[Aavso-photometry] Implications of weather for photometric accuracy

[Arne Henden]

Greg Crawford wrote:
> Can anyone point me in the direction of some resources for:
> 
> 1) Understanding the implications of atmospheric conditions for photometric
> accuracy; and 
> 
Not sure what you mean here.  For most differential photometry using CCDs,
the atmospheric conditions (do you just mean clouds?) are second order
effects:  signal/noise, varying transparency across the field, etc.
I don't recall seeing a paper on these effects, as they differ depending
on the conditions de jour.

> 2) Measuring such atmospheric conditions.
> 
This is usually done by just staying on one field and watching the
instrumental magnitude change.  Doing such measures in more than one
color will show any color changes as you go to higher airmass.

> At the moment I am being driven crazy with negative extinction coefficients. It
> has been suggested this may be created by a marine inversion layer and the fact
> that I "practically live in the ocean". Suggestions for reading material on this
> would be appreciated, especially as it relates to photometry. 
> 
I'll look around; I don't think there has been an article expressly
giving such information, but there might be a variable-star paper or two
that shows examples.  I doubt your negative-extinction coefficients are
due to marine inversion layer effects, especially if you are working at
airmasses smaller than X=2.5.  The coefficients for k(V-R) and k(R-I)
are typically very small, and least-squares can often give negative
results (perfectly legal mathematically).  Another effect is changing
seeing, which always gets worse at higher airmass anyway.  This effect
is often bundled into the extinction coefficients, but if you have
changing seeing through the night and not enough extinction measures,
you can be fooled.
Arne