[Comm2011] ROTATOR ANGLE and comments on OCAM2 daily log day 10

[Edwin A. Valentyn]

Thanks Gert,

few comments

On 16-6-2011 1:30, Gert Sikkema wrote:
>
> dear all,
>
>  I made a list of all 32 CCD pointings observerd so far:
>
> B,V, u, g(2x), i, z(2x)
>
> 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.
>
> 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.
>
> , I can think of several causes:
>
> -) it may be due to bad observing conditions (zeropoints of chips 
> fluctuating),
>
> -) the flat field used is not OK (only twilights, remember that the 
> dome flats are useless)
>
> -) 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.
>
> -) 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 )
>
> 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
>
True.
Indeed we are faced with large colour terms in u, and it will take some 
effort to work these out.
on
http://www.astro-wise.org/~omegacam/preparing_obs/throughput.jpg
you can see why this term are so large
>
> ---------------------------------
>
> ROTATOR ANGLE
>
> another point I would like to verify with you concerns the rotator angle:
>
> the illumation variations of the astronomical objects in a reduced 
> science frame are mainly caused by the (twilight) flat fields used.
>
> 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
>
> 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.
>
> 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...
>
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.
For making nice images with flat sky you point is also valid of course- 
but this is more cosmetic.

regards
Edwin
>
>  Gert S.
>
>
> On Thu, 16 Jun 2011 13:08:48 +0200, Gijs Verdoes Kleijn wrote:
>
>     Dear all,
>
>     Main points of what we have observed:
>
>     -domeflats r at 4 rotator angles to accompany their twilight
>     counterparts of yesterday.
>
>     -photometry with ADC in+out
>
>     -AO/guiding test observation, which includes pointings on Hercules.
>
>     -daily calibrations
>
>     Last afternoon we said goodbye to John.
>
>     Best wishes, Gijs
>
> dr. Gert Sikkema
> Kapteyn Institute Zernike Building, room 133,
> P.O. Box 800 9700 AV Groningen
> sikkema at astro.rug.nl
> homepage: http://www.astro.rug.nl/~sikkema
> 31-50-3634548
> Popular lectures on:
> - Einstein's theory of Relativity
> - Globular Clusters
> - Active Galactic Nuclei
>
>
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-- 
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Prof. Dr. Edwin A. Valentijn       University of Groningen
Coordinator Target                 www   : www.rug.nl/target
Head OmegaCEN                      www   : www.astro.rug.nl/omegacen
Kapteyn Astronomical Institute     tel   : +31 (0)50 3634011/4036 (secr)
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