One of the first lessons an astrophotographer learns is that buying a good refractor doesn't necessarily mean the telescope is ready for photography. Many small apochromats produce beautiful images in the center of the field, but the stars begin to stretch or distort toward the edges of a camera sensor. The usual solution is to purchase a separate field flattener, install it behind the telescope, and then work out the correct spacing between the flattener and the camera. Sometimes that process is straightforward. Sometimes a fraction of a millimeter seems to make all the difference.
The ZWO FF70 APO takes a more convenient approach. Its four-element apochromatic optical system is designed to produce a flat photographic field without a separate field flattener. All four optical elements work together to control color and field curvature, allowing the telescope to illuminate an image circle large enough for a full-frame astronomical camera.
With a 70mm aperture, 490mm focal length, and f/7 focal ratio, the FF70 is a compact instrument well suited to photographing nebulae, star clusters, and larger galaxies. The optical tube weighs only 5.5 pounds, and even with its mounting rings, handle, and dovetail plate, the complete assembly weighs approximately 6.6 pounds. That's light enough to make a portable astrophotography system practical without requiring a particularly large equatorial mount.
What I find appealing about the FF70 is the simplicity of the arrangement. Choose a compatible camera, connect it using the supplied adapters, reach focus, and begin imaging. You still need an accurate mount, and the usual skills of focusing, guiding, and exposure planning remain important. But one of the more frustrating mechanical adjustments associated with astrophotography has already been addressed in the telescope's optical design.
Four Elements Working Together
The FF70 uses a fully air-spaced, four-element apochromatic optical system that includes one element made from extra-low dispersion glass, commonly called ED glass. The manufacturer doesn't identify the particular ED glass formulation, but the purpose of the element is to reduce the tendency of different wavelengths of light to come to focus at different positions.
In a simple lens, blue light is refracted more strongly than red light. That difference can produce colored fringes around bright stars and reduce the sharpness of fine detail. An apochromatic design uses carefully chosen glass types and lens curvatures to bring different colors much closer to a common focus.
The FF70 takes that correction a step further by incorporating field flattening into the four-element optical arrangement. A conventional refractor tends to produce a curved focal surface. Your camera sensor, however, is flat. Even when the stars in the center are perfectly focused, stars near the corners may fall slightly ahead of or behind the best focus position.
A separate field flattener corrects that curvature, but it usually requires a particular distance between the flattener and camera sensor. With the FF70, the optical elements have been designed together to provide a flat image field directly, removing the need to purchase a separate flattener or experimentally adjust its spacing.
That's especially helpful if you use more than one camera or frequently change between imaging configurations. A camera with a filter drawer may have a different optical path length from one equipped with a filter wheel and off-axis guider. As long as the complete arrangement can reach focus within the telescope's available adjustment range, there's no separate flattener spacing requirement to solve.
Of course, mechanical alignment still matters. A camera that is tilted relative to the optical axis can produce distorted corner stars even when the telescope itself has a properly corrected field. The FF70 eliminates a common optical spacing problem, but it doesn't eliminate the importance of securely mounting the camera and keeping the imaging train square.
A Full-Frame Field at 490mm
The FF70 has a focal length of 490mm, making it a useful middle ground between a very short wide-field astrograph and a longer telescope intended primarily for smaller objects.
ZWO specifies a 44mm corrected image circle, large enough to cover the diagonal of a conventional 36mm x 24mm full-frame camera sensor. That gives the telescope a photographic field of approximately 4.2° wide by 2.8° high when paired with a full-frame camera.
To put that into perspective, the full Moon occupies roughly half a degree of sky. The FF70 with a full-frame camera can photograph an area more than eight full-Moon diameters across in its horizontal direction.
That's an attractive field for subjects such as the Andromeda Galaxy, the Pleiades, the Heart Nebula, and many of the large emission nebulae scattered along the Milky Way. You have enough field to provide context around the subject without making every nebula appear like a tiny patch of light surrounded by empty space.
An APS-C camera produces a smaller field, typically around 2.7° x 1.8°, depending on the sensor's actual dimensions. For example, a camera using 3.76-micron pixels records approximately 1.58 arcseconds of sky per pixel at the FF70's native focal length.
