Data processing of VST/OmegaCAM data: Frequently asked questions

• Are there any known problems with OmegaCAM data?

Answer: The quality control group keeps a list of know problems at

http://www.eso.org/observing/dfo/quality/OMEGACAM/qc/problems.html

• Where can I download the OmegaCAM pipeline to reduce my data?

Answer: ESO does not offer a data-reduction pipeline for OmegaCAM. The ESO OmegaCAM pipeline is exclusively used for quality control, i.e. it processes data to monitor the instrument's proper performance, and does not produce science-grade data products.

• My data shows an extended, bright feature - what can this be?

Answer: It is likely that some reflections from the moon are affecting your data. These reflections are unpredictable and have been found to be more frequent when the target is closer than ~40deg to the bright moon. You might also want to check:

http://www.eso.org/sci/observing/PublicSurveys/useful_links/OmegaCAM_images.html 

where we provide a collection of example OmegaCAM images, illustrating some special data features.

A comprehensive description of features visible in OmegaCAM detectors can also be found in the quality control web pages.

• A number of OmegaCAM filters show vignetting in their central regions. What are the causes for this and what are the dimensions of these features?

Answer: A number of OmegaCAM filters are not monolithic, but consist of four segments (each covering 8 detectors) that have a central cross-shaped support structure. This support creates a vignetting effect that will be visible in 20 detectors which is significantly larger than the gaps present between adjacent CCD's. The affected filters are: B_JOHN, V_JOHN,
H_ALPHA,  NB_659, NB_852,
and the effect looks like this (for the H_ALPHA filter):

When using these filters, one needs to take the vignetting into account when chosing a dither pattern. A conservative accounting for the total size of the vignetted area can be summarized as:

• Some OmegaCAM detectors show an odd central stripe in the bias and master bias images.

Answer: Two OmegaCAM detectors (ESO_CCD_#68 and #90) show a centrally elevated structure in their bias frames. This structure, however, remains constant with the underlying bias level and can be easily corrected.