Quote:
Originally Posted by Don Pensack
Let's do the math.
The exit pupil would be 14.47mm.
If your secondary mirror is 25% of the primary in diameter (likely at f/3.8), then the secondary shadow would be 3.62mm wide !
Assuming your dark adapter pupil diameter is, say, 6mm (common in people older than 40), then the shadow of the secondary would be 60% as wide as your pupil!
You would see a big black hole in the image.
Normally, you wouldn't go larger in focal length than 6 x 3.8 = 22.8mm focal length, but I think it's almost always OK to sneak a little past your maximum exit pupil.
My dark adapted pupil is only 4mm, but I use an eyepiece with a 5.2mm exit pupil and it works fine, so I will assume you can use an exit pupil up to 3.8 x 7 =26.6mm focal length eyepiece.
But seriously, focal lengths longer than that make no sense and significantly reduce the clear aperture of the scope and lead to visible secondary shadowing.
Don't even be tempted unless you have a scope of f/8 or longer.
Don
|
Thanks Don,
I have SIPs so my real focal ratio is 3.8x 1.15=4.37. Multiplying that by 7 gives me 30.59. I have the 31mm Nagler which may therefore be as long as I can go. Even the 41mm Panoptic would be pointlesss. I’ll hold off and just try it one night when a fellow observer has one to try.
Chris