CNC-Machined in the USA
Event Horizon EV3 — Standard Crayford Focuser for GSO Newtonians
The straightforward, no-frills way into true zero-backlash focusing. A single-speed friction-drive Crayford with a compression ring and a 2.75″ GSO bolt pattern — a drop-in replacement for the stock focuser on Apertura, Zhumell and other GSO Newtonians.
Drop-in replacement for GSO focusers — no drilling
The Event Horizon EV3 uses the same base footprint as the stock GSO focuser: 2.75″ (69.85 mm) mounting-hole spacing on a 3.5″ (88.9 mm) diameter base. On a GSO-built Newtonian the existing holes line up, so the swap is four screws out, four screws in — no drilling, no adapter plate, no focuser board.
2.75″hole spacing
(69.85 mm)
(69.85 mm)
3.5″base diameter
(88.9 mm)
(88.9 mm)
4 lbload capacity
1.4 lbfocuser weight
Not sure what you have? Measure center-to-center between the mounting screws on your current focuser base. If it reads 2.75″, the Event Horizon EV3 bolts straight on.
A masterpiece of engineering and workmanship, the JMI Event Horizon EV3 is a low-profile, friction-drive Crayford focuser for the discriminating astronomer. The Crayford design presses a metal drive shaft directly against a machined metal drawtube held by bearings on the opposing side — a direct surface-to-surface drive that eliminates the backlash and drawtube shift of rack-and-pinion designs and allows very fine adjustments. Zero image shift and zero backlash make it superb for both visual use and imaging.
Specifications
- ✓ Single-speed friction drive
- ✓ Compression ring for secure, mar-free holding of eyepieces and cameras
- ✓ Zero image shift, zero backlash — CNC-machined friction-drive Crayford design
- ✓ Drawtube length of travel is configurable at the time of order (see drop-down menu)
- ✓ Base diameter 3.5″ (88.9 mm) with 2.75″ (69.85 mm) mounting-hole spacing — matches the stock GSO focuser pattern
- ✓ Drop-in replacement on Apertura, Zhumell, Astro-Tech and other GSO-built Newtonians and astrographs — no drilling required
- ✓ Compatible with Newtonian telescope tubes 7.25″ in diameter or larger, as well as flat focuser boards
- ✓ Load capacity 4 lb · focuser weight 1.4 lb
- ✓ Individually assembled and tested in our own facility — 100% made in the USA
Will it fit my telescope?
Yes, if your Newtonian was built by GSO — which covers a large share of the imaging Newtonians and mid-to-large Dobsonians sold in the US. Confirmed drop-in fits include:
- Apertura Dobsonians — AD6, AD8, AD10, AD12 and the DT series
- Zhumell Dobsonians — Z8, Z10, Z12
- GSO imaging Newtonians and astrographs — 6″, 8″, 10″ and 12″ f/4 and f/5 tubes, including the Astro-Tech AT6IN / AT8IN / AT10IN and many GSO-built Orion imaging Newtonians
- GSO tubes sold under other badges — Revelation Astro, TS-Optics, Omegon, Bintel, Kson, Danubia and similar rebrands
- Any Newtonian or focuser board with 2.75″ mounting-hole spacing and a tube diameter of 7.25″ or larger
Telescopes built on a different footprint — many Sky-Watcher, Celestron and Synta-built Orion Dobsonians among them — use a different hole pattern, so drilling additional mounting holes in the tube or focuser board may be required. The opening in the tube is rarely the obstacle: the hole already cut for your existing focuser will almost certainly clear the Event Horizon EV3’s drawtube, so the work usually comes down to the mounting screws. Measuring your existing focuser base first takes thirty seconds and settles the question.
Fitting note: On a GSO-pattern telescope this is a true bolt-on swap. On any other Newtonian, drilling additional mounting holes in the tube or focuser board may be required — but the opening already cut for your existing focuser will almost certainly clear the Event Horizon EV3’s drawtube, so the job is normally limited to the mounting screws rather than enlarging the hole. Many publicly available tools and active community forums can help you plan it. Also check your tube balance afterward: at 1.4 lb the Event Horizon EV3 is light, and a lighter focuser can shift the balance point slightly on a Dobsonian.
