There comes a point when a small apochromatic refractor leaves you wanting just a little more telescope. A 70mm instrument is wonderfully portable and capable of photographing enormous regions of the sky, but smaller galaxies and nebulae can begin to look rather distant in the finished image. Moving all the way to a 130mm refractor brings substantially more aperture, but it also means a larger optical tube, a heavier mount, and considerably more equipment to handle every time the sky clears.
The ZWO FF90 APO occupies an appealing position between those two extremes. Its 90mm objective collects approximately 65% more light than a 70mm telescope, while its 609mm focal length provides additional image scale for medium-sized galaxies, nebulae, and star clusters. At f/6.8, it remains a practical instrument for deep-sky photography, without requiring an especially large mounting system.
Like ZWO's other FF-series refractors, the FF90 has another important advantage: its four-element apochromatic optical system produces a natively flat photographic field. There's no separate field flattener to purchase and no precise flattener-to-camera spacing to calculate. The telescope is designed to illuminate a 44mm image circle, large enough for a conventional full-frame astronomical camera.
The FF90 also includes a substantial 3" dual-speed rack-and-pinion focuser, a built-in camera rotator, retractable dew shield, mounting rings, carry handle, and dovetail plate. The bare optical tube weighs approximately 8.6 pounds, increasing to just under 10 pounds with the supplied mounting hardware.
For an astrophotographer who wants more aperture and focal length than a small travel refractor provides, but doesn't want to move into the weight and complexity of a much larger instrument, the FF90 is worth a serious look.
Four Elements, One Flat Photographic Field
One of the challenges of refractor design is making light of different colors come to focus at nearly the same point. A simple lens bends blue light more strongly than red light, producing the familiar colored fringes seen around bright stars in less sophisticated optical systems.
The FF90 uses a four-element, fully air-spaced apochromatic design incorporating one extra-low dispersion glass element. The combination of carefully selected glass types and lens curvatures helps control chromatic aberration, improving the definition of stars and other fine details.
ZWO has not identified the particular ED glass formulation, so it would be inappropriate to assume the telescope uses a specific premium glass type. What matters here is the performance of the complete optical system rather than the name of any single glass element.
Three of the optical elements are located toward the front of the instrument, with the fourth positioned farther back in the optical system. Together, they provide both color correction and the field flattening needed for astronomical photography.
That's an important distinction from a conventional refractor. Most refractors naturally form an image on a slightly curved surface, while the sensor in an astronomical camera is flat. When the center of the field is in focus, stars near the corners may be slightly out of focus, often appearing elongated or distorted.
The usual solution is a separate field flattener installed behind the focuser. That accessory can work extremely well, but it commonly requires the camera sensor to be positioned at a specific distance from the flattener. Adding a filter wheel, off-axis guider, or different camera can mean changing the spacing all over again.
With the FF90, field flattening is part of the telescope itself. As long as the camera can reach focus within the specified mechanical range, you don't have to find one exact flattener-to-camera distance to obtain the intended optical correction.
Good mechanical alignment still matters. A tilted camera sensor, loose threaded adapter, or flexing imaging train can produce imperfect stars even with an optically flat telescope. But eliminating the separate flattener removes one of the more common complications in assembling an astrophotography system.
More Aperture Without a Much Bigger Telescope
The difference between a 70mm and 90mm refractor may not sound particularly dramatic until you consider how aperture affects light collection.
A telescope gathers light according to the area of its objective lens. Since that area increases with the square of the aperture, the FF90 collects approximately 65% more light than a 70mm instrument, assuming comparable light transmission.
That additional aperture can be useful for fainter stars and fine details, and it provides greater theoretical resolving power. Atmospheric seeing, camera sampling, and tracking accuracy will still determine how much of that resolution appears in a finished photograph.
The FF90's 609mm focal length is also approximately 24% longer than the FF70's 490mm. Using the same camera, an object occupies a correspondingly larger area of the sensor. This can be helpful when photographing galaxies and smaller nebulae that look rather small through a very short refractor.
