Tools
Takahashi Imaging Train Calculator
Pick your optic and camera, add what sits between them, and get the spacing you still need — plus the parts that close it.
1 · Telescope
2 · Camera
Camera-mount route optional shortcut
A shortcut worth knowing. Where a Takahashi corrector terminates in a 35 mm camera bayonet, a ZWO or QHY filter wheel wearing the matching bayonet adapter lands on the required distance by itself — filter wheel included, no spacers to work out.
3 · In the light path
QHY adapter kit
A Combo kit is a parts bin, not a fixed stack — tick only the pieces actually in your train. Whatever you leave unticked, the calculator can use to close the gap.
4 · Filter
Glass in a converging beam pushes focus back by roughly a third of its thickness, so it adds to the distance you need. Cell and thread height are not part of this — count those under section 3.
The arithmetic
Takahashi back focus reference
Every figure the calculator above uses, set out in full. Takahashi publishes a back focus for each telescope and each corrector; these are those published figures, cross-checked between the 2025 Takahashi Astronomical Telescope & Accessories General Catalog and Takahashi's telescope backfocus table. The camera, filter wheel and off-axis guider figures come from each manufacturer's own specifications.
Telescope back focus by configuration 26 telescopes · 76 configurations
Back focus is measured to the sensor — from the rear of the corrector where one is fitted, or from the rear of the focuser at prime focus. 76 configurations across 26 telescopes.
| Telescope | Configuration | Focal length | F-ratio | Image circle | Back focus |
|---|---|---|---|---|---|
| FCT-65D | Prime focus | 400 mm | f/6.2 | — | 199.2 mm |
| FC/FS Multi-Flattener 1.04x | 416 mm | f/6.4 | Φ44 mm | 84 mm | |
| 65D RD NC 0.85x | 340 mm | f/5.2 | Φ44 mm | 56 mm | |
| 65D FURD 0.65x | 260 mm | f/4 | Φ44 mm | 56 mm | |
| FS-60CB | Prime focus | 355 mm | f/5.9 | — | 162 mm |
| FC/FS Multi-Flattener 1.04x (CA-60C ring) | 370 mm | f/6.2 | Φ44 mm | 87.5 mm | |
| 60C RD NC 0.85x | 300 mm | f/5 | Φ44 mm | 56 mm | |
| EX CQ 1.7x — becomes FS-60Q | 600 mm | f/10 | Φ44 mm | 158.4 mm | |
| FS-60CP | Prime focus | 355 mm | f/5.9 | — | 197.8 mm |
| 60C RD NC 0.85x | 300 mm | f/5 | Φ44 mm | 56 mm | |
| 60CP FURD 0.65x | 230 mm | f/3.8 | Φ44 mm | 56 mm | |
| FS-60Q | Prime focus | 600 mm | f/10 | Φ44 mm | 158.4 mm |
| FOA-60 | Prime focus | 530 mm | f/8.8 | — | 175.5 mm |
| FL 0.93x flattener | 495 mm | f/8.2 | Φ44 mm | 56 mm | |
| EX 1.7xR — becomes FOA-60Q | 900 mm | f/15 | Φ88 mm | 208.1 mm | |
