Scott Hesser
Prof. Chen
May 1999
Six Inch Rich Field Dobsonian
Over the course of the past semester I constructed a 6 inch, rich field, Newtonian telescope with a Dobsonian Mount using several commercially made telescope components and a wooden frame and aluminum mount of my own design. Before I began construction I researched amateur made telescopes in books, web-sites, magazines, and product flyers. The mass of information available regarding telescopes and the construction of telescopes was surprising. Even more surprising was the number of contradicting opinions regarding telescopes and their construction. My impression on this subject is that there are only a few guidelines to follow and an infinite amount of paths to take when constructing a telescope. I regard my experience of making my own telescope as a way of familiarizing my self with the instrument as a whole, and not the process of making a telescope. For this reason I am dedicating a substantial part of this report to recapping the concepts I became familiar with when I was deciding what type of telescope to build. In this discussion it is helpful to keep in mind that a telescope serves two main purposes. The first is to collect more light than you can with your own eye. In this manner, it acts like a light bucket. The second purpose is to offer more detail by making the objects appear larger which allows the observer to inspect them at higher resolution.
In the most broad sense, there are two types of optical telescopes, refractors and reflectors. The refractor uses a lens to refract, or bend, the light to a focal point. Most early telescopes were refracting, however, they have a number of inherent drawbacks. First, large refracting lenses are difficult to construct, and once built, their enormous size makes them difficult to mount. Lenses also have extremely long focal lengths, so a large lens requires an extremely long tube. Finally, lenses also suffer from chromatic aberration. This is a distortion in the image caused by an effect known as dispersion, where different wavelengths of light are refracted at different angles and therefore not all the light reaches the same focal point. Because of these faults most telescopes of apertures larger than 3 inches are reflecting telescopes. Reflecting telescopes are built with curved mirrors, usually in parabolic or spherical shape. These mirrors are more easily constructed at larger sizes and generally weigh far less. All large optical research telescopes, such as the Hubble Space Telescope, are now built with mirrors. The Hubble Space telescope has a 94.5 inch (2.4 meters) mirror on board. A lens of this size could never be put in space.
Mirrors do not suffer from chromatic aberration as lenses do. However, spherical and parabolic mirrors are not without their problems. Parabolic mirrors suffer from an effect called coma. Coma is a distortion in the shape of objects at the outer edges of the image. Stars at the outer edge will seem to have a teardrop shape. A number of people speculate that the name of this effect is somehow connected with the shape of a comet. Parabolic mirrors with shorter focal lengths are more concave and therefore have greater coma. Spherical mirrors do not have coma, but do suffer from a more serious aberration caused by different lengths from the mirror's surface to the focal point. Spherical aberration, as it is known can severely distort an image, but fortunately there are ways of correcting it. If a correcting lens is placed before the mirror, the light can be shifted to compensate for the aberration. The 12 inch Meade telescopes that are on Wheaton’s observation deck have spherical mirrors with correcting lenses in front of them.
My choice to use a parabolic mirror for my primary optic was not that difficult. A mirror is far cheaper and easier to deal with than a lens, and a correcting lens for a spherical mirror would be too costly. The next logical decision is what size should the mirror be. In terms of optical advantages from one size to another, there is only one rule to know, bigger is better. A common mistake is to judge a telescope by its magnification. This misconception also falls upon the myth that higher magnification is better, I will address this later. In truth, a telescope should be measured by its light gathering ability. Celestial objects such as the stars, planets, and the moon are so far away, that the light rays that reach the earth from them are traveling in parallel. You can imagine light traveling in such a manner as rain falling from the sky. If you wanted to collect as much rain as possible, you would use as wide of a bucket as possible. The same holds true for telescopes, the bigger the mirror, the more light that can be gathered (see Diagram 1).
What does more light mean for an observer? More light makes it possible to see dimmer objects. The larger the mirror and the more light that can be captured, the more quality one can see in an object. Light gathering power is increased from one mirror to another in proportion to their surface areas, therefore, it is proportional to the square of their diameters. For example, lets compare Wheaton’s 12inch telescopes to the 96 Hubble Space Telescope. The ratio of the square of their diameters is as follows: 95^2/12^2=9025/144=63. This means that the Hubble Space Telescope can collect 63 times as much light as a 12 inch telescope can. However, comparing a telescope to another does not get at the true purpose of any telescope, which is to collect more light than the eye can. For this we should use the human eye as the standard of comparison. The average pupil is roughly .25 inches in diameter at night. Comparing ratios for a 6 inch mirror we find that it collects 576 times more light than the human eye. This also means that the mirror can help see objects that are 576 times more dim, this translates to a lot more objects in the sky.
