[Aavso-photometry] Different ways of averaging data

[Radu Corlan]

> 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).

To have that many counts and still be linear i suppose you have a pretty 
large FWHM. in which case, you need to have large apertures, a large sky 
annulus (which is far enough from the aperture so seeing doesn't spread 
too much starlight into it).

Also, beware of using median sky estimation for 16-bit data with large 
apertures - a jump of 1 lsb in the sky can mean a lot across a large 
star aperture.

Radu

> 
> 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  
> 
>    You can still escape the "Gates" of Hell!
>                  Use Linux!
> 
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-- 
Radu Corlan       rcorlan _at_ pcnet _dot_ ro  

   You can still escape the "Gates" of Hell!
                 Use Linux!