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FORS2 Pipeline:
Known Problems

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Mis-identified standard stars (IMG)

Recently (2011-05-01) we noticed a problem with some standard star field results, where the pipeline mis-identified standard stars. Usually such mis-identifications are caught during the QC certification process, but a few such cases (≤5%) have slipped through. In such cases typically only few of the identified stars carry high weight.

Missing rows in wavelength calibration for MOS/MXU

Sometimes the pipeline does not find a wavelength solution for all CCD rows in MOS/MXU data. Interpolation across such missing rows sometimes does not work, resulting in missing rows (= flux identical to zero) in mapped science data. These are the narrow black lines seen in the lower slit in the screen shot below.

Association of PMOS spectra for different numbers of objects in different slits

If there is more than one active slit, and at least one slit has a number of objects different from the other slits, then the association between spectra belonging to the same object (in the two corresponding beams) was ALWAYS wrong for pipeline versions up to 4.7.3. As long as the number of objects per slit is the same for all slits (including the case of only 1 active slit) the association was correct.

Wavelength Calibration for 150I

For some offsets both in LSS and MOS the pipeline cannot correctly align the redmost part of the 150I arc spectrum. This can be done manually by changing the parameters of the pipeline slightly.

Cosmic correction/spectrum extraction

The correction for cosmic rays done in the new pipeline since 2006-10-01 can distort line profiles, esp. of narrow lines. Users are therefore urged to check their extracted spectra carefully.

Sky background in MOS/MXU observations

The assumption of the new pipeline (since 2006-10-01) that at least 50% of the pixels contain only sky background is violated in many cases, so that the sky will be overestimated. Comparing the mapped science frame and the mapped sky frame allows to check this effect.

Sky background in LSS observations

The use of a median to estimate the sky background (since 2006-10-01) does not correct for large spatial gradients.

Flat-fielding in MOS mode

Due to the problems induced by the stray light contamination, in some cases the pipeline may introduce small-scale, artificial patterns in the flatfielding. These patterns are mostly located close to the slit edges and are below the 3% level. Artifical patterns can also be introduced close to bad pixels regions.

The presence of these patterns does not hamper noticeably the accuracy of the wavelength calibration and the quality of the science data.


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