This page contains answers to questions Mr Sunspot received about Earth-bound telescopes. The questions are:
[138] How can I build a solar filter for my Celestron SCT 8" telescope for viewing the Sun's faculae, prominences, etcetera? paraphrased question from Mark. 6 May 1998
[40] How can I make a simple telescope? paraphrased question by Michael Theisen. 23 August/10 October 1996.
[15] I would like to use a small refractor to view the Sun. Should I purchase an aluminized mylar filter or a stainless steel coated glass filter? I am concerned about optical quality as well as safety.
Building Solar Filters
Unfortunately, to see solar activity such as faculae and prominences with a telescope requires an H Alpha filter. The reason these are so expensive is that they are interference filters, and it requires a bit of expertise to make them at the right wavelength and of sufficient optical quality.
I believe that years ago Richard (Dick) Dunn published an 8 part article in Sky and Telescope magazine on how to make a homemade narrow band filter from quartz elements. (I'm talking like 1958 here.) I don't believe anyone in the amateur realm ever tried to tackle that project, mainly due to the amount of work and accuracy required.
A typical H Alpha interference filter goes for about $1000, and that's cheap. The next step would be an interference filter/etalon combination, which allows an even narrower bandwidth selection.
Another route to approach is by constructing a spectroheliograph. These have a slit that scans across the solar image, and a corresponding exit slit scanning across the film/CCD plane. A homebuilt unit is feasible, and has been done. But it still takes time, and the device can be a bit cumbersome due to size. (I believe the book Amateur Telescope Making, Vol 3 has an article on how to make one, as well as ancient issues of Sky and Telescope.)
[BA 4 May 1998]
Making a Simple Telescope
Unfortunately, telescopes are one of those things where quality is very much directly related to cost. A simple telescope as you desire would be fraught with limitations. If you keep that in mind, then it shouldn't be too hard.
The simplest type of telescope would be similar to what Galileo made. This consisted of a single double convex lens, one curved outward on both sides, to collect the light, and a negative lens at the other end to allow the eye to see an image. Another convex lens could be used though in place of the negative lens, but this will invert the image, and require that the distance between the lenses be greater.
The big problem with any telescope is mounting it. A paper towel roll could hold two small lenses, but some way would be needed to adjust the distance between the lenses to focus, as well as mounting the whole assembly sturdily. Any small vibration will shake the image to the point of blurriness.
Furthermore, a small telescope like this will not enable one to see much more than the eye. (Although Galileo did quite a bit with his, he had the patience required.) There are limitations in resolution, image contrast, distortions of various types, as well as magnification.
Such a telescope would be fine for learning how telescopes and optics work, but nearly useless for any real observing of the sky. Generally, a good pair of binoculars would be much better in that regard. It's amazing what one can see with binoculars. And their mass production helps keep the cost down while usually providing very good optical quality.
If you want something a bit more, then I suggest making/buying a Dobsonian type telescope. These can be made very cheaply, relative to other commercial telescopes of similar size. Their optical quality can be excellent, as well as other important factors like resolution. The Dobsonian telescope is a Newtonian, which means that it uses a mirror at the bottom of a tube to collect the light. A diagonal mirror near the top of the tube redirects the light to the side, so that it can be viewed with an eyepiece. The mount can be made easily with common materials, such as plywood.
The mirror for a Dobsonian can be obtained from many sources, thus eliminating the problem of finding the right optics. Check out a recent issue of 'Sky and Telescope' magazine to find out about companies that sell parts for telescopes, etc. But then again, going this route will require spending at least a couple hundred dollors.
So whatever route you take, the first question you must answer is: How much do I want to spend on this? Factors that contribute to answering this include: How much will it be used? How much do I have? Where will the telescope be stored? How much time can be devoted to building it? What parts do I already have that I can use? What can I make and what must I buy?
See, it's not such an easy question to answer. I do know that there is a web page somewhere that discusses what kind of telescope can be bought for under $500 or so. Look for it for further ideas. (Sorry, I don't know where it is right now.)
[BA 24 August 1996]
Looking at the Sun through a Telescope
First of all, the safest way to look at the sun is NOT to! This may seem obvious, but remember that Galileo went nearly blind observing the sun. Here at the National Solar Observatory, we almost always use electronic cameras do the looking for us.
The safest practical way to see the sun is by eyepiece projection. Basically, the telescope is lined up on the sun, but instead of looking through the eyepiece, a sheet of white paper is held behind the eyepiece. The solar image is seen by observing the image projected onto the paper. You will notice that as the paper is moved farther back, the image becomes larger and fainter. Place the paper for a comfortable viewing contrast. This white light image of the sun allows a considerable amount of observing to be done.
To see a direct image of the sun requires a bit more work. The safest approach in this regard uses a telescope specifically designed for visual solar observations. These employ such tricks as using uncoated mirrors, Hershel wedges that direct most of the light into a heat trap, and other partially reflecting optical devices. The advantage of these systems lies in the fact that should an element break due to excessive solar heating, the light path to your eye becomes broken. This saves your precious eyesight in case of an accident. (It only takes an instant of concentrated sunlight to destroy an eye.)
The use of filters of any type is definitely the least safe of the safe approaches. That does not mean it isn't safe, just that extra care needs to be taken to be sure your eyes stay safe.
Coated metallic filters all suffer from the possibility of pinholes in the coating. When a material is vacuum coated with a metal, like aluminum, the thickness of the coating determines its opacity. (Mirrors use a thick coating, because as much light as possible must be reflected.) With the thinner coatings used for solar filters, it's possible for a speck of dust in the vacuum chamber to produce a pinhole for sunlight to pass through. For this reason, mylar filters usually consist of more than one layer of material.
Obviously, it would seem that a coated glass filter would also produce less optical distortion than a flexible mylar filter. (Imagine plastic wrap in front of your telescope compared to a glass window, and you'll get the idea.) Good quality plastic can overcome most of this difficulty, but glass will always be better in this regard. Don't ignore mylar though. You won't loose that much optical quality, especially for visual use, and it is less expensive. Both are probably equally safe, as far as they can be.
Another factor to remember is that mylar filters have a tendency to produce a blue solar image. (I'm not sure about coated glass. Most of the glass filters we use are interference filters designed to only transmit a particular wavelength of light, like 6563 Angstroms, the hydrogen alpha band.) If you use a glass filter, do NOT use a highly concentrated beam of light on them. They will heat up and crack! Also, keep the mirrored side toward the sunlight coming into it.
Also most solar filters, like the mylar, are designed to fit on the front of the telescope. This keeps a concentrated beam of sunlight from passing through them. This position helps the optical quality because the filter is not placed at a focal plane of the telescope. (By the way, a good way to check for pinholes is to use another lens behind the eyepiece, and project an image of the filter onto a sheet of paper while aiming the telescope at the sun. Any pinholes will show up as white spots on the filter image.)
And please, always check any filter before you use it, and each time you use it. All filters are delicate and can be ruined by the slightest mishandling. Don't use a filter you think may be damaged. Your eyesight is too valuable. [BA 30 April 1996]