Takahashi Back Focus Guide
Takahashi Back Focus Guide
Almost every Takahashi imaging question comes down to one number: how far behind the telescope your sensor has to sit. Takahashi publishes that number for every telescope and every corrector they make, which means there is no guesswork involved — only arithmetic. This page collects those published figures in one place, explains what they measure from, and shows how to add up an imaging train that lands on the right distance.
If you would rather not do the arithmetic, send us your telescope, corrector, camera and filter details and we will work the train out with you.
What Takahashi means by “back focus”
Takahashi defines back focus as the distance from the rear end of the focuser to the focal plane when you are at prime focus — or, when a corrector is fitted, the distance from the rear end of that corrector to the focal plane.
Two things follow from that definition, and both catch people out:
- The figure changes when you add a corrector. An FS-60CB has 162 mm of back focus at prime focus. Put the FC/FS Multi-Flattener 1.04x on it and the number becomes 87.5 mm — measured from the back of the flattener, not the back of the focuser. They are not the same measurement, so the two figures cannot be compared to each other.
- It is measured to the sensor, not to the camera. Your camera's own flange-to-sensor depth is part of the total. A typical cooled astro camera is 17.5 mm; most mirrorless bodies are between 18 mm and 20 mm. That figure comes out of your spacer budget before you add anything else.
A note on reference points. You will also see metal back distance on some Takahashi documents. It usually means the same thing, but not always from the same place — the Mewlon figures below are measured from the rear of the 50.8 adapter, and Takahashi's older backfocus table measures those two telescopes from a different reference and prints 162 mm instead of 67.2 mm. Neither is wrong; they start from different faces. Where sources disagree, this page follows the 2025 Takahashi General Catalog and says what the figure is measured from.
Telescope back focus, by configuration
All figures in millimetres. Image circle is Takahashi's own definition: the area where relative illumination is 60% or better and imaging performance holds to a stated level. FL = flattener, RD = reducer, EX = extender.
| Telescope | Configuration | Focal length | F-ratio | Image circle | Back focus |
|---|---|---|---|---|---|
| FCT-65D | Prime focus | 400 mm | f/6.2 | — | 199.2 |
| FC/FS MFL 1.04x | 416 mm | f/6.4 | Φ44 mm | 84.0 | |
| 65D RD NC 0.85x | 340 mm | f/5.2 | Φ44 mm | 56.0 | |
| 65D FURD 0.65x | 260 mm | f/4.0 | Φ44 mm | 56.0 * | |
| FS-60CB | Prime focus | 355 mm | f/5.9 | — | 162 |
| FC/FS MFL 1.04x | 370 mm | f/6.2 | Φ44 mm | 87.5 | |
| 60C RD NC 0.85x | 300 mm | f/5.0 | Φ44 mm | 56.0 | |
| EX CQ 1.7x (becomes FS-60Q) | 600 mm | f/10.0 | Φ44 mm | 158.4 | |
| FS-60CP | Prime focus | 355 mm | f/5.9 | — | 197.8 |
| 60C RD NC 0.85x | 300 mm | f/5.0 | Φ44 mm | 56.0 | |
| 60CP FURD 0.65x | 230 mm | f/3.8 | Φ44 mm | 56.0 * | |
| FS-60Q | Prime focus | 600 mm | f/10.0 | Φ44 mm | 158.4 |
| FOA-60 | Prime focus | 530 mm | f/8.8 | — | 175.5 |
| FL 0.93x | 495 mm | f/8.2 | Φ44 mm | 56.0 | |
| EX 1.7xR (becomes FOA-60Q) | 900 mm | f/15.0 | Φ88 mm | 208.1 | |
| FOA-60Q | Prime focus | 900 mm | f/15.0 | Φ88 mm | 208.1 |
| FC-76DCU | Prime focus | 570 mm | f/7.5 | — | 159 |
| FC/FS MFL 1.04x | 590 mm | f/7.8 | Φ44 mm | 75.0 | |
