What is Chromatic Aberration, and Do I Need To Worry About It?
Chromatic aberration, also known as false color or color fringing, is a common trait among optical telescopes, particularly in refractors. It occurs when one or more colors of light focus at different locations, causing bright objects to appear to have colored halos around them, including stars, planets, and the moon. Though it can be eliminated in higher-end refractors, chromatic aberration is commonly found in entry-level and some intermediate refractor telescopes. Find an example of what chromatic aberration looks like here:
If you're a beginner, you don't need to worry about chromatic aberration! Nearly all beginner refractors will feature it to some extent, and it doesn't affect the view too drastically. As amateur astronomers get more experienced, most generally prefer to make the jump from a doublet refractor to a triplet, as triplets will usually have much less of this color fringing. Using a quality refractor will also make dramatic improvements for astrophotography, as chromatic aberration is more present when imaging.
How Do I Know Which Refractor Telescope To Buy?
Generally speaking, we recommend that all beginners interested in the refractor design go with an entry-level doublet. There are many fantastic options for beginner doublet refractors out there, such as the smartphone-assisted StarSense Explorer series from Celestron.
As you grow with the hobby, you may want to consider upgrading to a higher-end doublet refractor or a triplet. If you plan to do serious deep sky imaging, you'll likely want to consider a triplet refractor like the Radian Raptor 61 or a quadruplet design as they can usually give better overall performance.
Are Refractor Telescopes Good For Astrophotography?
Yes, absolutely! Refractor telescopes can be some of the best-performing telescopes for capturing the night sky. The refractor is a renowned telescope design for imaging. So much so, in fact, that we recommend all beginner deep sky astrophotographers start with a small refractor due to their ease-of-use, high contrast images, and maintenance-free operation. All refractor telescopes are capable of astrophotography to some degree. Of course, this is highly dependent on the quality of the refractor, though. For entry-level and beginner refractors, you may only be able to use a smartphone adapter to take images through the eyepiece with your phone. This can give good beginner images of the moon and sometimes the planets. Higher-end refractors like APO doublets, triplets, and quadruplets, on the other hand, usually feature a larger 2" focuser that allows you to connect a DSLR or a dedicated astronomy camera for capturing even more detailed images of the night sky.
What is the Difference Between Achromatic vs. Apochromatic Refractors?
Achromatic and Apochromatic are terms that describe how much chromatic aberration a refractor telescope or lens suffers from. Achromatic, or achro, is a dated term to describe telescopes free of color when doublets first evolved from singlet refractors. In truth, achromats only correct for two of the three main colors. Apochromatic, also written as APO, correct for all three colors, among some other optical requirements. However, not all APO refractors are created equal. Some manufacturers will use the term APO to describe a higher-end doublet refractor, even though doublets usually suffer from chromatic aberration. Why is this? The answer lies with the type of glass used, which you can read more about below.
What Types of Glass Are Used in Refractors, and What Glass Qualities Should I Look For?
The type of glass used in a refractor has a significant impact on the quality of the view and image. For example, a doublet refractor with an ED (extra-low dispersion) lens element can often come close to the optical performance of a triplet with standard glass. For this reason, you may see some doublet refractors referred to as APO doublets if they use ED lens elements or other premium glass, even though the term APO is usually reserved for triplets and quadruplets. These higher-end doublet refractors usually have a price tag that compares them with lower-end triplet telescopes. Glass type is often obsessed over in the amateur astronomy community, but in truth, it is just one of many important aspects to consider when purchasing a high-end refractor telescope. Perhaps a more well-rounded specification for optical performance is comparing spot diagrams for different telescopes, as spot diagrams take into account the rest of the telescope design aside from glass like baffling. Regardless, below is a chart of some of the more common glass types and their general rankings as preferred by the amateur community.
|BK7 (Schott)||FPL-51 (Ohara)||FPL-52 (Ohara)||Fluorite/FL|
|FK61 (CDGM)||OK4 (LZOS)||FPL-53 (Ohara)|
|FCD1 (Hoya)||FPL-55 (Ohara)|
|FK51 (Schott)||FCD100 (Hoya)|