Data preparation

As a preparatory step, the data from the archive have been grouped into so called "associations" before processing them. An association is composed by a set of grism and direct images satisfying a specific set of rules, described below.

The process of building associations has been carried out in three steps: (1) identification of sets of grism images with similar properties, (2) addition of a direct image to each grism image, and (3) identification of complementary direct imaging in several bands.

1. They use the same guide stars
2. They have roll angle differences below 1 degree
3. They have an overlapping area larger than 50% of the whole field

We then used these criteria to define an equivalent relation among all grism images by adding a-posteriori a transitivity relation (i.e., two grism images are still compatible if there is a "chain" of compatible grism images). We then considered, for each grism frame, all imaging frames satisfying the same set of criteria described above. Among the various possible images, we selected the one showing the angularly closest one as "fiducial" direct image, to be used within aXe for the astrometric calibration of the grism frame.

Finally, we collected all images (in different filters) which have an overlap of at least 25% of the ACS field with the union of all grism frames. This set of images was used to create the reference image of the association and images in individual bands.