3 Extraction of astronomical objects

All sources (stars, galaxies, defects etc.) are extracted using the same algorithm as described in Paturel et al. (1996, Sect. 3.1), except that no attempt was made to share interacting objects which are simply flagged after visual inspection (Sect. 5). The reason is that we are interested first of all in well defined objects. At the end of this stage, we obtain for each frame a collection of matrixes (see an example in Fig. 2). Matrixes smaller than 17 pixels are rejected. They correspond, to the mean, of objects of 6 arcsec in diameter.

  

Figure 2: Example of a matrix for a small galaxy. One pixel is $2'' \times 2''$. The edge is outside the matrix

Astrophysical parameters are extracted for each matrix according to Paturel et al. (1996, Sect. 3.2). These parameters are the following:

• Weighted mean position of the center $x\rm _m$ and $y\rm _m$ (in pixels), where the weight is the pixel intensity f_ij
• major axis $\log D$ (D in 0.1 arcmin) at faint isophotal level (external diameter).
• axis ratio $\log R$ (log of major to minor axis)
• position angle of the major axis $\beta$ (in degrees, counted from North towards East)
• magnitude $I= -2.5 \log (\sum_{i=1}^{n} f_{ij}- f_{\rm bg})\rm + cst$(in arbitrary units), n is the number of pixels with intensity f_ij larger than the threshold.

We have now to perform astrometry (conversion of pixels positions to right ascension and declination) and then recognition of "galaxies'', "stars'', and "unknown objects''.