Its f/6.8 focal ratio is close to the FF70's f/7, so the two instruments have similar photographic exposure speed for extended objects at comparable image sampling. The principal advantages of the FF90 are its additional aperture, potential resolving power, and larger image scale rather than a dramatic reduction in required exposure time.
That's a sensible balance for a telescope intended to photograph many different kinds of deep-sky objects without specializing exclusively in either enormous nebula complexes or very small galaxies.
A 44mm Image Circle for Full-Frame Imaging
The FF90 is designed to provide a corrected photographic image circle measuring 44mm in diameter. Since a standard full-frame camera sensor has a diagonal of approximately 43.3mm, the telescope is capable of covering that format without an additional field flattener.
At 609mm focal length, a full-frame camera records a field approximately 3.4° wide and 2.3° high. That's large enough to frame a surprisingly diverse collection of astronomical objects.
The Triangulum Galaxy, M33, fits comfortably within the field, leaving useful surrounding sky. The Leo Triplet, comprising M65, M66, and NGC 3628, makes another excellent composition, with all three galaxies sharing the frame.
An APS-C camera produces a smaller field, approximately 2.2° by 1.5° for a typical sensor. With 3.76-micron pixels, the image scale is approximately 1.27 arcseconds per pixel.
That's a useful sampling scale for many deep-sky imaging systems. In practice, the amount of detail recorded will also depend on seeing conditions, guiding accuracy, and the optical performance of the complete arrangement.
The 44mm image circle makes the FF90 attractive to photographers using full-frame cameras, but it also places importance on proper camera alignment and clear optical connections. Vignetting can still occur with restrictive adapters or filters, and a small amount of camera tilt becomes easier to recognize as the sensor grows larger.
With a properly assembled imaging train, however, the FF90 provides the field coverage needed to enjoy a wide selection of astronomical targets without being limited to a smaller camera format.
A Substantial 3" Rack-and-Pinion Focuser
A good telescope for astrophotography needs more than well-corrected optics. Its focuser must hold the camera securely, move smoothly, and allow sufficiently small adjustments to reach critical focus.
The FF90 uses a 3" rack-and-pinion focuser with dual-speed adjustment. The coarse focusing control moves through the range quickly, while the 10:1 fine-focus mechanism makes the smaller corrections needed when you're close to the sharpest possible image.
The large focuser is especially welcome when using a cooled astronomical camera with a filter wheel, off-axis guider, and threaded adapters. A substantial mechanical assembly helps support that equipment, although the rigidity and alignment of the complete imaging train remain important.
At the rear is a built-in 360° camera rotator with an angular scale. This allows the camera to be rotated to compose the subject without loosening the telescope's mounting rings or changing the telescope's position on the mount.
That can make a meaningful difference when photographing objects with a particular shape or orientation. The Andromeda Galaxy, for example, extends diagonally across a wide area of sky, while the Leo Triplet benefits from careful framing to include its three prominent galaxies.
Once you've chosen the desired orientation, secure the rotator and verify focus before starting the imaging sequence.
The focuser also provides mounting provisions for a compatible ZWO EAF electronic focuser. With the appropriate electronic control system, an EAF can make small focus adjustments automatically throughout the night, helping compensate for changes in temperature and other factors that shift the point of best focus.
Flexible Camera Connections Without a Separate Flattener
The FF90 includes a photographic extension assembly and threaded adapters for common M54 and M48 astronomical camera connections. The supplied rear components also include a thread for a mounted 2" astronomical filter.
The native flat-field design means you don't need to reproduce an exact spacing behind a separate field flattener. You do, however, need to ensure the camera sensor can reach focus within the telescope's available focusing travel.
ZWO illustrates a usable photographic connection range of approximately 45mm to 75mm from the specified M54-to-M48 adapter shoulder.
The manufacturer also gives maximum rear connection lengths, measured with the focuser fully retracted, of 75mm from the base of the M48 male thread and 93mm from the base of the M54 male thread. The correct number depends on which adapters are installed, because those measurements begin at different physical reference points.
These are mechanical focusing limits, not mandatory field-correction distances. A camera system that exceeds the allowable connection length may be unable to reach infinity focus even though no separate flattener is involved.
