11 Superimposing and Contouring Images

This recipe is an example of superimposing two images in order to compare them. GAIA has two mechanisms for superimposing images: displaying two or more images and rapidly blinking between them (in a manner analogous to the ‘blink comparator’ used to compare photographic plates), and displaying one image as a set of contours superimposed on a display of the other. This recipe demonstrates the latter technique.

GAIA aligns images by using their World Coordinate Systems (WCS; see Section 4.1). Images that do not have a WCS are aligned by assuming that their pixel coordinates are coincident, which can be useful in some circumstances. Images extracted from the Digitised Sky Survey (DSS) have a WCS, and this recipe will use the DSS image of NGC 1275 retrieved in Section 9. Contours constructed from an X-ray image of the same region of sky will be drawn on top of it. The X-ray image is contained in file ngc1275hri.fits. It was obtained with the HRI (High Resolution Imager) on-board the ROSAT X-ray astronomy satellite (Röntgensatellit). The copy used here was retrieved from the LEDAS14 data archive service at the Department of Physics and Astronomy, University of Leicester. The image is in FITS format (see Section 4 for details of the data formats available to GAIA) and it too contains a WCS.

Proceed as follows.

(1)
It is useful to examine the X-ray image of the NGC 1275 region before trying to contour it. (You could skip these steps if you were already familiar with the properties of the image to be contoured). Start GAIA and load file ngc1275hri.fits.
(2)
Adjust the colour so that the very nucleus of the X-ray emission is clearly visible. For example, set the Auto Cut: to 100%. Then click on the View menu and select the Colors… option. A panel will appear. Set the colour scale algorithm to Logarithmic, the colormap to heat and the intensity to ramp. The display should now appear similar to Figure 6.

It is immediately obvious that the sky appears very different at X-ray and optical wavelengths. Also, by moving the cursor from one edge of the image to the other and noting the change in Right Ascension or Declination it is apparent that the image is rather larger than the 7${}^{\prime }$ field extracted from the DSS.

(3)
Aside: you might be interested to check that the X-ray image really is centred on NGC 1275.
(a)
Click on the Data-Servers menu, towards the right of the menu-bar at the top of the main window and choose the Catalogs option. Select the RC3 at CADC catalogue, which is a version of the Third Reference Catalogue of Bright Galaxies[17] provided by the CADC (Canadian Astronomy Data Center)15.

A window similar to Figure 5 should appear. Click on the Search button (in the bottom left of the window) and after a couple of moments the galaxies in the RC3 which overlay the X-ray image will be listed in the Search Results box and plotted in the main window.

(b)
One of the plotted objects is almost exactly coincident with the centre of the X-ray emission. If you click on this symbol the line for NGC 1275 is highlighted in the Search Results box.
(c)
If you double-click on the line for NGC 1275 in the Search Results box the corresponding symbol in the image is labelled as NGC 1275.
(4)
Before contouring an image it is useful to have some idea of the range of numbers that it contains. One possibility is to draw a slice through the image. Click on the View button on the menu-bar along the top of the main window and choose the Slice… item. You can interactively define a slice through the image which is then plotted as a graph (see Figure 17 in Section 14, below, for an example).

Alternatively, click on the View button on the menu-bar along the top of the main window and choose the Pixel Table… item. A table showing the values of a small grid of pixels centred on the current cursor position is displayed. You can move the cursor over the image examining the values. For the present purposes the largest grid permitted, 9x9, is the most appropriate.

You will probably want to experiment with these options for a while.

(5)
You are now ready to proceed with contouring the X-ray image on top of the DSS one. Load the DSS image ngc1275dss.sdf into GAIA. If you prefer you can substitute the DSS image that you retrieved whilst working through the recipe in Section 9; they should be very similar.
(6)
Set the colour table and magnification as in Section 9: click on the View menu and select the Colors… option. Set the colour scale algorithm to Linear, the colormap to ramp and the intensity to neg. Then click on the Close button.

Set the magnification by clicking on the Scale: button (in the bottom left of the control panel in the centre top of the window) and setting it to 2x.

The appearance of the display should now be similar to Figure 4.

(7)
Click on the Image-Analysis button on the menu-bar along the top of the main window and choose the Contouring… item. A window similar to Figure 7 should appear.

(8)
Click on the Choose file… button and load file ngc1275hri.fits.
(9)
Click on the Generate button (on the left hand side of the window, immediately below Levels). A window similar to Figure 8 should appear. Set the following values:
 Number: 5 Algorithm: linear Start: 100 Increment: 75

The window should now appear exactly like Figure 8. (In this recipe the required contour levels are already prescribed, however, the first time you contour an image you will probably need to experiment to find suitable levels. One way to do so is to set the Algorithm: to automatic and allow GAIA to generate the levels itself. If these levels are not satisfactory you can adjust them manually.)

Click on the Generate button and the appearance of the window should revert to that in Figure 7.

(10)
For all five of the specified contours click on the colour boxes and set them to red (which makes the contours easier to see if you are using the colour table set up above).
(11)
Click on the Draw Contours button. Contours of the X-ray image will now be drawn superimposed on the X-ray image. The appearance of the plot should be similar to Figure 9.

It is immediately obvious that the X-ray emission is centred on NGC 1275 and also that it extends beyond the optical limits of the galaxy (recall that the faintest contour drawn corresponds to quite a bright level in the X-ray image).

(12)
When you have finished inspecting the image click on the Close button and the contours will disappear.

11.1 Contouring the displayed image

Sometimes you might wish to plot contours generated from the displayed image, rather than superimposing those from another image. The procedure is very similar.

(1)
Display the required image.
(2)
Click on the Image-Analysis button on the menu-bar along the top of the main window and choose the Contouring… item, as before. A window similar to Figure 7 should appear.
(3)
Do not click on the Choose file… button to specify the image to be contoured. Rather, proceed immediately to specifying and generating the contour levels by clicking on the Generate button and then proceed as before.

14http://ledas-www.star.le.ac.uk/