Takahashi Back Focus Guide

Takahashi Back Focus Guide

Takahashi's system is built around one idea: set up one camera correctly, then move it between telescopes. You are not solving a new problem every time you change scopes. You are building the camera end once and reusing it.

That is what makes the range worth owning. Buy the camera you want, get its spacing right, and the same assembly screws onto an Epsilon, a TOA or an FSQ to give you a different focal length and a different focal ratio. The telescope changes. The camera end does not.

The number that matters

Most of the imaging range lands on the same output:

Telescopes Output Camera side needs
Epsilon-130D, Epsilon-160ED, Epsilon-180ED — with or without the extender M54 female 56.2 mm
FSQ-106EDX4 and FSQ-85EDX, on their 645 and F3 reducers M54 female 56.2 mm
TOA-130NS, TOA-130NFB, TOA-150B, on the TOA-645 flattener M54 female 56.2 mm
Mewlon-180C and Mewlon-210 on the M-FLRD, CCA-250 on its reducers M54 female 56.2 mm
New FSQ-80 and the FCT range M48 or M54 male 55 mm

The bottom row is the easy one. Those telescopes hand you a male thread at the standard 55 mm — assemble your camera, screw it on, take pictures. Nothing else to work out.

Building the camera end

Here is a real example. An ASI2600 Duo has a back focus of 55 mm. Add a Pegasus Astro M54 male-to-male spacer at 2 mm, and the assembly is 57 mm long.

That is longer than 56.2 mm — and that is fine. You want a little wiggle room. Filters from different manufacturers have different optical thicknesses, and glass in the light path pushes focus backwards. Leaving yourself a fraction of a millimetre in hand means you can change filter brands without rebuilding the train.

This is the part people get wrong by being too careful. The 56.2 mm figure is a design target, not a tolerance to hit on a lathe. Land in the 56–57 mm band with your filters accounted for and the corners will look after themselves.

What the spacer ring is doing

Once the camera end is built, the telescope end is a subtraction. Telescope plus its reducer or flattener presents a certain distance to the focal plane. A correctly sized spacing ring consumes most of it and leaves the 56–57 mm your camera occupies.

That ring is the only part that changes when you swap telescopes. Everything from the ring back — spacer, filter drawer, camera — travels with you.

Where the number is not 56 mm

Two situations put you outside the standard, and both are covered in the reference tables below:

  • The FC/FS Multi-Flattener 1.04x, which serves the whole FC and FS doublet range. Its working distance depends on which CA ring you fit — anywhere from 66.2 mm to 87.5 mm. The CA ring is the telescope-specific part and the flattener does not include one.
  • Extenders, which sit at 117.5 mm across the range, and prime focus on any telescope, where the figures run from 67 mm to 275 mm.

Why the system works this way

Metal back distance is a convention designed around film cameras thirty-five years ago, and carried forward through early CCD. The thinking was that you would buy one very good camera, set it up once, and swap it between telescopes to change focal length and focal ratio. That is still exactly how it works — and it is why a Takahashi you buy today will accept the camera you already own.

Send us your setup

Tell us your telescope, corrector, camera model, filter thickness and any spacers you already have, and we will tell you which parts close the gap. We stock the Takahashi adapter and ring range, and it is a shorter conversation than it looks.

Reference: telescope back focus, by configuration

Telescope names and product codes link straight to the part where we stock it. All figures in millimetres, measured to the focal plane from the rear of the corrector — or from the rear of the focuser at prime focus. Image circle is Takahashi's own definition: the area where relative illumination is 60% or better. 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. Where documents disagree on a reference face — as they do on the two Mewlons — this page follows the current catalog.

The FSQ-130ED also takes the 645RD and F3RD reducers. Takahashi's figures for those two combinations are not legible in the documents we hold, so they are left out rather than printed as something we cannot stand behind — ask and we will confirm them directly.

Reference: adapter and coupling light path lengths

Light path length is what each part contributes to the optical train — not 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.

Reference: CA rings for the FC/FS Multi-Flattener 1.04x

One optic covers the whole FC and FS doublet range. The CA ring is the telescope-specific part, and it is what sets the working 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.

Visual trains, and one thing about the Barlow

Back focus is an imaging concern. For visual use you have a diagonal and an eyepiece, and the focuser has enough travel to absorb them on almost every Takahashi refractor. The 1.25 inch prism diagonal adds 63.5 mm of optical path, which is the figure to check against your inward 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 as 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 — worth knowing when you work out the magnification an eyepiece will actually deliver.

Your telescope's system chart

Takahashi publishes a system chart for every telescope and every extender configuration of it: a diagram naming each adapter, ring and spacer by number with the distances marked, which turns a train into a parts list. The charts are attached to each telescope's product page here under the Downloads tab, and their numbered parts match the No. column above.

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.

© 2026 Ontario Telescope and Accessories. The selection, arrangement, commentary and worked examples on this page are our own work and may not be reproduced or republished without permission. The underlying specifications are published by Takahashi and remain Takahashi's; no claim is made to them. Takahashi and Takahashi product names are trademarks of their respective owner. Ontario Telescope and Accessories is an authorized dealer.