The Apochromatic Refractor Telescope
Apochromatic telescopes represent a significant advancement over the more commonly used achromatic telescopes in the visual astronomy community. While an achromat is acceptable for visual use, an ‘APO’ is recommended for deep-sky astrophotography.
By incorporating multiple lenses, these telescopes effectively eliminate the issue of chromatic aberration—where different wavelengths of light come to different focal points. A refractor that is truly color-free is called an apochromat.
Using an Apochromatic Refractor Telescope for Astrophotography.
This seemingly small enhancement makes a tremendous difference when photographing the night sky. It provides sharper and more accurate images, which is crucial for serious amateur astrophotographers.
Bright objects (such as stars or the moon) are a tough optical challenge for lenses and/or telescopes with inferior optics. As you can see in the example below, the color fringing created using an achromat is severe.
The difference between an achromatic refractor (left) and an apochromatic refractor (right). Insvla Astra
While a refractor is the most expensive telescope type in terms of dollars per inch of aperture, an apochromatic telescope offers superior image clarity compared to other telescope types (such as a Newtonian Reflector).
A compact apochromatic refractor telescope is lightweight and portable, making it an excellent choice for those familiar with camera lenses. The focal lengths, aperture, and lens diameter are often similar to those of a telephoto camera lens.
The William Optics RedCat 51 is a compact apochromatic refractor that feels like a camera lens.
Top Choices for Astrophotography
Here are some beginner-friendly apochromatic refractors for deep-sky astrophotography. These options would make an excellent choice for your first deep-sky imaging setup with a DSLR or dedicated astronomy camera.
Sky-Watcher Esprit 100 Super APO
When selecting your telescope, remember to consider the overall weight of your imaging equipment (including the camera, guide scope, and any other accessories). Your equatorial telescope mounts’ maximum payload capacity will determine the size and overall weight of the telescope you can use in your setup.
For example, a William Optics RedCat 51 (weighing just over 3 pounds) can be used on a portable star tracker mount. A heavier apochromatic refractor like the Fluorostar 132 needs a robust equatorial mount for successful imaging.
How They Work
Their ability to correct color aberration is particularly valuable in astrophotography, especially when capturing intricate details of large galaxies or nebulae. Unlike reflector telescopes that rely on mirrors, apochromatic refractors use lenses to deliver sharp, focused images directly through the eyepiece.
If you have attached a camera to your telescope, you will appreciate crisp images with excellent ‘color correction.’ This means that bright stars in the field will not display ‘color fringing,’ which can be a problem with the achromatic lens design.
The Apochromatic lens usually comprises three elements that bring light of three distinct colors to a common focus.
When connected to a camera, these telescopes produce color-accurate images with impressive contrast, making them ideal for capturing the night sky’s true colors.
Apochromatic refractors are also a fantastic option for beginners owing to their low-maintenance nature. Unlike other telescope types, they don’t require frequent lens realignment, known as collimation, as the lenses are securely sealed within a tube.
A typical deep-sky astrophotography setup with an apochromatic refractor on an equatorial mount.
This design protects the lenses from dust and moisture and ensures consistent optical performance. Additionally, these telescopes tend to have better heat dispersion than Newtonians, resulting in shorter cooldown times and quicker setup.
Another feature is their inclusion of full-frame sensors that deliver round star images even at the corners, a rare quality in smaller telescopes. They excel at observing and photographing bright celestial objects, like the full moon, offering crisp and clear views.
Glass Types
Telescope manufacturers use extra-low dispersion (ED) glass to reduce or eliminate chromatic aberration in an apochromatic refractor. This low dispersion glass is paired with either two or more glass elements (carefully spaced) for an optical design free of false color.
Japanese companies HOYA and OHARA make the most popular types of ED glass used in an apochromatic refractor telescope. The glass materials found in modern APOs include FPL-53, FPL-55, and FCD 100.
The following graphic shared by Stellarvue (who make some of the finest telescopes on the market), presents a fantastic overview of the optical glass types used in today’s refractor telescopes.
Optical Glass Types and Dispersion (Stellarvue.com)
Limitations
However, it’s important to note some of the limitations associated with apochromatic refractors. Due to the complexities of manufacturing larger lenses, these telescopes typically come in smaller sizes.
This size limitation means that the cost of larger apochromatic refractors can rise steeply. Furthermore, the smaller aperture restricts the level of magnification, which might affect the viewing experience of more detailed planetary surfaces or deep-sky objects.
They do not excel at solar system photography, as capturing planets requires a large aperture and long focal length for the best results. For this type of astrophotography, a Schmidt-Cassegrain telescope design is a practical choice.
Related Post: Best Astrophotography Telescopes Available in 2024.
Despite these drawbacks, apochromatic telescopes are versatile and user-friendly, appealing to both seasoned astrophotographers and newcomers to the hobby.
Their robust construction, compatibility with various accessories, and potential for future upgrades, like autoguiding systems, ensure that these telescopes can deliver professional-grade results, elevating your stargazing and astrophotography endeavors.
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