This Astro-Tech FCR68 focuser collimation ring is a useful and economical addition to Astro-Tech 6" and 8" Ritchey-Chrétien astrographs to improve your image sharpness when imaging with DSLRs and CCDs that use large format 35mm-class CCD sensors.
The focuser on 6” and 8” Astro-Tech Ritchey-Chrétien astrographs threads directly onto the rear cell of the central baffle tube/primary mirror assembly. Machining tolerances in the various components can sometimes lead to a very minor tilt in the alignment of the focuser and camera in relation to the scope’s optical path. This is rarely visible in images taken with an APS-C size imaging sensor (up to an approximately 23mm diagonal).
However, with large format 35mm-class DSLR and CCD sensors, image sharpness in the corners of your images will be improved if your camera is precisely aligned with the scope’s optical path.
This Astro-Tech FCR68 focuser collimation ring allows you to do just such a precise alignment, by letting you separately align your focuser/camera combination to the scope’s optical path completely independently of the baffle tube/primary mirror alignment.
The focuser collimation ring consists of two spring-loaded rings with push/pull collimation screws that let you tilt the rings slightly in relation to each other and then lock them firmly in place. The collimation ring fits between your scope’s rear cell and your focuser/camera combination. Once installed, you can then adjust the tilt of the collimation ring to zero out any tilt in your focuser during final scope collimation.
The telescope side of the collimation ring has M90 x 1 female threads that connect to the rear cell of your scope (or to any 90mm extension rings you have attached to the rear cell to provide the proper scope to camera spacing). The camera side of the collimation ring has M90 x 1 male threads for attaching your focuser. The collimation ring takes up only 15mm of back focus.
The focuser collimation ring will cover the mirror/baffle tube collimation screws on the rear cell of your R-C if the ring is attached directly to the rear cell. To give you enough clearance to gain access to the screws, at least one of the 90mm extension rings supplied with your scope needs to be installed between the scope's rear cell and the focuser.
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