ASToptics Variable M48 Extender 23-29mm Extension Tube
Getting the back focus right is one of the most important steps in astrophotography. Even a few millimetres off and your images will lose sharpness and correction across the field. Fixed extension rings can only get you so far - they come in set lengths, and the one you need is rarely in your drawer. The ASToptics Variable M48 Extender solves this cleanly: it adjusts continuously from 23 to 29mm, so you can dial in the exact back focus your optical train requires - no more stacking multiple fixed rings to hit the right value.
This is a practical tool for astrophotographers who work with different cameras or regularly switch between telescope, reducer, and camera combinations.
Standard M48 connection - fits straight into your setup
The extender uses standard M48 threads on both ends. It connects directly to a telescope or anywhere in the optical train where M48 is used. The compact outer dimensions of 56 x 56 x 26.3mm mean it adds very little bulk. Your setup stays mechanically stable and easy to manage, even when combined with other adapters.
One piece replaces a whole set of fixed rings
If you regularly experiment with different optical configurations, a single variable extender covers the full 23-29mm range that would otherwise require several fixed spacers. Adjusting it is quick. The result is consistent. Back focus adjustment becomes a straightforward task rather than a process of trial and error with assorted accessories.
Key features
- Continuously variable length: 23-29mm
- M48 threads on both ends
- Compact dimensions: 56 x 56 x 26.3mm
- Suitable for use anywhere in an M48 optical train
- Replaces multiple fixed extension rings
- Lightweight and mechanically stable
Technical specifications
| Parameter |
Value |
| Thread |
M48 (both ends) |
| Adjustable length range |
23-29mm |
| Outer dimensions |
56 x 56 x 26.3mm |
| Adjustment type |
Continuous (variable) |
Who is this extender for?
The ASToptics Variable M48 Extender suits anyone who works with M48 optical systems and needs precise back focus control. It is especially useful if you use different cameras, switch reducers, or simply want a cleaner and more flexible solution than a collection of fixed spacers. One compact piece handles the full adjustment range - ready whenever your setup changes.
