|
G'day Nigel. The lowest power eyepiece is mostly determined by exit pupil. Fully dilated your eye pupil opens to about 7mm. This reduces with age though. So you want to pick an eyepiece for low power work that gives an exit pupil no more than 7mm. Another thing to consider, if viewing from a light polluted site your pupil will not dilate to 7mm, so you will need to use an eyepiece with a smaller exit pupil, ie higher power.
To work out exit pupil for your eyepieces, divide the aperture by the magnification of the eyepiece.
So in your scope, a 35mm eyepiece will provide 43x. 305mm divide by 43 gives you a 7mm exit pupil. So 35mm is the longest useful focal length for your telescope. Though practically it would only be useful at a dark site where your eye can fully dilate. A more useful low power eyepiece might be closer to 6mm exit pupil. My 31mm eyepiece is close to that and works well in the same size telescope that you have.
Another factor affecting the size of the view is the apparent field of view of the eyepiece. Comparing a 110 degree eyepiece to a 30 degree one reveals a huge difference in how much you can see in the field of view.
|