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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.

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CA$461.00 - CA$461.00
CA$461.00
CA$461.00 - CA$461.00
CA$461.00

Askar 0.6x Reducer for 103APO

ASK-ASKRE10306
1 in stock

Transforms Askar 103APO to an f/4.08, 420.24mm focal length system Quadruplet optical design provides field flattening for full-frame sensors Stan...

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CA$461.00 - CA$461.00
CA$461.00
CA$461.00 - CA$461.00
CA$461.00
CA$291.00 - CA$291.00
CA$291.00
CA$291.00 - CA$291.00
CA$291.00

Askar 0.75x 71F Reducer

ASK-ASKAR71R

0.75x Reduction Factor Transforms Askar 71F to f/5.2 New Focal Length: 369mm Standard 55mm Back Focus Triplet Optical Design Description FAQ Sp...

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CA$291.00 - CA$291.00
CA$291.00
CA$291.00 - CA$291.00
CA$291.00
CA$368.00 - CA$368.00
CA$368.00
CA$368.00 - CA$368.00
CA$368.00

Askar 0.76x Reducer for 80PHQ

ASK-ASKAR80R

0.76x reduction factor specifically for the Askar 80PHQ Reduces focal ratio from f/7.5 to a faster f/5.7 Creates a new focal length of 456mm Gener...

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CA$368.00 - CA$368.00
CA$368.00
CA$368.00 - CA$368.00
CA$368.00
CA$538.00 - CA$538.00
CA$538.00
CA$538.00 - CA$538.00
CA$538.00

Askar 0.7x Universal Reducer for 107PHQ/130PHQ/151PHQ

ASK-ASF107130RE

0.7x Focal Reduction Factor 44mm Full-Frame Image Circle Maintains a Standard 55mm Backfocus 3-Element Air-Spaced Design with ED Glass Optimized f...

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CA$538.00 - CA$538.00
CA$538.00
CA$538.00 - CA$538.00
CA$538.00
CA$307.00 - CA$307.00
CA$307.00
CA$307.00 - CA$307.00
CA$307.00

Askar 0.8x Reducer for 103APO

ASK-ASKRE10308

Reduces telescope focal ratio to a faster f/5.44 Shortens 103APO focal length to 560.32mm for wider fields Triplet optical design provides field f...

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CA$307.00 - CA$307.00
CA$307.00
CA$307.00 - CA$307.00
CA$307.00
CA$306.00 - CA$306.00
CA$306.00
CA$306.00 - CA$306.00
CA$306.00

Askar 0.8x Reducer for 120APO

ASK-ASKRE12008

Reduces Askar 120APO to a fast f/5.6 focal ratio Shortens focal length to 672mm for wider fields of view Triplet optical design provides field fla...

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CA$306.00 - CA$306.00
CA$306.00
CA$306.00 - CA$306.00
CA$306.00
Askar 111F 0.74× Reducer
CA$419.00 - CA$419.00
CA$419.00
CA$419.00 - CA$419.00
CA$419.00

Askar 111F 0.74× Reducer

ASK-ASAPO111R

Reduces Telescope Focal Ratio to f/5.1 Creates a 566mm Focal Length Imaging System Triplet Optical Design for Field Flattening Provides Standard 5...

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CA$419.00 - CA$419.00
CA$419.00
CA$419.00 - CA$419.00
CA$419.00
CA$2,099.00 - CA$2,099.00
CA$2,099.00
CA$2,099.00 - CA$2,099.00
CA$2,099.00

Askar 111F Flat-Field Astrograph

ASK-ASKAR111F
1 in stock

111mm Aperture Quadruplet APO Optics 766mm Focal Length at f/6.9 44mm Full-Frame Corrected Image Circle Integrated Field Flattener Design 5.74kg N...

