Iris - Mosaic

The QM command (for QuickMosaic) assemble the image [name1] and [name2] in an unique image. QM is optimized for stellar images: The commun point between the two images is a star selected with the mouse (a simple click). QM is easy to use but the operation is rudimentary: only relative translation between the images is considered, not the distortion for example. The parameter [type] define the junction zone of the two images. If type=1 the image 1 is on image 2, if type=2 a pixel in the commun zone is the maximum of image 1 and 2, if type=3 a pixel in the commun zone is the minimum of image 1 and 2, if type=4 the commun zone is the mean intensity of image 1 and 2, if type=5 an interpolation is computed between images 1 and 2 for a more natural transition . For the majority of situation the type=5 option is preferred. QM can process black and white images and true colors images. Example, assemble the two images:

The two images. A correct commun star is selected (if possible bright but unsatured and isolated).

If the name of the two images is M27_1 and M27_2, the command is:

>QM M27_1 M27_2 5

The image M27_1 is first automatiquely displayed. Click on the selected star with the crossair. Then, the M27_2 is dispayed and select the same star. It is all.

QM2 is similar to QM but for non-stellar images. Iris use the intercorellation technique around the clicked zone. The syntax is:


First image. Click on a contrasted detail.	Second image. Select the same detail.

Like QM, the Mosaic dialog box (Geometry menu) allows you to combine a pair of images into a single, large image.

Before running a mosaic command, a crop operation is often necessary for elliminate unneeded areas in the commun parts of the images. For example, assemble these two images of Hercule galaxy cluster:

Select the valid area for the two images. For example drag with the mouse and run the Crop command in contextual menu (right click on the image). You can use also WIN or WINDOW (WINDOW2, WINDOW3, ...) familly commands.

Also, if the images there taken under varying conditions (variable integration time, variable atmpospheric transmission, etc), it may be necessary to balance the backgounds and dynamic range of each images prior building the mosaic. The commands OFFSET, MULT, NOFFSET (offset normalization of the background to a given value), NGAIN (gain normalization ot the background) are the basics tools for balance operations. This require also measuring the levels of background and/or stars in the overlapping regions. Use the cursor information, or statistics function (contextual menu or STAT console command).

Re-save the cropped file (field1 and field2 images for example), and open the Mosaic dialog:

Adjust the the position of field2 image relative to field1 image by acting on the arrow keys. The four arrow button will move the field2 image by the number of pixels indicated by the Step control. At a first stage the step increment can be large (20-30 pixels); At the final stage, you can select fractional values (0.2 pixel for example). For help you, select the Difference option: Iris compute the difference field1-field2 in the overlap region. The relative position of the two images can be edited manually (DX and DY controls).

For the final combination, select the Maximum, Mean or Interpolate option for the blending process (see QM console command). If the Interpolate option is selected, at each pixel in the area of the overlap, Iris calculates an interpolation function. This method gives the better results when residual tile background effects are presents.

Tip: The Save im2. button save the translated (or rotated if you use rotation button) Image 2. This function is very practical for align two images or a sequence of image relative to a reference image (the Image 1 term).