[Aavso-photometry] Why bother to calibrate?

[arne]

Michael Koppelman wrote:
> We do it because we are compelled to eliminate any systematic source  
> of uncertainty within our control to increase our confidence in our  
> measurements and to make our measurements as similar to those of other  
> observers as possible.
> 
> In a nutshell, although your random errors may be similar, the  
> systematic errors are now uncontrolled. Now your results are affected  
> by temperature, illumination issues and characteristics of the chip. I  
> think over time you'd start to recognize the influence of these  
> systematics during meridian flips, night-to-night zeropoint  
> differences and position on the chip.
> 
> Like you say, depending on what you are doing, it's possible that none  
> of these effect your ToM or ephemeris.
> 
> I admit I'm guilty of using old flats sometimes, but I always do bias/ 
> dark/flat. Always. :)
> 
Flatfielding is done to correct the response of all pixels so that
they are identical.  You have both defects internal to the CCD
(called Pixel Response NonUniformity, or PRNU) as well as optical
path differences such as vignetting or dust donuts.  Most of what
Tom is seeing is PRNU, which amounts to about 1-2percent between
pixels.  So if your seeing is 2-3 pixels fwhm, that means about
10 pixels contribute to the majority of your star signal, and a
2 percent random difference pixel-to-pixel will get averaged down
to something less than one percent correction.  As Michael states,
that might be a small effect for the scatter between measurements,
but it depends on how precise you need to be (example: exoplanet
transit work needs millimag noise levels), and whether the systematics
(like vignetting) are important for your photometry (usually *very*
important).

Can you get by without flatfielding?  Sometimes, and for some projects.
If you always flatfield, then you never have to worry.
Arne