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Event Horizon 3" Extreme  Telescope Focusers

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Event Horizon Extreeme 3" Crayford Focusers

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Event Horizon Extreme 3-inch Crayford Focusers

Model XT
Newtonian Version
without Motor


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Model XT
Newtonian Version
with Motor

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Model XT
Cassegrain Version
without Motor

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Model XT
Cassegrain Version
with Motor

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EV-XT Extreme Telescope Focuser

A 3" black-anodized (gold-anodized optional) aluminum "Extreme" focuser designed for use on large (10" to 16") Cassegrain Telescopes (EV-XTc) with standard output threads and Newtonian Telescopes (EV-XTn).  A 2" Eyepiece Adapter is included.  The Newtonian version includes a removable combination base (works on 10" radius up to a flat surface).  The manual version of the EV-XT lifts up to 8+ pounds and the motorized version up to 12 pounds.  Manual control is approximately 13 times finer than most rack-and-pinion focusers.  Regular motorized versions include a standard hand unit.

See the SCT Accessories Diagram, the Focuser Comparison Chart and the Focuser Accessories Page.  For additional equipment see the Smart Focus and DRO products.  (Specify all options and accessories when ordering.)

EV-XTn Newtonian version without motor, Part Number EVXTN
EV-XTn Newtonian version with motor, Part Number EVXTNM
EV-XTc Cassegrain version without motor, Part Number EVXTC
EV-XTc Cassegrain version with motor, Part Number EVXTCM


The EV-XT requires a special modified 1.25" eyepiece adapter (call for details).  The included 2" adapter does not accept the thumbscrew in a normal 1.25" eyepiece adapter.




All EV focusers include a compression ring in the drawtube for holding eyepieces and accessories without scratching the equipment.  The adjusting screw presses against the ring which tightens around the eyepiece or accessory.

What is a Event Horizon Extreme Focuser?

The Event Horizon Extreme Focuser (EV-XT) is a low-profile, friction-drive focuser designed for both the amateur and professional market.  The highly acclaimed Crayford design (see below) permits tolerances up to 100 times better than conventional rack-and-pinion focusers.  Zero image shift and zero backlash makes it fantastic for visual and photographic work and a  must for CCD imaging.  Add the optional variable-speed Motor and Smart Focus, Digital Read Out (DRO) or Digital Focus Counter (DFC), for the ultimate focusing system.  The EV-XT is truly a masterpiece of engineering and workmanship that you will be proud to own for years to come.

The EV-XTc is specifically designed for Cassegrain telescopes.  The most popular type of Cassegrain telescope is the Schmidt-Cassegrain Telescope (SCT).  These telescopes achieve focus by moving the primary mirror.  This design tends to create image shift which is especially problematic for CCD imaging.  The EV-XTc achieves focus by moving the eyepiece, camera or other equipment at the back of the telescope instead of the mirror.

Please see the Focuser Comparison Chart.

What is the Crayford Design?

The Crayford Eyepiece Mount (CEM) was invented  by John Wall.  The Crayford name comes from the Crayford Manor House Astronomical Society (UK) to which he belongs.  The Crayford design uses high pressure on a metal drive shaft pressing against a metal drawtube which is held by bearings on the opposing side.  This direct surface-to-surface drive mechanism eliminates backlash and shifting of the drawtube as seen in other designs such as the rack-and-pinion focuser and allows for very fine adjustments.

JMI recognized the benefits of this design and was the first company to bring it to the amateur market in a commercial product.  Mr. Wall, who chose not to patent his idea, continues to design and build Crayford Focusers.

Customer Comments

"Where did you get the great little motor?  Its the best little focuser.  Everything went together very easy.  All worked great.  Now I need a motor for my new ..."  (R. L. B.)

"I'm sure you have heard this before but I don't mind saying it again.  Your focusers are beautiful ! ! !"  (J. P.)

"My NGF-DX1 arrived the other day and everything was exactly as I had specified.  It is a fine example of superb workmanship and now my Nagler oculars are very happy!"  (J. L.)

"BTW, my 'scope is a pleasure to use with the new focuser.  Great product and great service.  Can't ask for more."  (P. L.)

"I am very impressed with the quality of the NGF focuser I purchased.  As a qualified first class machinist, I can appreciate the quality much more than most.  I am impressed, and that is no easy task.  (P. N. C.)

"I installed the [NGF-CM] yesterday and tested it out.  What a beautiful piece of engineering.  It's my second JMI focuser.  I have an NGF-DX1 on my homebuilt 18 inch dob.  You guys make great products!  Thanks again and Merry Christmas!"  (D. D.)

"I received the focuseróWOW, what a beautiful piece of machinery!  ...  You make a great product..."  (S. H.)

"Many thanks for the NGF-DX2 focuser which arrived safely.  I have to say I'm blown away by its quality and presentation.  ...  The foam contoured container is really something extra.  It is always a little daunting purchasing astro items from a picture in a magazine, especially flat black colored items.  The engineering and finish is excellent."  (P. W.)

"The [NGF] focuser [with Smart Focus] has made focusing more precise and infinitely easier, a very fine product!"  (Tony Hallas)

"It is marvelous to find that there still exist manufacturers of fine instruments that are capable of lasting a lifetime (longer, indeed, than many of the telescopes they are fitted to, I suspect) and are made to a superb level of precision and finish.  And you also obviously care about your customers and their satisfaction!"  (A. D.)

