Focal Reducers

Starfield Focal Reducers for Astrophotography

A focal reducer is one of the fastest upgrades you can make to an imaging refractor: it shortens focal length (wider field) and helps you reach target signal in less exposure time. Many reducer designs also act as a field flattener, improving star shapes toward the edges of the frame.

This page helps you choose the right Starfield reducer and links you directly to the correct products. For step-by-step setup and spacing details, use the official manual: Starfield Reducer & Flattener Manual (PDF) .

Quick Links

Choose Your Starfield Reducer

Starfield 0.8x Reducer & Field Flattener (Fixed)

A great “first reducer” for many refractors. This design is intended for refractors f/5.5 and longer with a 2" focuser, and typically uses 55mm back focus. It includes an M42 camera connection and a 48mm filter thread, and can be upgraded for larger camera threads with an optional M48 adapter.

Starfield - 0.8x Reducer and Field Flattener Starfield Optics

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Starfield Adjustable 0.8x Reducer/Flattener (70–102mm class)

Prefer to dial in spacing instead of stacking spacers? Adjustable reducers are built for exactly that—set your spacing, lock it in, and fine-tune with a twist if needed. This model uses M63 telescope-side threading and M48 camera-side threading, with an adjustable working distance.

Adjustable 2" Reducer for 70 to 102mm Telescopes

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Starfield Adjustable 0.8x Reducer/Flattener (102–155mm class)

For larger refractors and focusers, this adjustable reducer provides the same “set it once” approach with a tuning range. It’s designed as a corrector/reducer for refractors, with M63 telescope-side and M48 camera-side threads and adjustable spacing.

Adjustable 2" Reducer for 70 to 102mm Telescopes

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How to Choose (Beginner + Technical)

Step 1: Decide how much reduction you actually want

  • 0.8x is a balanced choice: meaningful speed/field gain without getting too aggressive.
  • 0.6x is more dramatic: widest field + faster imaging, but spacing and matching matter more. (Discontinued)

Step 2: Match your focuser and camera connection

  • M42 (T-thread) is common for DSLR/T-ring chains and many astro cameras.
  • M48 is often preferred as sensors get larger (more clear aperture and less vignetting risk).
  • Adjustable reducers often use M63 telescope-side and M48 camera-side threads.

Step 3: Respect back focus (this is where great imaging happens)

“Back focus” is the distance from the reducer’s rear reference point to your camera sensor. Many reducer systems are designed around 55mm spacing. Adjustable reducers simplify this by letting you dial in the correct spacing rather than building a spacer tower.

Focal Reducer FAQ

What does a focal reducer do, in plain language?

It makes your telescope behave like a shorter focal length instrument: you capture a wider patch of sky and reach your target signal faster. In practice, you can often reduce total integration time for a similar noise level.

Do Starfield reducers also flatten the field?

Yes—Starfield reducer/flatteners are designed to improve star shape across the frame, not just reduce focal length.

Is 55mm back focus always required?

Many fixed reducer/flatteners are designed around 55mm. Adjustable units are built specifically to make spacing easier by allowing controlled adjustment rather than stacking multiple rings.

M42 vs M48: which should I pick?

M42 is very common for DSLRs and many camera setups. M48 is often chosen for larger sensors and wider light cones, because it can reduce the chance of vignetting compared to smaller openings—especially when your imaging train is otherwise sized for M48.

Do you ship reducers across Canada?

Yes—Starfield Optics ships across Canada, with pricing in CAD and Canadian checkout. If you have a compatibility question before ordering, contact us and we’ll help you match threads and spacing.