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.

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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.