Section 2 of the ATM List FAQ
Telescope making

Document Table of Contents

2.0 Telescope making

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Aperture, focal length, and focal ratio, or designing your new scope

Before you jump into a telescope building project, you would be best to consider what you will use that instrument for and consider how the three primary characteristics of a telescope will affect your design.

The three primary optical characteristics of a telescope are its aperture, focal length, and focal ratio. These define what you can see, how much of it you can see, and often how well you can see it. Aperture is the size of the light-gathering element of your telescope, whether it be the primary mirror or lens (either being called the objective). The bigger the aperture, the more light your telescope can gather. Lenses or mirrors serve to bend the parallel rays of light from a distant object to a point called the focus. The distance from the objective to this focus point is called the focal length. Finally, the focal ratio is simply the focal length divided by the aperture. In the design of a telescope, all three are related and work together to define the characteristics and performance of your telescope.

The larger the aperture of your telescope, the more light that it can gather. Larger apertures also enable you to see finer details. So you want to get as large an aperture as you can. But, as aperture goes up, so does cost and weight. Cost is probably an issue for all of us. Weight might or might not be a problem for you. If you plan to take your scope to club events, travel to a dark site, or take it with you on trips, you better consider its weight. If you're lucky enough to be building an observatory to go along with your new telescope, you can relax a bit about its weight.

Focal length
The focal length, the distance between the objective and the focal point, determines, among other characteristics the magnification your telescope can produce. The longer the focal length, the higher the magnification (given a particular eyepiece) that your telescope will give. High magnification isn't necessarily the ultimate goal. If you will be viewing mostly planets, then you will want high magnification (in the 200-400 range or so). If you hope to search for deep sky objects, however, high magnification won't be your goal. Most deep sky objects are quite large; too much magnification and you won't be able to see as much of your targets and might not be able to see them as well (partly because as magnification goes up, brightness goes down). So generally, you want lower magnification (100 or less) for deep sky objects.

To determine the magnification for your scope, divide its focal length by the focal length of your eyepiece. For example, a 1000 mm focal length scope with a 10 mm eyepiece will produce 100 times (100x) magnification.

If you're considering a high-magnification scope, in other words a long focal length scope, you'll find there is another important way that focal length defines your scope. The focal length determines the minimum length of your telescope's optical tube assembly (OTA). A telescope with a 48 inch focal length must be about four feet long.

The shorter the focal length, the deeper the curve in the mirror. Such curves are hard to grind and figure, you must be more exacting. The longer the focal length, the shallower the curve (and within reason, the easier it is to grind a good curve). Focal length also determines the size of the secondary mirror in your telescope. Long focal length telescopes allow smaller secondaries; short focal length scopes require larger secondaries. Since diffraction from the secondary reduces the contrast of the image, a smaller secondary makes it easier to see the low-contrast details in a planetary (or any) image. (This is why refractors - and the Schiefspiegler and other unobstructed designs - can be good planetary scopes; there's no central obstruction to limit the contrast.)

The image area within the eyepiece that is in focus is much larger in a long focal length telescope. In a very short focal length scope, only a tiny spot in the middle of the eyepiece is in focus. Also, the depth of field of a long focal length telescope is greater than with a short focal length scope.

So, why would anyone want a short focal length telescope? First, they are smaller overall, thus easier to transport, lighter, and less costly to build and mount. Second, short focal length telescopes require shorter photographic exposure times than a similar aperture long focal length telescope. In fact, you will probably see scopes identified as "fast" or "slow." A slow scope is one with a long focal length (or a high focal ratio number). A fast scope is one with a short focal length or a low focal ratio number. Third and last, short focal length scopes (fast scopes) produce the low magnifications that are useful when searching for very large deep sky objects. These objects are often the target of astrophotographers.

One scope characteristic related to aperture, focal length, and focal ratio that I have left out so far is exit pupil size. This is the size of the image produced by a particular scope and eyepiece combination. If the exit pupil is too small, you won't be able to easily see your target. If the exit pupil is too large, say larger than your dark adapted eye or about 6 to 7 mm, then light is "wasted" because you can't take it all in at once. Of course, you could just move your eye around to see the rest of the image. But, the total light gathered by your scope doesn't all enter your eye at once in such a situation.

