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Focal reducers

Skyoptikst f/6.3 reducer-corrector for SCT

A four-element fully multi-coated reducer-corrector that threads onto the rear cell of a Schmidt-Cassegrain and converts f/10 to f/6.3.

Skyoptikst f/6.3 reducer-corrector with SCT thread

Skyoptikst f/6.3 reducer-corrector for SCT

A four-element fully multi-coated reducer-corrector that threads onto the rear cell of a Schmidt-Cassegrain and converts f/10 to f/6.3.

Reduction factor
f/10 to f/6.3, as the maker states it
Required back focus
The listing gives none
Image circle
Not published by the maker.
Corrector function
Reducer and field corrector with optics — four elements
Thread standards
Schmidt-Cassegrain rear thread
Vignetting
Not published by the maker.
Housing
Fully multi-coated, per the maker · Aluminium alloy; all optical glass elements per the maker
Amazon reference
B0CN9CVSBP (same reference on 6 Amazon marketplaces)
EAN
0741754671657
  • Four elements correct the field as well as reducing, which a two-element reducer does not
  • The conversion is published as a pair of focal ratios rather than as a bare factor
  • The required distance to the sensor is not published, and a reducer has no tolerance on it

Best for: A Schmidt-Cassegrain used for imaging, where f/10 makes exposures impractically long

≈ $95 / £93 / €109
Check price on Amazon

Specifications

Reduction factor
f/10 to f/6.3, as the maker states it
Required back focus
The listing gives none
Image circle
Not published by the maker.
Corrector function
Reducer and field corrector with optics — four elements
Thread standards
Schmidt-Cassegrain rear thread
Vignetting
Not published by the maker.
Housing
Fully multi-coated, per the maker · Aluminium alloy; all optical glass elements per the maker
Amazon reference
B0CN9CVSBP (same reference on 6 Amazon marketplaces)
EAN
0741754671657

What f/10 to f/6.3 changes

Exposure time, by roughly the square of the ratio — and field width, by the ratio itself. On a Schmidt-Cassegrain, whose folded design gives a long focal length in a short tube, that conversion is what makes deep-sky imaging practical at all.

Four elements, and why it matters

A two-element reducer shortens the focal length. A four-element reducer-corrector also flattens the field, which is what stops stars smearing towards the corners. The distinction is the main reason this costs several times what a simple 0.5x costs.

The number that is not here

The required distance between the reducer and the sensor. Every reducer has one, it has no useful tolerance, and this listing does not publish it. The back-focus page on this site explains what goes wrong on either side of it.

The compatibility list

Celestron Schmidt-Cassegrains at 12.7, 15.2, 20.3, 27.9 and 35.6 cm are named, along with one entry the listing prints as “9 1/10.2 cm”, which does not resolve to a size. The named apertures are the ones to rely on.

Availability

German, French and Italian Amazon stores ran from 109 € to 118 € on the day of the survey.

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

Spanish Amazon catalogue holds a record for it that nobody is selling.

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

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    Reduction factor
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    Required back focus
    Not published by the maker.
    Image circle
    Not published by the maker.
    Corrector function
    Focal reducer with optics
    Thread standards
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    Vignetting
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    Housing
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    EAN
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    • The bullet text is shared verbatim with another seller’s listing

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    Reduction factor
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    Required back focus
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    Image circle
    Not published by the maker.
    Corrector function
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    Thread standards
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    Vignetting
    Not published by the maker.
    Housing
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    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
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    Corrector function
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    Thread standards
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    Vignetting
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    Housing
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The rest of this rack

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    Neutral or tinted dimming
    Not published by the maker.
    Fixed or variable design
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    Thread size
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    Cell depth
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    EAN
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    Adjustment method
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    Steps
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    Load rating
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    Folded size
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    Weight
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    Foot design
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    Load rating
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All focal reducers