Data reduction

First VEGAS observations (2014-2016) have been processed with the VST-tube pipeline specialized for the VST-OmegaCam observations
(see Grado et al. 2012; Capaccioli et al. 2015)
Since 2018, the VEGAS data are reduced with the AstroWise data-reduction pipeline.

Dr. Marilena Spavone, at INAF-Astronomical Observatory of Capodimonte is leading and coordinating this task.

Both pipelines perform the standard pre-reduction steps, gain harmonisation of the 32 CCDs, illumination correction and, for the i band, defringing. Relative and absolute astrometric and photometric calibrations are applied before creating the final co-added image mosaics.

In addition, there are two tasks to perform the sky subtraction, depending on the adopted observing strategy:
- fitting of the background with a surface, typically a 2D polynomial, to the pixel values of the mosaic;
- for images acquired with the step-dither observing strategy, an average sky frame is obtained from exposures taken as close as possible, in space and time, to the scientific ones.

Observing strategies

Step-dither:
the brightest and most extended galaxies (with mB < 10 mag and a major axis diameter larger than 5 arcmin), a good background estimate is achieved by adopting the step-dither observing sequence (see Iodice et al. 2016, 2017, Spavone et al. 2017, Iodice et al. 2019).
It consists of a cycle of short exposures, 5 x 150 sec on the target (science frame) and 5 x 150 sec on the sky frame.

Standard:
For less extended objects, the sky background can be estimated on the science frame, by using a polynomial surface fit over the entire frame (see Capaccioli et al. 2015). In this case, the adopted observing strategy is the standard diagonal dithers to cover the CCD gaps.