Clouds ?

[Patrick Wils <patrickwils@yahoo.com>]

I looked a bit further into the large deviations in
the photometry at JD 2452312 (from Mhra2311530.cal). 
I have plotted the deviation from the average value of
the night against sky position for each image.  I have
included two such sequential images (2_61301.gif and
2_61478.gif; I have subtracted 2452310 from the JD to
get the file name) and a typical one (3_59152.gif)
from the next night (from Mhra2312524.cal) of the same
area of sky.  Each star is plotted as an arrow,
starting from its position in the sky.  If the arrow
points to the left, the star is measured fainter in V
on the image than the average of the night, if it
points to the right, it is brighter in V than on
average.  Similarly if it points up, it is fainter in
I than on average.  The larger the arrow, the larger
the deviation from the average (this is no indication
of the magnitude of the star).

It is clear that there are much less stars on JD
2452312 than on the next night.  In some parts of the
sky, no stars at all have been measured (on some
images of that night, there are virtually no stars). 
Besides that, the arrows generally point in the same
direction in the same area of sky on the first night,
while on the second night, the directions seem pretty
random.  In general, deviations in V are larger than
in I.  The arrows in the lower right part switch from
"too faint" on the first image, to "too bright" on the
second.  In the upper left part, no stars are visible
in the first image, while there are some that appear
on the second image, but much too faint.  I can only
conclude this is caused by high clouds passing by.  I
guess these images should probably be eliminated from
the data set, but I haven't come up with some sort of
automatic cloud filter yet.

Patrick



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2_61301.gz

2_61478.gz

3_59152.gz