Clear sky detector

From: Michael Waterman (
Date: Tue Jan 29 2002 - 18:02:24 EST

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    A few months ago someone said they would be interested in something
    to automatically detect when the sky is clear: very useful for
    satellite watchers in places with unpredictable skies. I did not reply at 
    the time, partly because I was having trouble getting my second CSD (Clear
    Sky Detector) working.
    A recent article (Vol 110,3 2000 = Jun 2000) in the JBAA = Journal of 
    the British Astronomical Association described a CSD (6 below), and 
    summarised several previous methods (1 to 5):
    Num Author      Method                 See
     1 P.L.Manly    Telescope pointed      CUP 1995 Unusual telescopes
                    at Polaris
     2 H.E.Mostert  Light reflected by     JBAA 93(5) p205 (1983)
                    cloud from streetlights
     3 D.M.Brierley Temperature difference JBAA 78(3) p191 (1968) and
                    between air & ground   JBAA 85(1) p30 (1974)
     4 M.C.Ashley & Cloud thermal radiation Proc Astron Soc Australia
       J.S.Jurcevic                           9(2) p334-335 (1991)
     5    -         Satellite weather maps  -
     6 P.Miles      Radiation upward       JBAA 110,3 p143 (2000)
    Number 3 (1974 version) above uses 2 thermistors (which are resistors
     highly sensitive to temperature, typically 4% per deg C) as sensors,
    one close to the ground, and one about 1.5m above ground (with a roof 
    to prevent it seeing too much sky). It is an improvement on the 1968
    A few tests showed that a thermistor with a mirror underneath detects 
    the sky, and one in a ventilated box detects air temperature. When it 
    is cloudy, both are at the same temperature, but when it is clear the 
    sky thermistor is about 4deg C colder.
    In my mark 1 CSD (1974) both are put one unit about 10cm big, with 3 wires 
    going indoors. The mirror (aluminised plastic) was protected by a thin 
    plastic cover which disintegrated and needed renewing every few months. 
    A thicker plastic or glass cover prevented the CSD from working. A circuit 
    of about 20 components whistled when the difference exceeded a chosen 
    value. It lasted several years, after which the mirror was too corroded. 
    I have almost completed a mark 2 CSD, with improvements. The sky 
    thermistor is above a shiny aluminium sheet. The whistling circuit
    (about 20 components assembled by me) is replaced by a computer. A PC 
    analog input is designed to go to a variable resistor in a joystick, with
     the variable pickoff input to the PC. Replace the joystick with the 2
    thermistors in series, with the joining of the thermistors as the pickoff,
    and the PC can read the voltage of the join, which corresponds to a 
    temperature difference: I will use the voltage corresponding to 2deg C
    The only part I have not done is a program which will wake me at certain
    times only if the sky is clear.
    Miscellaneous thoughts and problems.
    (a) Make sure the resistance of the thermistors is not too low (might
        damage the PC) or high (measured voltage erratic). My personal opinion
        is that between 1Kohm and 100Kohm should be OK.
    (b) Check your thermistors by comparing them to a fixed (indoors) 
        resistor, at several widely spaced temperatures. Test with both
        shielded from the sky, and with one exposed.
    (c) When testing thermistors, keep them (and your completed CSD) well 
        away from any heat source. I wasted a lot of time because my 
        thermistors were near a (closed) window.
    (d) My thermistors are quite large, about 2cm long and nearly 1cm diam.
        These take several minutes reacting to a change in sky.
        Smaller thermistors might be better.
    (e) It is convenient if the ratio of the resistances of the two thermistors
        is about the same (say within 5%) at all likely nighttime temperatures.
        If the discrepancy is not too large, it can probably be corrected by
        adding a resistance in series or parallel to one of the thermistors.
        Otherwise the PC will have compare each thermistor with a fixed
        resistor, and use your tables of test data to determine the 
        temperature difference.
    Of the 6 methods above:
    Method 1 might be practical using a long tube to keep the weather away
    from the telescope lens.
    Method 4 has a rotating chopper, which might not survive long -  
    Method 5 is not up-to-date and is not automatic. 
    Methods 4 and 6 have 50 to 100 electronic components, a bit complicated 
    for me.
    Probably the best is to use both 2 and 3, since 2 is effectively a cloud 
    More details of method 3 (1974 version and my variation) and 6 available
    on request.
    Mike Waterman
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