ASToptics M48 aluminium spacer rings - set of 3 (0.5mm / 0.8mm / 1mm)
Anyone who takes astrophotography or visual observing seriously knows that correct backfocus adjustment is fundamental. Extension tubes do a great job, but in practice there are situations where you need to shift the distance by just a fraction of a millimetre - and that is exactly where precision ASToptics spacer rings for M48x0.75 thread come in. This set of three aluminium shims gives you a flexible tool for fine-tuning your optical train without compromise.
What is included
The package contains a total of 3 aluminium spacer rings with M48x0.75 thread (standard 2″ filter thread), each with a different thickness. By combining all three rings you can achieve a wide range of values from 0.5mm up to 2.3mm in very small increments - simply by stacking the shims together.
| Piece |
Thickness |
| Ring 1 |
0.5 mm |
| Ring 2 |
0.8 mm |
| Ring 3 |
1.0 mm |
| Maximum combined |
2.3 mm |
Typical applications
The spacer rings are useful wherever standard extension tubes are not sufficient - for example, for fine adjustment of the camera sensor position relative to the rear principal plane of a focal reducer, field flattener or coma corrector. In practice you will encounter this problem when combining adapters from different manufacturers, where production tolerances cause differences in the order of tenths of a millimetre. Instead of machining metal parts, you simply add the right shim and the backfocus is exactly where it needs to be.
Precise spacing in the optical train is particularly critical for astrophotography - even a small deviation can cause uneven focus across the imaging field or degraded star shapes at the corners of the frame.
Common use cases
- Fine backfocus adjustment with focal reducers and field flatteners
- Compensating for manufacturing tolerances between adapters from different brands
- Correcting star shape at the edges of the imaging sensor
- Optimising the optical train for different cameras or eyepiece holders
Material and thread standard
Aluminium is the material of choice for a good reason: the rings are lightweight, corrosion-resistant and mechanically stable even with the temperature fluctuations typical of night-time observing. The M48x0.75 thread is today virtually a standard among astronomers - you will find it on filters, adapters, reduction rings, corrector cells and on most cameras designed for astronomy.
About ASToptics
ASToptics is a European manufacturer of astronomical accessories with a focus on precision machining and well-thought-out design. Their products are developed for real-world use both in the field and at the observatory, and are popular with beginners and experienced observers alike. These spacer rings are an excellent example of a simple but precisely executed accessory that solves a specific problem without unnecessary complexity. If you are looking for reliable accessories that simply work, ASToptics M48 spacer shims belong in the kit of anyone who takes their optical train seriously.
