what kind of optical abberation is this?

[coffee_prince]

Near the egdes of view the stars start to elongate away from the centre of view. I originaly thought it was coma, but from what I have read, coma appears comet like i.e triangular, whereas what I am seeing most definitely not, but more like a line than a comet like tail.

I can't find any website which offers real examples of chromatic abberations, only diagrams which aren't useful at all!

Thanks

[Moonshane]

the first link on this thread is good.

http://stargazerslounge.com/astro-lounge/125890-coma-astigmatism-other-aberrations-do-you-know-what-they-mean.html

[Capricorn]

Would guess at spherical abberation of the eyepiece.

May be caused by the image formed by the primary not being flat. The lower the f number of the scope the more curved in general this non-flat image plane is. That is why imagers use field flatteners - they flatten the image field to be a flat as the camera sensor. However as an eyepiece only pick out a section of the image created by the primary it is more likely to be the eyepiece.

Look for off axis abberations, since that is where the images that the eyepiece is picking up lie.

Not sure what the eyepiece is, so a better one may be required. Think that this shows up a lot on wide angle EP's and low f number scopes. WO have said that their SWANS are best suited for f/6 and greater, and I suspect that f/6 still shows problems but an amount that is considered to be acceptable.

[brantuk]

It might be tracking error if stars are appearing to elongate. Just to throw another idea in the pot

[Peter Drew]

Sounds like astigmatism to me, not an optical quality fault, more of an inherent effect of the optical design.

[coffee_prince]

thanks for the info, I am STILL using the supplied eyepieces, so they are modified achromats.

that link is especially useful, I will have a more closer look when i get the chance at the optical errors.