Field Flattener for Refractor Telescopes f/5 to f/7 - Astro

In this review the Field Flattener for Refractor Telescopes f/5 to f/7 is evaluated for astrophotographers who want sharper stars across the frame. The bottom line: this optic is aimed at users of medium-fast refractors seeking to reduce field curvature and star smear away from center, and it delivers a clear, engineering-focused solution. The reviewer found the device especially useful when paired with a dedicated camera and a precisely measured 55mm spacing, producing noticeably improved star shapes on the edges without changing the telescope's operation.

Key Features

• Minimizes star distortion: Corrects field curvature so stars remain round toward the edges, improving usable frame area for deep-sky and wide-field targets.
• Optimized focal ratio: Designed for refractors with focal ratios of f/5 to f/7 so performance is tuned for faster apochromatic and achromatic refractors commonly used in astrophotography.
• Precise spacing requirement: Requires approximately 55mm (+/- 2mm) from the flattener's rear to the camera sensor, providing a clear mechanical guideline for best optical correction.
• T-ring thread connection: Built-in T-ring thread allows a secure, camera-ready attachment using a user-supplied T-ring for reliable imaging setup.
• Fully multi-coated optics: Multi-coating maximizes light transmission and contrast, helping faint nebulae and galaxy details show with better clarity.
• USA support: Backed by manufacturer support based in Springdale, Arkansas, giving reassurance for warranty or setup questions.

Who It's For

The field flattener is best for amateur and semi-pro astrophotographers who use refractor telescopes in the f/5 to f/7 range and who want to get sharper stars across the whole sensor for both deep-sky and planetary imaging. It is particularly useful for those who already own a dedicated astronomy camera and can maintain or measure the required 55mm spacing.

People who should look elsewhere include users of much slower refractors (above f/7) or reflectors and those who cannot meet the specified backspacing, since the unit's correction relies on that mechanical tolerance. Casual observers using only visual eyepiece work will not benefit from this accessory.

Pros & Cons

Pros

• Improves edge star shape significantly when the correct spacing is used.
• Secure T-ring thread simplifies attaching cameras for consistent results.
• Fully multi-coated glass enhances contrast and light throughput for faint objects.

Cons

• Requires precise 55mm (+/- 2mm) spacing, which can add complexity to some setups.

Specifications

Our Verdict

The flattener is a practical, well-engineered accessory for refractor astrophotography at f/5 to f/7, delivering tangible improvement in star shape and image contrast when the 55mm spacing is respected. For users who can match the mechanical requirements and already image with a camera, it represents good value as a targeted correction tool that expands the usable portion of the sensor for astrophotography.

Frequently Asked Questions

Will this work on my refractor?
It will work if your refractor operates between f/5 and f/7 and you can provide about 55mm (+/- 2mm) spacing from the flattener rear to the camera sensor.

How do I attach my camera?
The flattener uses a T-ring thread, so attach your camera with a compatible user-supplied T-ring for a secure connection.

Does it improve light transmission?
Yes, the optical elements are fully multi-coated to maximize transmission and improve contrast for faint targets.