[Aavso-photometry] Sources of accuracy and precision inphotometric measurements

[Michael Newberry]

----- Original Message ----- 
From: "Justin Pryzby" <justinpryzby at users.sourceforge.net>
To: <aavso-photometry at mira.aavso.org>
Sent: Monday, December 19, 2005 5:09 PM
Subject: Re: [Aavso-photometry] Sources of accuracy and precision 
inphotometric measurements


> On Thu, Dec 15, 2005 at 12:10:07AM +0200, Radu Corlan wrote:
>> > There are 4 main areas which affect doing accurate, high-precision
>> > photometry:
>> >
>> > 1) the sky background estimate
>
>> > 1. Sky estimation:
>> > Mira offers 3 methods: Mean, Median, and Mode. Knowledgeable 
>> > photometrists
>
>> Mode does have good outlier-rejection properties. There is however one
>
> I just wanted to add that, no matter what algorithm you use, it is
> probably critical to use the statistical combination of multiple
> pixels.  Average+rejection does this, of course, but the textbook
> definition of median and mode do not.
>
    Agreed.

> For a "mode" algorithm, what you want isn't "the most frequent value",
> but something to the effect of "the weighted average of the values
> within some small range of the most frequent value, possibly computed
> iteratively", and for "median" something like "the weighted average of
> the values within some small range of the the center pixel value,
> counting duplicities, possibly computed iteratively".
>
    Agreed. The point is that "one man's mode is not another man's mode". 
The way a textbook defines these quantities is of limited use in real-world 
data analyis.

    And what is even more difficult to cope with is that any algorithm you 
use will not often come up with the value you would expect from carefully 
examining the image and doing it manually!

Cheers,

Michael

> -- 
> Clear skies,
> Justin
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