Hi Argonavis & All,
Argonavis wrote:
"I am not sure that using an exit pupil of 7mm or greater (as I use with my lowest power) is such a felony.
I find that the longer focal length eyepieces with consequent greater exit pupils will provide a much wider TFOV. Although you cannot "take it all in at once" you can move your eye around the field to take in any objects of interest."
Well, no, you can't actually -- it doesn't work that way. The exit pupil is the diameter of the beam produced by a telescope/eyepiece combination, that comes out of the eyepiece. It is found (approximated) by dividing the aperture in mm, by the magnification used. Alternately you can divide the fl of the eyepiece by the f/ ratio of the 'scope. The simple fact is, if the diameter of your fully dilated iris is less than the diameter of the exit pupil then your iris will vignette the beam coming out of the eyepiece before it gets to the retina, thereby acting like an aperture-stop.
For example, if a person hypothetically has a fully dilated iris of 6mm (common in the 35 to 50 age-group), and the exit-pupil employed is 8mm diameter, then your iris will be blockiing some of the light gathered by the 'scope. It won't matter how much you move your eye around, you can't fully fit an 8mm beam into a 6mm iris. As foghorn-leghorn was fond of saying "Son, yer can argue with me, but yer can't argue with figures".
But is it a felony?
No.
But you have to realise that to get that wider field at lower magnification, you _are_ effectively trading-off light-gathering power. There is no free lunch.
Accordingly, the biggest exit-pupil _I choose to use_ with my 18" is 5.5mm. I know my iris won't be vignetting the light-path, nor will I be using the extreme outer reaches of my cornea where aberrations are most likely to be found.
My earlier (attempted) sarcasm
regarding the ease with which you can measure your full dilation seems to have slipped under the radar. It isn't easy at all to measure at home (but it can be done). Therefore I proceed on the assumption that I have just less than average and then subtract a fudge-factor -- hence my self-imposed limit of 5.5mm.
Best,
Les D
Contributing Editor
AS&T