| FOA-60Q | Prime focus | 900 mm | f/15 | Φ88 mm | 208.1 mm |
| FC-76DCU | Prime focus | 570 mm | f/7.5 | — | 159 mm |
| FC/FS Multi-Flattener 1.04x (CA-76 ring) | 590 mm | f/7.8 | Φ44 mm | 75 mm | |
| 76D RD NC 0.85x | 486 mm | f/6.4 | Φ44 mm | 56 mm | |
| FC-76DP | Prime focus | 570 mm | f/7.5 | — | 207.1 mm |
| FC/FS Multi-Flattener 1.04x (CA-76 ring) | 590 mm | f/7.8 | Φ44 mm | 75 mm | |
| 76DP FURD 0.64x | 365 mm | f/4.8 | Φ44 mm | 56 mm | |
| FC-100DC | Prime focus | 740 mm | f/7.4 | — | 161 mm |
| FC/FS Multi-Flattener 1.04x (CA-100 ring) | 760 mm | f/7.6 | Φ44 mm | 71.2 mm | |
| FC-100DF | Prime focus | 740 mm | f/7.4 | — | 223 mm |
| FC/FS Multi-Flattener 1.04x (CA-100 ring) | 760 mm | f/7.6 | Φ44 mm | 71.2 mm | |
| FC-35RD 0.66x | 480 mm | f/4.8 | Φ44 mm | 74.1 mm | |
| FC-100DZ | Prime focus | 800 mm | f/8 | — | 225 mm |
| FC/FS Multi-Flattener 1.04x (CA-100 ring) | 820 mm | f/8.2 | Φ44 mm | 71.2 mm | |
| FC-35RD 0.66x | 530 mm | f/5.3 | Φ44 mm | 68.8 mm | |
| FSQ-85EDX | Prime focus | 450 mm | f/5.3 | Φ44 mm | 197.5 mm |
| FL 1.01x flattener | 455 mm | f/5.4 | Φ44 mm | 56.2 mm | |
| RD QB 0.73x reducer | 330 mm | f/3.9 | Φ44 mm | 72.2 mm | |
| EX ED 1.5x extender | 680 mm | f/8 | Φ44 mm | 117.5 mm | |
| FSQ-106EDX4 | Prime focus | 530 mm | f/5 | Φ88 mm | 178 mm |
| RD QE 0.73x reducer | 385 mm | f/3.6 | Φ44 mm | 72.2 mm | |
| 645RD QE 0.72x reducer | 380 mm | f/3.6 | Φ60 mm | 56.2 mm | |
| F3RD 0.6x reducer | 320 mm | f/3 | Φ44 mm | 56.2 mm | |
| EX Q 1.6x extender | 850 mm | f/8 | Φ44 mm | 117.5 mm | |
| FSQ-130ED | Prime focus | 650 mm | f/5 | Φ110 mm | 172.9 mm |
| EX Q 1.6x (50.8) extender | 1040 mm | f/8 | Φ44 mm | 117.5 mm | |
| TSA-120 | Prime focus | 900 mm | f/7.5 | — | 227.5 mm |
| TOA-35FL flattener | 880 mm | f/7.3 | Φ40 mm | 117.5 mm | |
| TOA-35RD 0.7x reducer | 635 mm | f/5.3 | Φ44 mm | 62.5 mm | |
| TOA-130NS | Prime focus | 1000 mm | f/7.7 | — | 222.5 mm |
| TOA-35FL flattener | 980 mm | f/7.5 | Φ40 mm | 117.5 mm | |
| TOA-645FL flattener | 990 mm | f/7.6 | Φ60 mm | 56.2 mm | |
| TOA-35RD 0.7x reducer | 698 mm | f/5.4 | Φ44 mm | 65.5 mm | |
| TOA-130NFB | Prime focus | 1000 mm | f/7.7 | — | 274.5 mm |
| TOA-35FL flattener | 980 mm | f/7.5 | Φ40 mm | 117.5 mm | |
| TOA-645FL flattener | 990 mm | f/7.6 | Φ60 mm | 56.2 mm | |
| TOA-35RD 0.7x reducer | 698 mm | f/5.4 | Φ44 mm | 65.5 mm | |
| TOA-150B | Prime focus | 1100 mm | f/7.3 | — | 275 mm |
| TOA-35FL flattener | 1080 mm | f/7.2 | Φ40 mm | 117.5 mm | |
| TOA-645FL flattener | 1085 mm | f/7.2 | Φ60 mm | 56.2 mm | |
| TOA-35RD 0.7x reducer | 775 mm | f/5.2 | Φ44 mm | 62.5 mm | |
| Mewlon-180C | Prime focus | 2160 mm | f/12 | — | 67.2 mm |