"[I] just returned from vacation and collected my JMI focuser.  It is superb!  Thanks for [your] very quick and efficient service.  I look forward to doing business again."  (J. A. O.)

"I recently purchased a deluxe NGF focuser unit for my LXD55 10" telescope.  Just today I received the motor upgrade kit and installed it.  It works great!  JMI, IMHO [in my humble opinion], makes one of the finest focuser upgrades for the money in this niche market today."  (J. P.)

"JMI provided the perfect solution to a common problem with [my] telescope.  The slide tube focusing arrangement that is standard equipment on [the refractor] proved to be somewhat problematic for people with different vision, especially eyeglass wearers.  JMI stepped in with a solution.  The NGF-miniS focuser..., outfitted with a 2" drawtube, proved to be the perfect solution to the focusing annoyance.  Lightweight and quality crafted, the motorized miniS works flawlessly for those fine tuning focusing problems.  Another great advantage is the ability to get a sharp focus quickly during imaging, no more loosening and sliding of the [refractor's] focuser!  My 'thanks' to Steve and the JMI crew for providing the perfect solution for my focusing problem."  (Jarrett R. DuBois)

Frequently Asked Questions (FAQ)

Q. Does JMI sell rack-and-pinion focusers?
A. No.  All of our focusers are based on the superior Crayford design.  They operate with friction between the drive roller and the drawtube created by high pressure.
Q. Can I add a Motor to my EV-XT at a later date?
A. Yes, but with older versions you may need to return it to us for installation.  There is a small charge for any after-purchase installation or upgrade performed by us.  See Motor for JMI NGF Focusers in our JMI Online Store.
Q. What is a Backup Plate?
A. A backup plate is a square flat piece of aluminum that can be bent to conform to the radius of a telescope optical tube.  It is designed to add support for an EV focuser when attached to a Sonotube (cardboard) or wood constructed optical tube.  Two backup plates are usually recommended, one for the inside and one for the outside of the tube as a base for the focuser.
Q. What do I do if the radius of the base doesn't match the curve of my telescope?
A. If your telescope tube has a slightly different radius than the focuser, you can add black foam weather-stripping tape under part of the focuser to block stray light while keeping a professional appearance (see diagram).

Q. What are leveling setscrews?
A. Leveling setscrews are installed in the base of all EV focusers.  They give you the ability to collimate the focuser with respect to the optical light path.  Correct alignment is critical for optical quality.
Q. Can a DRO be installed on my JMI focuser at a later date?
A. Yes.  The focuser may need to be returned to us for installation of a Digital Readout system.   There is a small charge for any after-purchase installation or upgrade performed by us.
Q. Do you have information on how your older NGF focusers and adapters work on SCTs?
A. Yes.  The SCT Accessories Diagram below reflects our NGF DX, Mini and XT focusers and their mounting adapters and output adapters.


Each focuser or accessory in the diagram above is labeled with its part number.  To see photos of each adapter, go to the Adapter Comparison Chart.

Generally, a medium SCT (Schmidt Cassegrain Telescope) is one with a 10" to 14" aperture.  A large SCT is one with a 16" or larger aperture (e.g. Meade LX200 16").  Although we refer to these accessories as SCT accessories, they will work on any Cassegrain telescope (e.g. Maksutov-Cassegrain) of the correct size with standard output threads.  Six of the mounting adapters (ADPT2SCT(L), ADPT3SCT(L), ADPT4SCT(L)) are for mounting the two NGF focusers (1.25" NGFMINC, 2" NGFC) specifically designed for Cassegrain telescopes.  Two of the mounting adapters (ADPT3XT, ADPT4XT) are for mounting the 3" EV-XTc focuser on a large SCT with mounting threads of 3" and 4" diameter respectively.

Most medium size SCTs come with an adapter that reduces the larger output threads to the smaller output threads found on the small SCTs.  These adapters usually cut into the light cone.  We designed the Large Format mounting Adapters (ADPT3SCT(L), ADPT4SCT(L)) to overcome this problem.  The larger version (ADPT4SCT(L)) is designed for the 4" threads on the Meade 16" LX200.  There are two more adapters (ADPT3XT, ADPT4XT) which are designed specifically for mounting the EV-XTc focuser.  All these adapters are non-vignetting.  In other words, they do not reduce the amount of light that gets to the eyepiece.

An "L" on the end of the part number indicates that a light shield is built in to the adapter.  The light shield portion of the adapter is shown in a light gray color in the chart above.  A light shield blocks any stray light coming through the bearing cavities and is normally not necessary unless you are using a CCD camera.  (The EV-XT design does not require any light shield.)

The threaded output adapter (ADPT2THREAD) duplicates the standard threads found on a small SCT.  The large threaded output adapter (ADPT3THREAD) duplicates the large threads found on the Meade LX200 16" telescope.  This allows you to use your standard SCT accessories on the back side of an EV/NGF focuser.  We have a standard 1.25" eyepiece adapter (ADPT2-1.25) to allow the use of 1.25" eyepieces in a 2" diameter drawtube.  The combination 1.25" eyepiece and T-thread adapter (ADPT2-1.25T) also works as a 1.25" eyepiece adapter but is designed for mounting cameras having a standard T-thread (including CCD cameras) into a 2" drawtube.

Future products for the EV-XTc may include special 1.25" eyepiece adapters to fit in the 2" eyepiece adapter (ADPT3-2).