Celestron M42 Spacer Ring Kit (#93618)
Achieve precision back-focus for sharper astroimages—especially at the corners.
In astrophotography, a millimeter can make the difference between pinpoint stars and smeared corners. The Celestron M42 Spacer Ring Kit is a precision toolset designed to help you fine-tune the back-focus distance between your imaging camera and telescope. Whether you're using a DSLR or a dedicated CMOS/CCD camera, these threaded aluminum rings allow you to match the optical back-focus your system demands—no guesswork, no flexure, no frustration.
Why Back-Focus Matters
Every telescope has an optimal back-focus distance—the exact spacing required between the telescope’s rear cell and the image sensor for sharp, fully illuminated images. If you come up short (or long), stars near the edges of a full-frame sensor will appear elongated or misshapen. This is especially critical when using fast optics like the RASA 8, where back-focus errors are amplified.
Example:
-
Required back-focus for RASA 8 (with M42 camera adapter): 25mm
-
ZWO ASI183MC Pro camera back-focus: 17.5mm
-
Spacer required: 7.5mm M42 ring (included)
The Celestron M42 Spacer Kit makes it easy to reach the magic number.
Versatile, Stackable, and Compatible
This kit includes three anodized aluminum spacer rings:
-
7.5mm (0.29")
-
12mm (0.47")
-
30mm (1.18")
Each spacer features standard M42 x 0.75 T-threads, so you can stack them as needed to reach your exact spacing. They’re compatible with virtually all DSLR T-rings and T-adapters, as well as astro cameras that use standard T-threads (ZWO, QHY, and more).
Simple Installation for Any Setup
It’s that easy. Each ring features a knurled grip surface for easy threading—even in cold conditions—and internal glare-threading for maximum contrast.
Key Features
-
Fine-tune your back-focus distance with confidence
-
Essential for achieving pinpoint stars across full-frame sensors
-
Three ring lengths: 7.5mm, 12mm, and 30mm
-
Compatible with any M42 x 0.75 T-thread system
-
CNC-machined aluminum with black anodized finish
-
Glare-threaded interiors and labeled exterior rings for easy ID
Technical Specifications
-
Thread Type: M42 x 0.75 (T-thread)
-
Included Spacer Lengths:
-
7.5mm (0.29")
-
12mm (0.47")
-
30mm (1.18")
-
Material: CNC-machined aluminum
-
Finish: Black anodized
-
Interior: Blackened and glare-threaded
-
Exterior: Knurled surface, laser-etched with length markings
Final Thoughts
A sharp image starts with proper spacing. The Celestron M42 Spacer Ring Kit offers a clean, rigid, and modular way to dial in your camera’s position for best performance—especially when imaging with fast optical systems like the RASA or f/5–f/6 refractors. Don't settle for "close enough" when you can get it just right.