You can calculate the size of the exit pupil by dividing the aperture by the magnification. Remember, magnification is the focal length of your scope divided by the focal length of your eyepiece. So, for a given aperture and eyepiece, a longer focal length telescope will produce smaller exit pupil. A shorter focal length will produce a larger exit pupil. Here's an example:

6 inch (150 mm) f/6 scope with 25 mm eyepiece = 36x magnification => 4.2 mm exit pupil
6 inch (150 mm) f/8 scope with 25 mm eyepiece = 48x magnification => 3.1 mm exit pupil
6 inch (150 mm) f/10 scope with 25 mm eyepiece = 60x magnification => 2.5 mm exit pupil
Focal ratio
So, I've left out focal ratio so far. This characteristic of your scope, usually noted as f/# where the number is the actual ratio, is important mainly because it is related to the other two. In most instances, you will not see people or companies mention the focal length of a telescope. Instead, they will give its focal ratio. Since you now know that the focal ratio is just the focal length divided by the aperture, you can use this number to calculate the focal length. (Most people and companies do mention aperture in addition to focal ratio.) Just multiply the aperture by the focal ratio and you know the focal length.

One final note, while there is no upper limit to the magnification that a scope can provide, there is an upper limit to the magnification that is useful. Beyond a certain point, the blurring effects of the atmosphere (called seeing) make any more magnification a waste. Magnification beyond that point is sometimes called empty magnification. Typically, you will see a figure like 50 times per inch of aperture as the maximum useful magnification for a telescope. However, given great optics and great seeing, there is no reason why (occasionally, when seeing permits) you can exceed this number. Just don't expect your $39 department-store refractor, with its plastic lenses, to come even close to meeting this figure, let alone exceeding it.

There is no perfect telescope. Telescopes optimized for visual use typically are not great photographic performers, and vice versa. Long focal length telescopes optimized for high magnification planetary observing typically perform worse on deep sky objects than short focal length low magnification scopes. Your scope will meet your needs and desires only if you consider each of these design parameters and compare them to your planned uses. Better yet, build two or three telescopes, each meeting a different design goal. Then, you really can have the best of all worlds!

For some additional background information, including another discussion of aperture, focal length, focal ratio, and magnification, visit the Telescope Vernacular page on the Star Ware Home page, provided by Phil Harrington (author of the book by the same name).

2.1: Buying versus making

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The debate of whether to buy a scope or to make your own, or whether to buy the components of your scope or to make all of the components (including the mirror) is a debate you must resolve for yourself. However, this debate usually centers around a couple of factors: cost and difficulty.

One of the most common questions asked by beginners is "Will I save a bundle by making my own?" Of course, answers will vary. But generally, making your own (whether it be a scope or mirror) is more the end than the means. In other words, most amateur telescope makers (ATMs) make their own for the pure enjoyment of making their own. Any money they save is just a side benefit.

As a just a point for comparison, this table compares the price of a mirror making kit from one of the major ATM suppliers (valid for early 1996) to the price range of a finished mirror. These prices are for comparison only, call the vendors directly for latest pricing. (See section 3.0 for information on suppliers and how to contact them.)

      Size             Kit Price       Finished mirror price
      4.25              $49.95              $110-140
      6.0               $69.95              $140-450
      8.0              $109.95              $185-890
      10.0             $219.95*             $350-1100
      12.5             $349.95*             $675-1450
 *Shipping extra

As you can see, these prices are significantly lower than the cost for a finished mirror. But, don't forget that to make a complete scope, you will need to add a mount, tube, focuser, cell, secondary, and so forth. Prices can add up quickly.

Depending on your ingenuity, resourcefulness, and persistence, you might be able to come up with most supplies for free and make most all the components of your telescope.

The second factor related to the "buying versus building" debate is complexity. In this regard I am referring to grinding your own mirror. Regardless of what you might have heard, grinding your own mirror is not impossible. Many good books, the ATM list, and hopefully a local club can help you through the process. Generally, persistence and patience, rather than special skills or talents, are what you need to successfully grind your own mirror.

By making your own mirror, you will be able to control the ultimate quality of your telescope: the primary mirror. You will gain detailed understandings of how your telescope works. You will gain immense pride at the work you can create with your own hands. And, you will be amazed at the precision that you can attain using what seem to be rather crude procedures. For example, you can achieve accuracy to within a few millionths of an inch, a small fraction of the wavelength of light, over the surface of your entire mirror!