| 76D RD NC 0.85x | 486 mm | f/6.4 | Φ44 mm | 56.0 | |
| FC-76DP | Prime focus | 570 mm | f/7.5 | — | 207.1 |
| FC/FS MFL 1.04x | 590 mm | f/7.8 | Φ44 mm | 75.0 | |
| 76DP FURD 0.64x | 365 mm | f/4.8 | Φ44 mm | 56.0 * | |
| FC-100DC | Prime focus | 740 mm | f/7.4 | — | 161 |
| FC/FS MFL 1.04x | 760 mm | f/7.6 | Φ44 mm | 71.2 | |
| FC-100DF | Prime focus | 740 mm | f/7.4 | — | 223 |
| FC/FS MFL 1.04x | 760 mm | f/7.6 | Φ44 mm | 71.2 | |
| FC-35RD 0.66x | 480 mm | f/4.8 | Φ44 mm | 74.1 | |
| FC-100DZ | Prime focus | 800 mm | f/8.0 | — | 225 |
| FC/FS MFL 1.04x | 820 mm | f/8.2 | Φ44 mm | 71.2 | |
| FC-35RD 0.66x | 530 mm | f/5.3 | Φ44 mm | 68.8 | |
| FSQ-85EDX | Prime focus | 450 mm | f/5.3 | Φ44 mm | 197.5 |
| FL 1.01x | 455 mm | f/5.4 | Φ44 mm | 56.2 | |
| RD QB 0.73x | 330 mm | f/3.9 | Φ44 mm | 72.2 | |
| EX ED 1.5x | 680 mm | f/8.0 | Φ44 mm | 117.5 | |
| FSQ-106EDX4 § | Prime focus | 530 mm | f/5.0 | Φ88 mm | 178 |
| RD QE 0.73x | 385 mm | f/3.6 | Φ44 mm | 72.2 | |
| 645RD QE 0.72x | 380 mm | f/3.6 | Φ60 mm | 56.2 | |
| F3RD 0.6x | 320 mm | f/3.0 | Φ44 mm | 56.2 | |
| EX Q 1.6x | 850 mm | f/8.0 | Φ44 mm | 117.5 | |
| FSQ-130ED § | Prime focus | 650 mm | f/5.0 | Φ110 mm | 172.9 ¶ |
| EX Q 1.6x (50.8) | 1040 mm | f/8.0 | Φ44 mm | 117.5 | |
| TSA-120 | Prime focus | 900 mm | f/7.5 | — | 227.5 |
| TOA-35FL | 880 mm | f/7.3 | Φ40 mm | 117.5 | |
| TOA-35RD 0.7x | 635 mm | f/5.3 | Φ44 mm | 62.5 | |
| TOA-130NS | Prime focus | 1000 mm | f/7.7 | — | 222.5 |
| TOA-35FL | 980 mm | f/7.5 | Φ40 mm | 117.5 | |
| TOA-645FL | 990 mm | f/7.6 | Φ60 mm | 56.2 | |
| TOA-35RD 0.7x | 698 mm | f/5.4 | Φ44 mm | 65.5 | |
| TOA-130NFB | Prime focus | 1000 mm | f/7.7 | — | 274.5 |
| TOA-35FL | 980 mm | f/7.5 | Φ40 mm | 117.5 | |
| TOA-645FL | 990 mm | f/7.6 | Φ60 mm | 56.2 | |
| TOA-35RD 0.7x | 698 mm | f/5.4 | Φ44 mm | 65.5 | |
| TOA-150B | Prime focus | 1100 mm | f/7.3 | — | 275 |
| TOA-35FL | 1080 mm | f/7.2 | Φ40 mm | 117.5 | |
| TOA-645FL | 1085 mm | f/7.2 | Φ60 mm | 56.2 | |
| TOA-35RD 0.7x | 775 mm | f/5.2 | Φ44 mm | 62.5 | |
| Mewlon-180C | Prime focus | 2160 mm | f/12.0 | — | 67.2 † |
| M-FLRD | 1760 mm | f/9.8 | Φ30 mm | 56.2 | |
| Mewlon-210 | Prime focus | 2415 mm | f/11.5 | — | 67.2 † |
| M-FLRD | 1961 mm | f/9.3 | Φ30 mm | 56.2 | |
| Mewlon-250CRS § | Prime focus | 2500 mm | f/10.0 | Φ40 mm | 147.9 |
| RD CR 0.73x | 1825 mm | f/7.3 | Φ35 mm | 72.2 | |
| EX CR 1.5x | 3780 mm | f/15.1 | Φ44 mm | 117.5 | |
| Mewlon-300CRS § | Prime focus | 2960 mm | f/9.9 | Φ40 mm | 189.5 |
| RD CR 0.73x | 2160 mm | f/7.2 | Φ35 mm | 72.2 | |
| EX CR 1.5x | 4475 mm | f/14.9 | Φ44 mm | 117.5 | |
| CCA-250 § | Prime focus | 1250 mm | f/5.0 | Φ88 mm | 193.9 |
| RD CR 0.73x | 910 mm | f/3.6 | Φ38 mm | 56.2 | |
| EX CR 1.5x | 1880 mm | f/7.5 | Φ40 mm | 117.5 | |
| 645RD CA 0.72x | 890 mm | f/3.6 | Φ60 mm | 56.2 | |
| Epsilon-130D | Prime focus | 430 mm | f/3.3 | Φ44 mm | 56.2 |
| ε-EX 130D | 650 mm | f/5.0 | Φ44 mm | 56.2 | |
| Epsilon-160ED | Prime focus | 530 mm | f/3.3 | Φ44 mm | 56.2 |
| ε-EX 160ED | 800 mm | f/5.0 | Φ44 mm | 56.2 | |
| Epsilon-180ED | Prime focus | 500 mm | f/2.8 | Φ44 mm | 56.2 |
| ε-EX 180ED | 780 mm | f/4.3 | Φ44 mm | 56.2 |
* Measured from the rear end of the adapter. † Measured from the rear end of the 50.8 adapter. ¶ Measured from the rear of the camera angle adjuster. § From Takahashi's telescope backfocus table rather than the 2025 catalog specification chapter, which does not cover these models. All values are Takahashi design specifications.
