[Aavso-photometry] Different ways of averaging data

[Radu Corlan]

> > Pedro Pastor wrote:
> > 
> > >I've done a high rate sampling on a Delta Scuti (NSV 3063). While
> > >processing data I saw that, although single point data precision is
> > >around 0.002 mag., data dispersion along the whole series is high.
> > > 
> > >As long as my sampling rate was high, I thought of averaging measures
> > >(every 5 points). Then, a big doubt rose in my mind. What would be
> > >better to average: differential photometry data or raw images (in order
> > >to raise SNR) and then calibrating the result image and get the
> > >differential photometry measure?

Pedro,

do you know why your dispersion was much higher than the expected 
precision? i would suspect there are two probable causes: either 
atmospheric scintillation, or thin clouds or other "accidents".

If it's scintillation (the calculated value is of the same order or 
magnitude as your error), i'd say you'll get very similar results 
whether you average images or reduced magnitudes. 

If on the other hand you had some transparency problems, you'll probably 
be better off by first getting the differential photometry data and then 
average that. If you plot the differential mag and a raw (instrumental) 
magnitudes vs time, the cause of the error becomes 
obvious: if both show about the same spread, and the graph appears 
uniform in time, you are probably scintillation limited. If increases in 
the spread of the differential value are accompanied by large changes in 
the raw magnitudes, you know you have thin clouds, a problem with flats, 
dust etc. 

Radu

> > > 
> > >I've done some tests in order to compare both methods, but my results
> > >don't shed any light on the question. Maybe both procedures are
> > >equivalent?
> > > 
> > >Could anybody provide me with some information (or results) about this
> > >point?
> > > 
> > >Thank you very much.
> > > 

-- 
Radu Corlan       rcorlan _at_ pcnet _dot_ ro  

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