The focuser of an Astro-Tech Ritchey-Chrétien is used by itself or (more typically) in combination with one or both of the thread-in extension tubes 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 each Astro-Tech R-C 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.

In some cases, 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.

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.