[Comm2011] seeing/psf for different bands

[Edwin A. Valentyn]

Jeffrey

great we can do this kind of things now so swiftly.!!
and very useful to have this for the standard fields.
I note a few things:
i) PSF systematically gets better towards red
ii) this trend in terms of the fractional difference, say between u and 
z stays more or less the same (eg compare good and bad seeing data.

i) is opposed to diffraction effects  and ii) is not what you expect 
from incorrect filter-focus offsets, which would more less propagate in 
squares.

So, it points to the colour dependency of seeing (so turbulance in the 
atmosphere), its well known this is worse at short wavelength, than at 
long (so trend opposed to diffraction). However, this trend  does not 
have such a strong wavelength dependency as we observed (expect 
something like lambda**(1/5) ). So its still somewhat puzzling and 
worthwhile to further persue. Likely, we are the first to be able to do 
this kind of studies on Paranal skies (and even when done for VLT it 
will also be diffrent for a 2.6m  telescope). So this work is worth 
publishing and alos of importance for our science.

Although the trend is not what is expected from wrong filter offsets, it 
would be re-assureing to make an observation which really demonstrates 
we are applying in practice the correct offsets. For instance, we could 
bracket a standard star field observation which fi will do u,g,r,i,z 
subsequently,  with preceeding an u focus run, and at the end a z focus 
run. What about that?
regards
Edwin






On 3-8-2011 2:30, Jeffrey Bout wrote:
> 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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Prof. Dr. Edwin A. Valentijn       University of Groningen
Coordinator Target                 www   : www.rug.nl/target
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