Celestron M42 Spacer Ring Kit Black. If the back-focus distance your camera consumes is not exact the performance at the corners of full frame imaging sensors will sufferstars will appear elongated and not as pinpoints. Example: RASA 8 with a ZWO ASI 183 MC PRO camera Required back-focus distance for RASA 8 with included M42 camera adapter: 25mm Back-focus consumed by ZWO ASI 183 MC PRO camera: 17.5mm To achieve equal back-focus distance a 7.5mm M42 spacer ring is required Body The M42 Spacer Rings are machined aluminum black anodized. The internal surfaces are blackened and glare-threaded to provide the highest contrast. Each spacer ring has a knurled surface providing a great gripping surface for threading/unthreading. The M42 Spacer Rings are compatible with M42 x 0.75 (T-thread) camera adapters. Celestrons M42 Spacer Ring kit includes three different length spacer rings to achieve the proper back-focus distance with our popular Celestron optical tubes and T-thread camera adapters. How to Use Depending your method of astrophotography (DSLR or CCD/CMOS) there are two ways to use the M42 Spacer Ring. If you are using a DSLR place the desired M42 Spacer Ring between your T-Ring and your T-Adapter for your optical tube. If you are using a CCD or CMOS camera place the desired M42 Spacer Ring between the camera and the T-Adapter for your optical tube. Specifications for Celestron M42 Spacer Ring Kit Telescope Accessories: Manufacturer: Celestron Color: Black Included Accessories: Kit includes 3 spacer rings of different lengths 7.5mm 12mm 30mm Quantity: 3 Weight: 8 oz Features of Celestron M42 Spacer Ring Kit Telescope Accessories Adjusts back-focus between your camera and telescope for the best astroimaging results Kit includes 3 spacer rings of different lengths (7.5mm 12mm 30mm) Threads to your T-Ring and T-Adapter with standard male and female T-Threads Package Contents: Celestron M42 Spacer Ring Kit Telescope Accessories Celestron 93618: M42 Spacer Ring Kit Black