Guiding in astrophotography may require the installation of a secondary telescope in parallel with the main instrument. For this purpose, Takahashi has designed a specific accessory, very close to a classic finder scope: the GT-40 guiding scope. Its very short focal length, 240 mm, allows a large field of view which facilitates the search for a guiding star.
The GT-40 has a 31.75 mm eyepiece holder, which is compatible with all commercially available autoguiding cameras. The image on the camera is focused by rotating the GT-40's lens cell with a thumbwheel. The metal back distance behind the refractor can therefore vary from 0 to 21.5 mm, depending on the needs of camera's backfocus.
It is possible to mount the GT-40 directly on Takahashi cradles and plates. Indeed, the steady rest has 2 sets of 2 tappings with 35 mm centres, with M8 and M5 threads. A Kodak thread (1/4"-20) is available in the centre of the mounting plate for users of other brands. However, it is recommended that M5 and M8 threads are used to avoid unintentional rotation of the mount on its base.
Takahashi proposes in option 2 devices facilitating the adjustment of the parallelism, in azimuth and in height: a standard L-clamp [TGT04010] and AZ Guidescope Mount [TAZ70120].
| Characteristic | Takahashi GT-40 guiding scope |
| Diameter | 40 mm |
| Focal length | 240 mm |
| Focal ratio | F/6.0 |
| Metal Back Distance | 0 to 21.5 mm |
| Characteristic | Takahashi GT-40 guiding scope |
| Bottom plate | 2 x M8 threads + 1 Kodak 1/4"-20 thread |
| Top plate | 2 x M5 threads |
| Eyepiece holder | 1.25" (31.75 mm) |
| Length | 272 mm (with dew shield) |
| Width | 58 mm |
| Heigth | 64 mm |
| Weight | 570 g |
| Focuser | Rotation of the front lens cell |
