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Go Back   IceInSpace > Equipment > Eyepieces, Barlows and Filters

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Old 15-03-2018, 02:47 PM
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Stonius (Markus)
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What makes eyepieces more prone to diffraction spikes

Are the factors that would make one eyepiece more prone to diffraction spikes than another? Just trying to account for some of the differences I'm seeing.

Is it due to EP focal length? Lens design? Blacking out inside the EP?

I'm not even sure it is actually diffraction spikes I'm talking about here. It looks like them, and they are roughly in alignment with the secondary vanes. It happens on the brighter stars. But then, I did get my eyes lasered years ago though Ive never noticed any ill effects from that. I suppose it's possible it shows up at the extremes of astronomical observing, which would make sense given that the culprit is a wide one which maxes out my exit pupil.

Anyone know? Is it my eyes or the EP itself?

Markus
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Old 15-03-2018, 06:26 PM
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, Difraction spikes are an artifact of the light passing the spider vanes , they are created by these , not the eyepiece ( the eyepiece only magnifies them along with the object being viewed and the atmosphere , tube currents etc. etc . ) .

A dirty , fogged or scratched eyepiece will show them more than a clean one with good coatings , like nearly all eyepieces today have no matter what the design .

Brian.
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Old 15-03-2018, 06:56 PM
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Hmm. Given that all the eyepieces I have are in good nic and not cheapies are there any other factors? Could longer focal lengths bend the light through more extreme angles and therefore pick up more of the light scattered from the vanes?
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Old 15-03-2018, 09:19 PM
Wavytone (Nick)
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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.

Last edited by Wavytone; 15-03-2018 at 09:58 PM.
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Old 16-03-2018, 09:41 AM
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Wow, wavytone, thank you so much for such a comprehensive reply!

I didnt thunk to rotate my head to see if the spikes stayed put. I'll try that when I next have it set up.

Since the EP is maxing out my pupil I'm beginning to suspect that its one of the latter things you mention.

Thanks again, Im blown away by the amount of knowledge on here. Thanks for sharing the good stuff.

Best,
Markus
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