Back focus and sensor spacing: how to add up a camera chain for reducers and flatteners

What back focus is

Celestron defines back focus as the distance from the end of the focuser drawtube to the point where the telescope forms its image, the focal plane. In its back focus article the company explains why a telescope that focuses fine with an eyepiece can refuse to focus with a camera: diagonals, reducers, cameras and filter wheels change the spacing, and if the sensor cannot physically reach the focal plane, no amount of focuser travel helps. Its rough figures by design: about 1–2 inches for Newtonians, about 127 mm for the C5, C6 and C8, and 55 mm from the rear shoulder of most refractor flatteners and reducers to the sensor.

There are really two requirements. One is reaching focus at all. The other is placing the sensor at the distance a corrector was designed for. William Optics separates them in its back spacing guide: “back focus” is a single distance with a very narrow tolerance, used by refractors with external flatteners or reducers, and getting it wrong distorts stars at the edge; “back spacing” is a usable range, used by Petzval designs with built-in correction, and getting it wrong means you cannot reach focus.

Why reducers and flatteners need an exact distance

Celestron’s focal reducer article states that every focal reducer works at a specific working distance, typically 55 mm for reducers made for f/7 or f/8 refractors, and that placing the sensor more than a few millimeters beyond it degrades the image, often as distorted stars near the edge. Its classic 0.63× SCT reducer has a working distance of 105 mm from the rear threads on the camera side. The reduction factor itself also depends on that distance, which the reducer calculator and the reducers, Barlows and spacing guide explain.

Published tolerances differ by design. Celestron’s spacing article asks for the sensor within half a millimeter of the correct distance. William Optics says its Petzvals need no millimeter-level tuning and recommends 55 ± 3 mm for the Cat 51 WIFD, to limit drawtube extension and the risk of light leaks. Askar lists a supported range of 50–60 mm for the SQA55, recommending 55 mm.

Celestron also gives a quick test: in a short exposure of a star field, stars that radiate outward from the center mean the camera is too close; stars that swirl around the center mean it is too far.

Published values in our data

Telescope	Published back focus	Measured from
William Optics Cat 51 WIFD	55 ± 3 mm (recommended)	rear mechanical reference surface
Askar SQA55	55 mm recommended, 50–60 mm supported	rear-end connection
Askar FRA400	140 mm	end of the rotator M68×1 thread
Celestron C8 with 0.63× reducer	105 mm	rear surface of the reducer
Celestron EdgeHD 8	133.35 mm	baffle tube lock ring
Celestron EdgeHD 9.25, 11, 14	133.35 mm or 146.05 mm	3-to-2-inch reducer plate, or 3-inch lock ring
Celestron RASA 11	72.8 mm	top of the threaded collar

Two notes from the sources. Askar says users of the FRA400 do not need to calculate back focus as long as the image is focused. Celestron says its 0.7× EdgeHD reducer shortens the EdgeHD 8’s back focus from 133.35 mm to 105 mm, while on the larger EdgeHDs the reducer keeps the same value, measured from its rear surface. The Askar Petzval page and the Celestron SCT page list the rest.

Cameras publish their own back focus, the distance from their front face to the sensor. ZWO’s ASI2600MM Pro manual gives 17.5 mm and a section of connection diagrams for reaching 55 mm. QHYCCD shortened the QHY268 to 14.5 mm, from 17.5 mm plus a 6 mm camera angle adjuster, to leave more room for a filter wheel drawer or rotator. DSLRs reach 55 mm through the T-ring: Celestron gives a Nikon F flange of 46.5 mm plus an 8.5 mm T-ring, and a Canon EF flange of 44 mm plus an 11 mm T-ring.

Adding up a spacing chain

The spacer length is whatever closes the gap between the camera’s own back focus and the telescope’s requirement. Celestron’s rule for filters, discussed below, adds a third of the filter thickness:

spacers (mm) = required back focus − camera back focus + filter thickness ÷ 3

Worked examples (computed):

Setup	Required	Camera	Filter	Spacers
ASI2600MM Pro on Askar SQA55	55 mm	17.5 mm	none	37.5 mm
Same, with a 1 mm Astronomik filter	55 mm	17.5 mm	+0.33 mm	37.83 mm
ASI2600MM Pro on EdgeHD 8 with 0.7× reducer	105 mm	17.5 mm	none	87.5 mm
Same, with a 3 mm filter	105 mm	17.5 mm	+1.0 mm	88.5 mm

Celestron’s own T-adapters show the method: the EdgeHD 8 adapter is 78.35 mm long and the camera side supplies 55 mm, making 133.35 mm; for the larger EdgeHDs, 91.05 mm plus 55 mm makes 146.05 mm. Changing the reducer also changes the field of view, so recheck framing after a change of focal length.

Filters and other glass

Glass filters in the light path add back focus. Celestron’s spacing article gives a rule of thumb: divide the filter thickness by 3, so a 3 mm filter adds 1 mm. Astronomik lists 1 mm glass for its narrowband filters, which by the same rule adds about 0.33 mm. Within the SQA55’s 10 mm range that is negligible; against Celestron’s half-millimeter tolerance behind a reducer, a 3 mm filter is not. William Optics adds that thick or stacked filters, extra optical elements and large temperature changes can all shift the focus position slightly.