Larger mirrors and more light also aid the second purpose of a telescope. Resolution is increases proportional to the amount of light gathered, and therefore proportional to the diameter of the mirror. Resolution is the ability to distinguish two objects that are small and close together. The closer and the smaller the objects that can be resolved, the better the resolution (see Diagram 2). The theoretical resolution of any size mirror can be determined by what is known as the Rayleigh Criterion. However, this theoretical value is of relatively little importance, specially for a small telescope. The atmosphere greatly decreases resolution, and because the atmosphere changes, there is no way of telling the exact resolution from location to location or night to night. The important thing to remember is that resolution is proportional to diameter. Therefore, a 12 inch telescope will have twice the resolution power of a six inch telescope. If a person wanted to find out the resolution of their telescope, it is perhaps best to do so experimentally by observing different celestial objects of known angular size.
Of course a large telescope would be nice, unfortunately I am bounded by a budget and space to put a scope, but more importantly a budget. The financial factor immediately limited my mirror size to 6 inches. However, there is no shame to this size. A 6 inch telescope is sufficient for observing most Messier objects such as the Ring Nebula, and is capable of resolving a good number of binary star systems.
The next major decision to be made was the focal length of the mirror. Focal length is the distance between the mirror and the image, or point where the light converges. This measurement is important because it directly affects the image produced by a telescope. When describing a mirror for a telescope, whether it be spherical of parabolic, one refers to its diameter and focal ratio, or F number, F/#. The focal ratio, or F/# is the ratio between the focal length and the diameter. For example a 12 inch F/10 mirror, such as the ones the school owns, have a diameter of 12 inches and a focal length of 120 inches. We can see where this number 10 came from if we examine the ratio of focal length to diameter, 120/12=10. Mirrors can be made with any focal length, however, most commercial optics companies have standardized F/#’s that they sell. The most common F numbers for 6 to 16 inch mirrors are F/5, F/6, F/8, F/10, and F/12. Mirrors above 16 inches tend to be custom ground and therefore have a bit more flexibility. I eventually decided on a 6 inch F/5 mirror, but only after some deliberation about a 6 inch F/6 mirror.
As stated before the focal length directly affects the image produced. To illustrate this I will run through the comparison I made of an 6 inch F/5 and F/6 mirror. However, before I do this, I should briefly introduce the role of the eyepiece of a telescope. The main mirror of the telescope is responsible for collecting light, however, the image produced by the mirror is difficult to look at. An eyepiece is used to steady and the main image and further refine this image to make viewing more pleasant. The eyepiece also gives the telescope its flexibility because it is relatively easy to change eyepieces. An eyepiece, in its simplest form, is a lens that further condenses the light rays to produce a smaller image. Unfortunately, today’s eyepieces are complex combinations of lenses. Analysis of these eyepieces requires a comprehensive understanding of optics. Fortunately only a few measurements of the eyepiece are required to calculate the general appearance of the image. These measurements are the focal length of the eyepiece and the apparent field of view, or field stop.
Before we compare these F/5 and F/6 telescopes, lets recap the measurements we have to work with. The mirror is defined by its diameter and focal length. The eyepiece is defined by its focal length and apparent field of view, or field stop. The eyepiece that I will be using has a 25mm focal length and a 70 degree field of view. The focal length is a common size, however, the field stop is unusually wide. I will use these measurements for the two telescopes.
Perhaps the first thing that is considered when examining the image is the magnification of the image. The degree of magnification is the apparent enlargement of the image and is often referred to as power. Telescopes with higher power make it easier to examine the image and show more contrast between light and dark regions. This makes high power telescopes ideal for planetary viewing. However, increasing the magnification reduces the telescope's field of view and reduces the amount of light per surface unit. However, this makes high power problematic when observing deep space objects such as star clusters and nebula, which is my intended use. The magnification can be calculated by objective focal length / eyepiece focal length. The magnification for the F/6 telescope is calculated as (36 inches ´ 25.4mm/in) / 25mm = 36.6´. The magnification for the F/5 is (30 inches ´ 25.4mm/in) / 25mm = 30.5´. We can see that the F/5 telescope has lower magnification and has a slight advantage for deep space viewing.
