Exit pupil: clarification please?

[The Warthog]

As my eps were misting slighty in the last few nights, I noticed that the light exiting the ep lit up a small circle on the eye lens. When I looked through the small circle, I could move my eye around to see all of the FOV, but it seemed to be 'contained' in that small circle. The small circle in my 4.3mm ep did seem to be bigger than the .45mm or so that would be the exit pupil, but I had no way to measure it. If my eye moved outside of that circle, I couldn't see anything useful.

Now, is this phenomenon that I saw related to 'exit pupil', and in what way? I am still trying to grasp exit pupil. My opthalmologist said he was going to look it up for me, but he'll probably forget. He did tell me that my pupil dilates to about 4mm without drops, so I'm thiking that long f/r telescopes with their smaller exit pupils make more and more sense for me.

[brianb]

Exit pupil is the smallest circle through which all the light leaving the EP passes. It's actually the image of the objective made by the eyepiece.

When the pupil of your eye is coincident with the exit pupil (or near enough) you can see the whole field of view without moving your eye. Too far back and you miss some of the edge of the field. Too near and you miss some of the light.

The better EPs often have an adjustable twist-up eyecup that helps you get your eye to the right place. Unfortunately these also promote dewing of the eye lens when approached by a warm moist eyeball ... which is why dew heater strips small enough to wrap round the EP barrel were invented.

[FLO]

In a nutshell, the eyepiece takes in the cone of light from the telescope, straightens it, then emits it as a straight circular beam. The diameter of that beam is the exit-pupil size.

[The Warthog]

So, is that lit up spot I see on the slightly fogged ep the exit pupil, or does the exit pupil exist in space some distance away from the eye lens?

[FLO]

Yes, that is the exit pupil illuminating the condensation as it exits the eyepiece.

[The Warthog]

I see. I think.

[FLO]

Actually, I am not so sure myself now so have edited my post.

My understanding is that an eyepiece takes in the focused light cone then straightens it. But, the light beam cannot be perfectly straight and parallel sided as it exits the eyepiece because your eye must be positioned at a fixed distance from the eyepiece (quoted as the the eye relief).

I am out of my depth so will wait for someone more knowledgeable to post...

[George Jones]

At what were you looking, the Moon, or a star or a planet?

What is the eye relief of the lens?

[Merlin66]

You're close to the mark.

The exit pupil is equal in diameter to the objective diameter / magnification. So x200 mag on an 200mm SCT would give you an exit pupil of 200/200 =1mm.

The "minimum useful magnification" is usually based on an exit pupil of 8mm, so on a 200mm scope the "Min" magnification would be 200/8= x25

The collimated rays of light coming out from the eyepiece cross at the position of "eye relief"; to get all the light from the telescope in the smallesr circle. ie the real exit pupil exists at the eye relief distance behind the eyepiece. If you hold a white card behing the eyepiece and focus on the moon, you will see the circle of light diminish in size until you reach the "eye relief" distance then it will start to get bigger.

Hope this helps.

[FLO]

The collimated rays of light coming out from the eyepiece cross at the position of "eye relief"; to get all the light from the telescope in the smallesr circle. ie the real exit pupil exists at the eye relief distance behind the eyepiece.

That's the part that was foxing me.

Thank-you

[George Jones]

That's the part that was foxing me.

Thank-you

Since the is a high magnification case, the field of view oft the eyepiece is needed, too.

Assumimg that it was the Moon, and assuming that you used the refractor in your sig, I think that the size of the illuminated circle on the eyepiece can be calculated if the length of eye relief of the eyepiece and the apparent field of view of the eyepiece are both known.

[FLO]

Thank-you George

The small circle in my 4.3mm ep did seem to be bigger than the .45mm or so that would be the exit pupil...

Merlin's post explains why it is larger as it exits the eyepiece, it reduces in diameter until it reaches the correct exit-pupil size at the eye-relief distance.

[George Jones]

Thank-you George

Oops, sorry. I meant this as a general reply, not person-specific. I clicked on Quote, but I though that I had clicked on Quick Reply.

Further general comments.

I have found a Java Applet for drawing simple ray drawings, and, after eating lunch and doing some work, I hope to use it to produce a series of diagrams and text, starting with how the eye sees stars, progressing through simple, two-lens refractors, and ending with an illustration of the stuff in Merlin66's post. Don't know when I'll finish. Hope it will be more informative than confusing.

[The Warthog]

Yeah, OK. As it happens, the AFOV of that ep is 67º. The fact that the cone of light passing through the eyepiece gets smaller between the eye lens and the eye relief distance sort of explains how I was able to 'look around' the circle on the eye lens to see different parts of the entire AFOV. The eye relief on these eps (Antares W70) is pretty good, somewhere about 12mm, I think.

This is all very interesting. Thanks, guys!

[ollypenrice]

Excellent thread. Thanks to those wise souls who have clarified matters so well.

Olly

[Macavity]

I ever find this article useful:

Tele Vue Optics Article Page

And the technique illustrated...

But I use things like "grease-proof paper" (ground glass even!), to reveal all sorts of

interesting / educational things re. what happens at the user end of the scope!

[The Warthog]

But I use things like "grease-proof paper" (ground glass even!), to reveal all sorts of

interesting / educational things re. what happens at the user end of the scope!

You are a true scientist!

[FLO]

Indeed.

Most camera shops have a dead film SLR camera or two that they are happy to offload, the focus screens above the mirror are perfect for experiments like these.