Optical Aberrations in Telescopes
Aberrations are defects in optical systems caused by the deviation of light. This can be a result of either a limitation in your design or a defect in your optics. Every telescope experiences some degree of aberrations, but by selecting the right telescope, you can reduce or eliminate specific aberrations.
What Are the Types of Optical Aberrations?
When a single ray of light is present, there are five aberrations that you may experience. These include spherical aberration, coma, astigmatism, field curvature, and distortion. When more than one ray of light is present, you may experience chromatic aberration.
How do Aberrations Affect Astronomy?
Each aberration presents a different issue. In spherical aberration, the image becomes blurry as you near the edges of your field of view. Coma and astigmatisms are off-axis aberrations, meaning they cause off-axis objects to appear blurred. Field curvature and distortion cause points to be misplaced on the optical axis due to differences in focal points. The final aberration, chromatic aberration, causes an outline or shadow of unwanted color.
How to Fix Aberrations in Telescopes
With modern design limitations, some degree of aberration is unavoidable. In general, higher-quality telescopes will experience fewer aberrations, but each has its own solution. This is because each aberration has a different cause.
Here are six types of aberrations and solutions to fix them:
Chromatic Aberration
Chromatic aberration occurs when a lens is unable to bring all of the colors to the same focal plane. If your image is in focus, and you still experience this aberration, you can purchase an Extra Low Dispersion glass to reduce or eliminate this issue.
Spherical Aberration
Spherical aberration is caused by rays of light failing to meet at the same image point. Therefore spherical aberration can appear in reflectors, refractors and catadioptric telescopes. In general, the easiest way to resolve this aberration is by using an aspherical lens. You can also minimize this optical error by employing additional glass elements, like doublets or triplets.
Comatic aberration
Comatic aberration, or coma occurs when rays from an off-axis object are brought into focus at different positions. The most common solution is to employ a diaphragm to eliminate the outer cones of rays.
Astigmatism
Astigmatism results from rays of light hitting the telescope obliquely. This causes light to focus on different planes at different times. The solution to astigmatism depends on the type of telescope, for refractor and catadioptric telescopes most likely you need to replace the lens. For reflector telescopes you can to adjust the mirror.
Field Curvature
Field curvature results from spherical focal planes, which cause rays of light to reach the center of the lens at a different time than the edges. Field curvature can be resolved by using a field flattener.
Distortion
Distortion is typically the least problematic when viewing celestial bodies because it doesn’t impact the focus of your image. Distortion is caused by an overall curvature greater or less than zero. This aberration is only noticeable in wide fields of view, but if you do experience distortion, it’s recommended that you ignore it or purchase an eyepiece with a smaller field of view.
