| This Astro-Tech 2” long x 90mm diameter extension ring can be installed between the rear cell of an Astro-Tech Ritchey-Chrétien astrograph and its Crayford focuser to more precisely match the back focus required for your particular complement of imaging components.
With Astro-Tech Ritchey-Chrétiens, the scope’s focuser is used by itself or (more typically) in combination with one or more of the thread-in 1” and 2” extension rings that are supplied with the 8” AT8RC (optional with the 6” AT6RC) to place the camera at the astrograph’s focal plane. The weight of very heavy equipment trains (camera, plus filter wheel, plus temperature-compensated electric focuser, adaptive optics system, etc.) may cause the focuser drawtube to tilt slightly when fully extended, affecting the focus.
The threaded 1” and 2” extension rings are installed singly or in combination between the astrograph’s rear cell and the focuser to provide a flex-free solid metal extension that changes the distance between the focuser and the rear cell. This lets you accommodate the varying back-focus requirements of DSLR-type camera imaging versus long equipment train CCD imaging, while minimizing the need to extend the focuser drawtube.
A little experimentation will be called for to determine what combination of extension tubes (either supplied, as with the AT8RC, or optional, as with the AT6RC) will be needed for your particular combination(s) of imaging components.
The ends of this extension ring have 90mm x 1mm metric threads (male on one side to thread into your existing extension rings or Crayford focuser, and female on the other to thread into your astrograph’s rear cell or existing extension rings). The extension ring is machined of aluminum, anodized black. The interior has anti-reflection threading. | 
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