30 Doradus/WFI Data Release: Version 1.0 (20 December 2006)
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This data release includes broad band and narrow band imaging of a one-square degree region around the 30 Doradus complex in the Large Magellanic Cloud. These data products are based on observations carried out by Joao Alves, Benoit Vandame and Yuri Bialetski with the Wide Field Imager (WFI) at the 2.2m telescope on La Silla (Programme ID 076.C-0888). These data consist of a 2x2 WFI mosaic in the bands B, V, and narrow band Hα and [OIII]. A spectacular colour composite image resulting from these observations is also included in this release and is the subject of an ESO Press Release. Note that observations were carried out in good seeing but non-photometric conditions; scientific applications will therefore require cross-photometric calibration.
Overview and field layouts
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Positions of the 4 fields overlayed on a colour mosaic derived from the FITS files in this release (an 8x8k pixel version (half the size of the full mosaic) is included in this release). All fields are covered in the bands [OIII], B, V and Hα |
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Content of this release
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The released optical WFI images from the 30 Doradus region reduced with the ESO/MVM pipeline are tabulated to the right, consisting of 16 astrometrically calibrated images in B/123 (4), OIII/8 (4), V/89 (4) and Halpha/7 (4) filters. The table gives: in column (1) the field; in column (2) the filter; in columns (3) and (4) the centre coordinates in RA and Dec; in column (5) the total integration time in seconds; in column (6) the number of frames that make up the final image; in column (7) the average FWHM measured on the image in arcsec ("seeing"); in column (8) the observation date. The same table is also available in ASCII format. |
Comparison to previous releases
Data reduction method
The WFI data were reduced using an improved version of the ESO/MVM image processing pipeline (v1.3.5), which was originally developed within the EIS project. This software package is publicly available in an earlier version, with the release of the latest version expected in the near future.
Each field is the co-addition of a number of sky-subtracted frames (NCOMBINE in the image header). Images are registered using an astrometric grid defined by the Two Micron All Sky Survey (2MASS) catalogue. After co-addition a constant sky value is substracted from each image.
Astrometric Solution
The astrometric calibration was derived using the 2MASS catalogue, as this is the only catalogue that provides sources within the entire area of the nebula. The RMS scatter in the astrometry is 0.2'' across the entire area, which is limited by the internal accuracy of the catalogue.
Photometric calibration
Data Format
The observations were carried out in highly non-photometric conditions, therefore no attempt to compute photometric zero-points was made. External photometric calibration will be required for scientific applications.
The data files in this release come in pairs: one is the science frame, the second one is the corresponding weight map (*.weight.fits) defined as a variance map (e.g. it should be used with SExtractor using the parameters: -WEIGHT_IMAGE weight_map.fits -WEIGHT_TYPE MAP_WEIGHT). All relevant parameters are included in the FITS headers (see a sample here), which have been designed to be consistent with future VO compliant releases of ESO Advanced Data Products.
In addition, an 8x8k (half-size) colour mosaic image has been created, derived from the data files in this release. Mosaic FITS images for the individual filters were created with ESO/MVM and then combined to create an RGB image (see also the ESO press release). The Hα mosaic was used for the R channel, the V and [OIII] mosaics were combined to form the G channel and the B mosaic was used for the B channel. Finally, a FITS RGB cube was created from this RGB image with proper WCS information added, which can be loaded directly into DS9 (e.g. 'ds9 -rgbcube ADP_WFI_30Dor_RGB_v1.0.fits') or Aladin.
The following file naming convention has been adopted for the data files:
ADP_WFI_xx_f_v1.0.fits
ADP_WFI_xx_f_v1.0.weight.fits
for the field name in the filter and the respective weight map.
Additionally, field names can be found in the OBJECT keywords in the FITS headers.
The 8x8k RGB mosaic is included in FITS cube and JPG formats:
ADP_WFI_30Dor_RGBcube_v1.0.fits
ADP_WFI_30Dor_RGB_v1.0.jpg
Data Retrieval
See here the complete list of data files belonging to this release as they can be retrieved via the data request page.
Acknowledgements
When using data products provided in this release, we request the following acknowledgement:
"Based on observations made with ESO Telescopes at the La Silla Observatory under programme ID 076.C-0888, processed and released by the ESO VOS/ADP group."