Most full-frame and APS-C DSLR and mirrorless cameras have an industry-standard 55 mm of internal back focus when paired with a lens or a T-ring. The thickness of the T-ring varies by camera model; however, when using the correct T-ring for your camera, the total internal spacing remains at the required 55 mm.
For example, a Nikon F-mount camera has a flange-to-sensor distance of 46.5 mm, so a Nikon F-mount T-ring is 8.5 mm thick to achieve the full 55 mm. A Canon EF-mount camera has a flange distance of 44 mm, so its T-ring must be 11 mm thick.
Celestron T-adapters are designed to work with this 55 mm internal back focus. The T-adapter for the 8-inch EdgeHD threads onto the 2-inch/24 thread on the baffle tube lock nut and is 78.35 mm long. The T-adapter for the 9.25-, 11-, and 14-inch EdgeHD models threads onto the larger 3.29-inch/16 thread and is 91.05 mm long. When combined with your camera’s 55 mm spacing, the T-adapter and T-ring place the camera sensor exactly at the required back-focus distance.
Back Focus Distances for Schmidt-Cassegrain Telescopes
| Schmidt-Cassegrain | Back focus distance from primary mirror baffle tube lock ring |
|---|---|
| C5 | 5 inches (127 mm) |
| C6 | 5 inches (127 mm) |
| C8 | 5 inches (127 mm) |
| C9.25 | 5.475 inches (139.07 mm) |
| C11 | 5.475 inches (139.07 mm) from 3-inch-to-2-inch reducer plate 5.975 inches (151.77 mm) from 3-inch baffle tube lock ring |
| C14 | 5.475 inches (139.07 mm) from 3-inch-to-2-inch reducer plate 5.975 inches (151.77 mm) from 3-inch baffle tube lock ring |
NOTE: The back focus for classic SCTs using the 0.63x focal reducer remains the same, as measured from the base of the threads on the rear of the reducer.
Back Focus Distances for EdgeHD Telescopes
| EdgeHD | Back focus distance from Primary Mirror baffle tube lock nut ring |
|---|---|
| EdgeHD 8" | 5.25 in (133.35 mm) |
| EdgeHD 9.25" | 5.25 in (133.35 mm) from the 3-inch-to-2-inch reducer plate 5.75 in (146.05 mm) From the 3-inch baffle tube lock ring |
| EdgeHD 11" | 5.25 in (133.35 mm) from the 3-inch-to-2-inch reducer plate 5.75 in (146.05 mm) From the 3-inch baffle tube lock ring |
| EdgeHD 14" | 5.25 in (133.35 mm) from the 3-inch-to-2-inch reducer plate 5.75 in (146.05 mm) From the 3-inch baffle tube lock ring |
Note: The back focus for EdgeHD using the dedicated 0.7x focal reducer for each model remains the same, as measured from the base of the threads on the rear of the reducer.
Back Focus Distances for RASA Telescopes
| RASA | Back Focus Distance |
|---|---|
| RASA 6" | ≈ 0.69 in (17.5 mm) with the included camera adapter |
| RASA 8" | ≈ 0.98 in (25mm) |
| RASA 11" | ≈ 2.87 in (72.8 mm) |
| RASA 36cm | ≈ 3.05 in (77.5 mm) |
How to Tell if Your Back Focus Is Correct
For the best photographic results, the camera sensor must be positioned within half a millimeter of the correct back focus distance. A quick check is to examine a short exposure of a bright star field. If the stars are round and crisp across the field, the spacing is correct. If the stars appear to radiate outward from the center of the field, the camera is too close and additional back focus is needed. If the stars seem to swirl concentrically around the center of the field, the camera is too far away, and less back focus is needed.
Cooled Cameras, Filter Wheels, and Off-Axis Guiders
Cooled deep-sky cameras typically have less internal back focus than DSLRs or mirrorless cameras. These cameras are often used in conjunction with filter wheels and off-axis guiders, which occupy some of the available back focus. Various length spacers with the same 42- or 48-mm thread as T-adapters are available to achieve the proper back focus spacing on a telescope.
Glass filters in the photographic light path add additional back focus to the existing telescope back focus. To calculate how much extra back focus a filter is adding to the image train, divide the filter thickness by 3. This 3 mm thick filter will add an additional 1 mm of back focus.
Back Focus on Newtonians and Refractors
Most Newtonian and refractor telescopes do not have a published back focus specification. Exceptions are premium refractors used exclusively for astrophotography. These instruments are called astrographs, and their manufacturers supply a range of adapters and spacers to properly position cameras at the correct back focus.
If no back focus specification is provided for a Newtonian or refractor, a quick way to approximate the back focus distance is to remove the eyepiece and point the telescope at the Moon. Rack the focuser all the way in and place a white card above the focuser. Move the card in and out until the lunar image is as sharp as possible. Measure the distance from the focuser to the card to approximate the telescope’s back focus specification.
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