Celestron 93635-A T-Adapter for NexStar 4GT

This T-adapter allows your film or digital SLR to attach to your telescope and take photographs. It is well constructed and easy to use. You will also need an adapter that fits on you camera, the two then link together. This adapter does not allow autofocus, but it didn't claim to do so. You focus via the focus device on your telescope.

it does the job as explained in the description
I have connected my Canon 40D to Celestron C90 using Celestron 93635-A T-Adapter and Adorama T-Mount for Canon EOS.

I took nice photos for the moon :)

The Celestron 93635-A T-Adapter connects the rear cell of a NexStar 4 or C90 scope to a T-ring (sold seperately) which is designed to fit your specific camera. There isn't much to the adapter -- just a threaded metal tube with a metal screw-on ring on one end. Note that you will need to use your camera in fully manual mode though the camera's internal light meter will probably function normally allowing you to set the shutter speed for a proper exposure during daylight hours.

While the T-Adapter does function as advertised, it suffers from a design flaw -- the connecting tube is too narrow causing severe vignetting (i.e., the corners of phtots are cut off and appear very dark or black) at least for a 35mm full frame camera mounted to a C90. I have not tried it with a C-sized digital camera yet. Due to the severity of vignetting, you might want to consider an alternative model of T-Adapter or, better yet, a Tele-Extender designed for Schmidt-Cassegrain telescopes such as the Orion 1.25" Standard Tele-Extender which can be used in place of a T-adapter on the C90, doesn't suffer from these sever vignetting problems and is much more versatile.

Note: A tele-extender is similar to the T-adapter but is designed to optionally allow you to place a telescope eyepiece inside - between the scope and the camera - for even higher magnification. For example, with the T-adapter, a C90 can function as a 1200mm manual camera lens. With a tele-extender, it can do the same or, by adding a 18mm eyepiece, act as a 5000mm or greater lens.