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Image Mosaics

Contents Introduction – Image Mosaics ...................................................................... 1 Downloads ..................................................................................................... 1 The Images..................................................................................................... 1 Making the Mosaic......................................................................................... 3

Introduction – Image Mosaics If imaging galaxies or clusters with large angular size, the whole galaxy or cluster may not fit into the field of view of the telescope. In this case, the way to obtain an image of the entire target is to take a number of smaller images with the telescope and then mosaic them together. The images need to overlap with each other so that there are indentifiable points that are common on adjacent images for registration (lining up the images with each other exactly).

Downloads From here you can download everything you need to get you started with the Image Mosaics project. 1. Download and install IRIS This software will allow you to line up the images with each other and save the resulting image as a new file. Follow the instructions for Configuring IRIS to set your working path to C:\data. 2. Download the image files Save these files to your hard drive in the directory C:\DATA 3180-1-b.fit, 3180-2-b.fit, 3180-3-b.fit and 3180-4-b.fit are four images of quadrants of the galaxy NGC 3180. The images were made through a B-band (blue) filter, but when you open them they will be in black-and-white.

The Images The images you are working with are shown in Figure 1. This tiled display was produced using ds9 software, but it is not necessary for you to download ds9 or to produce these tiled images. (This software will not be used to make the mosaic – IRIS will be used instead.) In IRIS, you can only work with two images at a time, so you will start with the top two images to make a new image called 3180top.fit. You will repeat the process with the bottom two images to make 3180bottom.fit and, finally, you will join the two new images together.

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Figure 1: The four images are displayed with their file names. Note that there are regions of overlap at the edges.

ASISTM Faulkes Telescope - Deep Space in the Classroom http://www.astronomy.mq.edu.au/deepspace/

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Making the Mosaic 1. Open the first two images in IRIS Open IRIS. Select Geometry > Mosaic. This will open a window that looks like Figure 2. You will need to type the file names of the first two files into the boxes labelled Image 1 and Image 2, omitting the .fit extension. You will initially set the Step to something large (10 or 20. Then click OK.

Figure 2: The mosaic window. The file names, omitting the .fit extension, are typed into the boxes Image 1 and Image 2. The step size may be set to any value. Choose a large step size initially (say 10 or 20).

2. Align the first two images

You should now see some kind of image in the display. If you zoom out with the icon, you may be able to figure out that the display is showing you both images superimposed over each other. The images may be easier to see if you set the Threshold limits to 1000 and 0 as shown in Figure 3. You now need to move Image 2 to make it line up properly with Image 1. You need to reopen the Geometry > Mosaic window. Then align the images by clicking the up, down, left and right arrows in the centre panel of the Mosaic window. When you do this, the images will automatically resize and you may not be able to see what is happening. If you scroll down so that you can see the centre of the galaxy (which is at the bottom of both these images) it may be easier to follow the movements. You can also try doing 5 clicks and zooming out again to note the changes.

Figure 3: When you zoom out you will see the centre of the galaxy in two places. The first two images are superimposed on each other.

Provided you have the same settings as shown in the Figures, clicking the left button should move Image 2 in the correct direction. You may notice the tiny white specks in the images. These are not stars. They are “hot” pixels on the CCD detector that appear in the same

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positions on all the images. The larger white spots are the stars you need to align with their counterparts in both images. As you get the images closer to their correct positions, you will notice that you can see the overlap region in the centre and, on either side, the parts where you are now seeing the individual images. These are shown in Figure 4. Also notice as you do this that the DX number on the Mosaic window is changing. This is telling you how far you have moved Image 2 from its original position.

Figure 4: Aligning the images step-by-step.

As the images get closer to the proper alignment, you will need to change the step size to 1 or even 0.2. It will also help if you check the Difference button on the Mosaic dialog box. This will show the stars as black on one image and white on the other, making them easier to match up. At first, you can get the large bright stars into approximate alignment. When these look pretty close, you may find it easier to look at the smaller, faint stars. When you are happy with your alignment, you can recheck the Maximum button and view the result by zooming out. It should look something like Figure 5.

Figure 5: The first two images are aligned.

3. Save the aligned images as a new file Go File > Save and save the new image as: 3180top.fit 4. Load the next two images Leave the settings in the Mosaic window as they are and load in the next two images 3180-3-b and 3180-4-b by typing the changes into the Image 1 and Image 2 boxes and pressing OK. If you zoom out, you will see that these two new images are roughly aligned already because the offsets (DX and DY) are similar to those for the first two images.

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5. Align the second pair of images You need to reopen the Geometry > Mosaic window and repeat Step 2 to obtain fine alignment. When this is done, save the new image as: 3180bottom.fit 6. Align the new images with each other Load 3180top.fit and 3180bottom.fit and reset DX and DY to zero. Align these two images as before (if you load 3180bottom.fit as Image 2, you will need to click the down arrow initially). When these two images are combined, you should be able to zoom out and see the whole galaxy as one complete image. If you select View > Modified Equalization, it should look something like Figure 6. Now save the new image as: 3180.fit

Figure 6: The final image of NGC 3180.