| M-FLRD reducer/flattener | 1760 mm | f/9.8 | Φ30 mm | 56.2 mm | |
| Mewlon-210 | Prime focus | 2415 mm | f/11.5 | — | 67.2 mm |
| M-FLRD reducer/flattener | 1961 mm | f/9.3 | Φ30 mm | 56.2 mm | |
| Mewlon-250CRS | Prime focus | 2500 mm | f/10 | Φ40 mm | 147.9 mm |
| RD CR 0.73x reducer | 1825 mm | f/7.3 | Φ35 mm | 72.2 mm | |
| EX CR 1.5x extender | 3780 mm | f/15.1 | Φ44 mm | 117.5 mm | |
| Mewlon-300CRS | Prime focus | 2960 mm | f/9.9 | Φ40 mm | 189.5 mm |
| RD CR 0.73x reducer | 2160 mm | f/7.2 | Φ35 mm | 72.2 mm | |
| EX CR 1.5x extender | 4475 mm | f/14.9 | Φ44 mm | 117.5 mm | |
| CCA-250 | Prime focus | 1250 mm | f/5 | Φ88 mm | 193.9 mm |
| RD CR 0.73x reducer | 910 mm | f/3.6 | Φ38 mm | 56.2 mm | |
| 645RD CA 0.72x reducer | 890 mm | f/3.6 | Φ60 mm | 56.2 mm | |
| EX CR 1.5x extender | 1880 mm | f/7.5 | Φ40 mm | 117.5 mm | |
| Epsilon-130D | Prime focus | 430 mm | f/3.3 | Φ44 mm | 56.2 mm |
| ε-EX 130D extender | 650 mm | f/5 | Φ44 mm | 56.2 mm | |
| Epsilon-160ED | Prime focus | 530 mm | f/3.3 | Φ44 mm | 56.2 mm |
| ε-EX 160ED extender | 800 mm | f/5 | Φ44 mm | 56.2 mm | |
| Epsilon-180ED | Prime focus | 500 mm | f/2.8 | Φ44 mm | 56.2 mm |
| ε-EX 180ED extender | 780 mm | f/4.3 | Φ44 mm | 56.2 mm |
Mewlon prime focus figures are measured from the rear of the 50.8 adapter; the FSQ-130ED from the rear of the camera angle adjuster. Where Takahashi's two documents disagree on a reference face, this page follows the current catalog.
Takahashi in-train parts and their light path 13 parts
Light path length is what a part contributes to the optical train, which is not the same as its physical length — measuring an adapter with calipers gives the wrong answer.
| Part | Product code | Chart no. | Light path |
|---|---|---|---|
| Aux. Ring | TKP20001 | 16 | 57 mm |
| Aux. Ring (FB, TSA-102) | TKA23250 | 50 | 52 mm |
| Vari-Tube (L) | TKA00586 | 61-A | 40 mm |
| Camera Angle Adjuster M | TKA23200N | 6M | 35 mm |
| Coupling (TW), CCA-250 | TKP86003 | 8A | 34.2 mm |
| Aux. Ring (S), FSQ-130ED | TKA38205 | 81S | 27.5 mm |
| Coupling (TW) | TKP31003 | 8 | 22.5 mm |
| Vari-Tube (S) | TKA00587 | 61-B | 20 mm |
| Coupling (L) | TKP00104 | 14L | 20 mm |
| Coupling (DT) | TKP20006B | 81 | 18 mm |
| Camera Angle Adjuster S | TKA21200N | 6S | 12.3 mm |
| Adapter (DT), M54×0.75 male | TKP73110 | — | 11 mm |
| Coupling (S) | TKP00103 | 14S | 10 mm |
Camera back focus — flange to sensor 69 configurations
69 configurations. Several bodies have more than one legitimate figure depending on which supplied ring is fitted, which is the most common cause of a train that will not come to focus.