The next important measurement of the image, which I just mentioned, is the field of view. The field of view, or true field, is the angular size of the sky that you can observe in the image. This should not be mistaken with the apparent field of view for the eyepiece, even though the two are related. The true field, in degrees, is calculated as the eyepiece field stop / objective focal length. A wide field of view makes it easier to find objects in the sky. This is helpful if you do not have an automated mount and drive system. A wider field of view also makes it easier to keep the telescope centered on the object during observing. The F/6 telescope’s true field is calculated as 70 degrees / 36 inches = 1.95 degrees. The F/5 telescope’s true field is 70 degrees / 30 inches = 2.34 degrees. Both of these fields are respectable, however, the F/5 field of view is 20% larger. This 20% can make a big difference when looking at an open star cluster for example.
The final characteristic to be considered is the exit pupil. This is the size of the image that leaves the eyepiece. The important thing to know about exit pupil is that the average size of the human pupil is 7mm in the dark. Why does this matter? If the exit pupil is bigger than the observer’s pupil, he or she is losing a portion of the light entering the telescope. Since the goal is to collect as much light as possible, an overly large exit pupil works against the large aperture. The overly large image also makes it difficult to keep the image centered, and the image will appear to move around as if you were observing from a moving boat. However, in the same manner, an overly small exit pupil is not good either because it is too difficult for an observer to see. In this case the size of the exit pupil is working against the objective of increased resolution. A good comfortable exit pupil is around 3-5mm. The exit pupil is calculated as eyepiece focal length / F/#. For the F/6 telescope, the exit pupil is 25mm / 6 = 4.2mm. The F/5 telescope is 25mm / 5 = 5.0mm. Both of these are quite satisfactory.
Although these calculations helped me determine which mirror to buy, the 6 inch F5 and F6 telescopes both work well for deep space viewing. However, the F/5 mirror falls under the category of rich field telescopes. By definition a rich field telescope provides a wide field of view and a good amount of light per image surface area. They tend to have focal ratios of F/3 to F/5, but some people will include F/6. These telescopes are good for deep space viewing and photograph. In order to see a true change in properties, one must look at a truly different F/#. For example, an F/10 scope with the same diameter and eyepiece would have a magnification of 61´ and a field of view of 1.15 degrees. This is a telescope more suited to planetary observing. Planets are relatively small and bright objects and require a great deal of contrast to see the detail.
Once I had decided upon what type of telescope I was going to build and exactly what its properties would be, it was time to actually design the telescope. As an amateur with a low budget building my first telescope, I am for the most part locked into building a Newtonian telescope with a dobsonian mount. A Newtonian telescope is perhaps the simplest type of reflecting design. It consists of the primary mirror, which is my 6 inch parabolic, and an elliptical secondary mirror (see Diagram 3). The secondary mirror is placed on axis in front of the primary somewhere before the focal point. The Dobsonian mount is a relatively recent innovation which uses Teflon pads as bearings for an altitude azimuth mount. This style mount is extremely cost efficient and I will go into it in some detail later.
The most difficult part of optical design of a Newtonian is determining the size of the secondary mirror. Because the secondary mirror is in front of the primary, it will block out some of its light. Therefore it is beneficial to keep the size of the secondary down. However, the secondary mirror has an elliptical shape and is responsible for catching the light reflected from the primary and directing it to the eyepiece. If the secondary is not big enough, it will miss some of this light. This causes an effect known as vignetting. Vignetting is the dimming of the edge of the image caused by lost light. As a rule, one should always expect some degree of vignetting. This means that we want to find a good compromise between the amount of aperture blocked and the amount of vigneting.
