Star Image Problem

[skytry]

hi All,
I require help please,
using my 102mm refractor and looking through a 50mm 2" eyepiece,
using the micro focus, the bright stars I focus on have a V shape
to the edge in the eyepiece,
is this coma?, I read under 150 magnification, it is possible,
with say 15mm and over 150 magnification, is this somewhat eliminated,

am I right?,

regards,
Peter.

[mental4astro]

Hi Peter,

What might the focal length of your refractor be? Reason being if the resulting focal ratio is fast (focal ratio is the focal length divided by the objective's/primary mirror's diameter), this may cause any inherant astigmatism in the eyepiece you are using to show up. The shape of the abberation you describe suggests astigmatism in the eyepiece.

Is your 50mm a GSO SuperView eyepiece? These are fantastic eyepieces for their price, but there is a limit to their performance, particularly in "fast" scopes. I have the 30mm SuperView, and while in my C8 the edge performance is ok, in my fast newtonians, f/4.5 & f/4, the astigmatism is very noticeable.

Coma it isn't as coma is a phenomenon related to reflectors only. The 'similar' abberation in fast refractors is chromatic abberation, due to the objective lens not being able to focus the whole spectrum of light to a single point. This is seen as a blue/violet halo around bright objects, particularly at high magnification.

Astigmatism can be controlled by using better quality eyepieces. The othe alternative is to understand and accept the eyepiece's shortcomings. I tolerate this in mine, and I'm happy with it.

Chromatic abberation is inherant to fast 'Achromatic refractors', and can be delt with by using violet minus filter, or by really splashing out and buying an 'Apochromatic' refractor.

Mental.

[skytry]

hi Alexander,
I am reading and understanding what you have
written,
great info, as it may not be the eyepiece alone too cause
the problem,
yes, I am operating with GSO eyepieces, also Celestron, and with the Vixen, Vixen eyepieces,
and there is more experimenting to be done with eyepieces
too achieve the best results with the refractor, eyepieces are cruital, I shall keep trying,

the refractor is a 102mm / 500mm skywatcher black diamond,

the other thing is my eyesight is not tops anymore, the aging process ya know,

regards,
Peter.