Focusing an image on a CCD camera can be a tricky task for the inexperienced. Certainly when I started out on this journey, I used to spend much of the available observing time acquiring and focusing the image.
One of the techniques I eventually found was to use a Hartmann mask. This is a simple device which can be bought under the trade name of Kwik Focus or made from basic materials. They can be made with 2, 3 or 4 holes and this description will apply to a 4 hole version.
When focusing is required, the mask is placed over the objective in the same way as the lens cap or objective cover. The telescope is then be pointed at a bright star or group of stars and a series of short exposures is taken with the CCD camera.
If the telescopes is not in focus, each star in the field will appear four times - once for each hole in the mask. Adjust the focus control and as the telescope is brought into focus, the four images will move closer together and will finally merge into one image. As the stars get closer together, it would be best to use dimmer and dimmer stars to judge the degree of merger. When the faint stars merge, the system should be very well focused.
Once this has been completed, the mask can be removed. Further checks can be made by checking the brightness readings of a particular star. When the value is at its greatest, the star is in focus.
Typical sequence using a Hartmann Mask
The mask itself is easy to make. I made mine from plywood. The disk should be made just slightly larger than the circumference of the scope body ( e.g. the diameter of the dust cover) so that the disk does not slide down into the scope. The outside edges of the holes are located very close to edge of the corrector plate or lens. The hole size is not critical but I made mine about 2" diameter to suit my 10" scope. As a guide, they should be approx 1/5th the diameter of the corrector plate or lens. Too big and the star images will not separate enough while too small and the images will be very dim.
To hold the mask in place I used two strips of Velcro tape. One end of the tape is secured to the mask while the other mates with its companion on the body of the scope.
I have also made one these masks for my 300mm camera lens which I use for wide angle images with my CCD camera.
The number of holes is not critical, and the photo shows one of my masks with 3 holes. It has been suggested that using triangular holes can give a more accurate in-focus position, as diffraction spikes are produced and, when they overlap, the system is in focus.
The photo below shows two of the masks I made, one in position on a 300mm camera lens.