To achieve the best astroimages, it is important to attain the proper back-focus distance for your camera relative to the telescope it is being used with. If the back-focus distance your camera consumes is not exact, the performance at the corners of full frame imaging sensors will suffer—stars 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

Celestron’s 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.

• 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

Below is a quick guide to help you determine which spacer ring to use for our EdgeHD, Schmidt-Cassegrain, and RASA 8 optical tubes: