[Aavso-photometry] RE: average vs. median

[Tom Krajci]

>From: "Tomas L. Gomez" <tomas.l.gomez at gmail.com>

>...If, instead of taking the average, I take the median,
>how is the error reduced?.

Here's one more consideration for median that can get you in
trouble...changing sky conditions.  If the moon rises or sets during your
series of images, or thin clouds move in or out, or you image over a long
enough time that airmass changes...or amount of light pollution changes...a
median stack of such 'disparate' image sets will not do much good.  If you
want rejection of way-out pixel values, you may want an algorithm like 'mean
of the median half'...where the pixels are sorted high-to-low...some highest
and lowest percentage of this sorted array is discarded (and extreme values
often are the cosmic ray hits, etc.)...and then the mean is taken of the
remaining values.

I hope this helps.

Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/



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