Why not boldly go where many astrophotographers have gone before and get the Orion M48 Extension Spacer Ring Set, to ensure your camera is at the proper back focus distance for your imaging telescope?
Setting the correct back-focus distance is crucial to achieving sharp, high-quality celestial images. If the back-focus distance is off, by a couple millimeters or more, the image may appear out of focus. Stars at the periphery of the image frame will appear elongated instead of pinpoint sharp. Focal reducers and field flatteners used in your imaging train have specific back focus requirements, so it is important that your camera's sensor be set at that exact distance from the reducer or flattener's rear mechanical flange.
The Orion M48 Extension Spacer Ring Set comprises four separate anodized aluminum rings of 5mm, 10mm, 20mm, and 30mm lengths, plus a Variable Spacer Ring that can be set to a length anywhere between 9mm and 14mm. Each ring has an internal (female) and external (male) M48x0.75 thread. Any two or more rings can be used in combination to provide the precise amount of extension you need beyond the individual fixed ring lengths. Maximum extension length of this set is 79mm.
The Variable Spacer Ring consists of a fixed spacer ring with a lock ring that threads onto the male thread of the fixed ring. By adjusting the lock ring's position over the male thread, you can vary the total spacing desired from 9mm to 14mm.
The outer diameter of the rings is 50.7mm (1.99"). Weight of entire set is 64g (2.3 oz.). For use with 48mm (2") accessories and cameras with a 48mm T-ring or thread mount.