How the Event Horizon focusers compare
| Model | Drive | Load capacity | Best for |
|---|---|---|---|
| EV3 (this focuser) | Single-speed friction drive | 4 lb | Visual observers who want zero backlash at the lowest cost |
| EV2 | Single-speed, rubber grip knobs | 8 lb | Heavier eyepieces and camera trains, focused by hand |
| EV1 | Dual-speed with 10:1 fine focus | 8 lb | Critical manual focus — planetary, high power, imaging |
| EVRZ | Single-speed + ZWO EAF mount (motor not included) | 8 lb | ASIAIR, N.I.N.A. and SGP autofocus routines |
Every Event Horizon Newtonian-base focuser shares the same fitment: 2.75″ mounting-hole spacing, 3.5″ base diameter, 1.4 lb, 2″ drawtube with compression ring, configurable travel, CNC-machined and assembled in the USA.
Frequently asked questions
Will this focuser fit an Apertura or Zhumell Dobsonian?
Yes. Apertura and Zhumell Dobsonians are built by GSO, and the Event Horizon EV3 matches the stock GSO focuser footprint exactly — 2.75″ mounting-hole spacing on a 3.5″ diameter base. The existing holes in the tube line up, so no drilling or adapter plate is required.
How do I confirm my telescope has the GSO focuser pattern?
Measure center-to-center between the screws holding your current focuser to the tube. A spacing of 2.75″ (69.85 mm) means the Event Horizon EV3 is a direct bolt-on replacement. Your tube also needs to be 7.25″ in diameter or larger, or you can mount the focuser to a flat focuser board.
Does it work on GSO imaging Newtonians and astrographs?
Yes. GSO f/4 and f/5 imaging Newtonians — including tubes badged as Astro-Tech AT6IN, AT8IN and AT10IN, and many GSO-built Orion imaging Newtonians — use the same 2.75″ focuser pattern as the Dobsonians, so the Event Horizon EV3 drops straight in.
What if my telescope is not a GSO?
You may need to drill additional mounting holes in the tube or focuser board to match the 2.75″ spacing of the Event Horizon EV3. The hole already cut in your tube for the existing focuser will almost certainly clear the drawtube, so the fitting work is normally limited to the mounting screws rather than enlarging the opening.
What is the difference between a Crayford and a rack-and-pinion focuser?
A rack-and-pinion focuser drives a toothed rack with a gear, and the tooth clearance produces backlash and image shift. A Crayford presses a smooth metal drive shaft directly against the drawtube, with bearings on the opposite side. That surface-to-surface contact has no gear lash, so the Event Horizon EV3 has zero backlash and zero image shift.
How much drawtube travel does it have?
Travel is configurable at the time of order — choose the length of travel from the drop-down menu on this page. Picking the right travel matters most on imaging Newtonians, where the camera sits far outside the tube.
Where is it made?
The Event Horizon EV3 is CNC-machined, individually assembled and tested in our own facility in the United States.
Should I get the EV3 or the EV2?
The EV3 and EV2 share the same single-speed friction drive and the same fitment. The EV2 adds rubber grip knobs and doubles the load capacity to 8 lb. Choose the EV3 for visual observing with ordinary eyepieces; choose the EV2 if you hang a camera, a coma corrector and a filter drawer off the drawtube.
Can I motorize the EV3 later?
The EV3 is a manual focuser. If you want motorized or automated focus, the EVRZ is built to carry a ZWO EAF, and the EVRP is the motor-ready alternative.
Optional Optical Adapters
- JMI Combination 1.25″ Eyepiece & T-Thread Adapter (ADPT2-1.25T) — for camera use. Mounts a CCD or other T-threaded camera into a 2″ drawtube, and doubles as a 1.25″ adapter. Uses a compression ring and includes 48 mm filter threads on the focuser end.
- Farpoint 2″ Focuser to 1.25″ Eyepiece Adapter (FAV100) — a visual 2″-to-1.25″ eyepiece adapter with a non-marring compression ring. Threaded for both 2″ and 1.25″ filters, with a stop to keep long eyepieces from damaging filters.
New to Crayford focusers? Read the Event Horizon Focusers guide. Want grip knobs and 8 lb capacity? See the EV2. Want 10:1 dual-speed fine focus? See the EV1. Have a Schmidt-Cassegrain instead? See our SCT Focusers.
You may also like
Recently viewed