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For over 30 years we have focused on the Astro T-2 thread system (M 42 x 0.75 mm); virtually every telescope manufacturer must offer this thread far ahead of the focal point of his instrument, such a universal T-ring (for DSLR cameras) can be connected.
The only exceptions are (still) Russian equipment, there the thread diameter is the same, but the pitch is still 1mm per revolution - rather than 0.75mm! Therefore, we offer among other things a "Russians adapter". At only 7mm additional optical length it can allow so many Russian lenses or telescopes to be compatible with an international Astro T-2 (M 42 x 0.75mm) connector!
For the sake of better recognition mostly are marked. See marking small 'a' or small 'i' in the respective indication! "a"; identifies an external thread (male); or, if Ø sign which a diameter, "i"; - a female is always (female) or - Ø character - an inner diameter.
The optical length describes the extension of the optical path caused by an adapter or accessory when inserted into the beam path of the telescope. The values listed in the system table rounded to 1/2 mm. For pure threaded adapters or tubes such as the T-2 ring (#25), the optical length arises directly from its geometric shape. Because the optical design lengths apply to the assembled, external threads are not included in the bill. This explains the extremely short optical length external thread adapters compared to female threaded adaptors.
Parts that contain optical elements (such as Zenit prisms and binocular) the optical length can vary strongly from the geometrical form of the housing. Hence the values for the optical length by such accessories was determined directly on the telescope. When used with the unique Alan Gee tele-compressor the optical length depends in addition to the distance from the primary focus.