The world of adapters is diverse and not easy to understand. This is due to the wide variety of thread and plug sizes that optics and accessories may have.
In astronomy, two plug sizes have become established, which are often indicated in the old unit inches ("):
- 1.25" corresponds to 31.8 millimetre
- 2" corresponds to 50.8 millimetres
Many eyepieces, but also cameras, have connectors in one of these two sizes, which can be inserted into the telescope’s focuser.
If a 1.25" connector is to be inserted in a focuser with a 2" connector, a 1.25"/2" adapter is required. This is an example of a reducer adapter: where a connection is reduced from a larger size to a smaller one. The reverse is called an extension adapter.
Another very important category of adapters is camera adapters. These are used to connect cameras to optics or to accessories. The T2 thread (M42x0.75), which enables a screw connection, is very well established for this purpose. Lighter cameras without many accessories can also be easily connected to the telescope via a socket.
For astrophotography it is important that the aperture is large enough to illuminate the sensor (no vignetting) and the optical length matches the focal point.

Omegon
2“ Extension tube, 35mm optical path
$ 29.90

Omegon
2'' extension tube with 50mm optical path
$ 39.90

Omegon
1.25'', 30mm extension tube
$ 19.90

Omegon
2'' extension tube, 80mm optical path
$ 49.90

Explore Scientific
Extension tube M48 set
$ 62.00

ASToptics
Extension tube M48 Fine tuning Kit - 0.5mm; 0.8mm; 1mm
$ 27.90

Omegon
Extension tube 45mm Extender 2''
$ 29.90

Omegon
40mm T2i/T2a T2 extension ring
$ 24.90

ASToptics
Extension Tube M48 Filter thread - 4mm Length
$ 22.90

Omegon
T2 extension tube - 30mm
$ 19.90

Omegon
7.5 mm T2i/T2a T2 extension ring
$ 19.90

ASToptics
Extension tube M48 - Filter thread - 5mm long
$ 22.90

Baader
Extension tube Hyperion Finetuning ring 14mm
$ 23.90

Baader
Extension tube Hypoerion Finetuning ring 28mm
$ 25.90

Omegon
2" extension tube
$ 19.90

Omegon
15mm T2i/T2a T2 extension ring
$ 24.90

Omegon
Extension tube Extender 30 mm T- thread
$ 29.90

ASToptics
M54*0.75 - 5MM extension ring
$ 33.90

ASToptics
T2 extension tube - 7mm long
$ 22.90

ASToptics
T2 extension tube - 6mm long
$ 27.90

ASToptics
T2 extension tube - 9mm in length
$ 27.90

ASToptics
Extension tube T2 Fine tuning Kit - 0.5mm; 0.8mm; 1mm
$ 27.90

Baader
Extension tube Fine Tuning Ring Set T2 0.3 / 0.5 / 1mm
$ 23.90

ASToptics
Extension Tube - M48 - Filter thread - 10mm Length
$ 22.90

ASToptics
Extension tube Variable M48 extender, 23-29mm
$ 77.00

ASToptics
Extension Tube M48 Filter thread - 11mm Length
$ 22.90

TS Optics
Extension tube M48 17-23mm
$ 42.90

William Optics
Extension tube Verlängerungsadapter 4" für Flat GT
$ 88.00

ASToptics
T-Thread extension kit / 7mm
$ 56.00

TS Optics
T2 extension ring, optical path 3mm
$ 23.90

TS Optics
T2 extension tube, 5mm optical path
$ 23.90

TS Optics
T2 extension ring, optical path 20mm
$ 24.90

ASToptics
Extension tube M48 (2") Fine tuning ring - 0.5mm (aluminium)
$ 10.90

ASToptics
Extension tube M48 (2") fine tuning ring - 2mm (aluminium)
$ 10.90

ASToptics
Extension tube T2 fine tuning ring - 1mm (aluminium)
$ 10.90
