Let’s build a large Lightweight Dobsonion Telescope
Part 1 of 4 by Lonnie Robinson LonRobie@surewest.net
John Dobson really started something with his Dobsonian “DOB” telescope design. He enabled the average Amateur Telescope Maker “ATM” to create a very large and very stable telescope with common workshop tools. Since then DOB’s have evolved from a boxy heavy structure to elegant lightweight designs with truly excellent optics. The DOB design performs best with the wide fields of lower magnifications, where nudging the scope for tracking is negligible. This design is very stable for high power viewing, but it requires a lot of nudging and motorized tracking, like Servo Cat, would be a great help. Building your own scope offers the opportunity to continually upgrade and customize over time. If there is one “must read” and “must own” book on building a DOB, it’s The Dobsonian Telescope by David Kriege & Richard Berry. This book will expertly guide you through every step of design, construction, and mirror making. I would like to share my personal thought process and my DOB building adventure with you, hopefully adding some additional fresh ideas to help inspire your new DOB telescope project.
WHAT SIZE IS BEST?
Let’s begin with what size DOB fits your needs. If you enjoy quickly setting up your scope at home for a short viewing session, then a six to ten inch size is very portable and is usually built with a one piece tube or trusses that can be left assembled for storage. Viewing conditions under city light pollution limits the effectiveness of larger scopes anyway. As you progress to higher and clearer viewing locations, the range of twelve to twenty four inches is doable. Bigger is better. My personal requirements involved the largest scope I could afford, one I could lift out of my car by myself, and one with a reasonable eyepiece height. It’s a lot of work to build your own scope, so I suggest choosing one large enough that will cure your aperture fever for many years. The DOB book says “my observing friends and I agree that we could live the rest of our lives observing with a 16” without the slightest regret”. For me 16” f5 was the maximum comfortable, usable, and affordable size.
DRAW YOUR DESIGN
Now that the issue of size is decided, let’s consider some basic design issues and make a rough pencil drawing of your new DOB. I spent a lot of time on the web looking at scope design. The ultra-light trend, in my opinion, borders being too fragile and too open to keep the optics properly aligned and protected. The heaver box-like designs seem too large for storage, transport, and easy setup. I really liked the style and graceful lines of the large altitude bearing ultra-lights, so I decided to design a middle weight with the best of both worlds. Some designs use only six trusses instead of the usual eight, but trusses should probably remain at eight with a medium weight cage and the longer length required for a light-weight design. Besides, six trusses make for a difficult mounting sequence at the base. My friend, George, asked a question, after I had pretty much finished roughing out my scope, “Will those longer poles fit in the trunk of your car?” I hadn’t considered that, and in a mild panic I was immediately motivated to check the fit. Yes, they fit! My upper cage design is only as tall as necessary, with enough area to block most stray light and protect the optics. Big altitude bearings add style and grace, allowing one to design a very short and strong mirror box and rocker box that are easy to carry and store. Large diameter altitude bearings have a higher surface speed, which enhances the Teflon’s buttery smoothness and speed regulation qualities. Stiction, resistance to initial movement, is nearly zero after applying silicone car wax. Designing the altitude and azimuth bearings nearly the same size equalizes the force required to move the scope in either direction. Larger bearing spacing and low to the ground rocker box make the scope exceptionally stable and a lot stronger. The bottom of the altitude bearing should not extend below the collimation bolts, preventing their use as feet. Don’t worry too much about designing in balance since it can easily be achieved later by changing the radius of the altitude bearings. Truss tube placement and truss connector styles are very difficult choices. Kriege & Berry like the trusses mounted on the outside of the mirror box for added stability, but that interferes with clean lines and the design possibilities. I’ll discuss the truss connectors in detail a bit later. I have a book on router techniques, and in it they described how to make decorative spindles with longitudinal decorative groves. This inspired my idea for the wooden hourglass shaped cage struts, handles, altitude bearing support, and the telescope name. To me, hourglass implies looking back in time with a glass optic. I carried the theme throughout the best I could. You can do the same with so many available wood turning design ideas. Here is my first rough drawing. I would be pleased if you should decide to copy it. Have fun and let yourself go with the design of your choice.
WHERE TO START
First, acquire all the optical parts: Main mirror, focuser, eyepieces, secondary mirror, and last the secondary spider. You will need them to establish focal length, fit, and balance. However, you could use dummy weights until the parts arrive.
Let’s talk about choosing the main mirror. Since we are building a large DOB you probably want brighter images for deep space objects, with low power wide field views. Image brightness is a sole function of aperture size and magnification. After choosing the biggest mirror you and your budget can handle, the first part of the image brightness equation is locked in. The second part, magnification, allows you to control how the available light is spread out. Higher magnification spreads the light over a larger area resulting in a dimmer image. Because the mirror size is now fixed, different focal lengths (F) change the magnification range and the resulting focal ratio (f). The focal length and f ratio can now directly relate to each other regarding magnification and image brightness. Focal ratio is usually used to describe your mirror, and lower f ratios provide shorter focal lengths, resulting in a lower usable magnification range, wider fields, and brighter images. Eyepiece height is a real issue and if the objective is very large (18” and up), a very low f ratio may be necessary to keep the viewing ladder short. Field of view is a basic function of magnification, but the eyepiece diameter, field stop, and field of view ultimately determine the actual true field of your telescope.
