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ZWO M42-M42 (T2-T2) 21mm Extender Ring

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in In stock (5)
0.15 kg
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ZWO M42-M42 (T2-T2) 21mm Extender Ring

This adapter might help you to achieve 55mm back focus with your camera.

Example 1:

ASI071MC-C has got a sensor to flange distance of 17.5mm, then you add this 21mm T2-T2 extension and then you add a 16.5mm M42-M48 adapter or a ZWO OAG (off axis guider) that has got the same thickness of 16.5mm. So basically if you use an OAG then you don't need this adapter, but if you don't use a ZWO OAG then this adapter/extension is necessary to achieve 55mm back focus. Basically, this adapter replaces the OAG and provides a M48 connection to the telescope.


Example 2:

The ASI1600MM-COOLED camera has got 6.5mm focal flange distance, add a 11mm T2 extension, add this 21mm T2-T2 extension and finally add a 16.5mm M42-M48 extension to achieve 55mm back focus to the sensor with M48 connection to the telescope.

Here is a bit of history...  Many optical elements like coma correctors, reducers, field flatteners etc. have been designed for Canon cameras that require 55mm back focus to the sensor, so this is how we ended up with this figure: 55mm, however we should note that a 55mm back focus is not always a requirement and some optical elements might require a different distance or in some telescope systems you might not even need to worry about back focus (i.e. when you don't use additional optical elements...)


  • Colour: black
  • Optical length: 21mm
  • Overall length: 25mm
  • Telescope side attachment: female T-thread (M42x0.75)
  • Camera side attachment: male T-thread (M42x0.75)

Images are for demonstration purposes only.

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