Catch the sun

You've probably read that you can't look directly at the sun without damaging your eyes. At an approximation, take that as a fact.

It's not exactly true, though. You can actually look at the sun briefly without burning a hole in your retina. A little longer and you will temporarily wipe out the part of your retinas that the image of the sun fell on. Longer than that and that part of your retina will be permanently out of operation. 

The problem is that everyone is different. If someone tells you that they looked at the sun for three seconds without lasting effects, your eyes might be able to take only one second. And that is why I suggest that you never look directly at the sun.

Your vision is precious. Don't risk it.

The image of the sun on the light sensitive part of your camera will also destroy it in a very short time.

But there are ways to observe the sun. You can get a blurry image with little cost. It's acceptable for viewing solar eclipses but not for detailed solar observation. For that, you will have to put out some cash.

I only do the inexpensive stuff here. This is my solar observation tool kit.

The most common means of viewing the sun is with a pinhole. That's what the silver rectangle is. I cut a small square out of two pieces of card stock (index cards are perfect, and folded a piece of light weight aluminum foil over one side. In the middle of the square hole, I used a pin to punch a tiny hole in the foil. To punch the hole, I placed the foil side down on a hard surface (I used a craft cutting board but a marble table top or similar surface will work fine) and pressed a pin point against it.

Holding the pinhole over another card and using it to project the sun's image, I got the following.

It's...uh, that tiny dot in the center of the black circle...you might have to enlarge the photo. You can move the pinhole nearer and further from the card. When you move it away, the dot gets bigger. The problem is that it also gets dimmer.

A large hole will provide a larger image but it will be dimmer and fuzzier. The main problem is that light from the surrounding area will wash out any details.

I have two inexpensive (but very cool) science kits that include pinhole projects. The white box is from the ScienceWiz: Light kit. I cut a hole, about a half centimeter, into one wall of the box opposite the side that isn't there (the box only has five sides. The missing side has been replaced with wax paper.) When I aim the pinhole at the sun, the sun's image is cast onto the wax paper. The box shields the image from glare.

The hole wasn't very round so the image came out sorta whompsided. A paper punch would have given me better results. But, if you do this project, don't expect to see a lot of details. The big hole is better suited as a pinhole camera for landscapes.

I got a much nicer image by replacing the lenses from a simple refractor telescope kit (the Project STAR telescope bought from Home Science Tools) with a foil pinhole (I punched a pushpin completely through the foil to create a larger pinhole) at one end and wax paper at the other (the kit instructions tell how to build the pinhole tube). 

With a pinhole tube, you can slide the telescoping cardboard tubes in or out to sharpen the image.

You can also use telescoping mailer tubes to create a pinhole tube.

A second way to inexpensively look at the sun is to use a #14 welder filter. It cuts out more than 99% of the sun's light. Eclipse glasses (which are really inexpensive) do much the same thing. Old science kits suggest that you use a candle to coat one side of a microscope slide with soot to create a solar filter. The problem is that it's very easy to scratch away a tiny section of soot and that's all the sunlight you need to blast your retina or a camera CCD into oblivion...not a good idea.

The second photo is zoomed. Zooming with a digital camera won't give you any more details but it will make the image larger (and fuzzier).

The pinhole phenomenon produces an interesting effect during an eclipse as spaces between leaves on trees act as pinholes to cast images of the sun onto the ground.

These methods will give you great images of an eclipse. (See the blog for August 21, 2017 for images of the last total eclipse in Colorado.) For observing the sun in detail, you need something that will either project a cool image (a lens will just start fires), or a special filter. You can buy a special telescope called a sunspotter for a little over a hundred dollars. With it, you can see sunspots and flares.

You can use a sun filter (or welder filter) with a scope but the filter has to go over the objective lens and it has to cover the objective completely. You can get a sun filter for most telescopes and some binoculars. Here's my Carson telephoto lens on my smartphone with a #14 welder filter between it and the sun.

Here, you can see the sun's corona. The bubble at the upper right of the image is an artifact, but you can just see a solar flare below it. This is about the best I can do with my set up. Any sunspots would be masked by the general brilliance of the sun's image.

You can project the image if the sun through a scope but keep in mind that things (the scope's optics, the surface you project onto, whatever's under that...) will quickly heat up.

If you want to seriously get into solar observation, the sunspotter telescope is one way to go.

Another is an H alpha filter. It filters out all light except a very narrow band from the hydrogen spectrum (thus, it's name). It's expensive but it will show you incredible solar details. It will also block light pollution in urban settings.

