[Comm2011] seeing/psf for different bands

[Jeffrey Bout]

I know I'm biased, must I must say astro-wise is a wet dream when
browsing telescope data for things like quality control:

I have been looking at seeing conditions for my Hercules data. In the
first attachment you see the result of a few lines of python-code. (in
our pipeline, the seeing/psf is calculated using sextractor and stored
in our database)
The plot shows the seeing for all the dithers (over 200) that have
been made for all the hercules fields over time. The x-axis shows
time, but not really the datetime, but instead the index of each
dither. This to get rid of the periods without observing. The grid
separates the different observing nights. The plot shows two black
lines, for two different chips near the center of the field of view.
The lines look almost identical.

One thing (of many things) I notice: during the third observing night
the seeing is stable, but not very good for the g band. Is there
something wrong with this band?

Idea by Edwin: make the same plot for standard fields: they are
observed often and with all the 5 band within half an hour. See the
second attachment.
And indeed: ugriz are imaged often, with the best seeing in z and the
worst in u.
I took a closer look at this as shown in the third attachment: it
shows the seeing as function of time since the first u band dither.
And again, z band seeing/psf is better.

Some possible causes:
- the active optics system does a better job in the redder bands?
- u band exptime is longer (300sec). However, g band exptime is only
60sec and for i 115 sec.
- bad focussing should not be a cause. This is done in the r-band,
which is halfway the spectrum.

Any other suggestions?

Jeffrey
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