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Project DR-01

Astrometric and photometric correction of dithered / jittered images


The purpose of this project is to provide a set of pipeline recipes based on the ESO Common Pipeline Library (CPL) for the stacking and mosaicing of imaging data. Currently a lot of observers split observations of a given target among different OBs. A set of CPL recipes will provide a seamless interface to the existing pipelines and to the execution of front-end tools esorex, Gasgano, and ESO Reflex.

Dithered images (several offsets around the target) are required to remove cosmic ray events and/or fill the gaps between CCD chips in a mosaic detector. Jitter imaging (microscanning) is an efficient observing mode to separate astronomical from sky signal by means of many small offsets around the target.

A number of existing packages have been used to process data from ESO instruments:

  1. A number of algorithms are already available as CPL functions (e.g shift-and-add algorithm, cpl_image_warp_polynomial, see http://www.eso.org/cpl/)
  2. A general set of algorithms is available in the MVM package (Multiscale Vision Model) which is defined as a sequence of operations required for automated image analysis on the basis of the wavelet transform. The MVM package and details on the MVM algorithms are available at: http://archive.eso.org/cms/eso-data/data-packages/eso-mvm-software-package
  3. The Terapix tools (SExtractor, scamp, swarp) include a suite of programs program that resample and combine FITS images in an arbitrary World Coordinate System (http://terapix.iap.fr/).


  • Vandame, B., 2003, New algorithms and technologies for the un-supervised reduction of Optical/IR images, astro-ph/0208230v1
  • Astro-Wise project: www.astro-wise.org, www.astrometry.net
  • Anconelli et al., 2006, A&A 460
  • Bertero et al., 2000, PASP 112, 1121
  • Lenzen et al., 2005, A&A 443, 1087

Applicable instruments and modes

This project is applicable to all ESO imaging instruments in optical and near-infrared (e.g. FORS, ISAAC, SOFI, VIMOS, WFI, etc..). This also applies to thermal instrumentation in N,Q bands (VLT:VISIR, ELT:MIDIR) and L,M bands (VLT: NACO, ISAAC,etc.).

Data from the survey telescopes (VISTA/VIRCAM and VST/OmegaCAM) shall be included in the list of applicable instruments as they become available. Considering their multi-detector nature, and the large gaps between the detectors (particularly in the case of VIRCAM), these instruments would profit the most from stacking and mosaicing routines.

Required archive, calibration or observation data:

A lot of observers split observations of a given target among different OBs. Data required for this project is available in the ESO archive.

Required algorithm and software developments

The development is divided into two tasks: DR-01is concerned with the determination of the photometry and astrometry solution to a set of data, while DR-02 is the task of combining these data into a mosaic. The task DR-01 involves the following steps:

  1. Define an interface to the pipeline products, namely the set of keywords and data structure that must be fulfilled for a pipeline product to be adequate for the task. One can assume that the input data for this task are pipeline reduced data, basically free of instrumental signature at the single-chip level. ESO will provide pipeline reduced data for the validation sets and the applicable instruments, and the Consortium will if necessary adapt these data to the defined standard data interface.
  2. Define an interface to astrometry and photometry catalogs, namely an input format into which the input catalog data (astrometry, photometry) must be formatted for use with the routine.
  3. Allow the user to provide an offset map and/or a fringe-correction map (e.g. as an output product frame of the task DR-08). This user input can be either at the single-chip or multi-extension level.
  4. Source detection will make use of the Terapix SExtractor task.
  5. During the phase of detailed specification, develop a prototype processing chain using existing software packages like the ESO MVM package or the Terapix tools (swarp, scamp). For an example of end-to-end processing pipeline making use of available packges see the Astro-Wise project (http://www.astro-wise.org). The prototype will be reviewed on the existing data sets before starting the coding in CPL.
  6. Design a method for determining a global astrometry and photometry solution to a set of data by querying catalog data. The method should be adequate for overlapping or non-overlapping images, images containing stars as well as extended sources, and for different fields of view.
  7. Investigate the possibility of calculating full astrometric solutions on mosaic images (including distortions) in a user-friendly way would be very welcome for single and mosaic data. (e.g. using WFI, VISTA/VIRCAM, VST/OmegaCAM, HAWK-I data).
  8. Specific issues will be considered for mid-infrared data: The possibility of obtaining a precise astrometry on VISIR data is small, however relative astrometry is of course possible. Data in the L, M, N and Q bands are usually obtained with chopping. A possible reference for reconstruction of chopped and nodded images is Lenzen et al. 2005, A&A 443, 1087.
  9. Implement the recipes in CPL, provide user documentation and Reflex workflows.
  10. The CPL routines shall deliver the photometry and astrometry solutions with error estimates, and quality indicators, in a format suitable for the execution of task DR-02.


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. A prototype based upon available software products shall be first developed and reviewed as additional part of the detailed specifications.



The astrometric and photometric correction on selected data sets (FORS, VIMOS, WFI, VISTA, HAWK-I, ISAAC, NACO, VISIR) will be compared to a standard package, e.g. the Terapix SExtractor/scamp suite of programs. ESO will process the raw data for the selected data sets and provide the corresponding pipeline products. A suitable test field will be the CHANDRA Deep Field South, for which both ESO data and published results are available.


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