Align stellar images (automatic methods)


The register commands are used to align a set of images. Several options are offered to carry out registration. All are compatible with 16 and 48 bits images.

For example, align the three preprocessed images of Messier 105, named i1, i3 and i3 (see here).

The simplest and the fastest method consists to identify an isolated star, common between images of the sequence. For example, load the first image


and define a selection rectangle with the mouse around an isolated star, bright, but unsaturated if possible. The registration of the images will be done with respect to that star. The size of the rectangle has to be large enough so that it contains the displacement of the star between two contiguous images (if this not the case, Iris could take a wrong star to perform the registration).

Open the Stellar registration command of Processing menu. Select the One star method:

The input sequence is i1, i2, i3. The ouput sequence (registered images) is j1, j2, j3. Click OK.

The equivalent console command is


Because only one star is concerned for register, images are aligned using shifting (at a fraction of pixel). No rotation or scaling is performed. The shift between images can be observed in the shift.lst file, automatically created in the working directory:

Note that the first image of the sequence is the reference; the shift is always null for this image. For many situation, registration with only one star is very powerful. It is the simplest and natural mode for align deep-sky images, useful for moderatly rich stellar fields.

You can now add registered images. For example

>ADD J 3

or activate Add a sequence command of Processing menu:

Try another mode for registration, more appropriate for crowded stellar fields. Select a zone containing several stars in the first image of the sequence, then

The method use a pattern recognition scheme between many stars in the selected zone. If the option Select a zone is not checked, the computation concern all the image. The geometric tranformation applied is a simple shifting, sufficient for the vast majority of the situations.

If images are affected by a field rotation phenomena for example, an affine transform is necessary. The software compute shifting, rotation and scaling for aligning the images of the sequences. A fast procedure for this consists to select the Three matching zone method:

Images are matched by looking for similar patterns in three automatically selected zones of the image. Size of each zone is defined by the parameter Zones size of the dialog box. Here is the position of the matching zone and the sigification of the size parameter:

The three matching zones method is  optimized for register large stellar DSLR images.

The global matching use similar triangle method for automatically identify commom stars in each image. It is possible of select an affine transformation, a quadratic transformation or a cubic transformation:

High degree transform (quadratic or cubic) permit to align for example images affected by differential optical distortion, or image taken with different telescopes.

An equivalent console command for register only two images by using an affine transform is COREGISTER.

For example, register there very different images of the galaxy NGC266 (one taken with a refractor, the other one taken with a reflector):




The image TELESCOPE2 (second parameter of the command) is transformed to match the image TELESCOPE1. The result is the current image in-memory:

New aspect of image TELESCOPE2

For exemple, it is now possible to add the image TELESCOPE2 to image TELESCOPE1:



For register NGC266 images from the dialog box, enter the parameters:

The equivalent console command for a global matching of the whole surface of images is COREGISTRER2. For example, for a quadratic transform enter:


For large image (DSLR) containing many stars, the procedure is time consuming. Prefer the dialog box Stellar registration and carry out registration on a zone of moderate size defined which the mouse.