Skip to content

Focal reducers

Astromania 0.5x focal reducer (RC05X)

A cemented two-element focal reducer of 85 mm focal length giving about 0.5x depending on spacing, with filter threads on both faces.

Astromania 0.5x focal reducer in black anodised housing

Astromania 0.5x focal reducer (RC05X)

A cemented two-element focal reducer of 85 mm focal length giving about 0.5x depending on spacing, with filter threads on both faces.

Reduction factor
About 0.5x, and the maker states it depends on the spacing
Required back focus
85 mm, as the maker publishes it
Image circle
22.5 mm, as the maker publishes it
Corrector function
Focal reducer with optics — two cemented elements
Thread standards
Filter thread on both faces; the maker writes it as 28.5 × 0.75 mm, where the convention on this shelf is M28.5 × 0.6
Vignetting
Not published by the maker.
Housing
Fully multi-coated, per the maker · Black anodised aluminium
Amazon reference
B01B2OR280 (same reference on 6 Amazon marketplaces)
EAN
6970019292991
  • Publishes its own focal length and clear aperture, which almost no reducer on this shelf does
  • States outright that the factor depends on spacing rather than printing 0.5x as fixed
  • The published thread pitch does not match the usual 1.25 in filter thread

Best for: A long-focal-ratio telescope with a small sensor or a narrow-field eyepiece

≈ $18
Check price on Amazon

Specifications

Reduction factor
About 0.5x, and the maker states it depends on the spacing
Required back focus
85 mm, as the maker publishes it
Image circle
22.5 mm, as the maker publishes it
Corrector function
Focal reducer with optics — two cemented elements
Thread standards
Filter thread on both faces; the maker writes it as 28.5 × 0.75 mm, where the convention on this shelf is M28.5 × 0.6
Vignetting
Not published by the maker.
Housing
Fully multi-coated, per the maker · Black anodised aluminium
Amazon reference
B01B2OR280 (same reference on 6 Amazon marketplaces)
EAN
6970019292991

Three published figures

Eighty-five millimetres of focal length, about 0.5x of reduction, 22.5 mm of clear aperture. Those three together describe a reducer properly — the first two say what it does optically and the third says what it will cover.

The factor depends on spacing, and the maker says so

A reducer shortens the effective focal length by an amount set by its distance from the focal plane. Astromania states that the 0.5x is approximate and varies with spacing, and offers spacers to change it deliberately. That is the correct account, and most listings simply print the factor as though it were fixed.

The thread that does not match

The maker writes the filter thread as 28.5 × 0.75 mm. Every 1.25 in filter on this site uses M28.5 × 0.6. Half a millimetre of pitch is the difference between a thread that engages and one that does not, and this page reports the figure as published rather than assuming which is right.

What it is not for

Fast telescopes and wide-field eyepieces — the maker excludes both. A simple two-element reducer cannot correct the steeper cone of a fast instrument, and a wide field will show the edge of its 22.5 mm aperture.

Availability

German, French, Italian and Spanish Amazon stores ran from 51 € to 53 € on the day of the survey.

American and British Amazon stores list it in local currency, which the euro range above sets aside.

Figures as Keepa reported them on the day this page was last updated. Stock and pricing shift without warning.

Prices quoted in this text were read on 17 September 2026. The live price is at the top of the page.

What this fits

More focal reducers

  • EBTOOLS 2 inch to T2 telescope camera adapter

    EBTOOLS 2 in to T2 adapter

    A 2 in nosepiece adapter carrying a T2 male thread for the camera and an M48 internal thread for a filter, with a fully matted bore.

    Reduction factor
    Not published by the maker.
    Required back focus
    Not published by the maker.
    Image circle
    Not published by the maker.
    Corrector function
    Not published by the maker.
    Thread standards
    2 in nosepiece for a 2 in focuser · M42 × 0.75 male — the T2 thread · M48 × 0.75 internal, at the front
    Vignetting
    “Full frame with 2 inch, no dark corners”, as the maker states it
    Housing
    Fully matted internal thread; black hard-anodised outside
    Amazon reference
    B07MCH2CVN (same reference on 5 Amazon marketplaces)
    EAN
    0719527383403
    • Carries an M48 filter thread at the front, so a 2 in filter fits in the same part
    • Fully matted bore, which matters immediately in front of a sensor
    • The no-dark-corners claim is made without naming a sensor size

    Best for: Putting a T2-threaded camera into a 2 in focuser with a filter in the path

    ≈ £15 / €15
    Check price on Amazon
  • ECSiNG M48 to M42 adapter ring in black anodised aluminium

    ECSiNG M48 to M42 double-male adapter

    An anodised aluminium ring with M48 × 0.75 and M42 × 0.75 male threads, blackened inside, containing no optical element at all.

