[Aavso-photometry] Different ways of averaging data

[Pedro Pastor]

Thanks to all of you for our responses (Arne, Wolfgang, Radu and Thom).

Responding to some points:

1)My exposure is 40 sec. for V and 50 sec. for B. It isn't that long
enough for averaging scintillation variations? In this point, the
question from Thom is quite adequate.

I'm talking of 0.02 mag. dispersion along the series for 0.002-0.003
mag. single point precision. My results are similar to those obtained by
a friend on a very different location with different equipment (but both
working on sub-urban skies).

I know I'm moving in very difficult arena: at mili-mag. scale any tiny
fluctuation of any environmental factor could produce such a dispersion.
The proper camera "stability" could contribute to that.

We have two points here:
 
- How could we get raw data with better data series dispersion?
- How can we diminish that dispersion by averaging the data once we got
the trouble?

2) From Arne comments, I understand that in case of not being
Poisson-noise limited in the individual frames, I would be better to
average frames than averaging individual measures. That's right?

I think Wolfgang made a very good point in his reasoning, but that is
not applicable in my case. Wolfgang is working with very low data
intensity in his examples, I'm working with >1,000,000 ADUs in my case.
(And I'm working in the linear area for my CCD, I presume, but any tiny
deviation from linearity could also be responsible for, at least, part
of the data dispersion).

Finally, I'll investigate where my data dispersion comes from, but I'd
like to know what's the best strategy for coping with "de facto"
situation. I expected there should be any experiences out there as long
as I think that's not an uncommon situation. A third possibility I'm
going explore is to perform a Wavelet smoothing on the data series
before applying any Fourier analysis for period determination.

Regards,

Pedro


-----Mensaje original-----
De: Radu Corlan [mailto:rcorlan at pcnet.ro] 
Enviado el: lunes, 06 de junio de 2005 0:56
Para: aavso-photometry at aavso.org
CC: Pedro Pastor
Asunto: Re: [Aavso-photometry] Different ways of averaging data

> > 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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