Telescope optical aberrations explained — coma, CA, astigmatism
You're looking at a astrophotograph you took last night. The nebula in the centre looks great — but the stars near the corners are stretched into little comet-shapes. Or there's a purple fringe around every bright star. Or everything looks slightly soft even though you focused carefully.
These are optical aberrations — imperfections in the way your telescope focuses light. Understanding them tells you whether your images can be improved with technique, or whether the optics themselves are the limiting factor.
This guide covers every major telescope aberration, what causes it, and exactly which EDISLA telescopes and astrographs address each one.
What aberration-free stars look like — and what to expect from different optics
Perfect star
(point-like)
(point-like)
Chromatic
aberration
aberration
Coma
(comet-shape)
(comet-shape)
Astigmatism
(cross-shape)
(cross-shape)
Field curvature
(bloated)
(bloated)
The four main aberrations — what they are and how to fix them
Chromatic Aberration (CA)
Purple/blue fringing around bright stars and planets
White light is made of many colours (wavelengths), each bending slightly differently when passing through glass. An achromatic (two-element) lens brings most colours to the same focus but leaves a residual colour fringe — usually purple or blue — around bright objects. This is most visible when photographing bright stars, the Moon's limb, or planets. Cheaper refractors (and many "professional" telescope listings) use achromatic lenses which exhibit visible CA.
Fix: Use an apochromatic (APO) refractor. Askar's ED glass triplets virtually eliminate CA — this is the primary reason Askar astrographs are worth their price for imaging. EDISLA stocks Askar 60F, 71F, 80ED, 91F — all apochromatic.
Coma
Stars near corners of image look like seagulls or comets pointing outward
Coma occurs when off-axis light rays (from objects away from the centre) don't converge to a single point. Stars near the edges of fast Newtonian reflectors (f/4–f/5) often show coma — they have a characteristic comet-like shape pointing toward the image edge. Coma is more pronounced in faster focal ratios and in reflectors without a coma corrector. Spherical mirrors in budget telescopes are more prone to severe coma than parabolic mirrors.
Fix: Use a parabolic mirror (standard in quality Bresser and Celestron reflectors). For fast astrographs, a coma corrector accessory (GSO, Baader) eliminates it. Flat-field astrographs like the Askar F-series have field correction built in.
Astigmatism
Stars near field edges show a cross or line shape — looks different inside vs outside focus
Astigmatism occurs when the optics focus light differently in two perpendicular planes. Stars appear as ellipses inside focus, rotate 90°, and become ellipses again outside focus. True circular stars exist only at exact focus, and they look better at focus than CA or coma. Astigmatism is often caused by a slightly warped mirror (especially in budget telescopes under mechanical stress), eyepiece tilt, or focuser tilt.
Fix: Check that the focuser and eyepiece are properly aligned. For persistent astigmatism in reflectors, check for mirror stress from overtightened clips. Quality Bresser mirrors use proper mirror cell designs to avoid this.
Field Curvature
Centre of image sharp but stars near edges are bloated — even if coma-free
The "natural" focal surface of most telescope designs is slightly curved — not flat. If your camera sensor is flat (which all are), stars near the edges of the image are slightly behind focus even when the centre is sharp. This is most problematic in astrophotography and less visible in visual observing. All standard Newtonian reflectors and most refractors have some field curvature.
Fix: A field flattener corrector. Askar's F-series (60F, 71F, 91F) include a built-in field flattener — this is the core engineering distinction of a "flat-field astrograph" vs a standard telescope used for imaging.
Which aberrations affect which telescope types
| Telescope type | Chromatic aberration | Coma | Astigmatism | Field curvature |
|---|---|---|---|---|
| Budget achromat refractor | High | Low | Low | Moderate |
| APO refractor (Askar ED) | Negligible | Low | Low | Low |
| Flat-field astrograph (Askar F-series) | Negligible | None (corrected) | Low | None (corrected) |
| Newtonian reflector (parabolic) | None | Moderate at f/5 | Low | Moderate |
| Dobsonian (visual) | None | Low-moderate | Low | Not relevant (visual) |
| Celestron EdgeHD | None (SCT) | Corrected | Corrected | Flat field (EdgeHD) |
The bottom line for Indian astrophotographers: If you're serious about imaging, the move from a regular achromatic refractor or basic Newtonian to an apochromatic flat-field astrograph like the Askar series eliminates three of the four major aberrations simultaneously — chromatic aberration, coma, and field curvature. This is why specialist imaging telescopes exist as a separate category from visual telescopes.
Frequently asked questions
What causes purple fringing around stars in telescope images?
Purple or blue fringing around bright stars is chromatic aberration (CA), caused by an achromatic lens failing to bring all wavelengths of light to the same focus. It's most visible in cheaper two-element refractors. The solution is an apochromatic (APO) telescope using extra-low-dispersion (ED) glass — such as the Askar series available at EDISLA, which reduces CA to negligible levels.
What is the difference between APO and achromatic telescope in India?
An achromatic (achromat) refractor uses two glass elements to bring two wavelengths of light to the same focus, leaving some residual chromatic aberration (colour fringing). An apochromatic (APO) refractor uses three or more elements, including special ED or FPL-53 glass, to bring three wavelengths to the same focus — virtually eliminating colour fringing. APO refractors are significantly better for astrophotography. All Askar astrographs at EDISLA are apochromatic.
Aberration-free imaging — the Askar astrograph range at EDISLA
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