| Brand | Model | Configuration | Back focus |
|---|---|---|---|
| ZWO | ASI2600MC Pro | Native, M42 flange | 17.5 mm |
| ZWO | ASI2600MM Pro | Native, M42 flange | 17.5 mm |
| ZWO | ASI2600MC Duo | Native, M54 flange | 17.5 mm |
| ZWO | ASI2600MM Duo | Native, M54 flange | 17.5 mm |
| ZWO | ASI6200MC Pro | Native, M54 flange | 17.5 mm |
| ZWO | ASI6200MM Pro | Native, M54 flange | 17.5 mm |
| ZWO | ASI533MC Pro | 11 mm T2 extender fitted | 17.5 mm |
| ZWO | ASI533MC Pro | Extender removed | 6.5 mm |
| ZWO | ASI533MM Pro | 11 mm T2 extender fitted | 17.5 mm |
| ZWO | ASI533MM Pro | Extender removed | 6.5 mm |
| ZWO | ASI294MC Pro | 11 mm T2 extender fitted | 17.5 mm |
| ZWO | ASI294MC Pro | Extender removed | 6.5 mm |
| ZWO | ASI294MM Pro | 11 mm T2 extender fitted | 17.5 mm |
| ZWO | ASI294MM Pro | Extender removed | 6.5 mm |
| ZWO | ASI1600MM Pro | 11 mm T2 extender fitted | 17.5 mm |
| ZWO | ASI1600MM Pro | Extender removed | 6.5 mm |
| ZWO | ASI183MC Pro | 11 mm T2 extender fitted | 17.5 mm |
| ZWO | ASI183MC Pro | Extender removed | 6.5 mm |
| ZWO | ASI183MM Pro | 11 mm T2 extender fitted | 17.5 mm |
| ZWO | ASI183MM Pro | Extender removed | 6.5 mm |
| ZWO | ASI071MC Pro | Native, M42 flange | 17.5 mm |
| ZWO | ASI2400MC Pro | Native | 17.5 mm |
| ZWO | ASI461MM Pro | Short ring | 17.5 mm |
| ZWO | ASI461MM Pro | Long ring | 22.5 mm |
| QHY | QHY268M | Short BFL body | 14.5 mm |
| QHY | QHY268C — 2023 onward | Current top part | 14.5 mm |
| QHY | QHY268C — pre-2023 | Incl. mandatory 6 mm CAA | 23.5 mm |
| QHY | QHY600M / QHY600C PH | Incl. mandatory 6 mm CAA | 23.5 mm |
| QHY | QHY600M / QHY600C SBFL | Short BFL version | 14.5 mm |
| QHY | QHY294M Pro | Native | 17.5 mm |
| QHY | QHY183M / QHY183C | Native | 17.5 mm |
| QHY | QHY533M | Native | 14 mm |
| QHY | QHY533C | Native | 17 mm |
| QHY | QHY411 / QHY461 | Without tilt adjust ring | 16 mm |
| QHY | QHY411 / QHY461 | With tilt adjust ring | 28.5 mm |
| Player One | Poseidon-C Pro | Tilt plate fitted | 17.5 mm |
| Player One | Poseidon-C Pro | Tilt plate removed | 12.5 mm |
| Player One | Poseidon-M Pro | Tilt plate fitted | 17.5 mm |
| Player One | Poseidon-M Pro | Tilt plate removed | 12.5 mm |
| Player One | Zeus 455M Pro | Tilt plate fitted | 17.5 mm |
| Player One | Zeus 455M Pro | Tilt plate removed | 12.5 mm |
| Player One | Artemis-C Pro | Tilt plate fitted | 17.5 mm |
| Player One | Artemis-C Pro | Tilt plate removed | 12.5 mm |
| Player One | Ares-M Pro | Native | 17.5 mm |
| Player One | Uranus-C Pro | Native | 17.5 mm |
| Altair / ToupTek | Altair Hypercam — TEC cooled | Range-wide figure | 17.5 mm |
| Altair / ToupTek | Altair Hypercam — fan cooled | Range-wide figure | 12.5 mm |
| Altair / ToupTek | ToupTek ATR series | Default | 17.5 mm |
| Altair / ToupTek | ToupTek ATR series | M42 external thread ring fitted | 12.5 mm |
| DSLR / mirrorless | Canon EF / EF-S | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Canon EF / EF-S | Direct bayonet adapter — flange only | 44 mm |
