### 10 Using World Coordinates

This recipe demonstrates the simple use of World Coordinates in GAIA. Recall that a World Coordinate System (WCS; see Section 4.1) relates the positions of pixels in an image to celestial coordinate systems on the sky. In practice it allows you to display and annotate images in terms of celestial coordinates.

Some images include a WCS as part of the auxiliary information that they contain; others do not. For example, ngc1275jkt.sdf, the JKT image used in the recipe in Section 8 does not possess a WCS, but images retrieved from the DSS (as in the recipe in Section 9) do. It is possible to use GAIA to add a WCS to an image, and the recipe in Section 12 is an example doing so. The present recipe merely gives some examples of using a WCS.

The recipe uses file ngc1275dss.sdf which is an image centred on the galaxy NGC 1275 extracted from the DSS. It will be very similar to the image that you created in the recipe in Section 9, and you could substitute your own image if you prefer.

Load file ngc1275dss.sdf into GAIA and adjust the colour table so that you can see the stars and galaxies. The display should look something like Figure 4.

#### 10.1 Reading off the Right Ascension and Declination

It is straightforward to read off the approximate Right Ascension and Declination of any object in the image.

(1)
The $x,y$ pixel positions and the Right Ascension and Declination of the current cursor position are shown in the boxes labelled: X:, Y:, $\alpha$: and $\delta$: in the control panel in the middle of the upper portion of the main window. To find the approximate coordinates of a star simply position the cursor over it and read them off.
(2)
In a DSS image J2000 equatorial coordinates are shown by default. It is possible to configure GAIA to display other celestial coordinates. Click on the Image-Analysis button in the menu-bar along the top of the main window and choose Change coordinates and then Celestial coordinates….

A window appears which allows you to specify the coordinate system. Choose the one required (perhaps ecliptic coordinates) and then click the Accept button.

Henceforth coordinates are displayed in the chosen system (but note that the coordinate boxes are still labelled ‘$\alpha$:’ and ‘$\delta$:’, even if ecliptic or Galactic coordinates have been selected).

Also, the coordinates are only updated when the cursor moves. Thus, if it is already positioned over the star that you are interested in, then you need to nudge it off the object and return it to the required position.

(3)
Depressing the Caps Lock key will prevent the $x,y$ and $\alpha$,$\delta$ values being updated as the cursor moves. This facility is useful, for example, if you wish to preserve the coordinates whilst moving the cursor to another window.
(4)
A list of positions in an image can be selected and saved as a text file by selecting the Positions… item from the Image-Analysis menu (in the menu-bar along the top of the main window).

#### 10.2 Showing celestial coordinate grids

GAIA can superimpose a celestial coordinate grid over the image.

(1)
Click on the Image-Analysis button in the menu-bar along the top of the main window and choose Overlay axes grid…. A window will appear with numerous options. You can change some of these if you wish, but there is no need to do so. Simply click on the Draw button and an axis grid is drawn in the current celestial coordinate system.

The grid is removed when you click Close to close the window.

(2)
To show a grid in a different celestial coordinate system click on the Image-Analysis button in the menu-bar along the top of the main window and choose Change coordinates and then Celestial coordinates…. A window appears which allows you to specify the coordinate system. Choose the one required and then click the Accept button.

Then repeat step 1, above, and a grid will be drawn in the new system.

#### 10.3 Measuring angular separation

To measure the approximate angular separation between two objects simply position the cursor over the first of them, and then click and hold down mouse button three. Continuing to hold down this mouse button, move the cursor until it is positioned over the second object.

A ‘rubber band’ cursor is drawn as the mouse moves. It shows the vector between the original and current positions and also the offsets in two orthogonal coordinates. The separation and offsets are all labelled with their size in minutes of arc, with a sexagesimal subdivision into seconds.

The cursor disappears when the mouse button is released.