VST image processing workflow

This image shows the process of going from the raw data captured by a telescope to a stunning astronomical image like the one featured here, showing the Vela supernova remnant as seen with the VLT Survey Telescope (VST).

1. The detector registers the light collected by the telescope. OmegaCAM, the camera attached to the VST, has an array of 32 detectors covering a large field of view.
2. The raw images contain artefacts and instrumental signatures such as dead pixels, shadows, or luminosity variations among detectors. These need to be corrected before the images can be used for scientific purposes.
3. Astronomers correct these effects using calibration data. This process of going from raw to science-ready data is called ‘data reduction’.
4. When an astronomical object is larger than the field of view one needs to stitch together  different images, typically called a mosaic. This also allows us to fill in the gaps in between the detectors. 
5. The brightness of the background can vary among different parts of the mosaic, especially if they were observed on different nights, because of changes in the phase of the Moon and other effects. For instance, the upper-left corner of image 4 is darker than the rest of the image. By comparing overlapping areas between different images this can be corrected for.
6. The mosaiced image is visually inspected, and any residual artefacts are corrected for. This includes, for example, imperfect seams between adjacent images.
7. Astronomical detectors don’t capture colour images. Instead, several images are taken separately through filters that let through light of different wavelengths. These images are then assigned different colours and combined into a final colour image.
8. The final colour image.