User manual

Aladin definitions

 An astronomical image is a rectangular array of values representing a field of

view of the sky. The astronomical image is usually provided with other information

about its origin and its calibration (sky position, pixel size, type of projection...);

 An astronomical catalogue is a table, or several tables, for which each row

contains information about an astronomical object called a "source" (ID, sky

position, physical measurements...);

 A graphical overlay is one or several graphical shapes (line, circle, polygon…)

associated to sky positions;

 A view is a projection of a image area on which has drawn catalogue source symbols

and/or graphical overlays;

 The sky position is considered as a couple of angles (RA - right ascension, DEC -

declination) specifying a celestial sphere position. Aladin does not manipulate the

concept of distance to the observer.

We will briefly describe the available Aladin operations on images, catalogues, graphical

overlays and views.

Image processing functions

 Pixel range adjustments (contrast);

 Symmetry (top / bottom, right / left);

 Image colour composition from 3 original

images;

 Image mosaicing;

 Cube generation from several images

covering the same field;

 Image resampling (image re-projection

according to the astrometrical solution of

another image);

 Image astrometrical calibration (by

parameters or by matching stars);

 Pixel computations (addition, subtraction,

multiplication, division, convolutions,

normalisation).

Note: In case of “huge images” (several

gigabytes), only the basic processing

functions are available (pixel adjustment,

symmetry)

catalogue processing functions

 Source measurement operations (selection, filtering, sorting, tagging….);

 Graphical symbol drawing according to some source measurement values (e.g. circles

proportional to the magnitude, arrows based on the proper movements, error

ellipses...);

 catalogue cross matching;