That sampling can be a practical match for many backyard imaging systems, although the final resolution will also depend on atmospheric seeing, guiding accuracy, and the camera's sensor characteristics.
It's worth remembering that ZWO's 44mm image circle describes the telescope's designed photographic coverage. The amount of illumination reaching the outer corners can still depend on the adapters, filters, and camera connections being used. A full-frame camera also places greater demands on mechanical alignment than a smaller sensor, so proper connections remain important.
No Separate Field Flattener or Fixed Correction Spacing
One of the FF70's most useful features is its flexibility when assembling an imaging train.
With many traditional refractor and flattener combinations, the camera sensor must be located at a precisely specified distance, commonly 55mm, behind the flattener. Adding a filter wheel, off-axis guider, or different camera can require recalculating every spacer in the optical path.
The FF70's native flat-field design doesn't impose that particular requirement. Instead, you need to make sure the camera's sensor falls within the available focusing range.
ZWO provides a photographic extension tube and several threaded rear interfaces, including M54 and M48 connections. The manufacturer specifies that focus can be obtained across a range of configurations, with a maximum accessory connection length of 82.5mm measured from the base of the M48 male thread, or 100.5mm measured from the M54 male thread, with the focuser fully retracted.
Its illustrated M54-to-M48 arrangement also shows an approximate 32.5mm to 82.5mm focusing range measured from the specified adapter shoulder. These measurements are referenced to different mechanical interfaces, so the appropriate dimension depends on which adapters are installed.
The important practical point is that these distances describe whether the system can reach focus. They aren't a single required spacing that must be matched to make the optical field flat.
For most photographers, that simplifies changing equipment considerably. Instead of spending an evening adding and removing thin spacing rings, you can concentrate on attaching the camera securely, reaching focus, and checking the resulting star images.
A Focuser Made for Photography
A good optical system deserves a focuser that can position a camera accurately and hold it securely. Even a small amount of unwanted movement can spoil the focus or tilt the camera enough to affect stars near the edge of the field.
The FF70 includes a 2.4" rack-and-pinion focuser with a dual-speed focusing mechanism. The coarse adjustment moves the camera quickly through its focusing range, while the 10:1 fine-focus control allows smaller movements when you're approaching the best focus position.
That fine control is particularly helpful when focusing on a star using a Bahtinov mask or a computer-assisted focusing routine. Astronomical cameras can detect very small changes in star size, and being able to make precise adjustments makes it easier to find the sharpest focus.
The rear of the focuser also includes a 360° camera rotator with an angular scale. This allows you to change the orientation of the camera without loosening the telescope in its mounting rings or moving the optical tube on the mount.
Camera rotation is more useful than it may initially appear. The Orion Nebula may look best in one orientation, while the Andromeda Galaxy or a long section of the Veil Nebula might benefit from another. Rather than accepting whatever angle the camera happens to occupy after assembly, you can rotate it to frame the target.
Once the camera is oriented correctly, secure the rotator according to the manufacturer's instructions and verify focus before beginning the imaging sequence.
The focuser also has provisions for installing a compatible ZWO EAF electronic focuser. With a suitable camera-control system, electronic focusing can make it possible to refocus automatically as nighttime temperatures change.
Built for a Portable Imaging System
The FF70's relatively small aperture and compact mechanical design make it well suited to a portable telescope arrangement.
Its retractable dew shield shortens the telescope for transportation and storage, while the CNC-machined tube incorporates internal anti-reflection treatment to reduce unwanted reflections and scattered light.
The supplied mounting rings, carry handle, and Vixen-style dovetail plate make it easier to install the telescope on a compatible equatorial mount. The handle also provides a convenient way to lift the instrument while attaching it to the mount.
Additional mounting positions on the handle and focuser area can accommodate compatible finder or guiding accessories. That is useful when building a small imaging system around a guide scope, guide camera, and electronic control equipment.
With the mounting hardware installed, the telescope weighs approximately 6.6 pounds before adding the camera, filter wheel, guider, and other accessories. Many compact equatorial and strain-wave mounts have enough capacity for an appropriately configured FF70 imaging system, but the complete equipment weight should always be considered rather than the telescope's weight alone.
The telescope itself is easy to transport. Accurate astrophotography still depends on a stable mount, careful polar alignment, and appropriate tracking or guiding, but the FF70 doesn't require you to build an unnecessarily large system around a small optical tube.
