Telescope Beginners Guide: Alt-Az mounts

Alt-Az Mounts

The altitude-azimuth system

Alt-Az mounts work in the altitude-azimuth coordinate system.  Although the words sound big, this is the one you usually use when you’re just talking to somebody. For example,a friend says “I saw this really bright light just above the horizon in the east this morning. Do you know what it was?” You reply “Of course! Venus is very bright and low in the east in the morning right now.” In this case, “low” is the altitude, and “in the east” is the azimuth.

Altitude

Altitude is in degrees above the horizon.  It is generally assumed that the zenith is at 90º, and the horizon  at 0º.  Of course what you can actually see can be affected by your location: a lot of trees may limit you to within a few degrees of the zenith. Colloquially, we may also use “high” or “low” for the altitude, but since different people have different ideas about where high stops and low begins, that’s not as helpful as actual degrees.

Azimuth

Azimuth is the direction along the horizon, such as east, or southwest. For more precise mounts, it is measured in degrees away from due north going clockwise. That means due east is actually 90º, and an azimuth of 200º would be SSE.

Altitude and azimuth are shown by the blue arrows. Altitude is measured in degrees from the horizon, azimuth from due north.

Examples of mounts

A telescope on an alt-az tripod.

This is one of the easiest mounts to make. The most common design is a turntable with an axis for altitude adjustment.   Since azimuth is along the horizon, it is important that the mount be horizontal, or level. The simplest version, or at least the cheapest, is probably cardboard box mount. You turn the box to rotate in azimuth, and rotate the telescope to rotate in altitude.

The most common place you are likely to encounter an alt-az mount is a tripod for cameras or small ’scope. a standard tripod has a mount that rotates in azimuth and has a handle for making quick adjustments in altitude.

A tabletop telescope, on a Dobsonian style mount with a single arm support.

A very popular mount, especially for do-it-yourselfers is the Dobsonian. A quick search for “home built dobsonian” turns up dozens of examples.  The body of a Dobsonian mount is actually just a box on a turntable.  Usually, a pivot attaches to the OTA and rests in a cradle in the box.  A carefully made Dobsonian may be accurate enough for setting circles and even a drive mechanism, which helps mitigate some of the disadvantages of alt-az mounts. One thing to note though is that these mounts really only work with Newtonian style telescope, where the eyepiece comes out the side of the tube near the top. If it’s a very big telescope, that may mean you’ll need a ladder to observe.

Most “tabletop” scopes are alt-az mounts. Dobsonians are common because the turntable can sit on the table without adding much of any height to the telescope. The OTAs are usually light enough that only one support arm is needed.

Advantages and Disadvantages

Advantages

• Easy set-up. Alt-az mounts are the easiest mount to set up. Dobsonian types you just plop down on a level surface, pop in an eyepiece, and you’re observing.  If it’s a tripod, you’ll have to extend the legs, but you can make adjustments if the ground isn’t level.
• Easy to operate. They work the same way you usually think about looking for things: up and over, so beginners usually find alt-az mounts much easier to aim.
• Cost. As with anything, you can get some very fancy, expensive alt-az mounts.  But you can’t beat a cardboard box for price.
• Easy to make. If you like building things yourself, a Dobsonian is the way to go.  You just have to make sure everything is square and you’ll have a working mount.

Disadvantages

• 

  Star trails over the Gemini observatory. Note that the length of the trail depends on the part of the sky the star is in. Longer trails indicate a bigger change in alt-az coordinates.

  Tracking is hard. Unless you are observing the pole, any target you set the ’scope on will move. Unless you are observing at the equator, the target will move at an angle, so both the altitude and azimuth will change. Even harder, the rate they change at depends on where the target is in the sky. A star close to the pole may only change 5 degrees in altitude over the course of the night, but a star that rises in the northeast may cover almost 90º over the course of the night. Images of star trails illustrate this. Near the pole, stars don’t move much over the time it takes to capture the image, so the trails are short, but farther from the pole the stars move more, and the trials are longer.  Modern telescope mounts overcome this problem by having a computer control the tracking, but that means if you want an alt-az mount that will track, you’ll have to spend the money on a computerized mount.

• Coordinates are location dependant. The north celestial pole is at about 45º altitude, 0º azimuth for people  along most of the US Canadian border, but only about 30º altitude for people on the US Mexico border. And of course, most objects change in azimuth as the night goes on.  This means if you want to get the coordinates of an object like a comet, you’ll have to know your latitude and what time you’ll be looking for it. Again, the computer controlled mounts can fix some of that.  The computer can calculate where it should point as long as it knows where you are and what time it is.

a cheap tripod alternative

Cardboard box mount

So you’ve just spent $15 on a new Galileoscope and need something to mount it on. Or maybe you just picked up your third used camera off eBay for $25 and you now have more optics than tripods. But you just can’t bring yourself to spend 3x more money on the mount than you spent on the optics.

Well you’re in luck. Here’s a cheap and easy solution: a cardboard box mount. This is a very basic alt-az style mount for any lightweight optics, and it really shouldn’t cost you more than a few cents to put it together. It may not be the most elegant mount, but it works.  And it doubles as a storage container!