Amazingly, dedicated amateurs have traditionally driven the retail telescope market. Two cases illustrate this best: the Schmidt-Cassegrainian and Dobsonian telescope designs. Both were developed and popularized by ATMs before the big manufacturers joined in.

For more information on making your own mirror, see section 3 of this FAQ.

2.2 Cutting circles in wood

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If you will be making a Dobsonian style mount, you will need at least two large circles of wood for the base. Methods for cutting these circles have been widely discussed on the ATM list. Here are some insights.

To cut the wood, you're going to need some sort of saw. You can use a jigsaw (Sabersaw) or a router to do this job. Jigsaws use a small thin blade, which moves rapidly up and down, to cut fine openings. You can cut tight curves with jigsaws due to the size of the blade. Routers also work nicely to cut circles. Use a "straight" bit with your router to do the cutting. In addition to the saw or router, you will need a circle cutter. This picture will give you an idea of what you need.

Circle cutter picture
Some jigsaws and routers come with circle-cutter attachments. Many stock circle cutters are fairly small and cannot be used to cut circles larger than a foot or so in radius. You can purchase circle cutters, including larger radius models, for your router or jigsaw from most mail order woodworking catalogs and most well-stocked hardware stores.

To make a circle cutter, you will need a thin board (tempered masonite works nicely) or plexiglass wide enough to mount your router (or jigsaw) to and a bit longer than the radius of the circle you wish to cut. At one end, drill a hole through which your router bit can protrude. Drill a couple of extra holes to match the general idea of the base plate that comes with your router. Remove the router base plate and screw your circle cutter in its place.

Insert the bit in your router (with the router unplugged of course). If you're cutting the outside radius of a circle (to make a disk), measure from the side of the bit towards your cutter the radius of your desired circle. Drill a small hole at that point for a nail or small bolt. This will be a pivot point for your cutter. If you're cutting the inside radius of a circle (to make a ring), measure from the side of the bit away from the cutter jig.

Now, on the wood stock from which you're going to cut your circle, measure in from the edges of the board a distance a bit more than the radius distance. (I would say find the center of the board, but if you're cutting a one-foot radius circle, that might waste a lot of wood.) Drill a small hole or hammer in a nail at that spot. Remove the nail. Place your circle cutter on the board, put the nail through the pivot hole in your cutter and through the stock. Make your plunge cut to a depth about half the thickness of the stock. Push your router around the circle. Flip your stock over so that your next cut will come from the other side. Plunge to cut through the board and push the router around again. There's your circle. If you're router has sufficient horsepower or your board is thin enough, you will be able to cut the circle in one pass rather than two as described here. Make sure to elevate your workpiece so you don't route a groove in your workbench.

Here's a low-tech approach: Drive a nail into your stock (the board from which you will cut your circle) at a point that will be the center of your circle. Tie a loop into both ends of a piece of strong string or twine, which is the length of the radius of your circle. Put one loop over the nail. Put a pencil through the other loop. Trace out your circle. Use a handsaw to cut following the line (a fine cutting saw like a scroll saw will work best for this but in a pinch any saw can work). Finally, sand the edge of your circle smooth.

These techniques will work for the ground board for your Dob mount. You can use the same circle-cutting techniques to cut the circular channels in which your side (altitude) bearings are supposed to ride. Or, as has been suggested on the list, you can cut a v-shaped notch in your side boards. The circular side bearings will ride as smoothly in a vee as they will in a semi-circular cutout.

Larry Manuel has offered two other techniques for cutting circles. He says:

I have cut outside circles very successfully by roughing the plywood [or whatever] to a circle, then finishing it by pivoting around a nail or screw. To turn the outside into a nearly perfect circle, I run it up against a belt sander - which is turned on its side, and temporarily clamped onto a workbench. It works like a charm, and is not dangerous, even for small circles - like a router would be. I have made circles as small as 2 inches in diameter.

Large [bigger than 12 inches diameter] circles can be cut very efficiently by pivoting the to-be-disc on a nail on a table saw. Clamp a big piece of scrap plywood to the saw's table to give something to hold the pivot nail. Start with the blade below the table. Raise it a bit, turn the work one whole revolution into the blade [do not let the work turn the same way as the blade rotation], raise the blade again, turn, etc. Never stop your secure grip of from the work. This only works to make discs - not holes. It is very accurate and fast.