| DSLR / mirrorless | Canon RF | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Canon RF | Direct bayonet adapter — flange only | 20 mm |
| DSLR / mirrorless | Nikon F | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Nikon F | Direct bayonet adapter — flange only | 46.5 mm |
| DSLR / mirrorless | Nikon Z | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Nikon Z | Direct bayonet adapter — flange only | 16 mm |
| DSLR / mirrorless | Sony E / FE | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Sony E / FE | Direct bayonet adapter — flange only | 18 mm |
| DSLR / mirrorless | Sony A | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Sony A | Direct bayonet adapter — flange only | 44.5 mm |
| DSLR / mirrorless | Micro Four Thirds | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Micro Four Thirds | Direct bayonet adapter — flange only | 19.25 mm |
| DSLR / mirrorless | Pentax K | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Pentax K | Direct bayonet adapter — flange only | 45.46 mm |
| DSLR / mirrorless | Fujifilm X | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Fujifilm X | Direct bayonet adapter — flange only | 17.7 mm |
| DSLR / mirrorless | Leica L | With nominal T-ring (T2 train) | 55 mm |
| DSLR / mirrorless | Leica L | Direct bayonet adapter — flange only | 20 mm |
Filter wheels, off-axis guiders and spacers 21 items
| Type | Item | Optical path |
|---|---|---|
| Filter wheel | ZWO EFW — mini, 8×1.25", 7×36 mm, 5×2", 7×2" | 20 mm |
| Filter wheel | ZWO EFW 7×50 mm square | 16 mm |
| Filter wheel | QHY CFW3S-SR (incl. 4 mm adapter) | 24 mm |
| Filter wheel | QHY CFW3S-US (incl. 4 mm adapter) | 19.5 mm |
| Filter wheel | QHY CFW3M-SR (incl. 2 mm adapter) | 22.5 mm |
| Filter wheel | QHY CFW3M-US (incl. 2 mm adapter) | 19 mm |
| Filter wheel | QHY CFW3L / CFW3XL | 21.5 mm |
| Filter wheel | Player One Phoenix 7×50 mm | 21 mm |
| Off-axis guider | ZWO OAG | 16.5 mm |
| Off-axis guider | ZWO OAG-L / OAG-L-68 | 17.5 mm |
| Off-axis guider | ZWO OAG-L 5 mm sensor tilt plate | 5 mm |
| Off-axis guider | QHY OAG (non-Pro, incl. 3 mm ring) | 13 mm |
| Off-axis guider | QHY OAG-M Pro / OAG-L Pro | 12 mm |
| Off-axis guider | Player One FHD-OAG MAX | 17.5 mm |
| Filter drawer | ZWO filter drawer — M42 | 21 mm |
| Filter drawer | ZWO filter drawer — M54 | 20 mm |
| ZWO spacers | ZWO 21 mm extender (T2-T2 or M54-M48) | 21 mm |
| ZWO spacers | ZWO 16.5 mm extender | 16.5 mm |
| ZWO spacers | ZWO 22.5 mm extender (M42-M48) | 22.5 mm |
| ZWO spacers | ZWO 15 mm M54 D70 adapter | 15 mm |
| ZWO spacers | ZWO 2 mm T2-T2 adapter (supplied with EFW) | 2 mm |
How much back focus a filter adds 7 filter brands
Glass in a converging beam shifts focus backwards by t(n−1)/n, where t is the glass thickness and n its refractive index. At n = 1.5229, a Schott B270-class filter substrate, that is close to a third of the thickness. Filter glass thickness is set by the brand, not by the filter size — mixing brands mid-set is the trap.
