Description
Diffraction spikes are the bright streaks that radiate around bright stars on telescopes with a central obstruction.
They are produced by the diffraction of light on the spider vanes that hold the secondary mirror in a Newtonian, Cassegrain, RC, or SCT with a suspended secondary. A 4-vane spider at right angles produces 4 spikes forming a sharp, symmetric cross; a 3-vane spider produces 6 spikes in a Star-of-David pattern; some curved or wave-shaped spiders eliminate spikes by spreading diffraction in all directions.
When spikes appear unequal in length, doubled, curved where they should be straight, or in an abnormal count, this betrays a faulty spider geometry: bent vane, unequal tension, a parasitic additional element (cable, heating wire), or a severely misaligned collimation that shifts the diffraction center. The defect is cosmetic but almost always indicates an underlying mechanical problem that should be corrected.
Visual signature
On a bright star, the spikes show one or more anomalies: unequal lengths among the 4 arms of a cross that should be symmetric; doubled spikes (two closely parallel streaks instead of one fine one); visible curvature on spikes that should be rectilinear; or the appearance of extra spikes for a total of 6, 8, or more on a setup that should produce only 4.
The pattern is identical on every bright star in the field, which immediately distinguishes it from a local defect. The orientation of parasitic spikes points directly to the mechanical cause: an extra spike betrays an unplanned obstruction in the light path.
The signature is best seen on stars of magnitude 0 to 3 against a black background, where spikes can extend 200-500 pixels.
Differential diagnosis
Distinct from normal spikes on a well-collimated Newtonian (a perfectly symmetric cross with 4 equal, straight arms: this is expected behavior).
Do not confuse with focus spikes that appear slightly on very bright stars even on a correctly focused refractor (pupil exit diffraction artifact, normal and symmetric).
Different from defocus, which produces fuzzy disks or donuts, not straight streaks.
Do not mix with a filter reflection halo (a circular bright halo around stars, not linear and radial).
A collimation offset can de-center the spike cross (no longer centered on the star peak) without distorting the individual spike geometry.
Also check that a parasitic spike is not actually a satellite or aircraft that crossed the field during the exposure: the trail then appears on a single frame only, not on all of them.
Probable causes
- Bent spider vane from impact, transport, or a screw over-tightened on one side
- Unequal vane tension, distorting the secondary mirror position
- Electrical cable (camera power, fan, dew heater) crossing the tube aperture and creating a parasitic obstruction
- Anti-dew heating strip on the secondary mirror poorly positioned and protruding into the light path
- Curved or asymmetric-profile spider incorrectly rotated during assembly
- Severely misaligned collimation shifting the beam center relative to the spider
- Secondary collimation screws overtightened on one side, deforming the support
- Extra obstruction inside the tube: forgotten clip, adhesive tape, unintended element
- Spider modified or replaced without recalibrating vane tension
- Out-of-round tube (mechanical deformation) altering spider geometry
How to correct it?
- Visually inspect the spider with a flashlight: check straightness and equality of vanes
- Equalize vane tension by hand or with a suitable key (feel the same resistance everywhere)
- Identify and re-route any cable or strip passing through the tube aperture: everything must be pressed against the walls
- Check for any parasitic element inside the tube with a flashlight (packaging residue, forgotten item)
- Re-collimate the secondary after any spider adjustment: beam center must equal geometric center of the spider
- On a thin-vane Newtonian (1 mm), accept long spikes but ensure they are symmetric
- To definitively eliminate spikes, consider a curved spider (Bob Royce models or reliable 3D printing)
- Document the spider geometry after a validated adjustment to reproduce it after disassembly
- On an already-affected image, CloneStamp or manual removal via mask is possible on bright stars, but fixing the cause remains the priority
- If spikes remain abnormal despite all checks, have the tube roundness inspected
The Doc's advice
A parasitic spike is your telescope pointing at something wrong. A fifth arm appearing is almost always a dangling cable in the beam: check your cable routing before suspecting the spider. If the vanes are unequal, it is a tension issue to correct, and five minutes with a headlamp is enough. The baseline rule: a well-adjusted Newtonian makes symmetric spikes as fine as pen strokes. If you have twisted spaghetti spikes, you have a mechanical problem, not an optical one.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
Why are my spikes not exactly perpendicular?
Because the spider vanes are not perfectly orthogonal, or because the tube has undergone some deformation. On a 4-vane spider that should produce two spikes at 90 degrees, a visible asymmetry (for example 88/92 degrees) betrays a slightly twisted assembly or a loose vane. Check the tension of each vane by lightly pinching it: they should all vibrate at the same frequency. An ovalised tube (rare but possible after a fall) can also shift the geometry without being correctable without reshaping the tube.
How can spikes be eliminated permanently on a Newtonian?
The radical solution is to replace the standard spider with a curved spider (also called a boat anchor spider), whose curved vanes spread diffraction around the full perimeter instead of concentrating it in 4 directions. The spikes then become an extremely faint, nearly invisible diffuse halo. Drawbacks: slight contrast loss, a less rigid secondary support (vulnerable to wind and vibrations), and the need for precise machining or reliable 3D printing. For most uses, clean and symmetric spikes remain preferable and are even aesthetically valued on cluster and wide-field images.
Does a cable crossing the tube really produce a visible spike?
Yes, and this is one of the most frequent causes of parasitic spikes. A 2-3 mm diameter cable placed in front of the primary mirror acts exactly like an additional spider vane and produces its own pair of spikes oriented perpendicular to its direction. On modern setups with cooled cameras, dew heaters, and fans, several cables may pass through the aperture. Solution: all cables must run along the edge of the tube, pressed against the wall via clips or velcro, never across the diameter.
Do spikes only affect bright stars?
Visually yes, but diffraction by the spider affects all stars: it is simply that the spikes of faint stars are below the detection threshold. In practice, spikes become visible from about magnitude 6-7 on a typical 180 s exposure, and clearly marked on stars of magnitude below 3. The contrast loss due to spider diffraction, however, affects the entire image including faint nebulae, which is the main argument for very thin vanes (1 mm or less) on Newtonians dedicated to scientific imaging.