Piggy-Back Quick Release Adapter
Piggy-back mounting a camera and lens on top of a telescope is a popular way of taking wide field astro-photographs but a suitable mounting system has to be devised. The SkyWatcher and Orion (USA) tube rings (and probably some other makes as well) have ¼" Whitworth camera mount bolts on the top of them and at first glance these look like a convenient way to mount your camera and lens. However, the reality is that although they do work, they are fiddly to use as the mounting platform supplied (a plastic knurled circular plate) cannot hold the camera that well especially with a long focal length lens fitted to the camera.
What I wanted was a mounting system that would allow me to readily install the piggy-back camera at the start of a session and easily remove it at the at the end of the session. I started by installing a Manfrotto 323 Rectangular QR (quick release) unit to the above mentioned mounting bolt but quickly realized that I had little control over collimation with the telescopes and although I could have used a small pan and tilt ball-joint unit, this would have meant that the camera was held on a small pillar which for astro work would be less than ideal.
The final solution was to cut a small piece off a spare dovetail bar and mount the quick release unit to this with a ¼" Whitworth bolt, washer and spring washer - this allows me to make fine adjustments to the horizontal axis collimation and then clamp the unit very tightly with a spanner. This unit is then bolted to the top of the tube rings with more ¼" Whitworth bolts giving me the opportunity to insert spacers to collimate in the vertical axis.
I then installed the male part of the quick release on the lens itself and this remains in place all the time. I can now rapidly install the piggy-back camera and lens on top of my guide scope and know that not only is it very secure but it will be in good collimation with the rest of the system.