From the Manufacturer
The Deluxe Tele-Extender for Schmidt-Cassegrain telescopes is a hollow tube that allows you to attach a camera to your telescope with an eyepiece installed. By using the Deluxe Tele-Extender in combination with an eyepiece, you can increase your instrument's effective focal length to well over 10,000mm. And this corresponds with a matching increase in image size. This technique is known as eyepiece projection photography. The following formula can be used to determine approximate effective focal length:
EFL = telescope focal length/eyepiece focal length x DF (the distance from the center of the eyepiece to the film)
The Deluxe Tele-Extender is used for high-power lunar, solar and planetary photography as well as for extreme terrestrial photography. It fits over the telescope’s eyepiece (even large eyepieces such as those in Celestron’s Ultima line), and connects to the visual back of the telescope. Optional counterweights will be necessary to properly balance the telescope when using the Tele-Extender.
To use the Deluxe Tele-Extender, remove the diagonal from your telescope and insert an eyepiece directly into the visual back. Place the Tele-Extender over both the eyepiece and the visual back, then attach your 35mm SLR camera to the back of the Tele-Extender using a T-Ring. The Tele-Extender’s built-in safety device will help prevent your eyepiece from becoming accidentally dislodged.
There are a few things to keep in mind when using this accessory. First, the image you'll see through your camera’s viewfinder will be upside-down. Secondly, due to the extremely high magnifying effect afforded by this accessory, you'll get best results by taking extra care to prevent camera and telescope vibration, to ensure accurate polar alignment, and to use under good seeing conditions.