Don't forget safety - wear proper eye and ear protection. Routers especially can be very loud and can send chips flying everywhere.

2.3 Best paint/material for tube lining

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To prevent stray reflections from the tube walls, focuser tube, spider vanes, and so forth, many telescope builders paint with black paint. Of course, the obvious question is "which is the best paint or material for stopping these stray reflections." Of course there are as many opinions on this as there are ATM list members.

Many have recommended Krylon Ultra Flat Black as their particular favorite. This is a common brand (at least in the U.S.), so it should be available at most hardware stores. Others have suggested Nextel Flat Black Velvet, formerly produced by 3M. However, 3M no longer produces that paint. Some have posted that the Illinois Institute of Technology's Research Institute sells that paint as MH2200, however, this is not confirmed. (The Red Spot Paint and Varnish Co. does not sell or produce Nextel Flat Black paint, contrary to some rumors.)

Other list members prefer different solutions. Baffling your tube is probably the most well-known solution for stopping stray reflections. Richard Combs has posted a good article on the ATM Page describing how to baffle a Newtonian telescope. Other list members suggest using flocking paper, sold by Edmund Scientific, Protostar, and others, to line tubes. Still others have combined the paint method with additional "rough" materials to produce an even blacker surface. For example, some have mixed sand or crushed walnut shells with the paint. Finally, some list members have sprayed paint onto 320 grit sandpaper. Then, applied the sandpaper to the inside of the tube.

Larry Manuel suggested a unique solution, that he claims produces wonderful stray-light blocking characteristics. He says:

I'd like to add my suggestion for a very rough, incredibly efficient, light absorbing tube interior: The inside of my tube is very rough, with ABS hair [10-20 mm long] epoxied on and sprayed black. I 'manufactured' this stuff by aggressively cutting up black domestic ABS drain pipe with a hand-held circular saw, and collecting the cuttings. Pushing the saw firmly into the work makes long, sturdy cuttings, instead of dust. I sifted the cuttings to remove the extra long pieces, using a kitchen colander [aluminum bowl with small round holes]. Then I painted a thick layer of WEST system epoxy on the tube interior. By pouring in the cuttings, covering the tube ends, and shaking the tube, I distributed the cuttings, and dumped the extras out.

When the epoxy was cured, I vacuumed the interior with a soft brush, removing the tiny percentage of fibers that didn't adhere well. Then I painted with black aerosol, spraying with the can duct-taped to a stick, and the push-button operated by string. The result is like flocking paper, about 1/2 an inch deep. To see this tube interior is to make one into a believer. Stray light is reduced to almost zero.

For an 8 inch diameter by 60 inch long tube, I used about 3 feet of 4 inch diameter pipe. Even with ordinary, not Krylon paint, the results are excellent.

2.4 Focuser placement

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It's not uncommon to see homemade telescopes with repairs where a mis-placed focuser was moved and the evidence (the hole) covered up. Even Newton himself misplaced the focuser on his first scope and had to re-position it. You can use this formula to calculate where your focuser should go.

Fp = Focuser Position (measured from the front of your primary)
Fl = Focal length
Tr = tube radius
Tt = tube thickness
Fh = focuser height
St = Spare focuser in travel

Assuming the field stop of your eyepieces is at the shoulder (the rim against which they rest when in the focuser).

Fp = Fl - Tr - Tt - Fh - St
The focuser height is measured with the focuser fully racked in. The "spare focuser in travel" builds in a buffer so that the focal plane isn't exactly at the top of the focuser when racked in fully. Typically, ¼" (or ~7mm) is sufficient, though some ATMs recommend adding ½ inch (12.6 mm). The term comes from the program Newt. From the documentation for that program:
The distance from the top of the focuser tube (when racked all the way in) to the focal plane (where the light from the primary mirror comes to a focus). Note: Some books say the focal plane should coincide with the top of the focuser tube. However, many eyepieces will NOT come to focus this way.

2.5 Spider construction

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I should have more here than this. But for now, check out Steve and Bruce Swayze's spider construction page at:

Proceed to section 3 right-facing arrow

This document, its contents, and its Web representation are Copyright ©1996, Tim Poulsen. For complete copyright information, including allowed uses of this FAQ, please see Section 8. Initially created on Thursday, September 12, 1996 by Tim Poulsen,