Align astronomical images
A science frame autoguiding and image alignment algorithm with sub-pixel precision, capable of guiding on defocused stars.
Project documentation: https://donuts.readthedocs.io/en/latest/
See the changelog for latest changes.
We operate or have access to several telescopes (NGTS, NITES, Warwick 1m, 1.5m San Pedro Martir) that require precise autoguiding. Sometimes we need to defocus a telescope but we would still like to autoguide. Donuts was designed to allow this. The algorithm had to be simple, fast and accurate. It has been shown to perform well as an autoguiding algorithm for equatorial telescopes (no field rotation).
The process of aligning apertures for photometry is essentially the same. Rather than correcting the telescope pointing, the apertures must track the drift of the stars. Donuts can therefore be used to track the stellar positions for CCD photometry also.
By default Donuts measures frame-to-frame translational offsets (X and Y) using all the stars in the image. The algorithm could be adjusted in the to select a specific region of interest (for extremely wide or distorted fields).
The algorithm has its limitations. It currently does not deal with rotation or very large drifts - where the field moves by approx. half a FOV or more. Our paper describing the details can be found here:
Below is a sample of 10 nights autoguiding residuals from NGTS while using Donuts. The upper plot shows the frame-to-frame error, while the bottom shows the drift which would have occured if not for Donuts. Aligning photometry apertures is essentially the same process and similar performance is expected under that scenario. We routinely achieve an autoguiding RMS of 1/20 pixels with NGTS.
Copyright (c) 2016 James McCormac & Simon Walker
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