Focusing a camera attached to a telescope has always been a challenge. Fortunately, focusing a CCD camera is simpler than focusing a film camera because the operator can get immediate feedback. However, to get useful feedback, the operator needs a little help. The round, white thing you see above is a Hartmann focusing mask for a C11 telescope. When it comes time to focus the CCD camera, the mask is placed over the objective in the same way as the objective cover. The telescope is then be pointed to a good clustering of stars of varying degrees of brightness and short duration focusing exposures are started.
When the telescopes is not in focus, each star in the field will appear four times - once for each hole in the mask. 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, one should use dimmer and dimmer stars to judge the degree of merger. If you are using a ST-7 camera, you can go into the Planet mode to speed up the operation. When the faint stars merge, you will have pretty nearly perfect focus. Perfectionist read on....
You can continue to refine the focus using the brightness readings of a selected star. This is the most common method of focusing. The difference is that you are starting with a nearly perfect focus. Even so, this process is tedious in anything less then perfect seeing conditions. If you have a JMI MotoFocus with Digital Read Out, you have a better choice. Using the MotoFocus controls, move the focus in one direction until the merged blob just begins to separate into four stars. Note the DRO reading. Now focus in the other direction until the merged blob once again just begins to separate. Note this new DRO reading. Set the DRO to the mid point between the two readings. Perfect!
Focusing example using Hartmann mask
The mask itself is easy to make. I use construction board from my local stationary store. One side of the board is white and the other is black. Obviously, it is best to have the black side facing into the telescope. The disk should be made just slightly larger than the circumference of the scope body 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, lens or mirror. The hole size is not critical. I use hole diameters about 1/5th to 1/6th the diameter of the lens or mirror.
A ruler and a protractor are used for layout. A sharp bladed Xacto knife does the job of cutting. Once you have the front of the mask, cut a long narrow strip of board for the side. Wrap this strip around the front of the telescope and trim its length so the ends meet in a loose fit. Use a small strip of Duct Tape to hold the ends together. Bring the mask up the sides and again using small strips of Duct Tape, secure the mask to side strip. The whole process will take you about a half-hour the first time. Subsequent masks will go quicker as you learn the techniques.
I have made these masks for scopes as large as the C11 and as small as a 50mm camera lens. In the case of the camera lenses, I skip the side strip and just drop the mask disk down onto the lens. For smaller lenses, often make two rather than four holes.
For another discussion about using Hartmann masks for CCD focusing, see The art of CCD imaging.
If you prefer not spend your time cutting a taping, you can buy a ready made, metal Hartmann Mask called Kwick Focus. These are available for 8,10,11,12,14 16 inch scopes with prices from $40 to $73. Pocono Mountain Optics carries these devices. Give Glenn a call at 800-569-4323.
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