[Aavso-photometry] Different ways of averaging data

[arne]

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?
> 
>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.
> 
>  
>
Since your Poisson error is in the 0.002 range, you are Poisson-noise
limited and sky/readnoise are negligible contributions.  This means
you will get the same results whether you stack frames and measure,
or do individual measures and average.  There is one exception
that I can think of immediately.  During stacking, if your software
averages the frames and then has a 16-bit frame as output, you lose
some signal/noise (you want either a 32-bit integer sum or a
floating-point result).
  The primary source of noise when taking high time cadence images
is scintillation.  Averaging several measures will decrease this
error at the cost of time resolution.
  I usually find averaging the individual measures to be easier
than to stack frames, as long as you are not skynoise or readnoise
dominated.  When the object gets faint, you usually have to be
more careful.
Arne