[Comm2011] images r band rotation: fixed rawdome dived by rawtwilights @ different rotation angles.

[Gert Sikkema]

 hi all,  

 I constructed the 4  images r band rotation.   

 These are: fixed rawdome at angle = 0 degrees divided by
rawtwilights @ different rotation angles: -166,-76,13,103 degrees
respectively.  

 see top 4 plots of:   


http://wiki.astro-wise.org/projects:omegacam:commissioning:flatfields
 

 the fits files can be found at:  

 /net/thomson/data/users/sikkema/DOWNLOAD/WD2/june/z/*deg*fits  

  cheers,  

 Gert S   

 On Thu, 16 Jun 2011 13:51:00 +0200, "Edwin A. Valentyn"  wrote:  
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  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 [1]
 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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Prof. Dr. Edwin A. Valentijn       University of Groningen Coordinator
Target                 www   : www.rug.nl/target [4] Head OmegaCEN    
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