My primary visual instrument is a 18" f5.5. The LE50 is totally unsuited to this. At f5.5, the LE50 projects a 9.1mm exit pupil. My 31mm Nagler on the other hand produces a 5.6mm exit pupil, theoretically almost perfectly matched to the maximum dark adapted dilation of my 57 year old eyes.
Back to the LE50. A friend of mine owns a Takahashi 180c 180mm f12. At f12, the exit pupil is 4.2mm and a good match to that instrument. Phil tried one out belonging to a mutual friend of ours and has been pining for one since. The new price was a bit more than the CFO was prepared to approve. 2nd hand LE50's don't come onto the market often in Australia. I saw the ad soon after it was posted but I knew my friend was in hospital that day having an adjustment made to his pacemaker so I pounced and bought it for him confident that he'd be happy to buy it from me, which he was.
Although I frequently use exit pupil to determine suitability of eyepieces etc, I had never really observed the effect of oversize exit pupil. This was a prime opportunity to test it out so I decided to use my 18" scope to satisfy my curiosity. I set it up and pointed it at the Eta Carina nebula.
Both eyepieces yield a true field of view of about 0.9 degrees.
Using the 18" f5.5 This comparison has nothing to do with eyepiece quality. Both eyepieces are definitely premium and both give gorgeous views when matched to the right
To the eyepiece! I started with the LE50, lovely sharp image and showed up Eta Carina beautifully. The eyepiece yields fantastic contrast, dark lanes are dark.
I swapped in the Nagler 31 and as I put my eye to the eyepiece, the thing that jumped out at me was that the image indeed was noticeably brighter. It was a stark difference. I swapped back to the LE50 and suddenly, by comparison, the image though still "Tak" sharp looked dull and lifeless by comparison.
So what's happening?
In the dark our pupils dilate. The amount that they dilate varies between individuals but mostly with age. The red line marks average dilation for my age group of around 5.3mm. If the exit pupil of the telescope is greater than the dilated entrance pupil of the eye, that light can't enter the eye and is wasted.
In my case, the (probable) 5.3mm entrance pupil matches the 5.6mm exit pupil with the Nagler 31mm pretty well so nearly 90% of the light is entering my eye. With the LE50, the amount of light entering the eye is the square of (5.3/9.1) or about 33%. So two thirds of the light is wasted and the view through my 18" reflector has the light intensity of a 6 inch reflector. My observation matches this theoretical value quite well.