A Refractor for Visual Astronomy, Too
Although the FF70 is clearly designed with astrophotography in mind, it can also be used for visual observing with suitable eyepiece adapters, a star diagonal, and eyepieces.
A 70mm apochromatic refractor can be a delightful instrument for exploring the brighter parts of the night sky. It won't collect as much light as a large Dobsonian or resolve the same fine planetary detail, but the combination of a modest aperture, good color correction, and a short focal length can provide attractive views of the Moon, double stars, open clusters, and broader Milky Way fields.
The Pleiades and Double Cluster are particularly enjoyable subjects for a small refractor. Both benefit from a wide field of view that places the stars in their surrounding sky rather than squeezing the entire cluster against the edge of the eyepiece.
Lunar observing is another natural application. The changing shadows along the terminator reveal crater rims, mountain ranges, and other features that can look surprisingly detailed in a well-corrected small refractor.
Visual use requires an appropriate diagonal and eyepieces, which are not included with the basic optical tube package. Make sure the chosen adapters and diagonal provide a configuration that can reach focus within the telescope's available travel.
Under the Night Sky
The FF70's 490mm focal length opens up some wonderful photographic possibilities throughout the year. It's long enough to show useful structure in many nebulae and galaxies, yet short enough to frame large objects without demanding a mosaic for everything you photograph.
Autumn brings the Andromeda Galaxy, M31, high into the sky for northern observers. At roughly three degrees across its visible photographic extent, it's an excellent subject for the FF70 with a full-frame camera. Its bright central region, extended spiral disk, dark dust lanes, and two prominent companion galaxies can all become part of one composition.
The Heart Nebula, IC 1805, in Cassiopeia is another attractive autumn project. Its broad regions of hydrogen emission are particularly suitable for narrowband imaging, while the nearby Double Cluster makes a beautiful subject for either photography or visual observing.
During winter, the Pleiades, M45, offer a different photographic challenge. Their bright blue stars are easy to record, but the faint reflection nebulosity surrounding them benefits from dark skies and a substantial amount of integration time.
The Rosette Nebula in Monoceros makes a wonderful combination of an extended emission nebula and the central star cluster NGC 2244. Its roughly circular structure frames nicely at this focal length, especially with a camera capable of recording a reasonably wide field.
Orion offers several excellent subjects. The region extending from the Running Man Nebula through M42 and M43 fits beautifully into a moderately wide field. A carefully exposed image can preserve detail in the bright Trapezium region while bringing out the much fainter surrounding nebulosity.
Summer brings the great emission complexes of Cygnus. The North America Nebula can be photographed as an individual subject, while the neighboring Pelican Nebula offers a second composition with intricate regions of glowing gas and dark dust.
The Veil Nebula is particularly interesting. The Eastern and Western Veil regions each make rewarding photographs, and a full-frame camera may allow much of the complex to be captured in a carefully oriented field. A mosaic can provide more comfortable framing for the entire network of filaments.
At the eyepiece, the Double Cluster, the star clouds of Cygnus, and the brighter open clusters scattered across the Milky Way remind us that a small refractor can be rewarding even when the camera stays indoors.
Observing Tip: Think About Where Your Filter Goes
The FF70 includes a photographic extension tube with a thread for a standard mounted 2" astronomical filter. This can be a convenient place to install a broadband light-pollution filter or UV/IR-cut filter when the rest of your imaging arrangement is compatible.
But if you're using an off-axis guider, think carefully about the filter's position. An off-axis guider takes a small amount of light from the telescope's optical path and directs it to a guide camera. If a narrowband filter is installed ahead of that prism, the guide camera may receive much less starlight, making it harder to find and maintain a suitable guide star.
In that situation, it is often preferable to place the imaging filter behind the off-axis guider, where it affects the main camera but not the guide camera. A filter wheel or filter drawer positioned appropriately in the imaging train can provide that arrangement.
This is one of those small details that can make an imaging system much easier to use. The FF70's built-in filter thread is a useful option, but the best filter location depends on the equipment connected behind the telescope.
Once the camera and filter arrangement are established, use the rotator to frame your target, secure the connection, and refocus. Take a short test exposure and examine the stars near all four corners before collecting several hours of data.
