This medium duty JMI Event Horizon 2” focuser is a precision-machined low profile Crayford-type focuser with variable speed pushbutton electric focusing. 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. Its compact and lightweight (only one ounce) DC motor drive fine-tunes the focus without your ever having to touch the scope, eliminating any possible image shake during critical high magnification visual use and photographic focusing. 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.
The linear voltage control hand unit contains a 9-volt alkaline battery (supplied) with a life expectancy of 6-8 months in normal use. A removable coiled cord with 1/8” pin jacks runs between the focuser and the hand control. Red pushbuttons on the hand unit control the direction of focuser travel. The hand control has a small black two-position slide switch that adjusts the motor’s slow speed for very precise fine adjustment of the focus when the directional buttons are pushed. With the slide switch set to the right, the focuser moves at a very slow speed. Set to the left, the focuser moves at a slow speed. The two slow speeds each ramp up to high speed if the directional button is held down for two seconds. This lets you make major focus changes quickly, without giving up the precision fine correction ability.
Although the focuser can be plugged into a Meade telescope’s drive base, allowing control from the Meade hand unit, you will have better control using the motorized Event Horizon’s supplied hand unit. The JMI hand unit includes quick-stop electromagnetic braking (not included in Meade systems), which stops the motor as soon as you stop pressing the keys to avoid overshooting the focus point at all speeds, plus linear voltage control for superior speed control. Also, older Meade LX200 telescopes do not have enough power to properly run the motor on the low setting.
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. The focuser can be used manually, but the motor drive gears must be disengaged by backing off the thumbscrew on the motor housing by turning it counterclockwise. To use the motors again, simply turn the thumbscrew clockwise until the gears engage again. 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 manual version of the focuser (#EV3C) is also available.