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.

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

Spacer Ring Size:	Works best with:
7.5mm	Provides proper spacing with RASA 8 for many ZWO cameras
12mm	Provides proper spacing with RASA 8 for many Atik cameras
30mm	Converts ZWO camera w/filter slider to 55mm back-focus standard for use with Schmidt-Cassegrain & EdgeHD

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.