Donut: extracting aberrations from a defocused image

An experienced optician can detect low-order aberrations by looking at the defocused image of a point source. A method to fit such image to a set of Zernike aberrations has been developed. Read the paper [PDF, 174K ] to learn more!

Reference: Tokovinin A., Heathcote S. Donut: measuring optical aberrations from a single extra-focal image. PASP, 2006, V. 118 pp. 1165-1175

Why use this method? No special wave-front sensor is needed to quantify the aberrations in the image - just the regular detector. No intervening optics - measurement refers to the image itself. Aberrations across the field can be studied. A similar method has been used to study the optics of the Hubble Space Telescope (J.E.Kirst & C.J. Burrows, 1995, Appl. Opt. 34, 4951)

Applications: Alignment and testing of astronomical imaging instruments and telescopes. Active optics. Fast focusing.

Screen shot

An IDL code with a GUI interface does the job. The screen-shot shows the defocused image extracted from a standard focus frame (pixel scale 0.27", 16x16 pixels) on the left and its fitted model on the right. One can edit the parameters (e.g. the number of the fitted Zernike terms), extract another star interactively from the same frame by pointing with a cursor or load a new FITS frame. After extracting the image, click 'Fit' and watch the process which takes few seconds.

The numbers below show the coefficients (standard Zernike, i.e. rms wave-front) in microns and seeing (also fitted). The rms residual error of this fit is 10.7%.

Get the code [tar.gz] and use it at your own risk! The distribution contains example of data with a parameter file and a very short instruction, to provide an easy start. Warning: the code is not a commercial product, it is given as a working example only. No user support will be provided.

Created: May 4, 2006 A. Tokovinin
Last modified: Sep 7, 2006