Two patterns are worth noticing. The Epsilon astrographs all sit at 56.2 mm whether or not the extender is fitted, which makes them unusually straightforward to plan around. And 56.2 mm recurs right across the range — the TOA-645 flattener, the Mewlon reducer, the F3 and 645 reducers on the FSQ bodies — because it corresponds to the widely used 55 mm imaging standard once a thin filter sits in the path. If you already own spacers cut for a 55 mm train, a good deal of the Takahashi range will accept them.
The FSQ-130ED also takes the 645RD and F3RD reducers. Takahashi's published figures for those two combinations are not legible in the documents we hold, so we have left them out rather than print a number we cannot stand behind — ask us and we will confirm them with Takahashi directly. The FCT-114DS, FCT-114DF and FSQ-80FC are not covered by either the current catalog or the backfocus table; if you own one, get in touch and we will source the figures for you.
Adapter and coupling light path lengths
These are the parts you add up to reach the back focus figure above. Light path length is what each part contributes to the optical train — it is not the same as the part's physical length, which is why measuring an adapter with calipers gives the wrong answer. The No. column matches the numbered parts on Takahashi's system charts.
| No. | Part | Product code | Telescope-side fitting | Light path |
|---|---|---|---|---|
| 6S | Camera Angle Adjuster S | TKA21200N | — | 12.3 mm ‡ |
| 6M | Camera Angle Adjuster M | TKA23200N | — | 35 mm |
| 8 | Coupling (TW) | TKP31003 | — | 22.5 mm |
| 8A | Coupling (TW) (CCA-250) | TKP86003 | — | 34.2 mm |
| 12A | Visual Adapter | TKP86005 | — | 17 mm |
| 13 | 50.8 Sleeve | TKP00113 | — | 5 mm |
| 14S | Coupling (S) | TKP00103 | — | 10 mm |
| 14L | Coupling (L) | TKP00104 | — | 20 mm |
| 15 | Visual Adapter | TKP20005 | — | 10 mm |
| 16 | Aux. Ring | TKP20001 | — | 57 mm |
| 46 | Eyepiece Adapter (31.7) | TKP00101 | M36.5 × 1.0 male | 29 mm |
| 50 | Aux. Ring (FB TSA 102 mm) | TKA23250 | — | 52 mm |
| 61-A | Vari-Tube (L) | TKA00586 | M42 × 0.75 | 40 mm |
| 61-B | Vari-Tube (S) | TKA00587 | M42 × 0.75 | 20 mm |
| 69 | 50.8 Adapter (SKY90) | TKP35110 | M55.9 × 0.75 female | 25 mm |
| 70 | 50.8 Adapter (normal refractors) | TKP27110 | M72 × 1.0 male | 40 mm |
| 70F | 50.8 Adapter (FCT-65D) | TKP11110 | M55.9 × 0.75 female | 39.5 mm |
| 70S | 50.8 Adapter (FSQ-106EDX4 / 85EDX) | TKP27110S | M72 × 1.0 male | 30 mm |
| 71 | Extension Tube (50.8) | TKP27112 | — | 62 mm |
| 71L | Extension Tube (L) (50.8) | TKP31112 | — | 73.5 mm |
| 81 | Coupling (DT) | TKP20006B | — | 18 mm |
| 81S | Aux. Ring (S) (FSQ-130ED) | TKA38205 | — | 27.5 mm |
| — | Adapter (DT) | TKP73110 | M54 × 0.75 male | 11 mm |
| — | 50.8 Adapter (EX-Q) | TKA00596 | M42 × 0.75 female | 39.5 mm |
| — | Ocular Adapter (50.8) | TKP82110 | M72 × 1.0 male | 14 mm |
| — | Adapter (TV) | TKA70110 | Mewlon-180C / 210, with locking ring | 20 mm shorter than the supplied 50.8 adapter |
| — | Adapter (DM) | TKA00111 | 50.8 mm sleeve, male | 11 mm |
| — | Extension Tube (S) | TKP00105 | M43 × 0.75 | 26 mm |
| — | Extension Tube (L) | TKP00106 | M43 × 0.75 | 34 mm |
| — | 50.8 Turret Extension Tube (S) | TKA00105 | 50.8 mm sleeve | 35.5 mm |
‡ Measured from the tip of the left ring to the base of the male thread on the right side of the camera angle adjuster; its outer thickness is 20 mm.
