low mag seems worse than high mag!

[puregeordie]

Hi, Just a quick Q if you don't mind, I have got it collimated the best I can get with what I have, so here goes.My telescope at low mgnfication at 25mm I cannot seem to get my eye in the right place in the eyepiece when its focused, or when it appears focused!

I seem to have to move my eye around to the side of the eyepiece to see it a bit better. Higher focus seems to be OK but lower focus getting my eye positioned correctly is a right pain for planets.

It must be something out, even my collimatation, but I've always seemed to have this problem.

Any ideas please,

Cheers, Si.

[richbyers]

hows it with other things- stars, binaries and clusters etc? can it focus any better?

[tjr]

Sounds like you're suffering from 'kidney beaning' with your eyepiece. Getting your eye in the right place is more important with lower mag eyepieces due to the larger exit pupil (hence them usually having greater eye relief). It's just something you have get used to when observing through them, although some EPs display this more than others.

[puregeordie]

Sorry for the late reply, and thanks for info.

Kidney Beaning ...!

cheers,

[umadog]

It indeed could be something like that. With Plossls the eye relief is proportional to the eyepiece focal length. Some long focal length Plossls can have astonishingly long eye relief (many cm). This can make eye positioning tricky because you don't have a physical guide nearby to indicate where you should be placing your eye.

[ismangil]

Another downside with low mag, especially with light pollution is contrast is very low, the sky doesn't seem dark enough, so the fainter stars are lost.

Try very hard to shield yourself from other light sources, eyepatch, hood over you and eyepiece etc.

[acey]

I find that with my lowest power eyepiece (32mm plossl, giving a 6.5mm exit pupil in my f4.9 scope) it can be hard to get the right viewing position when I'm at a light polluted site (i.e. I get blackout or "kidney beaning"), but at a dark site it's no problem. At the light polluted site, my eye pupil is presumably smaller than the eyepiece exit pupil, and I take this to be the cause of the problem.

Yet another reason why it's better to observe from a dark site!

[Tinker1947]

If there the EP's supplied with the scope, a upgrade will make a world of difference , name your budget and people here will spend it in no time at all.. but it will be good advice..

[umadog]

Here's a nice demonstration of kidney-beaning (scroll down): Kidney Beaning versus Vignetting in the Eyepiece - Astronomy Forums | Telescope Forums & Reviews | Astronomy Community

You can see clearly how the aberration gets its name. The exit pupil is the point at which rays with different angles converge. You want rays of different angles to converge in the same plane. Kidney beaning is an eyepiece aberration where this fails to happen. So some rays converge at a different plane and you see the blackout for those rays. Whilst kidney-beaning causes blackout, not all blackout is kidney-beaning. The eye leaving the exit pupil will also cause blackout. This is more likely in an long eye-relief eyepiece where it can be hard to position your pupil. It's quite possible to make a long eye-relief eyepiece with no kidney beaning (particularly if the apparent field of view isn't too wide) but you may still have blackout issues if you find it hard to position your eye.

[puregeordie]

Thanks Umadog for that detailed and understaing posts, Pain in the rear those beans!

And yes have to get some pennies and get a decent EP.

See how I get on for know.

Cheers.