Building my 12.5" Dobsonian
This article was published in the summer 2000 issue (issue 26) of Amateur Astronomy and was written by Gil Namur.
There are two reasons I have written this article. One is because as I did research to build my telescope, I received so much help from other ATMers who had taken the time to write articles and web pages. I told myself that the least I can do is to make a small contribution by sharing some of what I learned along my journey. By the way, this scope NEVER would have been built if not for reading Amateur Astronomy.
The second reason is that many people have not built a telescope for fear that it is beyond their abilities. Well, let the reader understand that I am NOT a craftsman. Far from it. I am a computer marketing professional and a musician. I have a few tools and have tinkered with things. Anyone who takes the time to properly research and plan a project and then thinks through each step can do this (or just about anything for that matter.) Take your time. After a few months of reading and planning, it took me about 2 months to finish this project.
As a child, I have had a fascination with the heavens but I did not get my first telescope till I was about 14. It was a department store special that was, in a word, awful. I managed the odd look at the moon and once saw Saturn dance around the eye-piece and that was about it. It could not be aimed and the tripod was wobbly and shaky. Years later, at 34, I purchased a Bushnell 565 60mm refractor that was "ok". I tinkered with it and the tripod and purchased a 1.25" eyepiece adapter after which it was far more useable. Believe it or not, I was able to see the smudges on Jupiter's surface after Shoemaker-Levy 9 slammed into it between July 16-22 1994. It was really exciting as for 3 days, the seeing was good and I had folks of all ages from up and down my street come to see Jupiter. Even though the view was not the greatest, it impressed all who looked and it was a real thrill to share my passion with others. At that time, my daughter was about 6 and her interest in the stars was born. She is now my astronomy partner and will spend hours with me looking at the heavens (she likes to chase down galaxies). A year later, I purchased a Sky Instruments Antares 515G 6" Newtonian which performs beautifully. I did rebuild the mirror cell in this telescope as it would not keep perfect collimation.
As my interests have always been more deep sky oriented, I decided it was time to build a larger aperture telescope. It came down to a choice between an equatorial mounted Newtonian or a Dobsonian. I had never used a Dob before but had read so much about their ease of use that I decided to take a chance and build one. I was NOT disappointed. The end result is so easy to use.
I toyed with the idea of building a truss tube scope but decided that for my first attempt, I would go with the classic Dob design as there is a ton of information available on building these. My design is largely based on the 10" plan detailed in Making Your Own Telescope by Richard Berry. I highly recommend this book. My son (15) helped me with the construction of this telescope and I treasure his company and involvement in the project. Every father and son should do something like this.
I decided on 12.5" as a compromise between aperture fever and having a telescope that would be easy enough to move thus get a lot of use. I also decided on an F5 as it allows me to keep both feet on the ground. I am NOT a huge fan of ladders. While my 12.5" is easy to get out and set up, it really does require 2 people to move the tube. It's not THAT heavy (about 60 lbs.) but it is awkward and I don't want to drop it.
I am not going to go into to much detail on my plan as most of it can be gleaned from Making Your Own Telescope by Richard Berry. However, there are a few points one should consider when building a telescope or anything for that matter.
Thoroughly consider your requirements. In my case, they were as follows:
Portability vs. Aperture:
I wanted to see lots of deep sky objects but I wanted a telescope that would get used a lot. Those larger apertures (16 and up) sound great BUT, will it fit your vehicle, can you physically handle it, where will you store it and can you afford the price of the optics?
Ease of use:
I wanted a telescope that would be easy to set up and easy to use. This telescope literally sets up in about 2 minutes including carrying it outside. It also loads into my van in about 1 minute. With the Telrad mounted, it is a joy to use. My poor 6" might start gathering dust ..(hmmm maybe I should Dob mount this one..yea that's it..that's the ticket).
I wanted a telescope that I would not be neurotic about using and that could stand the inevitable nicks and bumps it will get over its lifetime. If anything, I over built this.
While in the final analysis how the telescope functions should be your primary concern, how it looks was important to me as I wanted something that I could take pride in. Use your imagination. It's not difficult to take a little extra time to end up with a good looking, functional instrument.
Think about any special requirements you may have at present and any upgrade path you may have in mind for your telescope. Do you need power for fans or anti dew systems? Are you ever going to try a computer guided system of some type? Or maybe digital setting circles?
As this telescope was going to have a fan on the mirror cell and a Dry Eye Anti-Dew system, I wanted to try and avoid having cables running up and down the tube.
By the way, if you have not seen or heard of Mike Pendelton's Dry Eye Anti-Dew system, you need to check it out at: http://reely.com/darren/Astro/DryEye/index.html (it really works great. Nice work Mike.)
My telescope features 3 RCA connectors. One on each end ring and one on the tube just slightly forward of one of the spider support holes. The tube is wired internally and this allows me to run power from the back end up to the Dry Eye control unit that is near the focuser. I still have a few wires on the tube but they are short in length, look pretty tidy and no one is going to trip on them.
So if this is your 1st scope, plan, plan, plan. I once was given a piece of wisdom. It goes like this: People don't plan to fail but they often fail to plan..think about your needs.
