Carey Focus Masks
In 2009, George Carey, an amateur telescope maker and astronomer from the UK, published a modification of previous focus mask designs to make a much more sensitive version of the device. This focus mask allows you quickly, easily and precisely focus your DSLR camera, webcam or CCD camera for astrophotography. The method is simple, very intuitive, and the device has universally been named after the inventor: The “Carey Mask”.
Farpoint manufactures this “lifesaving” astrophotography tool out of virtually indestructible ABS plastic, which comes in two major types and in a range of sizes. The mask pictured below left is a general purpose mask designed to fit refractors and Newtonian telescopes. The mask pictured below right is designed to fit Schmidt-Cassegrain telescopes. Both types of mask are available in a range of sizes to cover your imaging needs.
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Carey Focus Mask Description and Use
(Thanks to George Carey for letting us plagiarize this information from his website at: http://www.geoastro.co.uk/careymask.htm )
The Bahtinov Mask requires you to judge when the central spike is midway between the two angle spikes.
This is quite easy to do, but there is still a little uncertainty in determining the exact centre alignment, especially when viewing in a DSLR viewfinder.
With the Carey mask the central spike idea has been abandoned. The angle between spikes has been considerably reduced. Instead of two sloping sets of slits and one central set, there are four sets of slits, with a small angle difference on each side of the mask. In the prototype I used 10 degrees and 12 degrees.
|12 degrees>||< 10 degrees|
The diffraction pattern produced is two overlapping ‘X’ shapes, with slightly differing angles and at perfect focus the Xs will be symmetrical.
Inside and outside focus the Xs will be displaced. Because the angle of each X is slightly different, it will be easy to spot when out of focus, and also which way focus should be altered.
The left X is inside focus. The middle X is in focus. The right x is outside focus.