(Image of the McMath Pierce Heliostat mirror at the Kitt Peak National Observatory. Credit: NSO/AURA/NSF )
If you can get past all the hype and hysteria, the August 21, 2017 total solar eclipse will be an event to remember, and it will be all the more special if you view it yourself. Seems like some people are just out to make a buck, though, selling memorabilia and fake observing glasses. But even if you were lucky enough to get your genuine and safe viewing “glasses,” the image you will see will be very small. If you want to share with a group, you’ll want something bigger.
Here’s the latest solar image from NASA’s Solar and Heliospheric Observatory (SOHO). The mirror projection method described here will not provide such a large and detailed image as this (neither will it be yellow-orange!), but it will display the partial phases of the eclipse quite nicely. And at other times, you will be able to see large sunspots.
By the way, this is the basic method used at large solar observatories. The image at the top shows the mirror at the McMath Pierce Heliostat at Kitt Peak National Observatory in Arizona. It allows highly detailed images of the Sun. Our instructions call for a somewhat more modest mirror.
Pinhole projection, which you will find suggested in many places, is a common method. This is good, and it works, but the small size of the image is still a bit disappointing. Instead, you can make a nice image 3 or 4 inches across or even larger using a variation with a mirror. It’s the same basic physics, except that you reflect the light onto a wall instead of a small piece of cardboard. [Interestingly, the two methods show the same difference as that between the two major types of telescopes, refracting (which depends on a lens) and reflecting (which naturally depends on a mirror.)]
Here is a basic graphic showing how this works, and instructions based an excerpt from an activity I give to my students that involves projecting an image of the Sun:
You will need a very small, flat (non-magnifying) mirror. You need something roughly a quarter inch across. Since such small mirrors are rare, you can use a larger mirror, but you must block out all but a small hole. Make a small round hole (about 6 or 7 mm, roughly one quarter of an inch, is good) in black construction paper and mount this on the front of the mirror so that the mirror only shows through the hole. In a pinch you could use a large index card for this, but black construction paper is preferable.
The image on the left is from a former student, Patti Sand, for her class project in which she measured the diameter of the Sun, not observe an eclipse. She attached a flat compact mirror on a tripod, with paper blocking out all but a small hole exposing the mirror.
You could also use a dental mirror, which is what I used for this example, but you will need to project the image maybe 35-50 feet to get a good, well defined image. The larger the mirror, the father you have to reflect the image to get good resolution. The key is experimentation before hand.
Determine how you will mount the mirror in order to be able to aim the reflection of the Sun onto the wall. It is preferable that the mirror be located at approximately the same height above the ground as the reflection will be on the wall. Using a tripod as illustrated is an excellent way to do this, although there are many other way you could do it. The exact method is left to you. Holding by hand is of course one way, but you have to have a steady hand.
Find a suitable location to perform the observation. The easiest way to do this is to project onto a North-facing wall around noon. The wall onto which you project needs to be in shadow so that you can see the image well. Furthermore, it needs to be round in shape rather than oblong. If you follow the instructions carefully, you will get a round spot on the wall that is, in fact, an actual image of the Sun. It may not be terribly sharp and detailed, but it is an image as opposed to simply a reflection of the shape of the mirror or cut out.
Experiment with sizes and distances to find something that works for you. But keep in mind that the ratio of the size of the reflection distance (“l” in the graphic) to the size of the mirror cutout should be in the order of 1200 or 2500 to one. A 6 meter reflection from a 7 mm mirror fits the lower end of this ratio. That is about 23 feet distance for a quarter inch hole. These are just suggestions. The exact measurements are not terribly critical here, but you need to be close to this ratio and you do need to be projecting onto a light surface in a darkened room.
Experiment with sizes and distances to find something that works for you. But keep in mind that the ratio of the size of the reflection distance (“l” in the graphic) to the size of the mirror cutout should be in the order of 1200 or 2500 to one. That’s a fairly large range. When using a 6-7 mm (quarter inch) mirror or cutout, it’s best that you do not use reflection distances (“l”) of less than 7 meters (about 23 feet) or more than 15 meters (nearly 49 feet). The reason for this is simply that if the projection distance is too short, the image will be bright but too fuzzy (out of focus); if the distance it too great, the image will be sharply defined, but dim and difficult to see. With a 6-7 mm hole, a distance of 7-15 meters is about the best range when projected onto a flat surface in the shade.
In general, use the shortest distance that provides you with an acceptable image. These are just suggestions. The exact measurements are not terribly critical here, but you need to be close to this ratio and you do need to be projecting onto a light surface in a darkened room.
Again, the larger the mirror, the father you have to reflect the image to get good resolution.
Of course, standard precautions hold. Do not look directly into the reflection and be careful not to reflect it onto people or animals. This does not intensify the heat like a magnifying glass, but it is still bright sunlight and should not be observed directly.
For the record, here is my quickly thrown together set up on Friday, August 17 using a tripod, a dental mirror and an index card cut out mask with a hole-punch hole (about a quarter inch). I reflected the sun from the tripod to the back of my garage onto a piece of white foam core. The distance was roughly 35 feet or approximately 10.5 meters.
As you can see, there is a distinct circular image. Unfortunately, the sunspots currently on the Sun’s surface are not big enough to resolve with this method. This is, of course a fairly crude set up. Still I have seen large sunspots before using a set up like this. During the eclipse, the crescent shape of the partially eclipsed Sun will be quite distinct.