CNC-Machined in the USA
Event Horizon EVRZ — ZWO EAF Crayford Focuser for GSO Newtonians
A precision Crayford focuser that mounts your ZWO EAF focus motor and drops straight into the 2.75″ GSO bolt pattern on Apertura, Zhumell and other GSO Newtonians. We supply the focuser mechanics — you supply your own ZWO EAF motor — for true zero-backlash, zero-image-shift autofocus.
Watch: a walkthrough of the Event Horizon EVRZ focuser and how the ZWO EAF mounts to it.
Drop-in replacement for GSO focusers — no drilling
The Event Horizon EVRZ 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)
8 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 EVRZ bolts straight on.
Bolting a ZWO EAF onto a stock Newtonian focuser just motorizes the backlash and image shift — they’re still there. The JMI Event Horizon EVRZ fixes that: it gives you a true friction-drive Crayford base, and your motor mounts to it with two screws using the couplers already included with your ZWO EAF kit. Now your autofocus routine drives a mechanism that actually holds focus.
Specifications
- ✓ Control focus from ASIAIR and ASCOM interfaces
- ✓ Autofocus with ASIAIR, N.I.N.A., Sequence Generator Pro, Ekos and other popular software
- ✓ 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
- ✓ Calibrated resolution: 2.54 µm of drawtube travel per ZWO EAF motor step
- ✓ Load capacity 8 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 EVRZ’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.
You provide the motor. The EVRZ ships as the focuser mechanics with the ZWO EAF mounting adaptation. It does not include the ZWO EAF motor itself.
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 EVRZ’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 EVRZ is light, and adding the EAF moves weight outboard of the focuser body.
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 EVRZ’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 EVRZ is light, and adding the EAF moves weight outboard of the focuser body.
What step size should your autofocus routine use? This focuser is calibrated at 2.54 µm per motor step. Use our free Autofocus Step-Size Calculator — pick your telescope and camera and it returns the exact step size for N.I.N.A., ASIAIR, Sequence Generator Pro, Ekos and more, with this focuser's calibration already loaded.
How the Event Horizon focusers compare
| Model | Drive | Load capacity | Best for |
|---|---|---|---|
| EV3 | 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 (this focuser) | 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 EVRZ 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 EVRZ 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 EVRZ 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 EVRZ. 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 EVRZ 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 EVRZ is CNC-machined, individually assembled and tested in our own facility in the United States.
Does the EVRZ include the ZWO EAF motor?
No. The EVRZ ships as the focuser mechanics with the ZWO EAF mounting adaptation built in. You supply the EAF itself, and it attaches with two screws using the couplers already in your EAF kit.
What autofocus step size should I use with the EVRZ?
The EVRZ is calibrated at 2.54 µm of drawtube travel per EAF step. Enter your telescope and camera into our free Autofocus Step-Size Calculator and it returns the step size to enter in N.I.N.A., ASIAIR, Sequence Generator Pro or Ekos, with this focuser’s calibration already loaded.
Why not just add an EAF to my stock GSO focuser?
A motor cannot remove mechanical backlash — it only automates a mechanism that still has it. A stock rack-and-pinion focuser will overshoot and drift during an autofocus run and lose focus as the tube swings. The EVRZ replaces the mechanism itself with a zero-backlash friction-drive Crayford, so the motor’s commanded position and the drawtube’s actual position agree.
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. Prefer manual focus? See the dual-speed EV1 and single-speed EV2. Have a Schmidt-Cassegrain instead? See our SCT Focusers.
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