APO & ED Refractor Telescopes for Astrophotography & Visual Astronomy

APO & ED Refractor Telescopes for Astronomy and Astrophotography

ED and apochromatic refractors are among the most popular premium telescope designs for visual astronomy and astrophotography.

Like all refractors, they use a lens objective to collect and focus incoming light. By combining specialised ED or other low-dispersion glass types with carefully designed optical systems, chromatic aberration can be significantly reduced compared with a traditional achromatic refractor.

The result is a sharp, high-contrast image with excellent colour correction.

APO and ED refractors are particularly suitable for:

• astrophotography
• lunar observation
• planetary observation
• double stars
• star clusters
• nebulae and galaxies
• wide star fields
• portable astronomy
• terrestrial observation

The ideal telescope depends on aperture, focal length, focal ratio, optical design and intended application.

What Is an ED Refractor?

ED stands for Extra-low Dispersion.

ED glass has special optical properties that allow different wavelengths of light to be brought to focus more accurately than with conventional optical glass.

As a result, an ED refractor can show significantly less chromatic aberration than a traditional achromatic telescope.

High-quality ED doublets combine an ED or SD glass element with a carefully matched second lens element.

Depending on the complete optical design, an ED doublet can provide excellent colour correction.

However, the ED designation alone does not define the overall optical quality. The mating element, glass type, focal ratio, optical design and manufacturing quality are equally important.

What Is an Apochromatic Refractor?

An apochromatic refractor, commonly called an APO, is designed to provide particularly high levels of chromatic correction.

Bright stars, the Moon and planets can therefore be observed with minimal distracting false colour and excellent contrast.

This colour correction is also highly desirable for astrophotography, where stars should remain small, sharp and free from obvious coloured halos.

APO refractors are therefore particularly popular among astrophotographers looking for excellent image quality in a relatively straightforward optical system.

ED, SD and APO – What Is the Difference?

The terms ED, SD and APO are not used completely uniformly throughout the telescope industry.

ED refractors use one or more elements made from low-dispersion optical glass.

SD refractors commonly use particularly low-dispersion specialist glasses. Depending on the manufacturer, these may include materials such as FPL53, FPL55 or comparable glass types.

APO refractors are complete optical systems designed to achieve a particularly high level of colour correction.

For this reason, the designation alone should not be the only factor considered when comparing telescopes.

Important specifications include:

• optical glass types
• number of lens elements
• optical design
• focal ratio
• lens coatings
• mechanical construction
• optical manufacturing quality

Doublet or Triplet APO?

Apochromatic refractors are commonly available as Doublet or Triplet designs.

Doublet Refractors

A Doublet objective consists of two lens elements.

Potential advantages include:

• lower weight
• faster thermal stabilisation
• compact construction
• attractive price-to-performance ratio
• excellent visual performance

High-quality ED and SD Doublets can provide excellent colour correction and are suitable for both visual astronomy and many imaging applications.

Triplet Refractors

A Triplet objective consists of three lens elements.

The additional optical element gives the designer greater freedom to correct different aberrations and can provide advantages in fast optical systems.

Triplet APOs are therefore particularly popular for demanding astrophotography.

They are generally heavier than comparable Doublets and may require more time to reach thermal equilibrium.

APO Refractors for Astrophotography

Astrophotography is one of the most important applications for modern ED and APO refractors.

Compact telescopes with focal ratios around f/5 to f/7 can provide relatively wide fields of view and are particularly suitable for many deep-sky objects.

Typical imaging targets include:

• emission nebulae
• reflection nebulae
• dark nebulae
• open star clusters
• large galaxies
• Milky Way regions
• comets

Excellent colour correction helps produce sharp stars with minimal unwanted colour fringing.

Field Flatteners for Sharp Stars Across the Sensor

Even a high-quality APO does not automatically produce a perfectly flat image field on every camera sensor.

Many refractors naturally show some field curvature toward the edges of the image.

A suitable field flattener corrects this curvature and improves star shapes toward the corners of the sensor.

This is particularly important when using larger APS-C or full-frame astronomy cameras.

Reducers for Wider Fields of View

A focal reducer decreases the effective focal length of a telescope.

