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Correctors and Reducers

Round stars in the corners are not a processing achievement — they are a spacing achievement. Field flatteners and reducers correct the curved focal plane and off-axis aberrations that every telescope has to some degree, and they only work when spaced to within about a millimetre of their design distance. David Astro carries field flatteners and reducers in Canada, shipping from Canadian stock.

This is the category where our back-focus calculation service saves the most frustration. Send us your telescope, camera and filters and we will specify the corrector and the exact spacers to go with it.

Filters

Orion Optics UK AG Wynne corrector to fit FLI 3" thread 18mm BFD with 4mm adj
$297.29 - $297.29
$297.29
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Orion Optics UK AG Wynne corrector to fit FLI 3" thread 18mm BFD with 4mm adj

OOUK-AG-WYNNE-CORRECTOR-FLI-3

4-lens ED corrector for the AG Astrograph range Machined to fit FLI 3" thread 18mm back focal distance Individually machined to order in the UK ...

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$297.29 - $297.29
$297.29
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$297.29
Orion Optics UK AG Wynne corrector to fit FW8STL using 33mm BFD+-4mm
$297.29 - $297.29
$297.29
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$297.29

Orion Optics UK AG Wynne corrector to fit FW8STL using 33mm BFD+-4mm

OOUK-AG-WYNNE-CORRECTOR-FW8STL-33MM

4-lens ED corrector for the AG Astrograph range Machined to fit FW8STL 33mm back focal distance Individually machined to order in the UK Descri...

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$297.29 - $297.29
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Orion Optics UK AG Wynne corrector to fit Moravian M68x1 using 31mm BFD
$297.29 - $297.29
$297.29
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$297.29

Orion Optics UK AG Wynne corrector to fit Moravian M68x1 using 31mm BFD

OOUK-AG-WYNNE-CORRECTOR-MORAVIAN-M68X1

4-lens ED corrector for the AG Astrograph range Machined to fit Moravian M68x1 31mm back focal distance Individually machined to order in the UK ...

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$297.29 - $297.29
$297.29
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Orion Optics UK AG Wynne corrector to fit SBIG using 35mm BFD with +-4mm adj
$297.29 - $297.29
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$297.29

Orion Optics UK AG Wynne corrector to fit SBIG using 35mm BFD with +-4mm adj

OOUK-AG-WYNNE-CORRECTOR-SBIG-35MM

4-lens ED corrector for the AG Astrograph range Machined to fit SBIG 35mm back focal distance Individually machined to order in the UK Descript...

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$297.29 - $297.29
$297.29
$297.29 - $297.29
$297.29
Orion Optics UK AG Wynne corrector to fit SBIG using 41mm BFD with +-4mm adj
$297.29 - $297.29
$297.29
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Orion Optics UK AG Wynne corrector to fit SBIG using 41mm BFD with +-4mm adj

OOUK-AG-WYNNE-CORRECTOR-SBIG-41MM

Wynne-Type Coma Corrector Optimized for Orion Optics UK AG Telescopes Designed for SBIG Imaging Cameras 41mm Nominal Back Focus Distance Provides ...

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$297.29 - $297.29
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Orion Optics UK AG Wynne corrector to fit STL dovetail using 12mm BFD with+-4mm
$297.29 - $297.29
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Orion Optics UK AG Wynne corrector to fit STL dovetail using 12mm BFD with+-4mm

OOUK-AG-WYNNE-CORRECTOR-STL-DOVETAIL

4-lens ED corrector for the AG Astrograph range Machined to fit STL dovetail 12mm back focal distance Individually machined to order in the UK ...

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$297.29 - $297.29
$297.29
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Orion Optics UK AG Wynne corrector to fit STL using 21mm BFD with +-4mm adj
$297.29 - $297.29
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$297.29

Orion Optics UK AG Wynne corrector to fit STL using 21mm BFD with +-4mm adj

OOUK-AG-WYNNE-CORRECTOR-STL-21MM

4-lens ED corrector for the AG Astrograph range Machined to fit STL 21mm back focal distance Individually machined to order in the UK Descripti...

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$297.29 - $297.29
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Orion Optics UK AG Wynne corrector to fit STL using 25mm BFD with +-4mm adj
$297.29 - $297.29
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Orion Optics UK AG Wynne corrector to fit STL using 25mm BFD with +-4mm adj

OOUK-AG-WYNNE-CORRECTOR-STL-25MM

4-lens ED corrector for the AG Astrograph range Machined to fit STL 25mm back focal distance Individually machined to order in the UK Descripti...

