FreeLists / az-observing / [AZ-Observing] Re: Photometric Night Definition

[Tom Polakis <tpolakis@xxxxxxx>]

---- Jeff Hopkins <phxjeff@xxxxxxxxxxx> wrote: 
> Those who have actually done photometry will understand that it is 
> not the brightness of the sky and it is not the extinction. What is 
> important is that the sky remains stable while the photometry is 
> being done.
> 
> It can be raining hard early in the evening and then clear and 
> provide an excellent sky for photometry. Then a few hours later it 
> could be raining hard again. Was it a photometric night? Indeed, the 
> only time that mattered was when photometry was being done...


Jeff,

While it may not be the best choice for a term, the accepted definition at the 
observatories of "photometric night" is a whole night that is clear from dusk 
to dawn.  In your rainy-night example, you may have experienced some hours in 
which you could do photometry, but that would not be considered a photometric 
night.

The term has widespread use.  Here's a summary table from 8 years of data at La 
Silla in Chile.

http://www.ls.eso.org/lasilla/weather/tablemwr.html

I am forecasting tonight to be a "useless" night.

Tom
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