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 is a general purpose mask designed to fit SCT telescopes.
Carey Focus Mask - Description and Use
(Description courtesy of Farpoint Astronomical and George Carey)
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 center 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 as shown in the image below.
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.
In this example the offset of the Xs causes a noticeable gap between the left hand spikes, but the right hand spikes overlap, appearing to be single spikes.The mask should always be placed on the telescope in the same orientation so that the user becomes familiar with which way to alter focus. In this example, because the left hand spikes are 'split' you will know that the focus knob must be rotated anti-clockwise.
Closer to focus, but the left hand spikes are still more split than the right.Rotate focus knob anti-clockwise a little more.
|The 'split' in the left hand spikes is now the same as the right hand side.
Sharp focus has been reached.
|The top part of the image in detail.
It only takes a very small shift in focus to make a drastic change in the appearance of the spikes.
|This example from a different trial shows that
a very small anti-clockwise turn is needed.
|This animation shows focus being reached.|
Manufacturer part number: FP431
Product Video: How to select and fit a focus mask for your telescope