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The cheapest fix for a focus that cannot be reached

The best extension tubes for 2026

An extension tube adds a measured length to the light path, and it exists because focus is a position, not a setting: a telescope brings an image to a sharp point at a fixed distance behind its optics, and if the eyepiece or the camera sensor cannot be placed there, no amount of turning the focuser will help. The problem appears most often when a camera replaces an eyepiece, when a binoviewer demands far more length than a single eyepiece, or when a diagonal is added or removed. The fix is a tube of the right length, and the right length is arithmetic rather than judgement. The fittings are where mistakes happen. Tubes come as barrels that slide into a focuser and as threaded sections that screw into an imaging train, and the threaded kind is rigid where the barrel kind can tilt under a heavy camera. Internal blackening is the specification that separates a good tube from a pipe: an unblackened interior reflects stray light straight into the field and lowers contrast in exactly the situations where contrast was the point. Read the clamping method if the tube takes an eyepiece, and read the rigidity if anything heavy hangs off the end. One consequence deserves stating: placing an extension between a Barlow and an eyepiece increases the amplification beyond the printed factor. We compare length, barrel and thread fittings, internal blackening, clamping method, rigidity and material.

The parts here, column by column

  1. Omegon 2 inch extension sleeve with spring-ring clamp

    Omegon 2 in extension sleeve, 50 mm optical path (55072)

    A 2 in extension sleeve of 50 mm optical path with a spring-ring clamp, a filter thread at the lower end and internal ribbing against reflections.

    Length
    50 mm, as the maker publishes it
    Barrel and thread fittings
    2 in · 2 in, at the lower end
    Internal blackening
    Ribbed inside against reflections, black anodised outside
    Clamping method
    2 in spring-ring clamp, per the maker
    Rigidity
    Not published by the maker.
    Material
    Not published by the maker.
    Amazon reference
    B074FQH48F (same reference on 6 Amazon marketplaces)
    EAN
    4049467550723
    • Publishes the optical path added, which is the only figure that matters for an extension
    • A spring-ring clamp grips the barrel evenly instead of denting it with a screw
    • Fifty millimetres is a large step; finer adjustment needs a set of thin rings

    Best for: A telescope whose focal plane sits too far out for its eyepieces to reach

    ≈ £36 / €39
    Check price on Amazon
  2. Explore Scientific set of five M48 extension sleeves

    Explore Scientific M48 extension sleeve set (0510381)

    Five threaded extension sleeves of 30, 20, 15, 10 and 5 mm, all M48 × 0.75, for fine adjustment of back focus in an imaging train.

    Length
    Not published by the maker.
    Barrel and thread fittings
    M48 × 0.75 on every sleeve
    Internal blackening
    Not published by the maker.
    Clamping method
    Not published by the maker.
    Rigidity
    Not published by the maker.
    Material
    Not published by the maker.
    Amazon reference
    B085HYZT9Y (same reference on 5 Amazon marketplaces)
    EAN
    4007922064154
    • Five published lengths that combine to any multiple of 5 mm up to 80
    • Threaded rather than push-fit, so nothing can tilt or slip in a long exposure
    • M48 only: a T2 train needs adapters at each end

    Best for: Dialling in the exact sensor spacing a reducer or flattener demands

    ≈ €55
    Check price on Amazon
  3. Bresser 37.5 mm M68 extension sleeve in aluminium

    Bresser Extension sleeve 37.5 mm for HEXAFOC (4940110)

    An aluminium extension sleeve of 37.5 mm with M68 × 0.75 male and female threads, intended for the 2.5 in HEXAFOC focuser.

    Length
    37.5 mm, from the title
    Barrel and thread fittings
    M68 × 0.75 internal and external, as the maker publishes them
    Internal blackening
    Matt black coated against stray light
    Clamping method
    Not published by the maker.
    Rigidity
    Not published by the maker.
    Material
    Aluminium
    Amazon reference
    B07PMKL9N3 (same reference on 5 Amazon marketplaces)
    EAN
    4007922060859
    • Both the length and the thread are published, which is everything an extension needs to state
    • M68 is the large-format thread that suits a 2.5 in focuser without narrowing the path
    • M68 × 0.75 is a specialist thread and will need adapters to meet T2 or M48

    Best for: A Bresser HEXAFOC focuser that needs its focal plane moved further out

    ≈ €27
    Check price on Amazon
  4. Topiky 60 mm 2 inch telescope extension tube

    Topiky 2 in extension tube, 60 mm

    A 60 mm 2 in extension tube in anodised aluminium with two brass-tipped locking screws to stop an inserted barrel slipping.

    Length
    60 mm, from the title · Not distinguished from the physical length in the listing
    Barrel and thread fittings
    2 in
    Internal blackening
    Black anodised, described as avoiding unwanted reflections
    Clamping method
    Two thumbscrews with brass locking tips, per the maker
    Rigidity
    Not published by the maker.
    Material
    Aluminium alloy
    Amazon reference
    B07VWQRT4P (same reference on 5 Amazon marketplaces)
    EAN
    0781593971556
    • Brass tips under the thumbscrews prevent an inserted barrel being marked
    • Two screws rather than one, which holds a heavy eyepiece more securely
    • The listing does not say whether 60 mm is the physical length or the optical path added

    Best for: A 2 in focuser that needs a substantial, inexpensive extension

    ≈ €24
    Check price on Amazon
  5. CHICIRIS 1.25 inch to 2 inch telescope eyepiece adapter

    CHICIRIS 1.25 in to 2 in adapter

    An adapter taking a 1.25 in eyepiece into a 2 in focuser, held by a brass compression ring rather than a bare thumbscrew.

    Length
    The listing gives none
    Barrel and thread fittings
    Holds a 1.25 in (31.7 mm) eyepiece in a 2 in (50.8 mm) focuser · Not mentioned
    Internal blackening
    Not published by the maker.
    Clamping method
    Brass compression ring, per the maker
    Rigidity
    Not published by the maker.
    Material
    Anodised aluminium
    Amazon reference
    B083587TGB (same reference on 5 Amazon marketplaces)
    EAN
    0738588852611
    • A brass compression ring protects the eyepiece barrel and keeps it centred
    • The conversion almost every 2 in focuser owner needs at some point
    • No length figure, and an adapter changes where the focal plane falls

    Best for: A 2 in focuser used with a collection of 1.25 in eyepieces

    ≈ €26
    Check price on Amazon
  6. ZWWAAZHF spotting scope to DSLR camera adapter tube

    ZWWAAZHF Spotting scope camera adapter tube

    An adapter tube for coupling a DSLR to named SVBONY spotting scopes, assembled by screwing a T-shaped adapter into a T adapter and then into the camera.

    Length
    Not published by the maker.
    Barrel and thread fittings
    Couples a DSLR to a spotting scope · None published
    Internal blackening
    Not published by the maker.
    Clamping method
    Not published by the maker.
    Rigidity
    All metal, per the maker
    Material
    Not published by the maker.
    Amazon reference
    B0H41D7DVW (same reference on 1 Amazon marketplace)
    EAN
    7862780235019
    • Names the exact scope models it is made for rather than claiming universality
    • Addresses focus wheel clearance, which is a real problem with spotting scope adapters
    • No thread designations of any kind are published

    Best for: Digiscoping with a named SVBONY spotting scope and a DSLR

    ≈ €15
    Check price on Amazon

Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.