Stockton Astronomical Society

Valley Skies - November 2000 Issue

The Telescope Nut
by Jeff Baldwin

Eyepieces!

People strive hard to make the most perfect mirror they can. They optimize the telescope with cooling fans, baffles, secondary mirror optimization and offset. They drive hundreds of miles to get the best seeing conditions--and then they gawk through the telescope with a second rate ocular.

Hmmm! What's wrong with this picture?

Some eyepieces are expensive beyond your wildest dreams, some are cheap, and the question always arises, "Do you get what you pay for?"

In most cases, yes.

Eyepieces are magnifying lenses that allow the observer to scrutinize the image that the telescope has created and is holding in the air just in front of the eyepiece. The telescope has caused the image to be upside-down (in most cases, not all), and when you look at this image with this magnifying lens, it will remain upside-down. Eyepieces do not flip the image--the ray-trace, or leverage that the telescope’s objective lens does to the light causes the image to appear upside-down. The eyepiece just makes it look larger/closer.

The magnification of a telescope depends on the telescope’s focal length and the eyepiece’s focal length. The quotient of the two gives you your magnification, or power. Example: a 60 mm refractor with a 900 mm focal length (f/15) and using a 25 mm eyepiece will have 900/25 magnification, or 36x. Changing to a 10 mm eyepiece will allow the scope to be used at 900/10 = 90x.

It is sad that department store telescopes are advertised using magnification as the leading information, especially now that you know that any telescope can use any power.

Just like a refractor telescope, an eyepiece needs lenses that converge light rays and correct for color. If there were only one element (one piece of glass) in the eyepiece, light of different colors would come to focus at different places. This would allow red to be focused and blue to be blurring, or visa versa. So, multiple elements, or more than one piece of glass, are required to be in an eyepiece. The overall collection of glasses must have a positive prescription so that the image the telescope is holding in the air can be focused by the observer. If there are a minimal number of glasses in the eyepiece all of which are spherical or flat in shape, then even though the image can be focused very well at the center of the field of view, the outer regions of the field of view will have some or all of the defects that come with eyepieces, namely astigmatism (different kind of astigmatism than with some mirrors), coma, and other stuff.

To keep the observer from seeing the poor images of the outer regions of the field of view, the eyepiece will have a field stop built into it. This field stop defines the field of view that the observer can see. So even though a Kellner might be able to see 80° , it is stopped down to only about 35° or maybe 40° to keep the observer from seeing the outer poor images. The better the eyepiece performs in the outer regions, the larger the field stop will be. Plossls have about a 55° field stop; PanOptics have about a 70° field stop; Naglers have about an 82° field stop. These more exotic eyepieces have multiple elements (some up to 8) and are aspherical in shape. That’s why they can run up to around $700 for a 31 mm Nagler (right, Monte?).

Eyepieces come in three standard barrel diameters. Japanese telescopes have a 29/32" standard. Unitron, Sears, and other Japanese telescopes with oculars of this size are pretty much non-existent now, but you run into them once in a while. The current standard sizes are 1 1/4" and 2". The reason for using more than one size is that the eyepiece is sampling a field of view that might be large or small. A 4.8 mm Nagler is only looking at a tiny field of view, so it doesn’t need a 2" barrel. However, a 31 mm Nagler uses the entire 1 7/8" that it has on the inside diameter of the 2" barrel. The 31 mm Nagler is the largest field of view eyepiece in today’s sizes.

I predict that someday a new standard will be formed that will allow larger fields of view to be used. Three inch or 4 inch barrels may be on the large Dobsonian telescope 20 years from now. Cool! Imagine a 45 mm Nagler. You’d need another car to bring it up!

Exit Pupil: There are two parts to this story.

Part I: Exit pupil is an important number in observational astronomy. Your eyeball’s entrance pupil is the size of the hole in your eye when light comes in. If you are 21 years old, it is most likely 7.1 mm in diameter when dark adapted (and intoxicant free). If you are 65 years old it is probably only about 4.5 mm across. You can interpolate your age with these numbers to get a good guess at what your entrance pupil is. You can also photograph yourself at night with a flash and a ruler against your face. Painful, but it works. The light that comes out of the eyepiece has a diameter, and this light bundle’s diameter is called the exit pupil.

Part II: The lower the magnification of your telescope the brighter the image looks. For example, if you look at the Square Nebula at 100x, then look at it with the same telescope at 50x, it will be only half the diameter and half the height, making it only 1/4 the size. Since it is illuminated with the same telescope and therefore has the same amount of light illuminating it, then it will appear 4 times brighter per unit of area. That means that as you decrease the magnification of the telescope, the image will get brighter and brighter. You ought to be able to keep going until it will drill a hole in your eye from being so bright.

Part I and Part II merge now.

The bundle of light that comes out of the eyepiece gets bigger as the magnification gets lower. It can become so big that it won’t all fit into your eyeball’s entrance pupil. The trick in maximizing the brightness that your telescope appears to create is to minimize the magnification up to an optimized point but not to exceed it. This point occurs when your eyepiece’s exit pupil equals your eyeball’s entrance pupil. That will make the image as bright as you can get it. When this happens, your telescope is said to be a Rich Field Telescope (RFT).

To calculate the exit pupil, you divide the focal length of your eyepiece by the f number of your telescope. For example, the exit pupil on a Celestron C-8, which works at f/10, using a Meade 32 mm Plossl, is 32 mm divided by 10 which is 3.2 mm.

OK. Which eyepiece will make a 16" f/5 work at Rich Field? If you are 21 years old, and your entrance pupil is 7 mm, then 5x7 = 35. A 35 mm eyepiece on an f/5 telescope has a 35/5 = 7 mm exit pupil. If you are 65 years old, then you would need a 5x4.5 = 22.5 mm eyepiece to get Rich Field. You can get the eyepiece that comes closest to these numbers. For example, I don’t think they sell a 22.5 mm Nagler, but a 22 mm Nagler will be close enough.

Another method of determining the exit pupil is to divide the aperture by the magnification. A pair of 7x50mm binoculars have an exit pupil that equals 50 divided by 7 = 7.1 mm. That’s why 10x70 mm, 9x63 mm, 8x42 mm, 7x50 mm, 6x42 mm and 5x35 mm are so popular with astronomers.

There is also a minimum exit pupil rule. If the exit pupil is 0.5 mm or smaller, then while you look through your telescope you will see the goobers that are floating around in your eye, a real gross-out, and not a good view of astronomical objects.