CA rings for the FC/FS Multi-Flattener 1.04x
The Multi-Flattener is one optic covering the whole FC and FS doublet range. The telescope-specific part is the CA ring, and it is what sets the correct distance. The flattener does not include one.
| Product code | CA ring | Telescope | Back focus |
|---|---|---|---|
| TKA01203 | CA-50 | FC-50 | 84.0 mm |
| TKA20203 | CA-60C | FS-60CB | 87.5 mm |
| TKA03203 | CA-65 | FC-60, FC-65 | 78.0 mm |
| TKA18203 | CA-76 | FC-76D, FC-76, FS-78 | 75.0 mm |
| TKA19203 | CA-100 | FC-100D, FC-100 | 71.2 mm |
| TKA07203 | CA-125 | FC-100DL, FS-102, FC-125 | 69.2 mm |
| TKA24203 | CA-128 | FS-128, FS-152 | 66.2 mm |
The Multi-Flattener does not fit the FOA-60 or the SKY90 — both use different optical designs, and the FOA-60 has its own 0.93x flattener.
Working out your train
Take an FS-60CB with the FC/FS Multi-Flattener 1.04x and a cooled camera with a 17.5 mm sensor depth, running a 2 mm filter in a filter drawer that adds 20 mm of path.
- Target back focus: 87.5 mm, from the rear of the flattener.
- Camera sensor depth: 17.5 mm.
- Filter drawer: 20 mm.
- Filter glass: a filter in a converging beam shifts focus back by roughly one third of its thickness, so a 2 mm filter accounts for about 0.7 mm.
- Running total: 17.5 + 20 + 0.7 = 38.2 mm.
- Spacers needed: 87.5 − 38.2 = 49.3 mm.
That last figure is what you make up with adapters and spacer rings from the table above. Get within about half a millimetre and corner stars will look the same as centre stars; drift several millimetres out and the corners are where you will see it first.
The filter allowance is a rule of thumb rather than a Takahashi specification — it assumes glass of about n=1.5, which covers most astronomical filters. Thick filters and unusual glass types are worth checking individually.
Visual trains, and one thing about the Barlow
Back focus is an imaging concern. For visual use you are working with a diagonal and an eyepiece, and the focuser has enough travel to absorb them on almost every Takahashi refractor. The Takahashi 1.25 inch prism diagonal adds 63.5 mm of optical path, which is the figure to check against your inward focus travel if you use a short tube.
The 2x Ortho Barlow is the exception worth knowing about, because its magnification depends on back focus. Takahashi publishes the relationship:
M = 2 + 0.0121 × b, where b is back focus in millimetres.
- Eyepiece inserted directly (b = 0 mm): 2.00x
- Photographic, with T-mount (b = 50 mm): 2.61x
- With the 31.7 mm prism diagonal (b = 63.5 mm): 2.77x
So a “2x” Barlow behind a diagonal is really giving you closer to 2.8x. That is worth knowing when you are working out the magnification an eyepiece will actually deliver.
Your telescope's system chart
Takahashi publishes a system chart for every telescope, and for every extender configuration of it. The chart is a diagram naming each adapter, ring and spacer by number, with the distances marked — it turns a train into a parts list. The relevant charts are attached to each telescope's product page here, under the Downloads tab, and the numbered parts on them match the No. column in the adapter table above.
If the arithmetic is not landing
Send us your telescope, corrector, camera model, filter thickness and any spacers you already own, and we will work the train out with you and tell you exactly which parts close the gap. We stock the Takahashi adapter and ring range, and it is a faster conversation than it looks.
Figures on this page are taken from the 2025 Takahashi Astronomical Telescope & Accessories General Catalog, Takahashi's telescope backfocus table, and Takahashi's published specification pages. All values are design specifications. Where sources differ, this page follows the current catalog and states what each figure is measured from.