Two 2" off-axis holes (with cover plates) in the original dust cover serve as a focussing aid for astrophotography.

Inserts can be used to stop-down the hole size to 1/2" diameter for precise CCD focussing - visual work needs the larger 2" holes.

A new focussing mask to fit the Meade dew-shield.

One method is to introduce large diffraction effects using twin parallel bars placed in front of the telescope objective (in front of the corrector plate in the case of an SCT). This produces two enhanced diffraction spikes which, when the image is in focus, are superimposed to form a single spike. However, to get distinct spikes requires a fairly bright star and optimal exposure (without blooming), and the 2 spikes are not far seperated at best.

A second method is the 'knife-edge' whereby a plane is setup at the exact same position in the focusser as the film plane or surface of the CCD chip would be, and a knife edge is then introduced into the light cone. It works much like the Focault test for mirror testing, and un-even image dimming indicates an out of focus position. Whilst very accurate, the disadvantage of this method is of course that the test plane then needs to be replaced by the actual image plane, whether it be the film camera body or CCD camera. This substitution can introduce errors.

Actual CCD image through 2-hole mask of an out of focus star (above) and a different star very nearly in focus (below)

By far the best method (in my own limited experience) is to use a 'Hartman Mask', which is an opaque mask with 2 holes at the extreme edges of the maximum aperture of the telescope, and placed over the front of the telescope. The view through such a mask consists of two distinct images, one from each hole, until the image reaches critical focus whereupon these two images are superimposed. For visual work, fairly large holes are required (or a very bright star) in order to get a sufficiently bright image from which to judge focus. A CCD camera on the other hand, being far more light sensitive, can take advantage of much smaller holes and/or increased exposure times. There is a very clear advantage to reducing the hole size - the effective base-line (or seperation) of the two out of focus images is increased, and it is thus much easier to visually judge critical focus position. For CCD focussing using my 10" LX200 and 416XT camera I use 1/2" or 3/4" inch holes the full 10" apart, together with an exposure time of between 100 and 200 milliseconds using a mag 3 or 4 star. This produces two very distinct, small circular images, and there is no doubt whatsoever when they are superimposed at the exact focus point. Very accurate focus is achieved in a couple of minutes using this method, and it can be done with the telescope pointing at a star very close to the target object to be imaged (there is a danger when slewing the telscope far from the target object for focussing purposes of introducing flexure and thus small loss of focus precision). Fainter stars can of course be used simply by increasing the exposure time up to several seconds.

The mask shown in the top picture was my prototype, a simple adaption of the original cover for the 10" LX200. I used 2" holes as an aid to visual focussing, and inserts for stopping down the hole size for CCD focussing. This worked fine until I purchased the Meade dew-shield, whereupon I found the original cover did not fit the end of the dew-shield. Removing the dew-shield each time to focus was a pain, so I made a second mask to fit the dew-shield. This is a more simple model, without stop-down inserts, and using 3/4" holes which I'd found to near optimal. I needed to fit a ring to the inside of the dew-shield about 2" from the front end to act as a ledge for the mask to rest against. To use, the mask is simply placed in the end of the dew-shield, focus, then lifted out again using the knob fitted to the center.

Back to Telescope Accessories page