Things you need for a cardboard box mount.

Here’s what you need:

• Something to mount;
• a cardboard box with some stuff in it to balance the optics;
• a 1/4-20 threaded thumb screw (a bolt will also work, but it is less convenient),
• 2 washers.

You’ll also need a pencil or pen and something pointy to put a hole in the cardboard box. The smallest screwdriver on a leatherman works realy well, though it is a bit smaller than the thread of the screw.Other good options include a small pocket knife, an ice pick, cork screw or  something along those lines.

How to assemble the mount:

1. Empty the box and open it up.
   

   Taping the flaps will add to the strength of the box.

   • You will want it open while you are using it as a mount, so if it is a box with flap lids, you’ll have to decide what to do with one of the flaps, especially if you want to use it for storage.
     • Folding the flap down will add more stability to the hole and more support for the telescope.
     • It is best to tape the flaps down to keep them out of your way while observing, but not if you want to close the box to store stuff.
     • After repeated use, the flap will tend to fall in and become useless as part of the box top, or it may tear off.
   • If your box is lidded, remove the lid and set it aside.
2. Mark the location with a pen.
   

   Mark the position for the thumbscrew at least 1" from any edge or corner.

   • The short side is better, since  that will make it easier to balance the telescope.
   • You want to to be a high as possible for a telescope, so if you are using something like a paper box, you may  want to stand the box on end and put the hole in the bottom.
   • Put the hole close to an edge to make it easier to get your head up next to the eyepiece. Make sure it is at least an inch (~2 cm) from any edge or corner so there is enough room to use the thumbscrew.
3. Punch a hole roughly the diameter of the 1/4-20 thumbscrew. If you decide to go with something like a  kitchen knife be very careful not to make the hole too big. Big holes may not have enough support for your optics.
   

   punch a hole just big enough for the thumbscrew.

4. Place a washer on the thumbscrew and push the screw through the hole from the inside of the box.  Place the other washer over the screw.


Place a washer on the screw and insert it into the hole

5. Attach your telescope/camera. Tighten the thumbscrew just enough to hold it in place. Too tight will wear out the box.
   

   Attach the telescope

6. If you can’t tighten it enough to hold the telescope in place, you may need to add a couple spacers.
   

   Add a spacer (or a few) to make the box thick enough to tighten the screw.

7. Add stuff to the box to act as a counterweight to the telescope. Load the side farthest from the telescope first.

Using the Mount

A Galileoscope on a cardboard box mount

1. To adjust the altitude (up and down), lossen the thumbscrew slightly, adjust your telescope, then tighten it down.  Moving it without loosening the thumbscrew will wear out the box.
2. To adjust azimuth (side to side), rotate the whole box.
3. Be sure to check the hole and the area around the washers frequently, or at least before every observing session, since there’s nothing worse than having your telescope fall off in the middle of an observing session. If the hole does wear out, simply make a new one at least an inch from the old one.

When your box wears out, or if you decide it wasn’t a very good box, you can simply move the hardware to another box. If you plan to use the box for storage, I recommend storing the hardware in a plastic zipper style bag in the box rather than leaving your telescope mounted to it.

Some tips for choosing a box:

1. A huge box may take up too much space and be inconvenient if you want to transport it somewhere.
2. A short box may not have enough clearance for high objects and long telescopes.
3. A tall skinny box may be to top-heavy to support a ’scope or heavy camera.

Telescope – Beginners Guide: Refractor

The white tube to the left and black tube near the middle are amateur refractors. Toward the right is a little orange reflector or the same diameter as the white tube telescope.

Refractors are the simplest telescopes in design. They use a large objective lens to collect the light, and an eyepiece to focus it.

There are two basic types of refractors. The usual type is a Keplerian, or some variation on it. The other type is called Galilean, because this was the design used by Galileo.

Galilean Refractors

A galilean telescope has a diverging lens for an eyepiece, so it must be placed close to the objective, before the focal point. The length of this telescope is actually the difference on the focal lengths of the two lenses, F-f.

In a Galilean refractor, the objective is usually a  long focal length lens, and the eyepiece is placed closer to the objective than the focal length. The eyepiece of a galilean telescope is a diverging lens, so it takes the converging light from the objective and makes it parallel, which makes it easier for our eyes to focus.

This is the usual design for spyglasses and day-time ’scopes because it produces an upright image.  However, it is a very narrow exit pupil and poor eye relief, so your eye must be perfectly aligned to see anything. Also, it has a very small field of view. Most galilean telescopes can’t fit the entire Moon in the field of view, even at the lowest magnification.

Keplerian and other Refractors

The keplerain telescope has a diverging lens for an eyepiece. The distance between the objective and eyepiece is the sum of the focal lengths of both lenses, F+f, so it is longer than a galilean telescope.

Kepler realized if he switched the  converging lens in the eyepiece to a diverging lens, he could get a wider field of view. This design also gives a wider exit pupil, so it is easier to look through the telescope.  However, the image is upside down, so this makes a poor terrestrial telescope.