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$297.29 - $297.29
$297.29
$297.29 - $297.29
$297.29
Orion Optics UK AG Wynne corrector to fit STL using 31mm BFD with +-2mm adj
$297.29 - $297.29
$297.29
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$297.29

Orion Optics UK AG Wynne corrector to fit STL using 31mm BFD with +-2mm adj

OOUK-AG-WYNNE-CORRECTOR-STL-31MM

4-lens ED corrector for the AG Astrograph range Machined to fit STL 31mm back focal distance Individually machined to order in the UK Descripti...

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$297.29 - $297.29
$297.29
$297.29 - $297.29
$297.29
Orion Optics UK AG Wynne corrector to fit STL using 35mm BFD with 4mm adj
$297.29 - $297.29
$297.29
$297.29 - $297.29
$297.29

Orion Optics UK AG Wynne corrector to fit STL using 35mm BFD with 4mm adj

OOUK-AG-WYNNE-CORRECTOR-STL-35MM

4-lens ED corrector for the AG Astrograph range Machined to fit STL 35mm back focal distance Individually machined to order in the UK Descripti...

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$297.29 - $297.29
$297.29
$297.29 - $297.29
$297.29
Orion Optics UK Wynne Corrector 3″ for AG Series Astrographs
$1,352.19 - $1,352.19
$1,352.19
$1,352.19 - $1,352.19
$1,352.19

Orion Optics UK Wynne Corrector 3″ for AG Series Astrographs

OOUK-AG20-AG-WYNNE-CORRECTOR-3

Corrects coma in fast parabolic (Newtonian) mirrors Delivers a wide, flat, and apochromatic (false-color-free) focal plane Advanced optical design...

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$1,352.19 - $1,352.19
$1,352.19
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Player One 2.1mm CS lens
$22.00 - $22.00
$22.00
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$22.00

Player One 2.1mm CS lens

POA-21LENS

2.1mm Focal Length CS-Mount Interface Designed for all-sky monitoring with planetary cameras 150° Field of View on 1/3" Sensors All-Sky and Wide-F...

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$22.00 - $22.00
$22.00
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$22.00
Player One 2.5mm CS lens
$22.00 - $22.00
$22.00
$22.00 - $22.00
$22.00

Player One 2.5mm CS lens

POA-25LENS

2.5mm Focal Length CS-Mount Standard Ultra-Wide Field of View Ideal for All-Sky Cameras Description FAQ Specifications In the Box Downloads War...

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$22.00 - $22.00
$22.00
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Player One ALL-SKY 2.1 KIT
$69.00 - $69.00
$69.00
$69.00 - $69.00
$69.00

Player One All-Sky Kit - 2.1mm

POA-AS2.1KIT

2.1mm Focal Length Lens 150° Field of View Optimized for 1/3" Format Sensors Ideal for All-Sky Monitoring Description FAQ Specifications In the...

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$69.00 - $69.00
$69.00
$69.00 - $69.00
$69.00
Player One ALL-SKY 2.5 KIT
$69.00 - $69.00
$69.00
$69.00 - $69.00
$69.00

Player One All-Sky Kit 2.5mm

POA-AS2.5KIT

Designed for All-Sky Monitoring Extremely Wide, Fish-Eye Field of View Optimized for select Player One CMOS sensors Fits the camera's M42 thread w...

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$69.00 - $69.00
$69.00
$69.00 - $69.00
$69.00
Player One CS-1.25 Adapter
$10.00 - $10.00
$10.00
$10.00 - $10.00
$10.00

Player One CS-1.25 Adapter

POA-CS125

Connects CS-mount lenses to 1.25" cameras Female Thread: CS-Mount Male Thread: M28.5x0.6mm (Standard 1.25" Filter Thread) Ideal for creating compa...

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$10.00 - $10.00
$10.00
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$10.00
$35.00 - $35.00
$35.00
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$35.00

QHYCCD Adapter Kit - Combo A3

QHY-020102

Connects QHYCCD cameras to Nikon F-mount lenses Enables wide-field astrophotography with camera lenses Camera-side thread: M54 Lens-side mount: Ni...

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$35.00 - $35.00
$35.00
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$35.00

QHYCCD Adapter Kit - Combo B2

QHY-020104

Connects M54 Threaded Cameras to Canon EF Lenses Provides a Precise 10mm Optical Path Length M54x0.75 Female Threads Secure Bayonet-Style Canon EF...

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$35.00 - $35.00
$35.00
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$35.00
$40.00 - $40.00
$40.00
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$40.00

QHYCCD Adapter Kit - Combo B3

QHY-020105
1 in stock

Connects Nikon F-mount lenses to M54 threaded accessories Provides a precise 12.5mm of optical path length Enables wide-field astrophotography wit...

