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OPERA (Objective Prism Enhanced Reduction Algorithms) is a package of software tools for the authomatic analysis of astronomical images. It is being developed by the Universidad Complutense de Madrid (UCM, Spain) Astrophysics researh group. It is copyright by C.E. García-Dabó. Contact C.E. García-Dabó at ceg@astrax.fis.ucm.es for further information


Introduction

Example Surveys of star-forming galaxies are of vital importance to constrain galactic evolution theories. One of the most successful searching methods is the objective-prism (OP) technique, which can register thousands of low resolution spectra in large areas. The UCM-CIDA project expects to cover 150 square degrees and detect ~2000 H_alfa emission line galaxies. OPERA is an automatic method to handle and analyze such amount of data. It goes trough the detection of the objects in the image, extraction of one-dimensional spectra and identification of the emission line feature. The main advantages of a method of this kind are: 1) to avoid subjectivity inherent to visual inspection used in past studies; and 2) the ability to obtain physical parameters without follow-up spectroscopy.

Description of the method

Example After common preprocessing (bias, flat, etc..) the first step is the identification of sources on both direct and OP images. This involves the previous determination of the local sky background and associated noise. Images are divided in user-defined small boxes to allow for sky background changes along the image. Inside these subsets the mode of the pixel values is assigned to the local background, while noise is assigned to standard deviation. Objects are found in the image following these steps: 1) the algorithm begins when it finds a pixel N sigma above the local background (N~1 usually). 2) neighboring pixels are added if the global S/N ratio is improved after adding them. This test is made for every surrounding pixel. 3) (optional) neighboring pixels cannot differ significatively from a gaussian seeing profile among each other. 4) (opt.) Minimum pixels for an object can be Star-galaxy separation is performed using the algorithm described in Sebok (1979; AJ 84, 10). However line search is performed on the whole catalog to allow for star-like objects like QSO's or ultracompact galaxies. Once the objects on the image have been identified and astrometric solutions have been computed, a cross-correlation may optionally be performed between direct and OP catalogs to eliminate scratches and other stuff in the images. Moreover, the final end of this correlation is to allow us to use an optimal spectra extraction. We also attempt to search emission lines in unpaired objects from the OP image for the case of very strong lines and weak continuum objects in the detection limit.

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References

Alonso O., Zamorano J., Rego M., Gallego J. 1995, A&AS, 113, 399
García-Dabó, C.E., Gallego, J., 1998 A&SS in press