The Primary Mirror:
The primary mirror comes from Steve Swayze at Swayze Optical. It is a 12.5 F5 (61 5/8" focal length) and is 2.1" thick. It weighs about 22 lbs. I had Steve center dot it to facilitate collimation with my Cheshire eyepiece.
Steve was extremely helpful. My mirror is of superb quality. Thanks a ton Steve..
You can check Swayze Optical out at: http://www.europa.com/~swayze
The Primary Mirror Cell:
The primary mirror cell comes from University Optics. I made several modifications to it. I replaced the collimation bolts with longer bolts with a higher thread count. I re-placed the springs with lawn mower valve springs (my son's idea) and made some seats for these springs so they would stay well positioned. I used nylon washers under the collimation wing nuts. I did not use the tube-mounting ring supplied with the cell as I wanted the cell mounted to the back of the telescope. I replaced the rubber pads with 1-1/4" diameter felt pads. Finally, I made new mirror clips out of brass (which I painted black) as the ones supplied with the cell were not long enough to hold the 2.1" mirror. The cell seems to work very well, keeps its collimation pretty close and my optics are not pinched. However, I am going to build a whole new cell with 9 point floatation, no clips, no sling ala Ed Stewart's cell design. It will incorporate a variable speed fan circuit.
The Secondary Mirror:
The secondary mirror is a 2.1" Pyrex with Enhanced AL. It came via Tom at Island Eyepiece who also has been very helpful to me in this project. Tom runs a great internet based business at: http://www.islandeyepiece.com
The spider is an offset spider made in Vancouver BC at Gary's Astro Fabricating by Gary Wolansky. I had already purchased some of Gary's other products and was very impressed with his work. This spider is superb. Gary's craftsmanship is outstanding and he also was a great help to me. I also purchased his quick-change adapter..a SUPERB MUST HAVE acessory. Gary's products can bee seen at:
Be sure to check out The Gary Wolanski Projects while you are at that site. VERY inspirational. Especially his 40lb 16" f/5..wow.
The focuser is a JMI NGF-DX3 Crayford style focuser. Upon opening it, it was not the smooth focuser I had been told it would be. HOWEVER, I had also made my decision to purchase a JMI based on the reputation that JMI enjoys due to their great service. I called JMI and they looked after my problem promptly. I am happy to report that this is a superb focuser. Thanks JMI for the great service.
JMI's web site is at: http://www.jimsmobile.com
Building The Tube:
The tube is a 14" inside diameter cardboard tube with a ¼ " tube wall thickness. This is not standard Sono tube. Call your local lumber / hardware store and ask if they will order you some Sono-tube minus the plastic and wax. The manufacturers of Sonotubes do in fact make this product and some call it Display Tube. It is available in a variety of wall thicknesses. I chose ¼". The tube ended up costing me about $1.25 more per foot plus about $10.00 shipping. For the extra twenty dollars, I ended up with a lot less hassle than I would have had scraping off all that wax and peeling the plastic plus I have the knowledge that the glue and paint will for sure bond well to the cardboard.
It is covered with 2 wraps of flat cut beech veneer. Before applying the veneer, the inside of the tube was painted with 3 coats of flat black paint. A 4th coat was applied after all of the tube construction (installing end rings, drilling out holes etc) had been finished.
The paint I used is a product called Corrostop Super Rust Paint 631. It's about the flattest tough sealing type paint I could find. To apply it, I used a 4" sponge roller on a roller extension handle. It was easy and did a great job. I do plan on further blackening the inside of the tube but am undecided as to what material to use. For now, the contrast is pretty good.
This tube is VERY strong..albeit somewhat heavy. It looks great!!! The tube end rings were made by cutting circles with my router and a plunge bit. I first made a simple jig out of plywood for my router that was used to cut all of the circles in this project. Buy a high quality router bit. It will cost you a few dollars more but, it will do a nice clean job and it will last much longer. After all, once you build one telescope, you may get the bug and want to build a few more.
Each end ring is made of 2 pieces of plywood. The outside diameter of the first piece was calculated to be 1½" larger in diameter than the tubes outside diameter. (Be careful here when you plan this ring dimension remembering that the tube ring will have to swing into the rocker box.) The ring's inside diameter fits over the tube's outside diameter. This ring is made of ¾ inch plywood. The second ring was cut to the same outside diameter as the first ring but the inside diameter was calculated to match the tubes inside diameter plus 1/16". It is made of ¼" plywood.
The two pieces where then glued and clamped. They form a cap over the cardboard tube's edge sealing it and preventing it from being crunched and nicked.
Before gluing and screwing it to the tube, any holes for mirror cells, dust covers or power connectors you need should be drilled first. Then glue the ring to the tube and screw it from the inside of the tube GENTLY counter sinking your screws into the cardboard. You don't need big screws here. Just enough to clamp the tube and ring tightly while the glue sets. I used a high quality wood glue with a slow setting time. This gave me time to properly position the ring, screw it and remove excess glue.
