Refractors

Doublets, Triplets and Glass Types

The purpose of an apochromatic design is to bring three wavelengths to a common focus, eliminating the violet halo that achromatic refractors show on bright stars. How well it does that depends on the glass and the number of elements.

• ED doublets: Two elements with one extra-low dispersion element. Lighter, less expensive, and excellent for imaging, with a small residual colour that is rarely visible in processed images.
• Apochromatic triplets: Three elements giving near-complete colour correction, preferred for high-contrast visual observing and for imaging bright stars without halos. Heavier and slower to cool.
• Petzval designs: Four-element optics with the field correction built in, so no separate flattener is needed and back-focus is fixed by the manufacturer — the simplest path to flat fields.
• Achromatic refractors: Two-element crown and flint designs, excellent value for lunar, planetary and general visual use where some colour fringing on bright objects is acceptable.
• Focal ratio: Fast refractors around f/5 give wide fields and short exposures; f/7 and slower give more image scale and are more forgiving of corrector spacing.
• Matched correctors: Most refractors need a flattener or reducer designed for that specific model, spaced to its exact back-focus.

Why Choose David Astro for Refractors

A refractor is usually bought as the centre of an imaging system, so it should be specified alongside the mount and camera rather than on its own.

• Corrector matched at the point of sale: We identify the flattener or reducer designed for your specific tube and supply the spacers to hit its back-focus exactly.
• Focuser load assessed: We check that the stock focuser will carry your intended camera train without slip, and recommend an upgrade only when it genuinely will not.
• Authorized dealer: Full manufacturer warranty and verified authenticity, with service handled inside Canada.
• Ships from Canadian stock: No duties or brokerage fees for Canadian buyers, and no extra border handling for a precision optic.
• Support after first light: Help with spacing, tilt and star shape diagnosis once the telescope is on the sky.

Refractors in Canada — Frequently Asked Questions

Doublet or triplet for imaging?

A good ED doublet is entirely sufficient for most deep-sky imaging and is lighter on the mount. A triplet shows its advantage on bright stars and in high-contrast visual use, at meaningful extra weight and cost.

Do I need a field flattener?

For imaging with anything larger than a small sensor, yes — a two-element refractor has a curved focal plane and corners will show elongated stars without one. Petzval designs have it built in.

How long does a refractor take to cool?

A small doublet is usable almost immediately. A large triplet can take 45 minutes or more to stabilize in Canadian winter conditions, and imaging before it does will show soft, bloated stars.

Is a small refractor enough for deep sky?

Yes, given integration time. A 60 to 80 mm apochromat on a good mount produces excellent wide-field images; total exposure time substitutes for aperture in imaging in a way it cannot for visual work.

Specifying a refractor?

Tell us your mount, camera and targets and we will match the tube and its corrector. Canadian stock, no surprise duties.