The best way to do this is to keep the distance between the secondary mirror and the primary as large as possible. For this discussion it will be helpful to refer to Diagram 3 above and Diagram 4 below. Because the light from the primary is being converged, the light path forms a cone, and the farther toward the tip of this cone, the smaller area is required to catch all the light. However, it must be kept in mind that the secondary has to reflect the light so that the tip of the cone, which is the focal point, up to the eyepiece. This means the farther the secondary is away from the primary, the closer the eyepiece will be to the secondary. The focal length has to equal the distance from the primary to the secondary and the secondary to the primary. Therefore, in order to keep the secondary as small as possible, we must keep the eyepiece as near as possible. However, there is a limit on this distance. If this distance is less than the radius of the primary, 3 inches, the eyepiece will block light. The eyepiece has to be even further to allow for movement when focusing. Also, the focal plane on most eyepieces is not at the end of the eyepiece, but some where in the middle. All of this leaves us with an average distance of 6 inches for most eyepieces. I used this length to calculate the size of my secondary. I did however, add a feature to my telescope that will allow me to adjust that distance, but I will discuss that later.
The formula I used is given below and makes reference to the diagram above,
where a is the size of the secondary, l is the length from the primary to the secondary, D is the size of the primary, d is the size of the fully illuminated focal plane desired, and f is the focal length. The desired focal plane refers to the diameter of the image you wish to project onto the eyepiece. Every eyepiece can handle a different size focal plane, however, .25 inches seems to be the convention. Using this measurement, the distance from secondary to the eyepiece as 6 inches, a 6 inch primary, and a 30 inch focal length, the formula calculates a secondary 1.4 inches long on the major axis. Unfortunately, elliptical mirrors come in certain standard sizes. The nearest sizes to 1.4 inches are 1.3 inches or 1.52. I decided to go with the larger secondary. I believe losing part of the reflection would cost more light than that which a larger secondary would block out
Once the optics are decided upon, you need to support them. It is possible to make holders for the optics on your own, however, a number of companies sell them also. A device that holds the primary mirror is referred to as the primary cell. This is actually a very important part of the telescope and deserves some thought before deciding upon one. The cell should not only support the mirror, but it should allow for easy access for when the mirror must be cleaned, easy adjustment for collimation, and proper ventilation for quick cooling of the mirror. If a mirror is not at ambient temperature it will generate thermal radiation that disrupts the air in front of the mirror that distorts the image. Therefore, it is important that a cell allow for good ventilation, and not retain heat itself. This was the problem I had with a cell I ordered from Orion Telescopes. It was constructed with a bulky particle board and had an overall shoddy appearance to it. I sent this back and ordered a cast aluminum cell from University Optics. I also ordered a four vein spider from this company. The spider is the part that holds the secondary mirror. Four veins refers to the number of arms which attach to the outside frame or tube. A three vein spider is the alternative.
Another part that you can either make, or purchase is a focuser. The focuser, holds the eyepiece and allows for small and fairly precise adjustment. There is a large variety and a large price range for this unit. There is everything from the electrical and motorized focusers to the most simple helical focuser. The only real guideline for this part, aside from price, is that it should be a low profile focuser. Low profile refers to the position the eyepiece has relative to the unit as a whole. Remember that it is important the eyepiece be kept close to the secondary in order to keep the size of the secondary down, therefore we want the eyepiece to sit low in the focuser, hence, low profile. Focusers, like eyepieces come in two dimensions, 2 inches or 1.25 inches. I decided to go with a 2 inch focuser so I could fit a higher quality eyepiece. The style I chose was a helical focuser. This is essentially a cylinder in which the eyepiece fits that is threaded on the outside. The threaded part screws into the housing. To adjust it you simply turn the housing. I chose this style for its low weight and low price.
The next step is to construct a frame that will hold all of these parts. The traditional way is to use a tube of some sort, however, the parts can be mounted in any fashion seen fit. For example most larger scopes use a truss system to avoid the weight of a large tube. However, if one does decide to use a tube, there are a number of inexpensive materials that will suffice. Those looking to build the most cost effective scope often use sono tubing. This is a composite cardboard like tube used for pouring foundations. It is cheap and easy to work with. However, I wanted a scope that would last a long time and age nicely over the years. I felt a nice hard wood frame would have an old fashion quality and I was sure that it would last a long time. I am also fairly experienced at wood working and I had the opportunity to have an experienced carpenter assist me.