Once the tube rings were attached to the tube, I gave the outside of the tube and rings 3 coats of Sico varnish. I used an indoor floor varnish with a drying time that allowed me to do 3 coats in 2 days.
My cradle is pretty standard stuff with a few exceptions. I wanted to store my telescope in an upright (vertical) position. After it was built and the tube was mounted, I found that even if I wedged the tube very tightly, it would gradually slip down the cradle, which of course messes up the balance point. To solve this, I placed an eye hook on each side of the cradle (inside) and made a nylon strap that connects to two turn-buckles so I can tighten the strap around another hook on the back of the mirror cell. By turning the turnbuckles, I can make minor adjustments in the balance point of the tube but only in one direction. I also put four rubber feet on the bottom of the cradle so that when I take the tube off the rocker, it can sit on these rubber feet preventing damage to the finish. Finally I plan to mount a piece of half round precisely across the middle of the bottom of the cradle so if I set it down on a 2X4 on the ground, I have a built-in rod to balance the scope. The first time I balanced my tube, I placed a dowel under the rocker to find the balance point. The procedure of balancing the telescope was a pain as I had to keep repositioning the dowel which also ended up scratching the bottom of the rocker.
Like the cradle, the rocker is standard issue. I used the AstroSystems Pivot Kit as it is precisely made to allow for a digital setting circle that I plan to use at a later date. I used hockey pucks for the feet. (I can't take credit for this one. I saw it in an article and I can't remember where. Great idea.) I used ebony star laminate and 1/8" Teflon that I cut into 1-1/2" by 1-1/2" squares placed directly over the feet on the ground board. I also placed one at the center of the pivot point. I waxed all the laminate with a silicone based car wax. The result is super smooth motion.
Each side of the rocker is stiffened on the outside with braces cut from rock maple stock and on the inside with a brace cut from spare plywood. The maple braces are glued and screwed while the plywood brace was glued and clamped. This makes for a very stiff side.
The front of the rocker has an eye-piece rack that holds five 1.25" and two 2" eyepieces. These are lined with felt to protect the eyepiece barrels. The felt also serves to hold the eyepieces in tightly.
The rocker dimensions were calculated to allow a 12 volt battery to be mounted on the inside of the rocker to power my fan and anti dew system.
The rocker and cradle were made from African mahogany marine grade ¾" plywood. The end rings and ground board are made from Honduras mahogany marine grade ¾" plywood.
Cutting the wood: If you don't have a table saw, do yourself a huge favor. Find a local cabinet shop, bring them the specific dimensions along with a drawing of your project and a cutting sheet. BE SURE to tell them that you need MATCHED pieces that are SQUARE. You will pay a few bucks for this but it will be money well spent.
Also, do not trust the stated thickness of plywood. For example, the African mahogany I used is listed as ¾" but is actually 11/16". This can really mess up your plans in a hurry if you cut before you measure.
Squaring up: BEFORE gluing and screwing any of the parts, be sure they are square. The more square you make your components, the better your telescope will work (and look).
Gluing: As you glue joint and end rings while building your project, have a few damp clothes on hand to IMMEDIATELY wipe off any excess glue that is forced out as you clamp your joints. This pays big dividends especially if you are going to varnish the parts. Any glue residue that is allowed to harden on the wood shows right through your finish. It only takes a few minutes to clean off the glue. I recommend that you clean off the excess with one cloth, then go over it again with another one to be sure you got it all.
Use good bits and practice drilling:
When you buy bits, try to buy the best ones you can afford. Cheap bits do not do as good a job as the higher quality ones. Have a few scraps of the material you are using on hand. When you are going to drill a hole, practice it first on a scrap to see how the drill will perform. Again, it only takes a minute to do this but it can save you work patching holes that are to big, or fixing gouges because the material chipped when you drilled it.
The skies opened up on Tuesday (Jan 18th, 2000) The seeing was average with moments of very good seeing. My daughter and I set up the scope that my son helped me build. We turn on the fan and anxiously wait about an hour before looking through the eyepiece. (Bear in mind here that the moon is super bright, and my eyes are nowhere near dark adjusted.)
Put in an 18mm eyepiece and pointed my Telrad at Jupiter. WOW..WOW WOW WOW WOW..as it floated across the field, you could CLEARLY see the banding..and every minute or so, the seeing would improve for a few seconds and the resolution was amazing. The time collimating is really going to pay off. Saturn was exquisite. Even as the seeing worsened, a division in its belt was clearly visible. Hey, are those its moons? My wife is very impressed.
Even with a glaring moon, the Orion Nebula is HUGE in this thing..I am a VERY happy guy.. :-) The double cluster in Perseus..gorgeous. Back to M1, the Pleiades, NGC 147, M31, M110, Jupiter, Saturn, M81, & M82 and a ton of doubles. Decided to look at the moon..HOLY COW..that sucker is big time bright in a scope this big. Actually hurts the head..where's my filters?
A few weeks later, another clear night with no Moon. My daughter and I stayed out till 1:45am. It was cold..brrrr..ice all over the telescope tube but the optics stayed dew free and dry.
My little astronomy partner saw over a dozen galaxies that night.
For more ( and bigger ) pictures...click here
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