Q:
According to my telescope manufacturer, those are the requirements:
First, a “B” setting which allows for time exposures. This excludes point and shoot cameras and limits the selection to SLR cameras, the most common type of 35mm camera on the market today.
Second, the “B” or manual setting should NOT run off the battery. Many new electronic cameras use the battery to keep the shutter open during time exposures. Once the batteries are drained, usually after a few minutes, the shutter closes, whether you were finished with the exposure or not. Look for a camera that has a manual shutter when operating in the time exposure mode.
The camera must have interchangeable lenses so you can attach it to the telescope and so you can use a variety of lenses for piggyback photography. (Ok)
You also need a cable release with a locking function to hold the shutter open while you do other things. Mechanical and air release models are available.