| Filter | Glass thickness | Back focus added |
|---|---|---|
| Astronomik | 1 mm | 0.34 mm |
| Astronomik XT clip | 0.5 mm | 0.17 mm |
| Optolong / ZWO 2" | 1.85 mm | 0.64 mm |
| ZWO 1.25" | 1.9 mm | 0.65 mm |
| ZWO 36 mm / Baader UHC-S / Antlia Pro | 2 mm | 0.69 mm |
| Chroma | 3 mm | 1.03 mm |
| Baader UBVRI photometric | 4 mm | 1.37 mm |
Below f/6 the calculator switches to the exact real-ray form, because at fast focal ratios the familiar "one third of the thickness" shortcut understates the shift.
Related
The Takahashi Back Focus Guide explains what these figures mean, how Takahashi defines back focus, and how to build a train by hand. Ontario Telescope and Accessories is an authorized Takahashi dealer — if the arithmetic is not landing, send us your telescope, corrector, camera, filters and spacers and we will work it out with you.
About this calculator
Ontario Telescope and Accessories built this, and we stand behind the numbers in it. Every telescope figure was taken from Takahashi's own published specifications — the 2025 Takahashi Astronomical Telescope & Accessories General Catalog and Takahashi's telescope backfocus table — and cross-checked between the two. Where those documents disagreed on a reference face, we followed the current catalog and said so. Every camera, filter wheel, off-axis guider and filter drawer figure came from that manufacturer's own manual or specification page. Nothing here is estimated, inferred from a similar model, or taken from a forum post. Where a manufacturer does not publish a figure, we left the product out rather than print a number we could not stand behind.
The filter calculation is real optics, not a rule of thumb. Glass in a converging beam shifts focus back by t(n−1)/n, and we use n = 1.5229 for a Schott B270-class filter substrate. Below f/6 the tool switches to the exact real-ray form, because at fast focal ratios the familiar "one third of the thickness" shortcut understates the shift by enough to matter.
One thing this tool cannot do, and we would rather say it plainly. It knows every distance, but it does not know which parts physically thread onto your telescope — that is set by Takahashi's system chart for your specific configuration. Treat the parts we suggest as candidates, confirm them against the chart on your telescope's product page, and if the chart and this page ever disagree, the chart wins and we want to hear about it.
If the arithmetic is not landing, or you own something this tool does not list, tell us your telescope, corrector, camera, filters and any spacers you already have. We will work the train out with you and tell you exactly which parts close the gap. That conversation is free, it is usually short, and you do not have to buy anything at the end of it.
© 2026 Ontario Telescope and Accessories. All rights reserved. The compilation, selection, arrangement, verification and cross-referencing of the data in this calculator, together with its logic, commentary and worked results, are our original work and are protected by copyright. They may not be copied, scraped, republished, mirrored, or incorporated into another site, tool, application or dataset — in whole or in part, and whether by a person or by automated means — without our prior written permission. Reasonable personal use by a customer planning their own imaging train is, of course, exactly what it is for.
The underlying manufacturer specifications remain the property of their respective publishers and are reproduced here for reference only; no ownership is claimed over them. Takahashi and Takahashi product names are trademarks of Takahashi Seisakusho Co., Ltd. ZWO, QHYCCD, Player One, Altair Astro, ToupTek, Astronomik, Baader, Chroma, Antlia, Optolong, Canon, Nikon, Sony, Pentax, Fujifilm, Leica and Pegasus Astro are trademarks of their respective owners. Use of these names indicates compatibility only and does not imply any endorsement or affiliation.
Ontario Telescope and Accessories is an authorized Takahashi dealer. All figures are design specifications published by the manufacturers and are provided in good faith for planning purposes. Manufacturing tolerances, running production changes and third-party adapters can all move the real number, so final spacing should always be confirmed against your own equipment and the applicable system chart before you commit to a purchase. Full background and the complete reference tables are in the Takahashi Back Focus Guide.