Question about Exit Pupil

[Utwo]

I have a Meade ETX-70 that I've borrowed from my in-laws. They haven't been able to get much use out of it, and so they lent it to me to see what I can do with it. They have 5 different eyepieces: a 6mm Plossl, 4mm Plossl, 25mm Modified Achromatic, 9mm Modified Achromatic, and a 2x Barlow.

Something that occured to me is that all but the 25mm seem useless. If the exit pupil is equal to the focal length of the eyepiece divided by the f-number of the scope (f/5), then the 25mm eyepiece has a 5mm exit pupil, and the rest are worse, with the 4mm having an exit pupil of just 0.8mm. I'm guessing that what this means is that my view through the 4mm is going to be heavily vignetted and basically useless.

However, this seems in contradiction to what I've heard about magnification vs. aperture, namely that I can expect to get 50X per inch of aperture. This scope is 70mm, or about 2.75 inches, and so I should expect to be able to go up to 137X, right? Well, the 4mm eyepiece is just 87.5X, which is well within that range.

Thing is, given the relationship between f-number, aperture, and focal length (namely, that the first is the quotient of the other two), it seems that no matter what the f-number of the scope, it's impossible to get 87.5X out of any scope with a 70mm aperture without the exit pupil being just 0.8mm. So, how does the 50X per inch of aperture rule hold? Is 0.8mm more acceptable than I'm envisioning? I won't actually have a chance to try it out until tonight.

Also, does the barlow lens affect the exit pupil? I wonder if I should try the 25mm with the barlow lens. All I really want to do is split Albireo, LOL.

[Hornblower]

I have a Meade ETX-70 that I've borrowed from my in-laws. They haven't been able to get much use out of it, and so they lent it to me to see what I can do with it. They have 5 different eyepieces: a 6mm Plossl, 4mm Plossl, 25mm Modified Achromatic, 9mm Modified Achromatic, and a 2x Barlow.

Something that occured to me is that all but the 25mm seem useless. If the exit pupil is equal to the focal length of the eyepiece divided by the f-number of the scope (f/5), then the 25mm eyepiece has a 5mm exit pupil, and the rest are worse, with the 4mm having an exit pupil of just 0.8mm. I'm guessing that what this means is that my view through the 4mm is going to be heavily vignetted and basically useless.

However, this seems in contradiction to what I've heard about magnification vs. aperture, namely that I can expect to get 50X per inch of aperture. This scope is 70mm, or about 2.75 inches, and so I should expect to be able to go up to 137X, right? Well, the 4mm eyepiece is just 87.5X, which is well within that range.

Thing is, given the relationship between f-number, aperture, and focal length (namely, that the first is the quotient of the other two), it seems that no matter what the f-number of the scope, it's impossible to get 87.5X out of any scope with a 70mm aperture without the exit pupil being just 0.8mm. So, how does the 50X per inch of aperture rule hold? Is 0.8mm more acceptable than I'm envisioning? I won't actually have a chance to try it out until tonight.

Also, does the barlow lens affect the exit pupil? I wonder if I should try the 25mm with the barlow lens. All I really want to do is split Albireo, LOL.

I believe you are misunderstanding the significance of the exit pupil. For the purpose of getting all of the light rays from the telescope into your eye, a small exit pupil is more forgiving than a large one.

All of the rays converge toward the exit pupil and then diverge behind it. At low magnifications the exit pupil may be the same diameter as your eye pupil, in which case you must place your eye exactly centered on the exit pupil to avoid cutting off some of the light. At higher magnification and a smaller exit pupil, you have some freedom of motion before the edge of your pupil starts encroaching on the beam. In addition, astigmatism in your eye is less troublesome with the smaller exit pupil.

At extremely low magnification the exit pupil may be larger than your eye pupil, in which case some of the light is lost no matter where you place your eye.

In a nutshell: A small beam of light (the exit pupil) gets through a large hole (your eye pupil) without a hitch. A large beam has trouble with a small hole.

If my words do not make it clear, try Googling "telescope" until you find some side view sketches of the ray tracing through the scope and into the observer's eye.

[Dave Mitsky]

As magnification increases, the exit pupil decreases for any telescope since exit pupil is also equal to aperture divided by magnification. This holds true whether shorter focal length individual eyepieces are being employed or longer ones in conjunction with a Barlow lens or Tele Vue Powermate. One potential problem with using an exit pupil below 1mm is that "floaters" present in interior of the eyeball become more noticeable. The only way of getting around this particular problem is to use a very long focal length telescope and longer focal eyepieces.

Some observers use long focal length oculars as "finder" eyepieces. In fast telescopes, these oculars produce overly large exit pupils (i.e., greater than 7mm) in oder to maximize field of view. However, the increased FOV comes at the expense of "wasting" aperture. This is not necessarily as bad as it may seem.

For more on the concept of exit pupil, see these sites:

http://www.skyandtelescope.com/howto...s/3304201.html

http://www.astromart.com/articles/ar...?article_id=30

Dave Mitsky

[Utwo]

Thanks, guys!

I took the scope out tonight, and I was able to get a nice view of Venus using the 9mm with the 2x Barlow. The rest of the sky was far too cloudy to see anything else. Is this a good time of year to view Saturn?

[redshifter]

This is a great time of year to view Saturn, though I believe the peak viewing time is past. However, it is located in the S/SW after sunset just on the E edge of Leo.