This generally provides:

• a wider field of view
• a faster effective focal ratio
• shorter imaging times for the same signal level
• improved framing of large deep-sky objects

Many astrophotography correctors combine focal reduction and field flattening in a single Reducer/Flattener.

Correct compatibility and sensor spacing are essential for achieving optimum results.

Choosing the Right Aperture and Focal Length

The ideal APO size depends strongly on your intended application.

Compact 50–72 mm Refractors

Small refractors are lightweight and highly portable.

They are particularly suitable for:

• travel astronomy
• large star fields
• extended nebulae
• portable astrophotography
• guide-scope applications
• selected terrestrial applications

Their relatively short focal lengths provide wide fields of view.

80–100 mm Refractors

This aperture range provides an excellent balance between light gathering, resolution, portability and mount requirements.

For this reason, 80 to 100 mm APOs are among the most versatile refractors for both visual astronomy and astrophotography.

Refractors of 110 mm and Larger

Larger refractors collect more light and provide greater resolving power.

They are attractive for demanding visual astronomy and imaging applications requiring greater image scale.

As aperture increases, however, so do:

• telescope weight
• physical size
• mount requirements
• transport requirements
• thermal stabilisation time

APO Refractors for the Moon and Planets

High-quality APO refractors are also excellent visual instruments.

Their high contrast and low chromatic aberration make them suitable for detailed observations of:

• lunar craters
• lunar mountains and rilles
• Jupiter and its cloud belts
• Jupiter's moons
• Saturn and its rings
• Mars
• double stars

Because a refractor does not require a secondary mirror obstruction in the light path, a well-made APO can provide exceptionally clean and high-contrast visual images.

APO Refractor or Achromat?

A traditional achromatic refractor is considerably less expensive and can provide excellent value for visual astronomy.

An ED or APO refractor, however, provides significantly improved colour correction.

This is particularly beneficial for:

• high magnification
• bright stars
• lunar and planetary observation
• colour imaging
• deep-sky astrophotography

Astronomers interested in imaging or particularly colour-neutral views will therefore benefit from an ED or APO refractor.

APO Refractor or Reflector Telescope?

Refractors and reflector telescopes offer different advantages.

APO refractors are valued for:

• high image contrast
• excellent colour correction
• closed optical tubes
• relatively low collimation requirements
• compact designs at shorter focal lengths
• excellent astrophotography performance

Reflecting telescopes generally provide considerably more aperture for the same budget.

For maximum light gathering in visual deep-sky astronomy, a Newtonian or Dobsonian telescope can therefore offer significant advantages.

For high-quality wide-field astrophotography, however, APO refractors remain one of the most popular telescope designs.

A High-Quality Focuser Matters for Imaging

For astrophotography, mechanical construction is just as important as the objective.

The focuser must safely support equipment such as:

• astronomy cameras
• filter wheels
• filter drawers
• off-axis guiders
• field correctors
• additional adapters

A fine-focus reduction such as 1:10 allows precise manual focusing.

Larger focusers and clear apertures can also be useful when working with large camera sensors.

Back Focus and Correct Sensor Spacing

Correct spacing between a field corrector and the camera sensor is essential in many astrophotography systems.

Flatteners and Reducers are usually designed for a specific working distance.

Camera adapters, filter wheels, filter drawers and off-axis guiders must therefore be included when calculating the optical spacing.

Correct spacing helps produce consistent star shapes across the entire image.

Choosing a Mount for an APO Refractor

For visual astronomy, compact APOs can often be used on relatively lightweight alt-azimuth or equatorial mounts.

Astrophotography places greater demands on the mount.

An imaging mount should provide:

• sufficient payload capacity
• accurate tracking
• good mechanical stability
• minimal backlash
• adequate capacity for the complete imaging system

For long-exposure deep-sky astrophotography, equatorial GoTo mounts combined with autoguiding are commonly used.

ED and APO Refractors at Optikshop24

Optikshop24 offers ED, SD and apochromatic refractors for visual astronomy and astrophotography.

The range extends from compact travel and wide-field refractors to premium ED Doublets and high-performance Triplet APOs for demanding imaging applications.

Choose a refractor according to your preferred focal length, aperture, camera sensor and observing or imaging requirements.