Professional astronomers use radio, ultraviolet, and infrared telescopes (in addition to their regular telescopes) to get their solar images. For a lot of cool images of our hot sun, check out Wikipedia (https://en.m.wikipedia.org/wiki/Sun).

As a curious astronomy observer, you don't have to spend a lot of money to watch space and most of the inexpensive pieces of equipment are also very portable so you can easily carry them on the trail. With a little more money, you can turn astronomy into a hobby that can grow to any level.

The sun is a fascinating object to track but be safe and enjoy it.

I am a camera

Actually, I am not a camera. That was a quote from "Into the Lens", a song by Yes. You should find it on the Internet and listen to it while you read this blog.. or not.

Astrophotography is a fun hobby. To get great photos, you need to put out some substantial funds, but to get nice shots, like my shots of Venus...

You just need a phone camera, a way to connect it to a tripod, and an inexpensive telephoto lens.

You also have to have an intimate knowledge of your scope and your camera.

For any photographic work, you need to know your camera's field of view and resolution, and if you don't have this in your phone's specs, you can easily determine them like I did for my phone camera. Here's my setup.

I carried my portable podium onto the patio with a half meter ruler held up by optical bench stands (those are from an inexpensive set I bought from Home Science Tools. Great company. You could probably make your own.) Under the podium, I stretched out 20 feet of a tape measure.

On the bottom photograph above, there's a plumb bob I threw together using a random piece of plastic I had lying around. I hung it from my phone tripod clamp with a 1/4 20 wing nut and cord. I clamped that to my phone so I could tell how far away I was from the ruler using the tape measure.

To figure out the angular field of view, I stood back until the ruler filled the camera view from one side of the frame to the other. 

That was right at 2 feet (27 inches).

Next, to determine the camera's resolution, the distance at which two close objects at a specified separation can just be seen as two separate objects, I moved back until the millimeter markings on the ruler just blurred into indistinguishable marks.

That was at 28 inches.

So, why would I want an angular field of view? Many terrestrial scopes, including binoculars, give their field of view in terms of width in feet at 20 feet. That's okay when you're working at distances that can be expressed in feet, or even miles, but astronomers work in distances from astronomical units (1 AU is the average distance from Earth to the sun) to light years (a light year is about 6 billion miles) to billions of light years.

If you draw a great circle around the Earth, at any distance, it is composed of 360 degrees. The moon, as seen from Earth, has a diameter of about half a degree (we say it "subtends" an arc of 0.5 degree.) So does the sun, although the sun is much bigger. That's why the moon can block out the sun in a total eclipse. Astronomers work with arcminutes (an arcminutes is one sixtieth of a degree) and arcseconds (60 arcseconds make up an arcminute, 3600 arcseconds make up a degree). Binary stars, as seen from Earth have a separation of from 20 to less than one arcseconds.

Next...the math.

I have set up a right triangle here. The angle from one end of the ruler to the camera, back to the center of the ruler is half the angular field of view. I know the distance from the ruler to the camera (d), and I know that the half ruler is 250 millimeters long (it's a half meter ruler). I can use trigonometry to figure out the angle.

I need everything to be in millimeters, so 27 inches is 685.8 mm. The tangent of my angle is 250 mm/685.8 mm, so the half angular field of view works out to be 20° and the full field of view is 40°.

Mount Evans, pictured here, is 40 miles away. The tangent of half my view angle is the half width of my view field divided by the distance. That means the half width is equal to the tangent of half the angular field of view times the distance, or 14.6 miles. My full field of view at 40 miles is about 29.8 miles.

I figure that my measured distance to the ruler could have been off by 2 inches in either direction, so I can calculate my error by recalculating my field of view at 29 inches and 25 inches and that error turns out to be about ±3°.

I can calculate the resolution of my camera using the same method but, instead of using half the ruler, I use half a millimeter. The distance from the ruler where I can just make out millimeter markings is 31 inches or 787.4 millimeters. That gives me a resolution of 0.07° ± 0.005° . That's a far cry from being able to see binary stars as two stars, but, at least, I can see the sun and moon as a disk instead of just a point source of light.

My camera's electronic zoom does not increase resolution at all but my telephoto lens does. If I wanted to check the resolution of my camera with an optical system like a telephoto lens, binoculars, or a telescope, I wouldn't use trigonometry, I would just see if I could see a pair of stars with a known separation.

Angular field of view is a more flexible measure than width of view at a given distance, but now you know how to find your camera's angular field of view.