Mike Moffat of the Tulsa, OK, area has produced a beautiful 6" f/8 wooden Dobsonian telescope and after my suggestion that he write about his project for our website, he sent me this story. I applaude his efforts and hope that others will take the time to submit their stories, too, especially if you have built as beautiful a telescope as Mike has. We would encourage anyone who has a story about astronomy, or telescopes, or observing, anything having to do with amateur astronomy, to contribute your story to this website. We'll edit it, and we only restrict profanities and "wild ideas" which can not be substantiated by the scientific method.
|Tulsan Mike Moffatt built this beautiful Coopered tube 6" f/8 Dobsonian telescope during his free time on weekends and evenings. Read his story about this project and then try to decide whether you might want to start your own similar project.|
• Am I glad I did? Absolutely!
• Will I build another? Already planning it.
• How does it work?
My pal Shane who has been doing this for 20 years says it rocks for a 6" scope. On two occasions I have resolved A through F in the Trapezium in M42 under my suburban skies. F was not solid but popping in and out as the seeing cleared momentarily as it often will. I am currently chasing after my Messier certificate, 43 captured so far, DSOs seem to have captivated my interest. Jupiter and Saturn of course look outstanding in this scope and the Mars transition this year was astonishing for this newbie.
I am a 44-year-old father of 4, grandfather of 2 that until October 2004 never thought much about the night sky. My daughter Sam developed an interest in the night sky and received a department store 70mm Bushnell refractor and all the frustration that went with it.
I didn’t know it at the time but a co-worker has been an amateur astronomer and ATM for more than 20 years. Shane brought a newly created 8” Newtonian to work one day for me to check out. Not only was I surprised to learn he was an astronomer I was blown away that someone could make a telescope. He pointed it at the moon that morning and I was captivated both by the view as well as by the workmanship required to make a telescope.
I called my wife later that day and asked why Sam didn’t use her scope any more (I had noticed she hadn’t taken it out in a while). She told me that it was too difficult to use and she had gotten discouraged and resorted to just laying in the yard and looking up.
An ATM project was born, but little did I know that it would be me that was bitten by the bug and she would move on to concentrate on her music and writing.
Click read more for the rest of this story and pictures detailing Mike's entire project
Planning the project
I spent several weeks visiting with my now avid observing partner Shane about astronomy in general and the strengths and weaknesses of different telescope types. His initial suggestion that a 6” f8 Newtonian on a Dobsonian mount to be the perfect beginner’s telescope has turned out to be absolutely accurate. It is a great all around combination with which I’ve split multiple stars, observed all manner of deep sky objects and is an excellent scope for the planets. I also enjoyed tracking comet Macholz for over a month shortly after its’ discovery.
At the time of planning the telescope I figured it was for my daughter and although I would “look through it” it was intended to be a keep sake worthy of a sitting room for her to keep for life. Quality woodwork was a must so a sonotube OTA was never considered. Shane had built a coopered tube with the instructions from Chuck Fellows on his web site and I decided to do the same. Newt for Windows by Dale A. Keller was used to calculate all the physical dimensions required for the telescope. The 6” primary mirror was purchased from Orion and Shane donated a secondary as well as the focuser, both also from Orion for the project.
Building the Mirror Cell and Tailgate
My buddy Shane told me the basics of a mirror cell and tailgate:
• Tailgate is same diameter of the inside of the tube and mounts in three places to the tube with screws.
• It must provide adjustment for the mirror cell, which is mounted on through bolts with springs between the tailgate and the cell.
• Must allow plenty of airflow for temperature stability of the primary mirror.
I cut a circle 7 7/16" in diameter for the tailgate, laid out and cut the "bio-hazard" symbol to provide airflow. Pretty simple to lay out, locate the center and the first line to the outer edge is your choice, with all other lines at 60 degrees from each other. Don't remember, but believe the center diameter left uncut was 1 1/2". All edges were routed with a 5/16" rounding bit.
