Project DR-08

Fringe correction of spectra and images

Fringes are interference pattern which are due to internal reflections when light enters a thinned CCD, optical fibre and/or other optical components. In spectroscopy, the fringes are a challenging calibration problem: they are not reproducible in VIMOS, and the new FORS1 CCD shows strong fringes with the red grisms. In UVES the fringe pattern is the cause of inconsistencies in the scientific measurements. For ground-based observations the correction might be obtained from sky fringes, flat-field and spectral observations strategies (e.g. nod and shuffle). The purpose of this project is to provide methods for the determination of fringe patterns in observation data sets, like the wavelet analysis of the fringe pattern. The main purpose is to characterize fringes in imaging data, in order to provide the fringe maps needed by the tasks DR-01 and DR-02. An optional extension of this project is to test the applicability of the methods to spectral data.

• Wavelet analysis of the fringe pattern: Rojo & Harrington 2006, ApJ, 649, 553
• Fringe removal features have been implemented in MVM: http://archive.eso.org/cms/eso-data/data-packages/eso-mvm-software-package

Applicable instruments and modes

• FORS1 imaging and spectroscopy with red grisms.
• VIMOS imaging and spectroscopy with red grisms.
• UVES

Required archive, calibration or observation data

Data are available from the ESO archive. Pipeline processed data will be provided by ESO.

Required algorithm developments

This task (DR-08) is related to the stacking/mosaicing tasks of DR-01 and DR-02, in that the recipe will provide a fringe correction map that can be applied to individual chips before stacking. Therefore strategies based on the analysis of data taken in different OBs (for different pointings or different nights) shall be investigated.

Two approaches will be compared, either based on calibration data, or alternative algorithms, e.g. based on wavelet analysis. Note that that the Rojo & Harrington method does not take the wavelength information into account. Therefore, it can only be applied on a specific frame, and other methods making use of the wavelength information could be investigated. However, an approach based on a physical model of the CCD detector fringes is excluded from this project. Different strategies are required depending the relative brightness of object and sky.

The following tasks are planned:

1. Retrieve archive data from the above mentioned instruments, perform a pipeline data reduction
2. Define a product interface (requirements on the pipeline products for this task)
3. Analyzing the data, testing methods
4. Writing/debugging/documenting CPL-based code 

Required software developments

All software will be developed following the guidelines of the VLT-SPE-ESO-19000-1618 document. The software delivery will include in particular the CPL-based recipes, test reports, user documentation and Reflex workflows.

Validation

The improvement in accuracy obtained with the fringe maps will be evaluated on different data sets, in particular for the data combined in the tasks DR-01 and DR-02 (e.g. FORS, VIMOS), and for spectral data (UVES).