For a simple camera connection, the supplied adapters may be all that's required. More elaborate arrangements with filter wheels, off-axis guiders, and additional adapters should be measured before everything is assembled.
Planning the connection lengths is particularly worthwhile with the FF90 because its available focusing range is narrower than that of the smaller FF70. A little preparation can save a considerable amount of frustration on the first clear night.
Portable Enough to Use Often
The FF90 is larger than a small 70mm refractor, but it remains a manageable instrument for one person to carry and mount.
Its optical tube weighs approximately 8.6 pounds, while the complete assembly with mounting rings, handle, and Vixen-style dovetail plate weighs about 10 pounds. A retractable dew shield keeps the telescope shorter during transportation and storage.
The precision-machined tube incorporates internal anti-reflection treatment to help reduce stray light. The detachable mounting rings and substantial carry handle make the telescope easier to handle when placing it into a mount saddle.
The supplied dovetail plate is approximately 9.1" long and follows the common Vixen mounting standard, making the FF90 compatible with a wide range of suitable equatorial and strain-wave mounts.
Mount selection should be based on the complete equipment arrangement, not just the optical tube's weight. A cooled camera, filter wheel, electronic focuser, guider, and associated hardware all add to the load. The telescope's length and exposure to wind also matter, particularly when imaging at fine image scales.
With an appropriately sized mount, the FF90 can form the center of a portable imaging system that is substantial enough for serious astrophotography without being overly demanding to set up and take down.
A Very Pleasant Visual Refractor
The FF90 is primarily presented as an imaging instrument, but its 90mm apochromatic objective can also provide rewarding visual views with suitable eyepieces and a star diagonal.
The Moon is an excellent subject. Along the terminator, low-angle sunlight reveals crater walls, mountain peaks, valleys, and other features in sharp relief. The detail changes noticeably from one evening to the next as the lunar phase progresses.
Jupiter's principal cloud belts and the changing arrangement of its four Galilean moons are within reach, while Saturn's rings remain a favorite visual target when the planet is favorably positioned. Although a 90mm telescope cannot provide the resolution of a much larger instrument, good optics and steady seeing can still produce satisfying planetary views.
Double stars are another enjoyable application, particularly pairs with contrasting colors. The FF90's relatively short focal length also makes it suitable for sweeping star fields and framing open clusters at lower magnifications.
Visual observing requires suitable adapters, a diagonal, and eyepieces, which are purchased separately. The FF90's rear assembly supports both 1.25" and 2" visual accessories with appropriate connections, subject to the telescope's available optical path length.
What Owners Say on Cloudy Nights
As a relatively recent addition to the ZWO telescope lineup, the FF90 doesn't yet have the long history of owner reports associated with more established refractors. However, early discussions on Cloudy Nights provide some useful perspectives.
In October 2026, an FF90 owner described taking his first astrophotograph using the telescope with a ZWO ASI585MC Air camera and AM3N mount. His subject was the Cocoon Nebula, IC 5146, and he collected approximately an hour of short exposures while learning the equipment and image-processing workflow.
He was encouraged to see the image develop and felt the stars appeared reasonably sharp and round. Other Cloudy Nights members discussed ways to improve the result through longer individual exposures, calibration frames, dithering, and more careful image processing.
His experience illustrates an important point: the telescope is only one part of producing a successful astronomical photograph. The FF90 can provide the optical foundation, but exposure settings, tracking, calibration, and processing all contribute to the finished image.
The same owner had previously sought advice about reaching focus with his particular camera and adapter arrangement, a useful reminder that a natively flat optical design still requires the imaging train to fit within the telescope's available focusing range.
For a prospective owner, these are worthwhile lessons. The FF90 removes the need for a separate field flattener, but learning to configure the rest of an astrophotography system remains part of the enjoyment and challenge of the hobby.
Under the Night Sky
The FF90's 609mm focal length is particularly appealing because it handles a broad range of targets throughout the year. It's long enough to give many medium-sized galaxies and nebulae a substantial presence in the image, yet short enough to retain a generous field of view.
