December 1999 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - December 1999 Issue

The Telescope Nut
by Jeff Baldwin

Pupils

Your eyeball has a hole in the front where the light goes in to be focused by the lens onto the retina. This hole is called the entrance pupil. It is small when the light is bright, and large when it is dark. When you are fully dark-adapted it reaches about 7mm in diameter. However, as you get older the entrance pupil is no longer able to open fully to 7mm, but rather only to about 5.5mm.

When you look through the eyepiece of your telescope, there is a "bundle of light" that comes out of the eyepiece. Imagine what would happen if that bundle of light were bigger than your eyeball's entrance pupil. Some of the light that your telescope is producing for you would not be seen, and it would go to waste. This can happen if you use too low a magnification. The bundle of light that comes out of your eyepiece has a diameter we call the exit pupil. It can be calculated by dividing the focal length of the eyepiece by the focal ratio of the telescope. If you have an f/10 SCT and you are using a 25mm Plössl, then your exit pupil is 25/10mm = 2.5mm.

Some deep sky observers and comet hunters use extremely low magnifications to hunt for little faint dudes. This is because if you look at, say, a nebula at medium power, then change to low power, the same amount of light is jammed into a smaller image, and that makes the light-per-unit-area increase. The object appears smaller, but brighter. You can reduce your power until you get to the point where it is as bright as you can make it by getting to the power that gives you a 7mm exit pupil.

Let's look at a 10" f/5.6 Newtonian. If you use a 20mm Nagler, the exit pupil will be 20/5.6 = 3.6mm. If you want to increase the apparent brightness of the object, reduce the magnification. If it is half the size, it will be 1/4 the apparent area, therefore 4 times the brightness-per-unit-area. Let's try a 40mm eyepiece. The exit pupil will be 40/5.6 = 7.2mm. The object now looks twice as bright per unit area, and you are still right at the entrance pupil for your eye. Let's try a 56mm eyepiece. Your exit pupil is now 56/5.6 = 10mm. The brightness-per-unit-area should now be 1.3 times brighter than with the 40mm eyepiece, but now the exit pupil is 10mm. That doesn't sound too much over 7mm, but here's the problem: 7 squared is 49 and 10 squared is 100, which makes the ratio of the exit pupil circles 49/100. That's less than half the light from the telescope that actually gets into your eye! Your goal for increasing apparent brightness is to lower your magnification as far as possible without having the exit pupil exceed your entrance pupil. This is called rich field.

OK, Eric is 42 and still has about a 6.5mm entrance pupil, and is looking at M42 with his 10" f/5.6 and a 35mm PanOptic. This gives him an exit pupil of 35/5.6 = 6.25mm, perfect. He says "Hey, Trevor, come and look at this!" Trevor steps over and looks through the scope. Trevor's entrance pupil is only 5.5mm, so the percentage of the light produced by this system that Trevor can see is (5.5/6.25)² = 77%. Even though it will look fantastic, he is only seeing 77% of the light that the telescope is sending him. He could get the same from an 8" telescope. An exit pupil that produces a rich field view for one observer may not produce a rich field view for another observer. It's all in the eye of the beholder.

There is a problem at the other end of the exit pupil game. If you use high magnification you may get an exit pupil that is too small. When the exit pupil is 0.5mm or less, the bundle of light entering your eye may be obstructed by floaters, those little crappy junky dudes that are floating in your eyeball. They may be larger than 0.5mm, or at least large enough to set up diffraction patterns. These floaters goof up otherwise great high-power views of planets and the Moon. If you have a 4" f/15 refractor and you are using a 3mm eyepiece to look at swirls in Jupiter's belts, your exit pupil is 3/15 = 0.2mm. You will certainly see some of these floaters roaming in your view. Sometimes it's a problem, sometimes it's not. I'm only saying that this problem exists.

I have violated the rich field rule: I've used Dave Schamber's 35mm PanOptic on my 24" f/3.7 Newtonian, giving me an exit pupil of 9.5mm. That means that I was only seeing 7/9.5 = 74% of the diameter of my 24", or 17.7" of mirror. It was still wonderful, but it wasn't a 24" telescope anymore, only 17 1/2". It still looks great--you can get huge fields of view--but your eye isn't going to grasp all the light coming from the mirror.

For binoculars and spotting scopes, the exit pupil can be calculated by dividing the objective lens aperture by the magnification of the optics. 7X50mm means that the objective lens is 50mm in diameter and the optics are 7 power. 50 divided by 7 = 7.1mm, perfect for astronomy. 5X35mm are also good, 6X42, 10X70. The other binoculars are also good, but they are not rich field for young viewers. More mature viewers should consider 10X50mm, 6X30mm and other binoculars that offer a 5mm exit pupil.

How big is your entrance pupil?

I know of three ways to measure your personal entrance pupil sizes. One way is to buy the little device with holes in it that you look through and see where the light circles meet. This doesn't work real well in the dark, and it is the dark adapted pupil you want to measure. The other way is to have yourself photographed in the dark with a flashing camera (ouch) and a millimeter ruler placed on your forehead. Analyze the photograph later to measure both your entrance pupils and your interpupilary distance. I guess the third way is to have a doctor give you eyedrops and measure your pupils, but not being an astronomer, he/she may not be concerned enough to measure that as accurately as you want. As awful as it sounds, I think the best method is to shoot yourself with a camera and flash and check it out later when you can see again.

Clear Skies...Jeff Baldwin
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