examples.rst

Examples

Very few stars on the field

Note

The minimum number of stars necessary to find a transformation is 3

If your field has few stars on the field, of about 3 to 6, you may want to restrict astroalign to only pick that number of stars, to prevent catching noisy structures as sources.

Use max_control_points keyword argument to do so:

>>> import astroalign as aa
>>> registered_image, footprint = aa.register(source, target, max_control_points=3)

This keyword will also work in find_transform.

Faint stars

If your stars are faint, they may not be bright enough to pass the 5σ threshold. If you need to lower the detection σ used in the source detection process, adjust the detection_sigma keyword argument:

>>> import astroalign as aa
>>> registered_image, footprint = aa.register(source, target, detection_sigma=2)

This keyword will also work in find_transform.

Avoiding hot pixels and other CCD artifacts

If your CCD is dominated by persistent defects like hot or dead pixels, they may be taken as legitimate sources and output the identity transformation.

We suggest cleaning the image first using CCDProc's cosmicray_lacosmic to clean the image before trying registration:

>>> from ccdproc import cosmicray_lacosmic as lacosmic
>>> clean_source = lacosmic(myimage)
>>> registered_image, footprint = aa.register(clean_source, clean_target, min_area=9)

Another quick fix can be increasing the expected connected pixels in order to be considered a source. Increment min_area from default value of 5:

>>> import astroalign as aa
>>> registered_image, footprint = aa.register(source, target, min_area=9)