    Reduction factor
    Not published by the maker.
    Required back focus
    Not published by the maker.
    Image circle
    Not published by the maker.
    Corrector function
    Not published by the maker.
    Thread standards
    M48 × 0.75 and M42 × 0.75, both male
    Vignetting
    Not published by the maker.
    Housing
    Aluminium, black anodised, matt internal wall
    Amazon reference
    B0C773T289 (same reference on 5 Amazon marketplaces)
    EAN
    4895243556683
    • Both threads named with their pitch
    • A matt bore, which reduces the reflections a shiny adapter introduces
    • It reduces nothing: the category is wrong and this page says so

    Best for: Joining a female M48 component to a female T2 one

    ≈ £10 / €12
    Check price on Amazon
  • Oumij1 0.5x focal reducer with M28.5 thread

    Oumij1 0.5x reducer, M28.5 × 0.6

    A 0.5x reducer for 1.25 in eyepiece noses in an aluminium housing, with the same stated exclusions as its twin listing.

    Reduction factor
    0.5x, per the maker
    Required back focus
    Not published by the maker.
    Image circle
    Not published by the maker.
    Corrector function
    Focal reducer with optics
    Thread standards
    M28.5 × 0.6 — the standard 1.25 in filter thread · Screws onto the nose of a 1.25 in eyepiece
    Vignetting
    Not published by the maker.
    Housing
    Aluminium alloy
    Amazon reference
    B093ZSLSDJ (same reference on 4 Amazon marketplaces)
    EAN
    9182438203992
    • The thread is named correctly, which several neighbours on this shelf fail to do
    • The exclusions are published rather than discovered
    • Shares its entire description with another listing

    Best for: A second reducer, or a first one where price decides

    ≈ €21
    Check price on Amazon

The rest of this rack

  • Omegon Pro CLS 2 inch light pollution filter

    Omegon Pro CLS 2 in

    A 2 in broadband CLS filter that the maker states suits telescopes from 70 mm upwards and darkens the object only slightly.

    Blocked wavelengths and published curve
    The listing gives none as wavelengths
    LED and sodium behaviour
    Not published by the maker.
    Visual or photographic design
    CLS broadband filter, 2 in thread
    Colour cast
    Not published by the maker.
    Thread size
    Not published by the maker.
    Focal ratio limits
    From 70 mm, as the maker states it
    Amazon reference
    B016O66DY0 (same reference on 6 Amazon marketplaces)
    EAN
    4049467472919
    • The maker states a minimum aperture, which almost no filter listing does
    • A 2 in thread suits wide-field eyepieces and 2 in focal adapters
    • No wavelengths and no transmission percentage appear anywhere

    Best for: A 2 in eyepiece or focal adapter on a telescope of 70 mm or more, under a town sky

    ≈ £83 / €93
    Check price on Amazon
  • SVBONY 1.25 inch CLS light pollution filter

    SVBONY CLS 1.25 in (FUSF9155A)

    A CLS broadband filter publishing 90 per cent transmission at the hydrogen, oxygen and sulphur lines and 0.1 per cent at the sodium and mercury lines of artificial lighting.

    Blocked wavelengths and published curve
    90 % at Hα 656 nm, O-III 496 and 500 nm, S-II 672 nm and Hβ 486 nm, as the maker publishes it
    LED and sodium behaviour
    0.1 %, named at Na 589 and 578 nm and Hg 435 nm
    Visual or photographic design
    CLS broadband filter, threaded for 1.25 in eyepieces and camera noses
    Colour cast
    Planetary rotation coating system, for uniformity — the maker’s own description
    Thread size
    Not published by the maker.
    Focal ratio limits
    Not published by the maker.
    Amazon reference
    B079BWD595 (same reference on 6 Amazon marketplaces)
    EAN
    0609378745577
    • The only filter on this shelf to publish both in-band and off-band transmission with named wavelengths
    • The blocked lines are named by element, which is what allows the claim to be checked against physics
    • A broadband filter helps less than a narrowband one under heavy light pollution

    Best for: Imaging or observing emission nebulae from a town, with a colour camera or by eye

    ≈ $32 / €46
    Check price on Amazon
  • Celestron 94107 variable polarising moon filter

    Celestron Variable Polarizing Moon Filter (94107)

    A two-element polarising filter for 1.25 in eyepieces with transmission adjustable from 1 to 40 per cent by rotating its lower ring.

    Transmission percentage
    1 % to 40 %, adjustable, as the maker publishes it
    Neutral or tinted dimming
    Not published by the maker.
    Fixed or variable design
    Variable polarising filter, threaded for 1.25 in eyepieces · Rotate the lower ring, per the maker · Threaded on both faces, so other filters can be combined with it
    Thread size
    Not published by the maker.
    Glass flatness
    Not published by the maker.
    Cell depth
    Not published by the maker.
    Amazon reference
    B07N99RW5K (same reference on 6 Amazon marketplaces)
    EAN
    0050234941075
    • A published transmission range rather than a single fixed figure or a colour word
    • One filter covers what several fixed neutral filters would
    • A polarising pair costs some light even at its most open setting

    Best for: The Moon across its whole cycle, where a fixed filter is right at only one phase

    ≈ $43 / £42 / €38
    Check price on Amazon

All focal reducers