[News] Potential Data Problems

[Roberto Silvotti]

Hi Mark and Konrad,

A few more comments on fringing correction (see below)

Regards,
           Roberto


>> 3/ Fringe Pattern Correction:
>>
>> This can be relatively straight-forward when the observing conditions 
>> are stable and photometric.  It can be a nightmare when they are not.
>> For light of a given wavelength, the observed fringe pattern can be 
>> very stable, being determined by the thickness of the chip at each 
>> pixel.  The effect is not very strong for continuum sources, since 
>> the fringe pattern is smeared out. However, the fringe pattern seen 
>> in flattened images is the result of a few strong atmospheric 
>> emission lines (mostly OH and O). Since it is caused by the sky, and
>> not inherent in the sensitivity of the detector to the stellar flux, 
>> the fringe pattern should be treated as an additive term remaining 
>> after the flat-fielding. The problem is that under variable sky 
>> conditions the OH and O emission varies in intensity apparently 
>> independently from one another.  Since they emit at different
>> wavelengths, the strength of the fringe pattern will vary 
>> substantially relative to the sky brightness from exposure to exposure.  >> I have heard the claim that this can be modelled and corrected 
>> (WFI users/Meisenheimer?) but I have not seen the results and am not 
>> familiar with the exact technique.
 
> The claims I have heard concern INT WFC data (similar detectors). Mike
> Irwin has developed a technique which robustly fits the amplitude of the
> fringe pattern and subtracts it out. He claims that for i' and bluer, a
> single fringe pattern works very well (down to better than 1% of the
> original amplitude), but that in z' (or I_c) there are indeed different
> families of lines tat vary independently. He also developed a technique to
> fit these simultaneously, but it was not as successful. We should try this
> out on WFI data, though the lack of an i' filter is a pain (WFI I filters
> extend much further into the nasty near IR sky lines).



For what concerns the fringing correction we have improved our
technique (the one described in Alcala et al. 2002).
In the previous version we were simply normalizing the amplitude 
of the fringing pattern to the mean background.
Now, before calculating the mean level of the background, we remove
the stars. In this way the results further improve.
Roeland has seen the results during his last visit here with WFI 
fields at high galactic latitude.
We have further tested the method even with more crowded fields
and it still works fine.
The good point of this method is that it is very simple and gives
good results: the residual fringing variations are of the order 
of 1% or less, even with WFI Z filter.
Note that the correction improves when we have a high number of
scientific frames in differents fields and we use all them together
to derive the fringing pattern (see also point 1/ of the comments
by Konrad).

By the way we have checked that the WFI fringing pattern is very 
stable, not only from one night to the next, but even from 
one year to the next !
Therefore it can be convenient, when we have a small number of 
scientific images, to use frames of different nights in order
to derive a more precise fringing pattern.


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         Dr. Roberto Silvotti
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INAF (Istituto Nazionale di AstroFisica)
Osservatorio Astronomico di Capodimonte
via Moiariello 16, I-80131 Napoli, Italy
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tel/fax:  +39-081-5575583/456710
e-mail:   silvotti@na.astro.it
web:    http://www.na.astro.it/~silvotti
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