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Thanks Gert,<br>
<br>
few comments<br>
<br>
On 16-6-2011 1:30, Gert Sikkema wrote:
<blockquote cite="mid:05551a5a22a989ab80dc3cd180f2ed3e@astro.rug.nl"
type="cite">
<p>
dear all,
</p>
<p>
I made a list of all 32 CCD pointings observerd so far:
</p>
<p>
B,V, u, g(2x), i, z(2x)
</p>
<p>
g and z were observed two times, but the first of each of these
bands was done with cirrus in the sky, this can at least be seen
in my analysis of g. </p>
<p>
I also analyzed the u band data of SA113, but the resulst of
this do not make any sense ( I included both gain and/or QE
corrections), I do not see a smooth picture, but many discrete
off-sets of magnitudes between chips. </p>
<p>
, I can think of several causes:
</p>
<p>
-) it may be due to bad observing conditions (zeropoints of
chips fluctuating), </p>
<p>
-) the flat field used is not OK (only twilights, remember that
the dome flats are useless) </p>
<p>
-) the QE has a large gradient as a function of wavelength at
low wavelengths, my correction based the 360 nm part of table
2.A.1 of Fabrice his thesis may not be a good estimate of the
QE.
</p>
<p>
-) another possibility is that the standard star catalog is not
really good here (however the scatter was only ~0.05 mags for
each chip using ~30 standard stars )
</p>
<p>
So...if you have some time left it may be a good idea to
re-observe in the u band in <very> good conditions at
SA107 </p>
</blockquote>
True.<br>
Indeed we are faced with large colour terms in u, and it will take
some effort to work these out.<br>
on<br>
<a class="moz-txt-link-freetext" href="http://www.astro-wise.org/~omegacam/preparing_obs/throughput.jpg">http://www.astro-wise.org/~omegacam/preparing_obs/throughput.jpg</a><br>
you can see why this term are so large<br>
<blockquote cite="mid:05551a5a22a989ab80dc3cd180f2ed3e@astro.rug.nl"
type="cite">
<p>
---------------------------------
</p>
<p>
ROTATOR ANGLE </p>
<p>
another point I would like to verify with you concerns the
rotator angle:
</p>
<p>
the illumation variations of the astronomical objects in a
reduced science frame are mainly caused by the (twilight) flat
fields used. </p>
<p>
This means that the illumination correction is only dependend on
the rotator angle of the twilight flat field and not on the
rotator angles of the science frames </p>
<p>
Not that a varying illumination will also occur in the sky
background --> this DOES depend on the rotator angle of the
science frames. Deviding this with an (twilight) flatfield with
another rotation angle will cause some strange effects on the
sky background. </p>
<p>
So if we want to use a default illumination correction frame in
the future, this should mean that the twilight frames in
observing mode should always have a similar rotation angle as
the ones used for the twilight flatframes that were used to
calculate the default illumination correction frame... </p>
</blockquote>
I'm afraid you are right, in case we make nightsky flats, but not if
we simply apply twilight flats and let the sky- substraction in
Sextractor do the rest for cataloguing.<br>
For making nice images with flat sky you point is also valid of
course- but this is more cosmetic.<br>
<br>
regards<br>
Edwin<br>
<blockquote cite="mid:05551a5a22a989ab80dc3cd180f2ed3e@astro.rug.nl"
type="cite">
<p>
Gert S.
</p>
<p>
<br>
On Thu, 16 Jun 2011 13:08:48 +0200, Gijs Verdoes Kleijn wrote:
</p>
<blockquote style="padding-left: 5px; border-left: 2px solid
rgb(16, 16, 255); margin-left: 5px; width: 100%;">
<p> Dear all, </p>
<p> </p>
<p> Main points of what we have observed: </p>
<p> -domeflats r at 4 rotator angles to accompany their twilight
counterparts of yesterday. </p>
<p> -photometry with ADC in+out </p>
<p> -AO/guiding test observation, which includes pointings on
Hercules. </p>
<p> -daily calibrations </p>
<p> </p>
<p> Last afternoon we said goodbye to John. </p>
<p> </p>
<p> Best wishes, Gijs </p>
<p> </p>
</blockquote>
<span id="_rc_sig">
<pre>dr. Gert Sikkema
Kapteyn Institute Zernike Building, room 133,
P.O. Box 800 9700 AV Groningen
<a class="moz-txt-link-abbreviated" href="mailto:sikkema@astro.rug.nl">sikkema@astro.rug.nl</a>
homepage: <a class="moz-txt-link-freetext" href="http://www.astro.rug.nl/~sikkema">http://www.astro.rug.nl/~sikkema</a>
31-50-3634548
Popular lectures on:
- Einstein's theory of Relativity
- Globular Clusters
- Active Galactic Nuclei
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