Re: Analyzing Data

[Arne Henden <aah@nofs.navy.mil>]

(Boy, I wish all maillists would decide one way or another
as to how Reply To: is handled. TASS requires Reply All in order
to get tass@ included.)

 > ...One difference is that I am throwing out data based on the median sky
 > value.  I have watched data going by and set different levels for V and
 > I based on losing about equal numbers of frames to the rejection.  I set
 > it to lose a small fraction of the frames when the data looks clear.
 > This does a good job of clearing out dawn, dusk, and cloud data.
 >
 > I am now looking at a data set go by where it is keeping most of the V
 > data and rejecting all the I data (so far).  As Paul Bartholdi points
 > out, the Clear Sky Detector sees the water vapor in the atmosphere, as
 > does the I filter, I suppose.
   The water vapor is not a uniform sheet; it looks like cirrus.
Ic sees this nicely, especially with wide field cameras; you can
watch the vapor bands move across your images.  On the other hand,
why your Ic frames are getting rejected is strange to me.  You probably
just have the threshold set too high.  Are you rejecting all points
below <sky> + noise, or <sky> + n*noise?  The usual threshold is
something like n=3 for star finding, though of course all data is
used when you go back and do photometry of the detected objects.
   Clouds, on the other hand, will affect both V and Ic about the
same since cloud extinction is basically "grey".
> 
> Now suppose that a few high water vapor nights get mixed into a big pool 
> of data.  These will have more noise, and thus might skew the 
> distribution of the aperture photometry.  But I don't see how.  Any 
> comments from the experts???
   I'd like to see evidence that (a) there were no clouds, but (b) there
were large variations in the water vapor, and (c) you can correlate
(a)+(b) with increased noise in your Ic images.  I've had lots of
nights where the satellite water vapor map had definite structure over
Arizona, yet the night reduced as photometric from U through Ic.  Water
vapor, on the other hand, is not "grey"; you have definite absorption
bands.  Water vapor is the major noise source in photometry for JHK
and is why you cannot get better than 0.05mag absolute accuracy on
a given night, even if the Poisson statistics indicate much better
precision.
Arne