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$40.00 - $40.00
$40.00
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$50.00
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$50.00

QHYCCD Expansion Kit for QHY5III462C

QHY-120001

This kit contains a methane filter, an AllSky lens and a lens adapter. It's made for the QHY5III462C camera only.

$50.00 - $50.00
$50.00
$50.00 - $50.00
$50.00
$35.00 - $35.00
$35.00
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$35.00

QHYCCD M42(M) Canon 10mm Adapter

QHY-020072

Connects Canon EF Lenses to M42 Female Threads Provides a Fixed 10mm Optical Path Length M42x0.75 Male "T-Thread" Interface Enables Wide-Field Ima...

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$35.00 - $35.00
$35.00
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QHYCCD M42(M) Nikon 10mm Adapter

QHY-020074

QHYCCD M42(M) Nikon 10mm Adapter

$35.00 - $35.00
$35.00
$35.00 - $35.00
$35.00
QHYCCD M42/M54 Threads-Canon EF/Nikon F DSLR Lens Adapter
$35.00 - $35.00
$35.00
$35.00 - $35.00
$35.00

QHYCCD M42/M54 Threads-Canon EF/Nikon F DSLR Lens Adapter

QHY-020098

Connects Canon EF and Nikon F mount DSLR lenses to astronomy cameras Features both M42 (T-thread) and M54 female threads for camera-side connectio...

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$35.00 - $35.00
$35.00
$35.00 - $35.00
$35.00
QHYCCD SWIR Lens for QHY990/991
$395.00 - $395.00
$395.00
$395.00 - $395.00
$395.00

QHYCCD SWIR Lens for QHY990/991

QHY-070005

Engineered for Short-Wave Infrared (SWIR) wavelengths Optimized for QHY990 and QHY991 SWIR cameras High-transmission coatings for the SWIR band De...

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$395.00 - $395.00
$395.00
$395.00 - $395.00
$395.00

Flatteners, Reducers and Coma Correctors

Each type solves a different optical problem, and using the wrong one degrades the field instead of correcting it.

  • Field flatteners: Correct focal plane curvature without changing focal length, so framing stays the same and only corner star shape improves.
  • Focal reducers: Flatten and shorten focal length, typically to 0.8x or 0.7x, widening the field and increasing the effective focal ratio for shorter exposures.
  • Coma correctors: For Newtonian reflectors, correcting the comet-shaped off-axis stars inherent to a parabolic mirror, essential below about f/6.
  • SCT reducer-correctors: Typically 0.63x units for Schmidt-Cassegrains, which both widen the field and considerably reduce exposure times at long focal lengths.
  • Telescope-specific versus universal: A corrector designed for your exact tube will outperform a universal one, particularly at the corners of large sensors.
  • Back-focus specification: Every corrector has a design distance from its rear surface to the sensor, commonly 55 mm. Missing it produces elongated corner stars pointing either inward or outward.

Why Choose David Astro for Field Flatteners and Reducers

A corrector is one of the few products where the accessory list matters as much as the product itself.

  • Back-focus calculated and spacers supplied: We provide the full component list including fine shims, and account for the focal shift your filter glass introduces.
  • Star shape diagnosis: If your corners are wrong, the direction of elongation tells us whether you are over- or under-spaced. Send us a corner crop and we will tell you which way to move.
  • Telescope-matched selection: We identify the corrector designed for your specific tube rather than defaulting to a universal unit that will underperform on a large sensor.
  • Authorized dealer: Genuine correctors with full manufacturer warranty, held in stock for fast domestic shipping.
  • Ships from Canada: No duties or brokerage fees for Canadian customers.

Field Flatteners and Reducers in Canada — Frequently Asked Questions

How precise does back-focus have to be?
Within about one millimetre for most correctors, and tighter at fast focal ratios. That is why filter thickness matters — a 3 mm filter shifts focus by roughly 1 mm, which is the entire tolerance.
My corner stars are elongated. Am I over or under spaced?
Stars elongated radially, pointing away from centre, generally indicate too much spacing. Tangentially elongated stars generally indicate too little. Send us a corner crop and we will confirm and give you a number.
Flattener or reducer?
Use a flattener if your framing is already right and only the corners are wrong. Use a reducer if you also want a wider field and shorter exposures, accepting that your image scale drops.
Will a universal corrector work on my telescope?
Sometimes acceptably, but a corrector matched to your specific tube will correct better at the edges, especially with an APS-C or larger sensor. Tell us your tube and we will check what exists for it.

Chasing corner stars?

Send us your imaging train and a corner crop. We will calculate the spacing and specify the parts. Held in Canada, shipped fast.