ZWO FF90-APO 90mm Quadruplet Refractor F/6.8

A 4-Element Air-Spaced Apochromatic Design

The optical core of the FF90-APO is its four-element, air-spaced quadruplet design. One Extra-Low Dispersion (ED) glass element reduces chromatic aberration for improved color correction and image clarity. All four elements work together to deliver a flat imaging field without an additional field flattener.

This design simplifies your imaging train by removing the need for a separate flattener and the complex back-focus calculations that come with it. At a focal ratio of f/6.8, you can acquire deep signal on targets like the Eagle Nebula (M16) or the Triangulum Galaxy (M33) with shorter sub-exposure times than a slower instrument would require.

A 44mm Flat-Field Image Circle for Full-Frame Sensors

The FF90-APO has a 44mm image circle, which covers a full-frame (36x24mm, 43.3mm diagonal) sensor. You can confidently pair this telescope with high-resolution DSLRs, mirrorless cameras, or dedicated full-frame astronomy cameras to capture wide, sweeping vistas of the night sky.

Because the field is natively flat, no complex back focus calculations or spacing adjustments are required: attach your imaging setup and start capturing once focus is achieved.

The 3" Dual-Speed Rack & Pinion Focuser

The FF90-APO carries a rigid 3" dual-speed rack and pinion focuser for cameras, filter wheels and off-axis guiders. Its 1:10 coarse and fine adjustment helps you reach critical focus with high-resolution sensors.

For framing, a 360° field rotator with a scale sits at the rear of the focuser. This allows you to rotate your entire imaging train to compose your shot without loosening the camera from the focuser. The focuser body also has mounting holes for a ZWO EAF (Electronic Automatic Focuser, sold separately), enabling automated focusing routines for remote or unattended imaging sessions.

45-75mm Focus Range and M48/M54 Connections

The FF90-APO offers flexible connectivity for a wide range of accessories. The included photography extension tube terminates in standard M54x0.75 and M48x0.75 male threads, which are common connection sizes for astronomy cameras, filter wheels, and off-axis guiders. The M48 adapter also incorporates a thread for standard 2" filters.

Instead of a single, fixed back-focus distance, the system is designed to achieve focus with your camera sensor placed anywhere within a 45mm to 75mm range (measured from the M48x0.75 male thread). This range accommodates a variety of imaging train configurations without requiring a large collection of extension tubes. The telescope also includes 2" and 1.25" adapters for visual use with diagonals and eyepieces.

CNC-Machined Tube with a 230mm Vixen Dovetail

The FF90-APO's optical tube is precision CNC-machined and features a high-quality painted finish. A retractable dew shield helps block stray light and prevent dew formation on the objective lens while also reducing the telescope's length for transport. ZWO's manual describes the tube interior as lined with matting felt (its web page says an internal anti-reflection coating) to reduce stray light and improve contrast.

The telescope includes mounting rings, a carrying handle, and a 230mm Vixen-style dovetail plate for attachment to a wide range of equatorial mounts. The handle has a finder base slot and two more finder bases sit on either side of the focuser, so a finder scope can be moved without disassembly and other accessories can be mounted.

A 9.97 lb. Imaging Platform

The FF90-APO strikes a balance between light-gathering power and portability. The bare optical tube weighs 8.64 lb., and the total weight comes to 9.97 lb. with the included handle, mounting rings, and dovetail plate. When choosing a mount, add the weight of your camera, guide scope and other accessories to the telescope's 9.97 lb and compare the total with the mount's rated payload, leaving headroom for long exposures.

With its dew shield retracted, the tube measures just 513mm (20.2 inches) long, making it relatively compact and easy to transport to dark-sky sites. The FF90-APO is lightweight and compact, designed for both visual observation and astrophotography.

Camera Pairing and Visual Observation

While designed primarily for astrophotography, the FF90-APO is also a capable visual instrument. Its 90mm apochromatic objective gives high-contrast views of the Moon, planets and bright deep-sky objects with well-corrected color. For visual use, you will need to supply your own 2" or 1.25" star diagonal and eyepieces, which attach to the included adapters.

For imaging, the 44mm image circle covers full-frame sensors, as well as smaller APS-C and Micro Four Thirds chips. At 609mm a full-frame sensor covers about 3.4° x 2.3°, which frames large nebulae like the North America Nebula (NGC 7000), broad galaxy groups, and expansive star clusters. The M48 and M54 threads provide direct attachment points for most dedicated astronomy cameras or DSLR/mirrorless T-ring.

ZWO FF90 vs. FF70: 90mm Aperture and Image Scale

When choosing between ZWO's quadruplet refractors, the primary differences are light-gathering ability and image scale. The FF70 is lighter and offers a wider true field of view for the largest nebulae. The FF90, however, has a distinct advantage in light grasp and resolution with its 90mm aperture.

• Light Gathering: The 90mm objective of the FF90 gathers approximately 65% more light than a 70mm objective, allowing for shorter exposures or the capture of fainter details.
• Image Scale: The FF90's 609mm focal length provides greater magnification, framing smaller deep-sky objects like the Whirlpool Galaxy (M51) more effectively than the shorter focal length of the FF70.
• Weight: The trade-off for the increased aperture is weight. The FF90 weighs 9.97 lb. fully configured, so check the total against your mount's rated payload.