Autumn brings M33, the Triangulum Galaxy, high into the evening sky. Unlike the bright central bulge of Andromeda, M33 has relatively low surface brightness, with loosely wound spiral arms that benefit from dark skies and generous integration time. The FF90 can frame the galaxy comfortably while providing enough image scale to begin revealing its structure.
In Ursa Major, M81 and M82 make a beautiful photographic pair. The broad spiral shape of M81 contrasts with the narrow, irregular appearance of M82, while the smaller galaxy NGC 3077 adds another interesting object to a wider composition.
Spring offers the Leo Triplet, consisting of M65, M66, and NGC 3628. These three galaxies fit comfortably in the FF90's field, and their contrasting appearances make them a rewarding project for a photographer using either an APS-C or full-frame camera.
Summer brings a different collection of subjects. The Eagle Nebula, M16, combines an open star cluster with glowing hydrogen gas and dark structures within the nebula. The famous Pillars of Creation are small features within this larger region, and recording meaningful detail in them depends on seeing, image scale, and careful processing.
Farther south, the Lagoon and Trifid Nebulae make a particularly attractive pairing in Sagittarius. Their emission, reflection, and dark-nebula structures provide a colorful view of a very active region of the Milky Way.
The Elephant's Trunk Nebula in Cepheus offers another excellent challenge, with its curved dark structure appearing against the surrounding hydrogen emission of IC 1396. Narrowband imaging can be particularly effective in bringing out the delicate gas clouds surrounding it.
Winter brings the Cone Nebula and Christmas Tree Cluster in Monoceros, surrounded by an extended complex of hydrogen emission and dark dust. The region is rich enough to reward repeated imaging sessions, with plenty of fine structures that become more apparent as additional exposures are combined.
For visual observing, the Double Cluster in Perseus, the rich star fields of Cygnus, and the Moon provide wonderful opportunities to enjoy the FF90 without using a camera at all.
Observing Tip: Measure Your Imaging Train Before Dark
The FF90 doesn't require a separate flattener, but the camera still needs to be positioned where the focuser can bring the image to focus.
Before assembling the equipment, make a simple diagram of everything installed behind the telescope. Include the camera's sensor depth, filter wheel, off-axis guider, filter drawer if used, and the thickness of each threaded adapter or spacer.
Measure the total distance from the appropriate FF90 rear connection to the camera sensor and compare it with ZWO's specified focusing range. Remember that the published M48 and M54 maximum lengths begin at different reference points. Don't add the dimensions from different reference points together.
If your imaging train is too long, the camera may remain outside the reachable focus position even with the focuser fully retracted. If it's too short, an extension may be useful, depending on the particular configuration. The goal is to place the sensor within the telescope's available focus travel, not to reproduce an arbitrary 55mm field-flattener spacing.
Once everything is assembled, use a bright star and a short camera exposure to find focus. You can begin with the coarse adjustment, then use the 10:1 fine-focus control or a compatible electronic focuser to refine the star image.
Finally, take a test photograph and examine all four corners at full resolution. If one corner is noticeably worse than the opposite corner, investigate camera tilt or mechanical alignment before assuming the optical design is at fault.
A few minutes spent planning and testing can save an evening of experimenting with spacers when you'd rather be collecting photographs.
FAQ
- Should I choose the FF90 or FF70 APO?
- The FF90 collects approximately 65% more light than the FF70 and has a longer 609mm focal length, providing additional image scale for medium-sized deep-sky objects. It also has a larger 3" focuser. The FF70 is lighter, has a wider field at 490mm, and offers more flexibility in its available focusing range. Choose according to your preferred targets, camera, and mounting system.
- Does the FF90 need a separate field flattener?
- No. The four-element optical design provides a natively flat field with a 44mm image circle. There is no separate field-flattener spacing requirement, although the imaging train must fit within the telescope's available focusing range.
- Will the FF90 work with a full-frame astronomical camera?
- Yes. ZWO specifies a 44mm corrected image circle, sufficient to cover a standard full-frame sensor. Camera tilt, adapter clearances, and vignetting should still be considered when assembling a full-frame imaging system.