For all purpose viewing, f5 to f6 seems the best compromise. I decided on a 16” f5 for the best all around viewing without using coma correctors, great high power planetary views, low power wide field performance, and reasonable eyepiece height.
Planetary high power viewing is best accomplished with a smaller mirror (6” to 10”) with a long focal length. High f ratios of f6 to f15 and over, provide higher magnifications, long in-focus travel, narrower field of view, require a smaller secondary for less obstruction, and provide clear high contrast images. Longer-focal-length eyepieces can be used with their increased eye relief.
Should you decide to make your own mirror, f5 and higher presents the best chance of making a really good mirror. Between f4 and f5 the in-focus range is extra sensitive, and the steeply angled light cone requires highly accurate figuring. Coma is two times worse in f4 than f5! The higher the f ratio, the easier it is to figure. Polish and figure your mirror first so you can measure the final focal length to set the truss length, assemble the scope, and star test on Polaris before aluminizing. Polaris works because it moves very little, and the optical tube assembly can be propped up on the ground in a fixed position. You can get some really great views of bright objects without the aluminum! The total uncoated light grasp is about 4%, making an uncoated 16” about equal to an aluminized 3.2” (20% of diameter) but with a 16”s resolution. My 16” showed amazing detail on the moon without a filter, and Saturn was tack sharp. The DOB book divides mirror makers and telescope makers, saying it is difficult to do both. They even indicate if you make mirrors you may never observe again. I did both, and even though it took a lot of time, the reward has been awesome. Looking at a celestial object with your own mirror is very satisfying, and in many cases you can make a better mirror than a professional simply by spending more time to perfect it. However, there are many great commercial mirrors available and used by DOB telescope companies.
The focuser is a fun choice. I really like the 2” Feather Touch and Moonlight crayford units, with a nod to the Feather Touch. Moonlight offers some great colors. Feather Touch has taken the crayford design to the next level. They use dovetail like stainless steel rails to run the upper ball bearings on, resulting in a very durable, smooth, and tight system. They both have an excellent gear reduction knob for fine focus, and cant adjustment screws in the base for optically aligning the focuser. They both have motorized focus available. I highly recommend compression rings for holding eyepieces and equipment. The ring holds securely, centers, and doesn’t mar your eyepieces. They really help when using heavy eyepieces, bino viewers, cameras, and aligning with laser collimators. There is a complaint that the new recessed security groves on newer eyepieces catches on the compression ring, but I haven’t experienced a problem with it. Be sure and choose a fairly low profile focuser with at lease a 2” range of focus. If it is too tall, it will change the secondary mirror requirements. Focusers have come a long way in recent years relating to smoothness, accuracy, and toughness.
Secondary mirror size is a difficult choice. What you want overall is the smallest secondary you can get away with to catch all the available light from the main mirror, with a minimum obstruction, and adequate eyepiece illumination. The size and focal length of the eyepieces you want to use, and their field of view, will affect the required secondary size. Your largest, widest field, and lowest power eyepiece requires the largest illumination area, so use that eyepiece as a basis to decide the secondary size. High power 1-3/4” eyepieces can use a slightly smaller secondary. Eyepieces with very large fields of view, like 80 deg Naglers, require a slightly larger secondary mirror. Check out the DOB book for details on eyepiece illumination, but it should be between ½” and ¾”. Try to keep the secondary mirrors maximum obstruction to 19% or less on a linear basis. Lower than 19% achieves very little visual improvement, and higher gradually degrades the image resolution and contrast. Cassegrain obstruction can approach 34% linear, so you can see how much wiggle room there is! My experience is that smaller is not necessarily better, since a little extra obstruction is pretty much undetectable visually. There is a series of standard sizes available. I found my exact choice to be in-between standard sizes, so I simply opted for the next larger size making sure not to waste any precious light from the primary mirror. The larger size fully illuminates a 3/4” field for my favorite, 65 deg, 2”, low power, wide field Tele Vue eyepiece. Planetary views at 300X are amazing even with a 19% obstruction!
I have come to really appreciate the secondary spider from Proto Star. It has a nylon center mount bolt that stretches, applying constant spring like pressure on the collimation screws as you adjust the mirror. This allows each collimation screw to be adjusted independently, without affecting the other two adjustment screws. It also keeps things snugly locked in place after adjusting. I made my own version of Bob’s Knobs from three stainless steel 10-24 bolts with small knobs and loctite. Optical alignment literally takes seconds with a laser collimator, Proto Star spider, and these knobs. It couldn’t be easier! Well perhaps having the significant other set it up for you would help.
OK, we have completed Part 1 with our drawing and have acquired all the optical components.
Next we will discuss:
Part 2- Mirror Cell & Mirror Box – Upper Cage
Part 3- Truss Poles & Truss Connectors – Optical Tube Balance – Rocker Box – Ground Board &
Pivot Bolt – Powered Ground Board – Encoders
Part 4- Altitude Bearing Design & Installation – Wood Finishing – Final Assembly, Collimation &