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CA$2,099.00 - CA$2,099.00
CA$2,099.00
CA$2,099.00 - CA$2,099.00
CA$2,099.00
Askar 131F 0.74× Reducer
CA$419.00 - CA$419.00
CA$419.00
CA$419.00 - CA$419.00
CA$419.00

Askar 131F 0.74× Reducer

ASK-ASKAR131FRD

Reduces telescope focal ratio to f/5.6 Achieves a 733.6mm final focal length Triplet optical design for field flattening Standard 55mm back focus ...

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CA$419.00 - CA$419.00
CA$419.00
CA$419.00 - CA$419.00
CA$419.00
Askar 185APO 0.8x Full-frame Reducer
CA$460.00 - CA$460.00
CA$460.00
CA$460.00 - CA$460.00
CA$460.00

Askar 185APO 0.8x Full-frame Reducer

ASK-ASKRE18508
1 in stock

Reduces Askar 185APO to f/5.6 Resulting Focal Length: 1036mm 4-Element Quadruplet Optical Design Standard 55mm Back Focus Illuminates Full-Frame S...

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CA$460.00 - CA$460.00
CA$460.00
CA$460.00 - CA$460.00
CA$460.00
Askar 185APO 1x Full-frame Flattener
CA$460.00 - CA$460.00
CA$460.00
CA$460.00 - CA$460.00
CA$460.00

Askar 185APO 1x Full-frame Flattener

ASK-ASKFL18510

Maintains Native f/7 Focal Ratio Maintains Native 1295mm Focal Length Triplet Optical Design Standard 55mm Back Focus Includes M68, M54 & M48 ...

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CA$460.00 - CA$460.00
CA$460.00
CA$460.00 - CA$460.00
CA$460.00
CA$307.00 - CA$307.00
CA$307.00
CA$307.00 - CA$307.00
CA$307.00

Askar 1x Field Flattener for 103APO

ASK-ASKFL10310
1 in stock

Corrects field curvature and coma for the Askar 103APO Maintains the native 700.4mm focal length and f/6.8 ratio Triplet optical design delivers s...

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CA$307.00 - CA$307.00
CA$307.00
CA$307.00 - CA$307.00
CA$307.00
CA$306.00 - CA$306.00
CA$306.00
CA$306.00 - CA$306.00
CA$306.00

Askar 1x Field Flattener for 120APO

ASK-ASKFL12010
1 in stock

Designed specifically for the Askar 120APO Telescope 1x Magnification Factor Triplet Optical Design Maintains Native f/7 Focal Ratio Standard 55mm...

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CA$306.00 - CA$306.00
CA$306.00
CA$306.00 - CA$306.00
CA$306.00
CA$384.00 - CA$384.00
CA$384.00
CA$384.00 - CA$384.00
CA$384.00

Askar 2.5" F/3.9 Full Frame Reducer for FRA400 and FRA500

ASK-ASKAR7256RD
2 in stock

Reduces focal ratio to a fast f/3.9 0.7x reduction factor 44mm image circle fully illuminates full-frame sensors Standard 55mm back focus for easy...

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CA$384.00 - CA$384.00
CA$384.00
CA$384.00 - CA$384.00
CA$384.00
CA$615.00 - CA$615.00
CA$615.00
CA$615.00 - CA$615.00
CA$615.00

Askar 3" F/3.9 Full Frame Reducer for FRA600

ASK-ASKAR656RD

Reduces FRA600 focal ratio to F/3.9 Designed for full-frame sensors Maintains a standard 55mm backfocus Features integrated M48 (2-inch) filter th...

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CA$615.00 - CA$615.00
CA$615.00
CA$615.00 - CA$615.00
CA$615.00
Askar 60F 0.75x Reducer
CA$279.00 - CA$279.00
CA$279.00
CA$279.00 - CA$279.00
CA$279.00

Askar 60F 0.75x Reducer

ASK-ASKAR60R
1 in stock

0.75x Focal Reduction Reduces Focal Length to 311mm Creates a Fast f/5.1 Focal Ratio 44mm Full-Frame Image Circle Standard 55mm Back Focus Triplet...

