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
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.
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.
It is straightforward to read off the approximate Right Ascension and Declination of any object in the image.
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 ‘:’ and ‘:’, 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.
Caps Lockkey will prevent the and , 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.
GAIA can superimpose a celestial coordinate grid over the image.
The grid is removed when you click Close to close the window.
Then repeat step 1, above, and a grid will be drawn in the new system.
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.