User manual

Computing expression

To create a new column, one needs to indicate the computing expression that Aladin will

use to create values for this new column. The writing is done in the central box called

“Expression”. The syntax uses usual conventions for algebraic expressions. References to

other columns keep the syntax:

${column_name}

To avoid writing mistakes, the main window gives a list of buttons showing the column

names for the chosen catalogue. A click with the mouse on one of them inserts its

reference at the cursor’s location in the expression-writing box.

It is also possible to insert operators (+, -, *, /…) and arithmetic functions (abs, cos, sin,

rad2deg, …) by clicking on the respective buttons and selectors.

Generating a column

Once generated with the button “Add a new column”, the new values will appear on the

right of the measure table. They are displayed in orange-brown in order to be easily

distinguished from the original measures. This new column can be used as any other

column (sort, filter…) if saved; it will be kept with other columns.

5.8Astronomical calibration window

Menu: Image => Astronomical calibration …

catalogue => Astronomical calibration …

When Aladin overlays sources on an image, it uses the astronomical solution associated

to the image. This is information that makes the link between each pixel in an image an

image and a location on the sky, and vice-versa. There are several possibilities that

describe a projection method from a part of the celestial sphere onto a plane (tangential,

sinusoidal, …). Aladin deals with most current projections used in astronomy (SINUS,

TANGENTIAL, AITOFF, ZENITAL_EQUAL_AREA, STEREOGRAPHIC, CARTESIAN,

NCP, ZPN)

Information on the astrometrical calibration

Aladin uses 3 methods to obtain the astrometrical calibration for an image:

 When dealing with FITS format image, information on the calibration will be

obtained with the key words in the FITS header (according to the WCS standard);

 For a JPEG image, Aladin also knows how to recognize an astrometrical calibration

given in its comment segment (according to the JPEG standard). This calibration has

to follow the syntax with FITS keywords, followed by a ‘\n’ at the end of each line;