- What is the back-focus requirement?
- ZWO illustrates a 45–75mm focusing range from the specified M54-to-M48 adapter shoulder. It also lists maximum connection lengths of 75mm from the M48 male thread or 93mm from the M54 male thread with the focuser fully retracted. These are focusing limits rather than a single required optical-correction spacing.
- What mount is suitable for the FF90?
- The telescope weighs approximately 10 pounds with its mounting rings, handle, and dovetail, before cameras and other accessories. A suitably rated equatorial or strain-wave mount with adequate tracking performance and stability is recommended for astrophotography. Include the complete imaging train when calculating the payload.
- Can I add a ZWO electronic focuser?
- Yes. The FF90 focuser includes mounting provisions for a compatible ZWO EAF electronic focuser. The EAF and any required model-specific mounting components are sold separately.
- Can the FF90 be used visually?
- Yes. The telescope can accept suitable visual accessories, including 1.25" and 2" eyepiece arrangements. A compatible star diagonal and eyepieces allow observation of the Moon, planets, double stars, star clusters, and brighter deep-sky objects.
What's Included
- ZWO FF90 APO 90mm f/6.8 flat-field apochromatic refractor optical tube
- Detachable mounting rings with carry handle
- Vixen-style dovetail mounting plate
- Photography extension assembly with mounted 2" filter thread
- M64x1 to M54x0.75 threaded adapter
- M54x0.75 to M54x0.75 threaded adapter
- M54x0.75 to M48x0.75 threaded adapter
- 1.25" and 2" visual connection adapters
- Manufacturer documentation
Equatorial mount, imaging camera, filters, guide equipment, star diagonal, and eyepieces are not included.
Optional Accessories
ZWO Electronic Automatic Focuser (EAF): Provides motorized focusing and supports automatic focus adjustments through compatible imaging software or control equipment.
ZWO Off-Axis Guider and Guide Camera: Allows the guiding camera to observe through the main telescope, reducing the potential for differential flexure during long exposures.
2" Astronomical Filters: Compatible mounted filters can be installed in the photography extension assembly. When using an off-axis guider, consider placing restrictive filters behind its pickoff prism to preserve guide-star brightness.
Electronic Filter Wheel or Filter Drawer: Makes it easier to switch between broadband and narrowband filters when photographing different types of astronomical objects.
Star Diagonal and Eyepieces: Allow the FF90 to serve as a capable visual refractor for observing the Moon, planets, double stars, and open clusters.
Final Thoughts
There's a reason many astrophotographers eventually become interested in refractors around 90mm to 100mm in aperture. They're large enough to provide useful resolving power and image scale, yet small enough that setting up for a clear evening doesn't necessarily become a major undertaking.
The ZWO FF90 APO fits comfortably into that category. Its 90mm objective gives it noticeably more light-gathering ability than a 70mm refractor, while the 609mm focal length provides an attractive field for galaxies, star clusters, and medium-sized nebulae.
What makes this particular telescope interesting is the integrated flat-field optical design. There's no separate field flattener to purchase, and changing cameras doesn't automatically mean starting another round of spacing adjustments. The substantial 3" focuser, camera rotator, and included photographic adapters make it practical to build a serious imaging system around the telescope.
It's not the right instrument for every situation. Someone whose primary interest is photographing the largest emission nebulae may prefer the wider field and lighter weight of the FF70. An observer concentrating on very small galaxies may eventually want considerably more focal length and aperture.
But for someone who enjoys photographing a little bit of everything, the FF90 offers a very sensible compromise. It provides enough reach to make medium-sized subjects interesting, enough field to keep larger objects comfortably framed, and enough portability that you won't dread carrying it outside.
And if you occasionally want to leave the camera indoors, there's still a well-corrected 90mm refractor waiting to show you the Moon, the planets, and the brighter parts of the night sky.
Sometimes the most useful telescope isn't the smallest or the largest one you own. It's the one that strikes the right balance between capability and convenience, and makes you want to take advantage of every clear night.
