Eye positioning

[GreyhoundMatt]

Just been out with my scope for the first time looked at the moon which was good. Have a 6"dob with a 10 an 25mm then looked at Jupiter and saw 2 bands and all 4 moons was made up :-) anyways I know conditions arnt perfect and its my first time out but the focus was very tempeimental. I have read about exit pupil things and I wear glasses but take them off to look thru scope and put them on to use finder scope.

Anyways sounds daft but what's the technique for looking thru the scope? Should I be holding the scope or eye piece ? Should my eye be pressed up against it or should I hover away from it?

Also should the eye peace be slid right into the focuser or can you have it half way in with the screws tightened?

[Lee M]

If the 'seeing' is poor you'll find focus coming & going. I find conditions like that hard going. My poor old eye muscles going ten to the dozen trying to focos on something that's just not playing ball.

[umadog]

Focus the scope then leave it apart from when you need to push it. If you're touching it the whole time it will vibrate.

The exit pupil is shown here: http://www.telescope-optics.net/eyepiece1.htm Light comes out of the back of the eyepiece in parallel ("collimated") rays. The diameter of these ray bundles is more or less equal to the exit pupil, as you can see in the diagram. The rays come from different directions too (the diagram shows two directions). When you look around the field of view you're looking at these different directions. Eyepieces with a wider apparent field of view have rays coming in from a wider range of directions. All of these directions intersect at the exit pupil (as you can see in the diagram). The distance between this intersection point and the back of the eye lens is known as the eye relief. As you can see, placing your eye at this point is the only way you will see all of the rays and so the full field of view offered by the eyepiece. If you're closer in or further back you will miss some of the rays and get a smaller field of view.