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CA$279.00 - CA$279.00
CA$279.00
CA$279.00 - CA$279.00
CA$279.00
Askar 91F 0.75x Reducer
CA$279.00 - CA$279.00
CA$279.00
CA$279.00 - CA$279.00
CA$279.00

Askar 91F 0.75x Reducer

ASK-ASKAR91R
1 in stock

Reduces focal ratio to a fast f/5 0.75x reduction factor for wider fields of view Triplet optical design provides field-flattening correction Stan...

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CA$279.00 - CA$279.00
CA$279.00
CA$279.00 - CA$279.00
CA$279.00
CA$401.00 - CA$401.00
CA$401.00
CA$401.00 - CA$401.00
CA$401.00

Baader Alan Gee Telecompressor Mark II

BAA-2454400

Dramatically widens the field of view of Schmidt-Cassegrain telescopes Engineered to produce a sharp, flat field for visual observation Creates a ...

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CA$401.00 - CA$401.00
CA$401.00
CA$401.00 - CA$401.00
CA$401.00
CA$1,063.00 - CA$1,063.00
CA$1,063.00
CA$1,063.00 - CA$1,063.00
CA$1,063.00

Baader Fluorite Flatfield Converter

BAA-2458200

Genuine Calcium Fluorite (CaF2) Two-Element Cemented Design Corrects for Field Curvature to Produce a Flat Imaging Plane Variable Magnification Fa...

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CA$1,063.00 - CA$1,063.00
CA$1,063.00
CA$1,063.00 - CA$1,063.00
CA$1,063.00
CA$1,056.00 - CA$1,056.00
CA$1,056.00
CA$1,056.00 - CA$1,056.00
CA$1,056.00

Baader M68 Field Flattener

BAA-2458260

Corrects field curvature for astrophotography Designed for Baader APO 95/560 Travel Companion Compatible with Zeiss APQ apochromatic refractors Fe...

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CA$1,056.00 - CA$1,056.00
CA$1,056.00
CA$1,056.00 - CA$1,056.00
CA$1,056.00
CA$271.00 - CA$328.00
CA$271.00 - CA$328.00
CA$271.00 - CA$328.00
CA$271.00

Baader MPCC Mark III (Multi Purpose Coma Corrector) - 2"

Options available

Corrects Coma in Newtonians as Fast as f/3.5 Zero Change in Focal Length or Field Size Fits Standard 2" Focusers Features Baader Planetarium Phant...

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CA$271.00 - CA$328.00
CA$271.00 - CA$328.00
CA$271.00 - CA$328.00
CA$271.00
Baader Planetarium Alan Gee Universal 2" Telecompressor (UAG II)
CA$286.00 - CA$286.00
CA$286.00
CA$286.00 - CA$286.00
CA$286.00

Baader Planetarium Alan Gee Universal 2" Telecompressor (UAG II)

BAA-2454405

Unique telecompressor designed for visual use with binoviewers Based on the flat-field Alan Gee design by Roland Christen of AstroPhysics Enables ...

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CA$286.00 - CA$286.00
CA$286.00
CA$286.00 - CA$286.00
CA$286.00
Baader Planetarium Glasspathcorrector 2" / 1.7x for Newtons
CA$316.00 - CA$316.00
CA$316.00
CA$316.00 - CA$316.00
CA$316.00

Baader Planetarium Glasspathcorrector 2" / 1.7x for Newtons

BAA-2456300

1.7x focal length multiplier Enables binoviewer use in Newtonian telescopes Integrated coma corrector for sharp edge-of-field stars Designed for 2...

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CA$316.00 - CA$316.00
CA$316.00
CA$316.00 - CA$316.00
CA$316.00
CA$277.00 - CA$277.00
CA$277.00
CA$277.00 - CA$277.00
CA$277.00

Baader RCC I Rowe Coma Corrector

BAA-2956800

Triplet optical design corrects coma in fast Newtonian telescopes Engineered for a long back focus to accommodate complex imaging trains Ideal for...

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CA$277.00 - CA$277.00
CA$277.00
CA$277.00 - CA$277.00
CA$277.00

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.