Description
Coma is an optical aberration that deforms stars into small comets whose tail points radially outward from the field center. The word "residual" indicates that it persists despite a corrector, or in its absence.
Its signature is radial: the image center stays sharp, and star elongation worsens progressively as you move toward the corners, each comma pointing toward its nearest corner.
This is a defect inherent in fast optics, particularly fast Newtons (F/4 to F/5), but also some refractors without a field flattener. A coma corrector reduces it significantly, provided it is set to the correct backfocus.
It is to be distinguished from a collimation error, which deforms the entire field in a single direction (center included), whereas coma is radial and leaves the center clean.
Visual signature
At the image center, stars are sharp and round. Elongation appears and worsens only as you move toward the edges.
Corner stars take on a comet or fan shape, the tip pointing toward the center and the tail fanning outward toward the corner (radial orientation).
The degradation is symmetric and progressive: all four corners are affected equally, and intensity increases continuously from center to periphery.
The faster the instrument's focal ratio, the more pronounced the coma and the earlier it invades the field. An incorrect corrector backfocus can leave asymmetric residual coma.
Differential diagnosis
Not to be confused with a collimation error: collimation deforms the entire field in a single direction and affects the central star, whereas coma is radial and leaves the center clean.
To be distinguished from astigmatism: astigmatism stretches stars into crosses or lines depending on focus, without the fan-shaped tail oriented toward a corner that is characteristic of coma.
To be linked to incorrect backfocus: an incorrect corrector-to-sensor distance prevents the coma corrector from working and leaves deformed corner stars.
To be separated from sensor tilt: tilt makes the deformation asymmetric (one corner worse than its opposite), whereas pure coma affects all four corners equivalently.
Probable causes
- Intrinsically comatic fast optic (Newton F/4 to F/5)
- No coma corrector on a fast instrument
- Incorrect corrector backfocus (corrector-to-sensor distance wrong)
- Corrector mismatched to the focal ratio or tube
- Refractor used without a field flattener
- Misaligned optical train, amplifying edge aberrations
Course of action
- Mount a suitable coma corrector (Paracorr, GPU, MPCC, field flattener)
- Set the corrector backfocus precisely (value per model: MPCC ~55 mm, Paracorr per its adjustment), using calibrated spacing rings
- Verify collimation first, as it is the most common source of confusion
- Choose a corrector compatible with the instrument's focal ratio
- Slightly crop the worst-affected corners if needed
- Correct mild residual coma in processing (BlurXTerminator)
- On a refractor, add a matched field flattener
The Doc's advice
Coma on a fast Newton is not a malfunction, it is physics: at F/4 without a corrector, your corners will be full of comets, that is normal. The real question is the coma corrector (Paracorr, GPU, MPCC) and above all its backfocus, whose value depends on the model (MPCC ~55 mm, Paracorr per its adjustment), to be respected to the millimeter with precise rings, otherwise you correct poorly or introduce other aberrations. Also check that everything is well collimated before blaming coma, it is the number one confusion. And as a last resort, BlurXTerminator handles mild residual coma, but it is no substitute for a properly set-up optical train.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
Coma or collimation error: how to tell?
Look at the star at the image center. Coma leaves the center clean: if the central star is round and sharp and only the corners elongate radially, it is coma, inherent to the optics. A collimation error deforms the entire field in one direction and also affects the central star. Another reference: coma is radial (each comma pointing toward its nearest corner), collimation is uniform (all commas in the same direction). If in doubt, redo collimation first: it is free and it is the most common confusion.
Does a coma corrector fully eliminate coma?
A good corrector (Paracorr, GPU, MPCC, field flattener) reduces coma very substantially, subject to two conditions: it must be matched to the instrument's focal ratio, and it must be mounted at the correct backfocus. An incorrect corrector-to-sensor distance leaves residual coma or introduces other edge aberrations. Essential prerequisite: perfect collimation, since a corrector cannot compensate for a misaligned telescope. With all of this in place, a very slight residual coma in the extreme corners may remain on the fastest optics.
What backfocus does a coma corrector need?
The most common value is 55 mm between the corrector exit face and the sensor plane, but it depends on the model: always check the manufacturer's specification. Tolerance is tight, on the order of plus or minus 1 mm, beyond which corner stars degrade. It is adjusted with calibrated spacing rings, accounting for filter drawer thickness and sensor window. The final judge is the appearance of stars in all four corners after a test exposure.
Can coma be removed in post-processing?
Only partially. A tool like BlurXTerminator handles mild residual coma well by recentering star shapes, but it does not recreate lost signal and can introduce artifacts if pushed on severe coma. The right approach remains treating it at acquisition (corrector, backfocus, collimation). As a last resort, cropping the image slightly to remove the worst-affected corners gives a clean result without relying on software.