Exit pupil

[scuffer]

I'm trying to understand exit pupils , am I correct in saying that using the info below when I view Saturn I will be seeing an image 1.67 mm in size ?

FL = 1200 , EP=10mm , Aperture = 200m

So magnification = FL / EP = 120

and Exit pupil = Aperture / Magnification = 1.67

[michael.h.f.wilkinson]

The maths are right, but the interpretation is wrong. The exit pupil diameter is not the size of the image, it is the diameter of the parallel beam coming from one of the points in the field of view into your pupil. When you measure the apparent size of things, you express it as an angle, in degrees, minutes and seconds of arc. At its closest point, Saturn is some 20". Multiply that by 120 and you get 2400" = 40' or 2/3 of a degree, which is larger than the apparent diameter of the moon with the naked eye (1/2 degree).

[scuffer]

Thanks Michael , knew I had something wrong but didn't know where

[umadog]

The equation is right. You can also calculate it as eyepiece focal length divided by telescope focal ratio. In your case: 10/6=1.67

It's true that the exit pupil is a disk of light 1.67 mm in diameter but it's misleading to say that you're seeing an image 1.67 mm in diameter. It's misleading because it doesn't describe what you actually see. Partly this is because if you draw a circle that size, how large it appears depends on how far away it is. Furthermore, Saturn will only occupy a small portion of the field of view. Thinking about it in terms of angles (Angular diameter - Wikipedia, the free encyclopedia) makes more sense:

There are 90 degrees between zenith and horizon. Let's take the example of the moon, which is half a degree across (so you could chain 180 moons end to end from horizon to zenith). If you look at the moon through a telescope you are magnifying it (let's not worry about how). If you magnify the moon 20 times it now occupies 20x0.5=10 degrees of visual space. This is now 1/9 of the distance between the horizon and the zenith (so 180x(1/9)=20). You can work out the same thing for any other object by knowing its true size and the telescope's magnification. Saturn is about 20 arc seconds across when it's closest to us, which is about 0.005 degrees. So if you magnify it 100 times it will look about the size of the moon viewed with the naked eye.

The exit pupil is very relevant for one particular thing: image brightness. When the exit pupil of the telescope fills your eye's pupil you will see the brightest possible image. As you magnify the exit pupil becomes smaller and the image dims. If you play with the formulae you will see that the consequence of a larger objective is that you can magnify the image more without it becoming too dim. Thus, a larger telescope doesn't really make the image brighter. Instead, it allows you to magnify more without the imaging becoming overly dim.