This medium duty JMI Event Horizon 2” focuser is a precision-machined low profile Crayford-type single-speed manual focuser. It is designed for use on the rear cell of 6” to 16” catadioptric (Schmidt-Cassegrain and Maksutov-Cassegrain) telescopes. You can also use it with many refractors having a 2” focuser drawtube if the refractor has enough back focus available to install the 3” tall focuser in the optical train. In addition to making high magnification visual observing a precision joy, this friction-drive focuser’s zero image shift/no backlash design is a must for critical CCD imaging and astrophotography. It totally eliminates the image-shift inherent in any catadioptric telescope that moves the primary mirror to focus.
The four-pound vertical lift capacity of this focuser is sufficient for most visual observing and casual photographic use. If very heavy 2’ eyepieces and 2” star diagonals are to be used, or very heavy imaging chains (CCD camera, filter wheel, adaptive optics system, etc.), the pusher bearing design eight pound capacity heavy duty #EV1C or EV2C would be a better choice.
The body and rotating base are machined of black anodized aluminum, with tolerances up to 100x better than rack and pinion focusers. Both drawtube and drive shaft bearings use sealed stainless steel ball bearings for optimum smoothness. There is a locking drag brake thumbscrew and three focuser rotation thumbscrews, all made of nylon. Stainless steel hex head screws (with the appropriate hex head wrench) are provided for a more permanent rotation lock. A brass drag brake thumbscrew is also provided to replace the nylon thumbscrew for locking the drawtube position when heavy accessory loads are used.
The focuser’s rotating base threads onto the standard 2” diameter thread rear cell used on 6” to 16” catadioptric (Schmidt-Cassegrain and Maksutov-Cassegrain) telescopes. A 2” adapter is provided that fits into the focuser drawtube to duplicate the scope’s 2” rear cell threads so you can use SCT-style accessories with the focuser. You can replace the standard base with an optional base adapter that lets you use the focuser on larger diameter SCT rear cells. This lets you remove the rear cell to 2” thread step-down adapter ring used on 10” and larger scopes and use the focuser directly on the 3.5” and 4” diameter SCT rear cells of the larger scopes. Adapter #EV-BASL is for 3.5” rear cell threads (10”, 11”, and 14” SCTs).
For use on a refractor, simply insert the supplied 2” to SCT thread adapter into the refractor’s 2” focuser drawtube, then thread the focuser onto the adapter’s SCT threads. As mentioned above, the refractor must have enough back focus available to accept the 3” tall focuser (fully extended) and still reach focus with your normal visual and photographic accessories attached.
Once the Event Horizon focuser base is threaded firmly onto your scope, the focuser itself is rotated on the base to a convenient angle and locked into place with the three focuser rotation thumbscrews. You first approximately focus your telescope using the scope’s regular focus knob; then all subsequent fine-focusing is handled by the JMI focuser without the need to touch the scope’s focus knob again. This eliminates the possibility of the scope’s primary mirror tilting during fine focusing, so that there is no danger of a celestial object accidentally shifting off the small imaging sensor of a CCD camera during critical focusing.
With the focuser’s 2” drawtube retracted, the focuser body is 2.4” tall from base to the top of the drawtube, with no penetration of the drawtube into the telescope body. With the drawtube extended, the height increases to 3”. Inserting the supplied adapter that duplicates the standard 2” Schmidt-Cassegrain rear cell thread (to allow the use of SCT visual and photographic accessories) increases the fully-extended focuser height to 3.5”. Drawtube travel is 0.6”, sufficient for most fine-focusing needs.
The 2” diameter drawtube terminates in a soft non-marring brass compression ring adapter that holds 2” star diagonals and accessories, instead of an ordinary thumbscrew that can scratch your diagonal and accessory barrels. No 1.25” adapter is provided, as this is normally supplied with a 2” star diagonal.
There are two satin-anodized knurled aluminum focus knobs, for operation with either hand. One turn of the knob moves the drawtube 0.58”, about one and a half times the precision of most rack and pinion focusers. Moderate finger-pressure tightening of the nylon drawtube tension screw allows the drawtube to be locked in place, once a photographic focus has been reached. Varying the tightness of the tension screw also allows you to match the focuser friction to accommodate a variety of eyepiece weights and photographic loads.
A motorized version of the focuser (#EV3CM) is also available.