A simple usage example¶
Suppose we have two images of about the same portion of the sky, and we would like to transform one of them to fit on top of the other. Suppose we do not have WCS information, but we are confident that we could do it by eye, by matching some obvious asterisms on the two images.
In this particular use case, astroalign can be of great help to automatize the process.
After we load our images into numpy arrays, we simple choose one to be the source image to be transformed, and the other to be the target.
The usage for this simple most common case would be as follows:
>>> import astroalign as aa >>> registered_image, footprint = aa.register(source, target)
registered_image is now a transformed (numpy array) image of
source that will match pixel to pixel to
footprint is a boolean numpy array,
True for masked pixels with no information.
- If instead of images, you have lists of bright, reference star positions on each image, see Finding the transformation.
astroalign.registerwill also accept as input, data objects with
Numpymasked arrays. See Objects with data and mask property.
- Check this Jupyter notebook for a more complete example.
Flux may not be conserved after the transformation.
If your image requires special care see Examples.
Mask Fill Value¶
If you need to mask the aligned image with a special value over the region where transformation had no pixel information,
you can use the
footprint mask to do so:
>>> registered_image, footprint = aa.register(source, target) >>> registered_image[footprint] = -99999.99
Or you can pass the value to the
>>> registered_image, footprint = aa.register(source, target, fill_value=-99999.99)
Both will yield the same result.
Finding the transformation¶
In some cases it may be necessary to inspect first the transformation parameters before applying it,
or we may be interested only in a star to star correspondence between the images.
For those cases, we can use
>>> transf, (source_list, target_list) = aa.find_transform(source, target)
target can be either numpy arrays of the image pixels,
or any iterable of (x, y) pairs, corresponding to star positions.
Having an iterable of (x, y) pairs is especially useful in situations where source detection requires special care.
In situations like that, source detection can be done separately and the resulting catalogs fed to
find_transform returns a scikit-image SimilarityTransform object that encapsulates the matrix transformation,
and the transformation parameters.
It will also return a tuple with two lists of star positions of
source and its corresponding ordered star postions on
The transformation parameters can be found in
and the transformation matrix in
If the transformation is satisfactory, we can apply it to the image with
Continuing our example:
>>> if transf.rotation > MIN_ROT: ... registered_image = aa.apply_transform(transf, source, target)
If you know the star-to-star correspondence¶
estimate_transform from scikit-image is imported into astroalign as a convenience.
If for any reason you know which star corresponds to which other, you can call
Let us suppose we know the correspondence:
- (127.03, 85.98) in source –> (175.13, 111.36) in target
- (23.11, 31.87) in source –> (0.58, 119.04) in target
- (98.84, 142.99) in source –> (181.55, 206.49) in target
- (150.93, 85.02) in source –> (205.60, 91.89) in target
- (137.99, 12.88) in source –> (134.61, 7.94) in target
Then we can estimate the transform:
>>> src = np.array([(127.03, 85.98), (23.11, 31.87), (98.84, 142.99), ... (150.93, 85.02), (137.99, 12.88)]) >>> dst = np.array([(175.13, 111.36), (0.58, 119.04), (181.55, 206.49), ... (205.60, 91.89), (134.61, 7.94)]) >>> tform = aa.estimate_transform('affine', src, dst)
And apply it to an image with
apply_transform or to a set of points with
Applying a transformation to a set of points¶
matrix_transform from scikit-image is imported into astroalign as a convenience.
To apply a known transform to a set of points, we use
Following the example in the previous section:
>>> dst_calc = aa.matrix_transform(src, tform.params)
dst_calc should be a 5 by 2 array similar to the
If your image is stored in objects with
such as ccdproc’s
or a NumPy
you can use them as input for
In general in these cases it is convenient to transform their masks
along with the data and to add the
footprint onto the mask.
Astroalign provides this functionality with the
propagate_mask argument to
>>> from astropy.nddata import NDData >>> nd = NDData([[0, 1], [2, 3]], [[True, False], [False, False]])
and we want to apply a clockwise 90 degree rotation:
>>> import numpy as np >>> from skimage.transform import SimilarityTransform >>> transf = SimilarityTransform(rotation=np.pi/2., translation=(1, 0))
Then we can call astroalign as usual, but with the propagate_mask set to True:
>>> aligned_image, footprint = aa.apply_transform(transf, nd, nd, propagate_mask=True)
This will transform
nd.mask simultaneously and add the
footprint mask from the transformation onto
>>> aligned_image array([[2., 0.], [3., 1.]]) >>> footprint array([[False, True], [False, False]])
Creating a new object of the same input type is now easier:
>>> new_nd = NDData(aligned_image, mask=footprint)
The same will apply for
CCDData objects and
NumPy masked arrays.
See Module API for the API specification.