Re: Analyzing Data

[Tom Droege <tdroege2@earthlink.net>]

Nomenclature is always a problem for me since I don't have enough contact 
with the field to learn the language.

Arne writes:

>Are you rejecting all points
>below <sky> + noise, or <sky> + n*noise?  The usual threshold is
>something like n=3 for star finding

I look at a bunch of data and note the lowest median values for the sky for 
the V and I frames.  This is about 1500 ADU for the V channel and 2500 or 
so for the I channel.  I have the limits set at 3500 and 6000.  So any 
median value above these values get trashed.  This puts me at an n of 2.4 
per the above.  I have just been guessing and am interested in wisdom on 
this subject.

I am also trying to look for clouds.  The idea is to divide the image into 
36 sub images and throw out any image where the median of any sub image is 
3 sigma *below* the median of the whole image.  This works for patchy 
clouds.  I also throw out images where there are more than 3 sub images 
where the median of the sub images is above the median of the whole 
frame.  This catches things like sticky shutters without losing very many 
frames with multiple bright stars.

This patch rejection should also work for the water vapor if the variation 
is large enough??

>though of course all data is
>used when you go back and do photometry of the detected objects.

Why?  Seems to me that this increases the noise of the measurements??  I 
certainly have enough frames to throw away those that might be questionable.

Tom Droege





At 12:06 PM 2/8/03 -0700, you wrote:
>   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".