Optical Aberrations

If our telescopes were perfect optical tube systems, they would be able to capture all the light across the entire field of view and be able to focus it all at a point, without any errors across the entire camera sensor.  Unfortunately, I’ve yet to find such a telescope optical system as all that at least I’ve found have problems because they are imperfect.  These imperfections are generally called aberrations and the type of aberration depends on the type of optical tube you are using.  There are 4 main types of aberrations you can encounter.

Chromatic Aberration - generally, you see this problem in refractors as they have lens elements that focus light to a point.  This issue occurs where different wavelengths and colors of light do not converge at the same distance and the result is a color fringing around the edge of stars.  The primary cause is that shorter wavelengths of light are refracted by lenses more than longer wavelengths of light.  Mirrors generally don’t have chromatic aberration but if you have a hybrid system that contains lenses with mirrors such as Catadioptric Telescope, you could see this problem.

Spherical Aberration - This is where the light entering the system from the edge of the field is focused at a point closer than the light from the center of the field.  The problem affects both lens and mirrored systems, affects the entire field, and there is no compromise focus that can be reached. This aberration is the most important one that telescope designers plan and control for.  For reflecting systems, parabolic mirrors are free of this type of aberration but a spherical or hyperboloid mirrors must be corrected for it.  For refractors, spherical aberration is minimized by using multiple lenses which the lens curvatures are offset.   

Coma - this aberration is where stars that are off the main axis of the optical tube appear deformed; they look like small comets or wedges where the tails point toward the center.  Coma is found in mirrored systems and is caused by incoming light that is focused at different distances depending on where the light is reflected on the mirror.  Coma is related to the optical tube focal ratio - fast reflectors will have worse coma than slower systems.  Coma is usually corrected with a device called a coma corrector.  Refractors do not suffer from coma but it can be a limiting factor with catadioptric telescopes.

Field Curvature - simply put, this is where the focal plane is not flat; the outer edges and corners of the image field show some vignetting and looks curved.  Imaging sensors are flat, so if the center of the field is in focus, the edges are not and vice-versa.  Field flatters are used to correct for this and they're usually used when you’re imaging with a refractor.  Field flattners are generally designed to work with a specific focal ratio since this type of aberration is dependent on the focal ratio of the optical system.