just received 250px

[sam mayo]

i heard about the xit pupil a few times what is it and how does it afect viewing

[RikM]

Exit pupil is the size of the focused beam of light that exits the eyepiece and goes into your pupil. You can work it out by dividing the eyepiece focal length by the focal ratio of your scope. The focal ratio is the scope focal length divided by the aperture (diameter of the main mirror). 1200 / 254 = f4.7 for your Dob, so your 25mm eyepiece would give you 25 / 4.7 = 5.3mm exit pupil.

Everyone's eyes are different but generally your pupil will only open to about 7mm. Perhaps a bit more when you are young but it decreases as you get older. So, any more than 7mm (or however wide your pupil opens) and the light is wasted, you may also see a dark blob in the centre of your vision from the secondary mirror. As the exit pupil size increases, the apparent brightness of the view increases and the magnification decreases. You need to balance the magnification, brightness, field of view and contrast between the object you are observing and the background sky.

I can't use a 40mm in my Dob, I see the shadow from the secondary but then my eyes don't open very wide. I have a 30mm wide angle eyepiece as my widest.