David's Astronomy Pages Sky Conditions Graph - Explanation Back   Home Page

• Airmass of image. Purple Line
  1 is zenith, with higher number indicating positions progressively towards the horizon
  

• Sky Brightness.  Red circles/line.  
  Sky Magnitude in mag per sq. arc sec. Higher numbers (towards top of chart) indicate darkest skies.
  

• Elongation.  Red circles/line.  
  Elongation (long axis length/short axis length). 1 = circular star image.
  

• Mag Zero Point. Green circles/line.   
  For each Clear Filter image this is the average difference between the catalog magnitude of stars and their instrumental raw magnitude normalized to a 1 sec exposure.  Only sharp star-like objects are used in this calculation.  Larger numbers (towards top of chart) indicate more transparent skies .   Lower numbers indicate either a less transparent sky or the formation of condensation/dew on the front lens of the telescope.
  

• ZP. Small green circles
  These are magnitude zero points for individual stars in Clear Filter images. The range is due to a number of factors including catalog magnitude error, star colour, impact of star brightness on S/N
  

• FWHM (clear). Blue diamonds
  There are the Full-Width Half Maximum measurements of individual stars in Clear Filter images.  Only sharpest star-like objects above a S/N threshold are shown. Lower number (towards top of chart) indicate best seeing (most stable atmosphere) and best focusing.  Note that seeing & focusing may drift over an observing session.  
  

• FWHM (RGB). Orange diamonds
  There are the Full-Width Half Maximum measurements of individual stars in Colour Filtered (RGB) images.  Only sharpest star-like objects above a S/N threshold are shown. Lower number (towards top of chart)
  

• FWHM (RGB). Small white circles
  These are objects that fail to meet sharpness & S/N thresholds.  They might include galaxies or low S/N stars 

Example : Sky Condition Graphs

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