Correctors and Reducers

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.