The three images do not necessarily need to be from the same image source. The 100 micron band is defined as red, 60 microns as green and 25 microns as blue, giving the result shown in Figure 19.7, “A three-colour composite image display for the blazar OJ287”.įigure 19.7. A three-colour composite image display for the blazar OJ287 from IRAS data after applying the dialogue in Figure 19.6, “The three-colour composite image display dialogue” - the 100 micron image is mapped into red, the 60 micron image is mapped into green, and the 25 micron image into the blue layer. The dialogue to create a three-colour plot for the blazar OJ287 from IRAS data (identified with a small box produced using the "show current fixed target" overlay) is shown in Figure 19.6, “The three-colour composite image display dialogue”. For example, to produce an rgb plot from 2MASS data, you should select 2.17 microns for the red image, 1.65 microns for the green image and 1.25 microns for the blue image. This results in a menu from which you can select the next image in the composite. To read in another image and map it into another colour plane, for instance for the green layer, select the "Add Colour: green" item in the layer control box shown in Figure 19.6, “The three-colour composite image display dialogue” for the base image. Select a colour and the image to read in as that colour. Then, select "Make This a 3 Colour Plot" in the "Three Colour Plots" box in the "Image" window. To produce a composite, first select an image from the "Images" menu. The three images are read individually into the 'red,' 'green,' and 'blue' (RGB) colour planes to produce the composite. HSpot provides you with the ability to produce three-colour composite image display. This feature can be used to blink frames to compare images. An icon will appear in the top right hand corner of the screen (see Figure 19.5, “The image display icon (circled in red) that allows you to blink between several images in the same frame.” - the icon is highlighted here with a red oval) that allows you to pass between images every time you hit the left or right arrow. Just one tab will appear in the bottom bar - that of the last image to be displayed, in this case the 2MASS K image - and the other two bands will be hidden. However, HSpot also allows you to place the three images on top of each other in a single layer. This option is useful for displaying images of the same object at different wavelengths to compare them. This places each image side-by-side in the top layer and each image may be controlled seperately by a tab in the bottom bar. The default option is "Panel plots 1 per frame". HSpot allows you several display options, as shown in Figure 19.4, “The image display dialogue set to show three images in the same frame.”. You can move it between other overlays in the frame by clicking on any part of the layer control box and dragging it between layers to where you want it. The Base Image layer does not need to be the bottom layer. Each overlay introduces a new layer control box on the side bar. You can create several layers and then hide or show them with the show/hide layer icons without having to recreate them each time. The side bar contains the controls of image and overlay layers. The colour table control icon brings up a dialogue allowing you to adjust the colour table for the image. This is very useful if you have a second image as an overlay in a frame (not 'loaded' in the same frame, overlaid as in Overlay and then Image Overlays from the HSpot menus). This allows you to control how opaque will be the image layer.
The opacity control icon brings up an image opacity control dialogue. If you click the delete layer icon, the image layer in the frames will be deleted. If you click the show/hide layer icon, HSpot hides or shows the image. The side bar to the right of the image frames ( Figure 19.3, “ The HSpot screen after downloading a 2MASS K-band image of M51.”) shows a box that says "Base Image" with show/hide layer, delete layer, opacity control, and colour table control icons in it (hold the mouse over each icon for a short time to see the tooltip indicating its function). 19.2.4. Layer control and blinking images
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