Checking Camera Focus

The focus of the camera is adjusted by jacking the spider cage toward or away from the mirror using the jam nuts on the Invar bars spacing the spider from the mirror. To test the adjustment of the focus on a Schmidt camera by making star photographs can take quite a long time. Aligning the film holder to be perpendicular to the main optical axis can also be a vexing problem.

Fortunately, there is a purely optical method for adjusting the focus. The main requirement for this method is another telescope of the same, or nearly the same aperture. To focus the 8-inch Schmidt, an 8-inch Schmidt-Cassegrain telescope will work quite well. The procedure involves locking the telescope at its infinity focus, placing it nose-to-nose with the Schmidt camera, and using it to examine the appearance of the film holder in the Schmidt camera. It sounds more complicated than it is and will be fully explained shortly.

The telescope used to check the focus of a Schmidt camera must be focused for "infinity". Eyeglass wearers can find the infinity focus either with or without glasses as long as focus tests on the Schmidt camera are performed the same way. The easiest way to focus on infinity is to use the moon as a focusing subject. This can even be done in daylight. Do not use any terrestrial object that is quite far away, because for example when focusing a point that is 2 kilometers away with a large Schmidt-Cassegrain telescope there is a shift from the real infinity focus point of 12mm. Use a medium sized eyepiece, such as a 25 mm. Alternately, a 12.5 mm crosshair guiding eyepiece can be used to help eliminate accommodation focus by the eye. If you use a Schmidt-Cassegrain telescope for this, it is more convenient to use a star diagonal to achieve a comfortable viewing position. Before setting the infinity focus, bottom the star diagonal all the way into the visual back on the telescope. Next, make sure the eyepiece is not bottomed all the way into the star diagonal. Leave it pulled out about a half inch. Use a felt tip pen to mark the circumference of the eyepiece where it enters the drawtube of the star diagonal. This will verify that eyepiece has not been moved from its infinity position when the camera focus tests are being performed. We will see later why it is important to have the eyepiece partially drawn outward instead of fully bottomed into the star diagonal.

Before attempting to focus the camera using the adjusting nuts on the Invar rods, the film holder should be squared with the optical axis of the instrument. By using a bubble level or spirit level to do the initial squaring, a lot of work can be saved over performing this initial step photographically or by viewing the film holder through an infinity-focused telescope. First, place the level on the corrector to get the camera level, and then use it on the film holder to level it. This operation will have to be done by working through the film-loading door. The film holder faces the mirror, so in order to place the level on the film holder the camera will have to be placed in a nose-down attitude. This in turn means the level will have to be placed on the corrector from the inside of the camera as it rests on its nose.

Once the camera is mounted nose-to-nose with the infinity telescope, the camera's focus can be checked. Load the film holder with a clip of film. To be sure the telescope and the camera are on-axis with each other, shine a small bright light, such as a MiniMaglite, into the telescope eyepiece. This will project a small spot onto the film holder. Adjust the jackscrews on the handling fixture to center the spot in the middle of the film holder.

This focusing eyepiece view is of the center of the crosshair target in the film holder at the above right. Notice the high magnification showing the fibers of the paper

To aid in navigating around the face of the film holder as viewed through the high magnification of the telescope, use a needle to scratch a small cross in the center of the film and engrave the numbers one through four in the four corners of the film chip. Make these numbers small because you will be surprised at how small an area of the film holder you will be viewing through the telescope. If the film emulsion surface is too smooth to accurately determine if it is in focus as seen through the telescope, lightly dust the film surface with talcum powder. This will create enough relief on the surface of the film to give you a target to focus on.

If using a film chip as a focusing target proves inadequate, an alternative target is a high contrast lithographic reproduction of a fine multisided dot pattern on glossy stock. When the camera is in focus, the smaller of these dots will reproduce the appearance of stars showing Airy disks when viewed with the focusing telescope.

Go to the previous page ---- Special Equipment for Focusing the Celestron/Epoch Schmidt Camera
Go to the next page -------- Adjusting Camera Focus
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