The construction of the tube is more a subject of carpentry than it is of telescopes so I will omit the woodworking details. I can say a bit about the design however. The original shape was an exercise in geometry. I had to find a shape that had boards that ran perpendicular to the veins of the spider and the bolts of the cell. This would be easy if the cell and the spider had the same number of connectors. The cell however had three bolts with which to connect it and the spider had four. I also had to have a large enough board to hold the focuser. To accommodate all of this I designed a symmetrical shape with eight sides of different lengths. When I cut the holes to hold the primary, I made them slits so that I could change the length between the primary and the secondary and therefore the distance between the secondary and the eyepiece as well. I did this in case I ever wanted to use a different eyepiece or attach a camera that sat differently in the focuser. I started with a distance of 24 inches from the primary to the secondary, this was the focal length minus the 6 inches up to the eyepiece. I then made some room forward and back. I also painted the inside of the tube black to keep down glare.
Once the tube was finished, the next step is to build the mount. The mount always comes last because it requires that the center of mass of the telescope is known. As mentioned before, the mount I will be using is a Dobsonian mount. This is an altitude azimuth mount. It will allow the telescope to move left and right and up and down. This is great for hopping around from one part of the sky to the other, but the manual tracking makes it less than ideal for long photo exposures. The heart of the dobsonian mount is its Teflon bearings. Teflon allows the mount to have friction in the bearings so that a simple push will not send the telescope spinning wildly. A common mistake is for people to make a mount with ball bearings or something else with low friction. This makes it difficult to keep the scope steadied on one part of the sky. Friction is needed because you want the scope to stop moving once you stop pushing it. Teflon is great for this because it provides friction, yet it has an extremely low coefficient of static friction. This allows for nearly instantaneous motion as soon as force is applied.
The altitude bearings consist of circular pieces of wood attached to both sides of the telescope. This circular piece is lined with a kitchen counter top laminate. The laminate happens to provide the perfect amount of friction. Some telescope builders recommend a few certain styles of laminate. Mine happens to be “ebony star” by Willamart. These circular pieces are positioned at the scope’s center of mass. I cut adjusting slits for these also for in case I changed eyepieces. The piece sit on two pads of Teflon which are supported at 45 degrees to the horizontal. An easy push allows the telescope to move up and down without any kind of backlash. The supports for the altitude bearings run down to a circular base. This circular base is pivoted with a half inch bolt to another base board which has legs that stand on the ground. On the bottom of the upper base is a sheet of laminate. This sits on the Teflon pads which are connected to the lower base. This is the azimuth bearing. For my mount I designed an aluminum yoke that would support the altitude bearings and would act as the upper portion of the azimuth mount. A family member who owns a sheet metal fabrication company was kind enough to cut and bend the metal for me. I then pivoted this yoke on a knee high stool to give the telescope a good height for me to observe while standing. I am slightly unhappy with the aesthetics of the mount and I do believe I will build another in the future.
Once the bearings are working and the mirror and eyepiece are all together, it is time to collimate the scope. Collimating the scope refers to the process of aligning all the mirrors on axis. If the mirrors are not collimated, stars will look slightly distorted, as if they have tails. There are a number of devices used for collimating, such as certain eyepieces and lasers. These devices are almost a necessity for large dobsonian telescopes. A number of articles have also been written on collimating, and each one seems to say a different thing. The technique I prefer is to look at a star in the sky and bring the image out of focus. This allows you to see a donut shaped object. The dark center is caused by the secondary. Adjusting the mirrors till the donut seems perfectly round and with a centered hole will bring the telescope into collimation (see Diagram 5).
Once the scope was collimated, it was time for a test run. I was quite pleased with the initial results. On the first night I was able to observe Venus, Mars, M13 (a globular star cluster in the Hercules constellation), and a number of passing satellites. As expected, Venus and Mars were nothing more than bright points of light. However, the quality of the image of M13 was quite surprising, especially considering the amount of light pollution were I was observing. I believe once I can observe from a darker place I should be able to see some of the nearby galaxies such as the Andramada galaxy. Yet, the most pleasantest of surprises was the ease at which I could track a satellite moving swiftly across the sky. This is testament to the ability of the Dobsonian mount. The only draw back I found to the scope was a heavy degree of coma. This is expected of a shorter focal length, however, I believe the problem is more in the eyepiece. I had the opportunity to replace my University Optics eyepiece with a 20mm Type II Nagler eyepiece. This Nagler is perhaps the finest eyepiece made, and its price says so. Using this eyepiece the amount of coma was greatly reduced. However, at $390, its purchase is currently out of my league. For now I am happy to say that I am quite satisfied with my telescope.
.