" views or captured images.  I have a 
    technical paper somewhere where NASA used a largish telescope - I think it 
    was a 16 inch - to try and determine the photometric brightness of the ECHO 
    1 100 foot diameter balloon satellite and they used proper photometric 
    equipment and, if memory correct, the results were not very encouraging.
    
    MAXIMDL will indicate the fluctuations in brightness along a satellite trail 
    but will not give an absolute value. MAXIMDL is pretty expensive so I would 
    instead recommend a program called Makali'i produced by the National 
    Astronomical Observatory of Japan - it will do exactly what MAXIMDL does and 
    is FREEWARE. (I dont believe in supporting expensive programs when a cheaper 
    or freeware option is possible.)    The program may be downloaded from 
    www.nao.ac.jp/E/Education/Makali.  There is an online registration 
    requirement but that is merely a formality and one is free to use the 
    program. It is really intended for FITs images but will do what you want 
    with JPG's. In addition its a relatively small file.
    
    The astrometry program APEX also provides photometric magnitudes of 
    satellites but again I would not rate it more accurate than perhaps 0.5 
    magnitude at best.
    
    The estimation of visual magnitudes is somewhat similar to that of observers 
    of variable stars - you try and match the brightness of the variable star 
    against several stars of known magnitude in the region of the star. Skilled 
    variable star observers can probably get magnitudes to 0.1 magnitude but the 
    stars of known magnitude in the area of the star have to ideally match the 
    same spectral class as the variable which invariably ( no pun intended!) is 
    a "red" star, so trying to use a young blue star as a comparison for an 
    ageing red variable star is not a good idea.  So for a satellite- in real 
    time when it appears as a "stellar source" you should use comparison stars 
    of a similar spectral type . What is the spectral class of a satellite ? --  
    I dont know if there is a single answer for this - it shines by reflected 
    sunlight so is it a "yellow" spectral class like the sun ? -
    
    In trying to use photographic or video means for matching magnitudes one has 
    to take into account the spectral response of the recording system - eg 
    video cameras are normally most sensitive in the near infra-red. Commercial 
    cameras such as the DSLR are equipped with infra-red blocking filters. 
    Anyway the whole point is that there is no simple practical way to get 
    accurate satellite magnitudes, except perhaps by experience. Ive observed a 
    great many satellites - and done a lot of variable star photometry to 
    milli-magnitude level in my professional career - and I would not reate ANY 
    of my satellite magnitudes as better than 0.5 magnitudes. Any visual 
    observer who says they do better is kidding himself in my opinion.
    
    Cheers
    Greg
    
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