Problems Common to all Refractors

All refractors have a few problems in common.  There are the obvious problems, such as imperfections in the glass or the shape of the lens.   Large lenses can also sag under their own weight, so very large refractors are particularly hard to build. Light is also absorbed and reflected by glass (think about trying to look outside at night from a brightly lit room). Thin lenses mitigate many of these problems, but the thinner the lens, he longer the focal length, which makes for really unwieldy telescopes.

Spherical Aberration

Spherical aberration: Light coming through the edges focuses farther from the lens than the light passing closer to the center

The objective is normally a spherical lens (meaning each surface is shaped like a section of a sphere, like the overlapping area in a Venn diagram with two sets.) The focus of a spherical lens isn’t actually a point, but a line. The light passing close to the center of the lens focuses at a shorter distance than light passing farther out, so a sharp image isn’t possible.

The shorter the focal length of the lens, the worse the aberration is, so one solution is to make q very long focal length lens. Either choosing a different type of glass, or  making the lens very thin will increase the focal length. Unfortunately, this will result in a very long tube.  Some early refractors were so long, they took several supports, and several people to operate the telescope.

Another option is to use non-spherical or compound lenses. Non-spherical lenses are very hard to make, so they tend to be prohibitively expensive.  Compound lenses are easier, and can solve the other major problem, chromatic aberration, as well.

Chromatic Aberration

Chromatic aberration: red light bends more than blue, so it focuses closer to the lens than blue light.

The shorter the wavelength (the more blue) the light is, the less it bends when it passes through a lens, so the farther away from the lens it focuses.  This means if you bring the red light into perfect focus, but the green and violet light will be out of focus, and usually results in a blue or purple halo around the image.

Achromatic compound lenses were the first solution to this problem, developed in the 18th century. Each half of an achromatic lens is made of a different material. One half of the lens works best with one color, usually red, and the other half works with another color, usually blue. The two different colors then focus at the same point. Many of the great refractors of the 19th century were this design, and this is the common solution for good refractors.

Apochromatic lenses combine several lenses made of different materials designed to bring three colors, usually red, green and blue, into focus at the same point. This produces a clearer image than the achromats. Most are also designed to correct spherical aberration as well.  However, the extra elements make them undesirable for large refractors. Not only are they usually heavy, but every element will absorb some of the light, making the image slightly dimmer. They are generally found in high-end amateur telescopes, especially those designed for photography. These are frequntly the favorite ’scope for amateurs who prefer imaging solar system objects.

Telescope – Beginners Guide: Reflector vs. Refractor

There are two basic types of amateur optical telescopes, reflectors and refractors. The  difference is in how they collect the light:  reflectors use a mirror, and refractors use a lens.

Even if you can’t look inside to see how the light is collected, you can usually tell them apart. In refractors, the light must travel all the way through the tube, so refractors are long for their width.  Since reflectors use a mirror, the light travels back and forth in the tub, so most reflectors are short for their width.

Basic Structure

Refractors

Refractors are generally long and narrow, with the wide objective at the front and small eyepiece at the back

Refractors always have a large lens at the front. You point this lens at the object you want to view, so it is called the objective lens. The light travels through the tube to the eyepiece, where you put your eye to view the object. If you look the wrong way through a refractor, you can still see the object you’re looking at, but minimized instead of magnified.

The refractor is the original and stereotypical telescope.  If someone wants to include a telescope in a movie (or other popular media), it is usually a refractor. Galileo and latter Kepler used refractors.  Spyglasses, prevalent in pirate movies, are refractors. However, they are usually not the best astronomical telescopes because they are limited in size and expensive to build compared to reflectors.

For more on refractors, see the entry on refractors.

Reflectors

Reflectors have a big mirror at the back, so they remain wide. The eyepiece can be located in several different places., so it is not shown here.

Issac Newton realized the limitations of refractors, and developed the first reflector.  Reflectors all have a primary mirror at the bottom of the tube. However, there are many different types of reflectors with eyepieces in back, or the side, or pure imaging ’scope with a camera mount in the middle of the tube. Some reflectors even have lenses on the front. No matter what the configuration, if you look in the wrong end you see yourself rather than the object you’re trying to look at.

Reflectors are the more common type of telescope, and all large professional telescopes are reflectors, because they are cheaper and easier to build. There are many different types, and each have their own benefits and drawbacks.

For more on reflectors, see the entry on reflectors.

Differences

Large mirrors are much lighter than large lenses, and are much easier and cheaper to make. Mirrors only require one perfect surface,  they don’t need perfect glass or supports, and they don’t need to get thicker as they get larger. Lenses must be perfect all the way through and have two perfect surfaces. Wider lenses should also be thicker, or they’ll have ridiculously long focal lengths. Mirrors can be supported all across the back surface, but lenses can only be supported around the edges so large lenses may actually sag.

There are of course plenty of other differences, and each type of ’scope has its own set of problems.  See the individual pages for each ’scope.