| 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 both of the thread-in extension tubes that are supplied with the scope 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 two threaded extension tubes supplied with your scope, installed singly or in combination between the astrograph’s rear cell and the focuser, 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.
In some cases, however, depending on the number of imaging components used, it may not be possible to precisely match the focal planes when using only the two extension tubes supplied with your Astro-Tech R-C. This 2" extension ring can be added to the rings supplied with your scope, or used in place of one or both of the rings, to match the needed back focus. When firmly threaded into place, it provides a flex-free solid metal extension that changes the distance between the focuser and the scope’s rear cell.
A little experimentation will be called for to determine what combination of extension tubes 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 tube is machined of aluminum, anodized black. The interior has anti-reflection threading.
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