Back focus is the distance from the end of your focuser’s drawtube to the point where your telescope forms an image—the focal plane. When your telescope is used in its default configuration, such as with the eyepiece it came with, the system is designed to reach focus without any special effort.
Back focus becomes important when you change that setup. Adding or swapping accessories, such as diagonals, focal reducers, cameras, filter wheels, or binoviewers, changes the physical spacing behind the telescope. If your modified setup can’t physically place the eyepiece or camera at the correct distance, the telescope will never come to focus, no matter how much you adjust it.
This is why a telescope that works perfectly for visual observing can suddenly refuse to focus when you add a camera or additional accessories. Nothing is wrong with the optics. The focal plane is simply no longer where your equipment can reach it.
Understanding back focus helps you choose compatible accessories, avoid frustrating trial-and-error setups, and get better results, especially when moving from visual observing to astrophotography.
Back Focus by Telescope Type
Back focus varies widely depending on the optical design. Schmidt-Cassegrain telescopes, such as the C5, C6, and C8, all offer approximately 5 inches (127 mm) of back focus, while the C9.25, C11, and C14 provide slightly more. EdgeHD models offer about 5.25 inches (133.35 mm) of back focus when measured from the reducer plate, or 5.75 inches (146.05 mm) from the 3-inch baffle tube lock ring. These generous distances make Schmidt-Cassegrains and EdgeHDs compatible with a wide range of accessories.
Maksutov-Cassegrain telescopes also offer long back focus, often around 5 inches or more, depending on the model. Newtonian reflectors, by contrast, have very short back focus—typically only about 1–2 inches—because their focal plane sits close to the tube wall. Refractors vary widely depending on the focuser design, but most flattener or reducers for refractors follow a standard 55 mm spacing from the rear shoulder of the corrector to the camera sensor.
Finally, Celestron’s Rowe-Ackermann Schmidt Astrograph (RASA) offers a wide range of back focus distances depending on aperture. It’s just 17.5mm with the included camera adapter for the RASA 6, approximately 25–29mm for the RASA 8, 72.8mm for the RASA 11, and 77.5mm for the RASA 36 cm.
How Back Focus Affects Your Choice of Accessories
Back focus directly impacts your ability to use accessories with your telescope. A telescope with limited back focus may prevent you from using a Barlow lens, focal reducer, binoviewer, DSLR camera, or 2” eyepieces.
Newtonians are affected the most. Two common solutions are to move the primary mirror cell forward in the tube or to install a low-profile focuser. For DSLR photography on a Newtonian, the Celestron Barlow T-adapter is especially helpful because its built-in Barlow lens pushes the focal point farther outward, creating the space needed for a camera.
Combining accessories can also reduce available back focus. For example, focal reducers move the focal plane inward. When you pair a reducer with a DSLR that needs about 50 mm of sensor spacing, you may run out of room—even with an SCT’s long back focus. Spacer kits, such as the M42 Spacer Kit, help fine-tune this spacing for proper astroimaging results.
Refractor Back Travel and Other Focus Limitations
Refractors sometimes have the opposite issue: too much inward travel is required for straight-through viewing. Many refractors are designed with the assumption that you will use a star diagonal. Without one, an eyepiece or small camera may sit too far back to reach focus at infinity. In these cases, adding extension tubes in place of the diagonal fills that extra distance.
Another situation you may encounter is when certain accessory combinations make the focal plane inaccessible. For example, some 2” diagonals place the focal plane too far down inside the diagonal for short-focal-length 1.25” eyepieces to reach. Switching to a 1.25” diagonal typically solves this.
Spotting scopes typically have very little back focus and are not designed to accommodate accessories such as binoviewers or 1.25” camera adapters.
Why Back Focus Is More Forgiving for Visual Observing and Planetary Imaging
Back focus requirements are far less critical for visual observing. Simply inserting an eyepiece into a Newtonian focuser, or using a star diagonal on a refractor or SCT, usually places the eyepiece near the correct position. Visual observing uses only the central part of the telescope’s field of view, where spacing errors are less noticeable. Planetary cameras behave similarly because their sensors are small and only capture the center of the image circle.
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