[Aavso-photometry] average vs. median

[bailyhill at aol.com]

Hello Thomas;

The rule of thumb that I have heard, and I think its in the Maxim 
manual, is that the noise std dev for a normal ccd image will be about 
20% lower for the average, than the median.

Median combine has the advantage that it eliminates outliers, ie like 
cosmic ray hits, etc.

I always use averages, and typically either 36, or 49 images of darks, 
flats and bias'.  This seems to be adequate to do exo-solar planet 
transits at the 1-2% level.  Typical std dev's of the check stars in a 
group of 20-40 images, runs from .003 to .010;  depending upon 
magnitude of the comp star being measured.  Ideally, if its really a 
constant comp star, this gives an error representative of one's whole 
process, not just the "theoretical 1/(S/N)", which is the best that the 
detector can do as a result of an integration.

Hope this Helps
Gary





-----Original Message-----
From: Tomas L. Gomez <tomas.l.gomez at gmail.com>
To: Aavso-photometry at mira.aavso.org
Sent: Fri, 20 Jan 2006 11:39:47 +0100
Subject: [Aavso-photometry] average vs. median

   If I have N measurements, each one with error sigma,
and I take the average, then the error will be reduced
to sigma/sqrt(N).

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

Knowing this could be very useful to decide whether
to use the median of the average when stacking images,
for instance.

Thank you in advance for any help anyone can give.

-Tomas L. Gomez (GOT)

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