ZWO FF131-APO 131mm Quadruplet Refractor F/7.5

No. The ZWO FF131-APO uses a 4-element Petzval optical design that has a built-in flattener. You do not need to purchase an additional flattener or calculate any back-focus spacing; as long as you can reach focus, the field will be flat.

Due to its 20.7 lb. weight, the ZWO FF131-APO requires a medium to heavy-duty equatorial mount for astrophotography. For reliable tracking, a mount with a payload capacity of at least 35-40 lbs is recommended, such as a Sky-Watcher EQ6-R Pro, ZWO AM5, or a larger class of mount.

The 983mm focal length is excellent for medium-to-small deep-sky objects. On a target like the Whirlpool Galaxy (M51), it provides enough image scale to resolve individual dust lanes and star-forming regions, especially when paired with cameras that have small pixels (under 4µm).

At 20.7 lbs for the optical tube alone, plus the required heavy-duty mount, the ZWO FF131-APO is generally considered a backyard or observatory-class instrument rather than a "grab-and-go" portable setup. While transportable, it requires more effort to set up than smaller refractors.

The FF131-APO offers a 20% increase in aperture over the 110mm model, resulting in higher resolution and the ability to capture fainter detail. It also has a longer focal length (983mm vs. 766mm), making it better suited for smaller targets. The main trade-off is weight; the FF131-APO is nearly 6 lbs heavier and requires a more robust mount.

The ZWO FF131-APO includes a photography extension tube with standard M48x0.75 and M54x0.75 male threads. You connect this to the focuser's drawtube, and then attach your camera using the appropriate T-ring or adapter for your camera's bayonet or thread size.