FAQ
- Does the FF70 APO need a separate field flattener?
- No. Its four-element optical design incorporates field flattening, with a manufacturer-specified 44mm image circle. You do not need an additional flattener or a fixed flattener-to-camera spacing. The imaging system still must fit within the telescope's available focusing range.
- Will it work with a full-frame camera?
- Yes. ZWO specifies a 44mm image circle, sufficient to cover a conventional full-frame sensor. Proper camera alignment, adapters, and filter clearances remain important for the best corner performance.
- What back-focus distance does it require?
- Unlike a conventional telescope with a separate flattener, the FF70 doesn't require one exact correction spacing. ZWO specifies maximum accessory lengths of 82.5mm from the M48 male-thread base or 100.5mm from the M54 male-thread base with the focuser fully retracted. The appropriate measurement depends on the adapters being used.
- What mount should I use?
- The complete optical tube assembly with mounting rings, handle, and dovetail weighs approximately 6.6 pounds before adding imaging accessories. It is suitable for a variety of small equatorial and strain-wave mounts with adequate capacity and stability. Include the weight of the complete imaging train when selecting a mount.
- Can the FF70 be used for visual observing?
- Yes. With suitable visual adapters, a star diagonal, and eyepieces, the FF70 can be used for observing the Moon, star clusters, double stars, and other brighter objects. Visual accessories are sold separately.
- Does the FF70 work with the ZWO EAF electronic focuser?
- Yes. The focuser provides mounting provisions for a compatible ZWO EAF. An electronic focuser is available separately and can be used for automated focusing with compatible control software.
What's Included
- ZWO FF70 APO 70mm f/7 flat-field apochromatic refractor optical tube
- Mounting rings with carry handle
- Vixen-style dovetail mounting plate
- Photography extension tube with 2" filter thread
- M64x0.75 to M54x0.75 threaded adapter
- M54x0.75 to M54x0.75 threaded adapter
- M54x0.75 to M48x0.75 threaded adapter
- Manufacturer documentation and inspection materials
Equatorial mount, camera, electronic focuser, guide equipment, star diagonal, eyepieces, and filters are not included.
Optional Accessories
ZWO Electronic Automatic Focuser (EAF): Adds motorized focusing and supports automated focus adjustments through compatible control software.
ZWO 30F5 Mini Guide Scope and Guide Camera: Provides a compact guiding arrangement for long-exposure astrophotography.
2" Astronomical Filters: The photographic extension tube accepts compatible mounted 2" filters. Choose broadband or narrowband filters according to the intended targets and imaging arrangement.
Filter Drawer or Filter Wheel: Allows convenient filter changes and can place filters in a suitable location behind an off-axis guider when necessary.
Star Diagonal and Eyepieces: Allow the FF70 to be used for conventional visual astronomy with an appropriate connection and focusing arrangement.
Final Thoughts
There's something appealing about a telescope that removes a problem instead of asking you to purchase another accessory to solve it. The ZWO FF70 APO is a good example. The four-element optical system is designed to provide its own flat photographic field, eliminating the need for a separate field flattener and the sometimes frustrating process of adjusting the spacing behind it.
That doesn't make astrophotography effortless. A good mount, careful focusing, accurate guiding, and sensible exposure planning still matter. A poorly aligned camera can still produce misshapen stars, and the sky will continue to have its own ideas about what constitutes a good night for imaging.
But the FF70 offers a thoughtful combination of features for a relatively small refractor. Its 490mm focal length gives you access to a wide variety of attractive deep-sky subjects, the 44mm image circle supports full-frame imaging, and the dual-speed focuser, camera rotator, and included adapters make it practical to build a complete photographic system around the optical tube.
At only 5.5 pounds for the bare tube, it's also a telescope you're likely to use frequently. It doesn't demand a large mount or a complicated transportation plan, making it equally appealing for backyard imaging and trips to a darker observing site.
And if you occasionally want to leave the camera behind, a suitable diagonal and eyepiece turn it into a pleasant little wide-field visual refractor.
For the astrophotographer who wants a compact, capable instrument without adding a separate field flattener to the shopping list, the FF70 APO makes a great deal of sense. Sometimes the most enjoyable equipment is the equipment that lets you spend less time adjusting hardware and more time collecting light from the sky.
