Eyepieces not the normal question

[naked"I"]

Sorry people I've searched for hours.

I can't seem to grasp the basics, Doh.

1/ When an EP is classed as 6mm 10mm 15mm etc what is that measure for ?

2/ what affect does the length of the EP have for planets DSO?

3/ FOV is that the part you look through or what you see via the scope.

4/ (a) My scope has a 2" Dual-speed Crayford focuser, with a 1.25" adaptor, would a 2" Barlow be best or a 1.25"?

5/ Why would I need a 40mm EP? I assumed that the smaller the EP mm the deeper into space you could see.

Again sorry if its an easy one, and it been asked over & over

Dave

[Ags]

1/ When an EP is classed as 6mm 10mm 15mm etc what is that measure for ?

It is the focal length of the eyepiece (just like the telescope has a focal length so does the EP.) Magnification is the telescope focal length divided by the EP focal length.

2/ what affect does the length of the EP have for planets DSO?

You want short focal length EPs for planets as they give high magnification - but be careful you don't want too much magnification, which just looks fuzzy and dim.

3/ FOV is that the part you look through or what you see via the scope.

The EP shows you a disc of sky - the width of that disc as it appears to your eye is the Apparent Field of View (AFOV). The actual amount of sky shown (the true Field of View) is the AFOV divided by the magnification. So if you have 50 times magnification and an EP with 50 degrees AFOV, you are looking at 1 degree of sky.

4/ (a) My scope has a 2" Dual-speed Crayford focuser, with a 1.25" adaptor, would a 2" Barlow be best or a 1.25"?

No idea.

5/ Why would I need a 40mm EP? I assumed that the smaller the EP mm the deeper into space you could see.

The longer focal length EPs show you a wider field of view, so you can take in big objects like the pleiades.

[MorningMajor]

I won't attempt to answer your first 4 questions - I'll leave those to someone far more knowledgeable than me - but point as for point 5

Why would I need a 40mm EP? I assumed that the smaller the EP mm the deeper into space you could see

. I was observing Pleiades the other night and through a 25mm EP it looked wonderful - loads of stars that appear as a blur to the naked eye - crystal clear and amazingly bright - however when I went down to a 9mm EP all that happened was that I lost sight of a lot of the stars and just "focussed in" on a few - I wasn't seeing the whole picture - in fact the effect would have been better had i used (or owned) a 32mm or greater EP because I would have seen more of the whole cluster - forget trying to magnify individual stars - they're far too far away - unless you have some serious kit

[Astro_Baby]

Its also worth mentioning the following

Eye relief....the distance between your eyeball and the lens. Short eye relief can be uncomfortable, too long an eye relief and its like trying to look down a ball point pen from six feet away.

Exit pupil...calculated from the ep focal length divided by the focal ratio of the scope. If its too big you will end up losing contrast at the centre of the eyepiece FOV and you will start to see the secondary mirror in a reflector. For a 200 scope 32mm is the upper limit on ep focal length.

I have a 38mm and I dont have problems with it in a 200 but my eyeballs are weird anyway