I have read a lot about matching eyepiece exit pupil size to the dark adapted pupil of the eye size when looking for faint objects. My question is: How do I determine my fully dark adapted pupil size? The reason I ask this is if I have a maximum pupil size of 5mm, why would I purchase an eyepeice with a 6mm exit pupil?
I recently purchased an XT8 withe a 2-in focuser and all my current eyepieces are 1.25 in.
Any education you can provide would be appreciated!
Clear and steady skies,
OK, when you use an eyepiece whose exit pupil matches the pupil of your eye, you achieve the brightest image that scope can give you.
If you use an eyepiece that yields a larger exit pupil, some of the light doesn't go in the eye, but the magnification is also lower.
The light loss perfectly matches, mathematically, the brightening that comes from a lower magnification.
Hence, the image is exactly as bright as it was using the eyepiece whose exit pupil matched the eye.
So, what are possible reasons to not go with larger exit pupils?
1) You may not need a lower power or larger true field.
2) The aberrations of your own eye might overwhelm the image quality
3) coma in the wider field might appear worse (if a newtonian used without a coma corrector).
4) the shadow of the secondary might become obtrusive.
The last reason is a big one, since the percentage of the field covered by the secondary shadow in your eye increases as the exit pupil gets larger.
since your pupil does not get larger, the secondary shadow might become a noticeable problem in the field as the magnification goes down.
This also points out that, because there is no secondary obstruction n a refractor, there is also no lower limit to magnification or size of exit pupil.
My dark adapted pupil is 4.5mm, yet I have used a 41mm eyepiece in my f/7 refractor to gain the widest possible field: 3.7° at 17x, with a 5.9mm exit pupil.
Edited by Starman1, 22 October 2019 - 12:35 PM.