[Wavytone]

While looking through an eyepiece with this problem, rotate your head a tad. If the spikes stay fixed with respect to the scope, they are indeed diffraction spikes from something in the light path. If they rotate with eyeball, they're produced within your eye.

One easy way to tell if its your eye is to go outside at night, cover the eye you don’t use and look at a distant bright streetlight - or car headlights. If its a nice sharp dot, your eye is good. If you see spikes, astigmatism, double images and other issues in low light, consider seeing a good optometrist who knows how to do a proper examination - not the cheapass ones that plonk you in front of a laser for 10 seconds.

Now... causes.

In a telescope the ideal (no spikes) only occurs when the entrance pupil is a perfect circle and nothing obstructs with the light cone along the way to the image. The amount of light scattered into the diffraction spikes is proportional to the area of the light cone obstructed by the defect.

The suspects are:

1. The optical perimeter of the primary mirror is not a smooth perfect circle. For example
- if there are retaining clips to stop the mirror falling out and these protrude over the edge, each will cause a pair of diffraction spikes.
- if the mirror has a significant chip on the edge of the front surface, this will also cause a pair of diffraction spikes.
- defects in the reflective coating at the edge.

2. Spider vanes that are straight. Reducing the spider to just 2 or even 1 curved vane with a radius of curvature equal to half the mirror diameter has an interesting result - no diffraction spike ! It still causes diffraction but the energy is scattered into the diffraction rings around the star, rather than a spike.

3. Things like the bottom of the focusser, or heads of nuts, bolts inside the OTA protruding into the light cone will case spikes.

4. Secondary mirror holder. If the shadow of this is not perfectly circular - eg small nuts/bolts sticking out sideways into the light cone, these will cause diffraction spikes.

Lastly, your eye can cause a different problem - if you use a very low power eyepiece such that the diameter of the exit pupil (ie the cone of light entering your eye) is wider than your iris. This is because for most people the iris is not a perfect circle - look at it closely with a magnifier and you will find the edge is rather ragged. The lens in the eye is quite poorly corrected when at its widest aperture - the surface errors often exceed 2 wavelengths and it gets worst near the edges.

While it is possible to have the lens surgically replaced with an artificial silicone one that may be better (or may not, if unlucky), at the moment there's not much that can be done about the iris or optical "floaters" - junk in the aqueous humor inside your eyeball. Not until the Mark 1 Digital EyeBall Replacement, anyway.