#10-24 brass threaded inserts were used on each of the three legs for mounting. I installed the tailgate in it's location, 1" in from the end of the tube, and located the legs centered on a flat of the coopered tube and then drilled pilot holes through the tube into the tailgate to locate the holes. Dimension in from the end of the tube for the holes will be the distance from the end of tube to the back face of the tailgate plus half the thickness of the material used to make the tailgate. Devise a method of marking one leg of the tailgate to a location on the tube to make assembly easier in the event that your holes are not perfectly 120 degrees from each other.
The mirror cell was cut from 3/4" material as well, 6" in diameter to match my primary mirror. The same layout, 60 degrees apart like on the tailgate was laid out and then a circle drawn 1" in from the edge to provide additional strength to this piece as it is not mounted on the edges. See the photos. Inside triangles were cut with a jigsaw leaving a 1" support rim around the outside edge. Next I drew a circle with a compass 70% of the diameter on the tailgate to locate the adjustment screws. The holes were located 120 degrees apart, centered in each of the triangles. The mirror cell was centered and clamped to the tailgate and the cell was used as a template to drill the holes for the adjustment bolts to slide through. I then counter sunk the holes in the cell and epoxied brass flat head 5/6" x 18 machine screws in place. Assemble the cell to the tailgate to hold bolts square while epoxy is drying.
Lowe’s had a combo package of various springs, one size of which was conical in shape, which allows the springs to compress smoothly, and more completely when installed.
The tailgate and cell were painted black of course. NOTE: The first time I mounted my mirror to the cell it fell off because the adhesive for the rubber pads I used did not stick to the paint. I subsequently sanded off the paint in the mount locations on the cell and re-attached the pads.
The mirror was mounted using round adhesive backed clear rubber pads purchased at Lowe's on a circle 70% of the diameter. Be sure and sand the paint underneath the pad mount locations so adhesive attaches to bare wood. If you have been paying attention you are probably thinking the pads are adhered directly over the bolts. That probably would have worked fine but I rotated the layout slightly so they were adhered to the wood and not the epoxy of the screws. The factory adhesive was removed from the pads and they were epoxied in place. The mirror was mounted to the pads using a drop of 100% pure silicone adhesive on each pad. A strip of duct tape was cut and attached loosely but around the mirror and the cell for a safety catch in case it fell off again.
Building the Secondary Holder and Spider
I wanted to make the secondary mirror holder out of aluminum but could not find any 1 1/4" round bar stock in town for less than the minimum cut fee of $175.00. After spending 4 hours one Friday driving to all of the metal yards in Tulsa and not finding any scrap I decided to make it out of a hardwood dowel.
Learned a big lesson on this one. While it is ok to make the holder itself out of hardwood, DO NOT make the support/adjustment wafer out of wood! I used bicycle spokes, an idea from my buddy Shane, for the spider and while tightening them in the OTA the wooden wafer they clip into broke. Shane had a brass disk 1/4" thick in his toolbox and we remade that part of the mount.
I cut a 1 1/4" hardwood dowel 1 1/2" long for the mirror mount and cut a 45 degree angle on one end. The photo should be self explanatory as to the construction of this part.
The spider mount is made from a 1/4" thick brass disk. Three holes are drilled 120 degrees apart and tapped 6-32 for the adjustment screws. Three more holes were drilled 60 degrees off of these also 120 degrees apart for the spokes to clip into. I purchased black anodized spokes from a local bicycle shop complete with nuts. The standard nuts used in a bike rim work very well to attach the spokes into the tube. Simply calculate the distance from the spoke mount holes to the outside of the tube with the wafer centered and cut spoke lengths sufficient to bend a 90 degree end and clip it into the wafer. I calculated spoke length to come out flush with the outside edge of the tube when inserted in the holes. This provides about 1/4" of adjustment in either direction to center the secondary in the OTA. Close tolerance on the drill size will keep the spokes aligned straight. Be sure and mount the spokes pointing out the end of the tube to prevent the secondary holder from falling onto your primary mirror if the spokes are loosened.
Remember, these pictures show a wooden spider mount but in fact I used brass. Aluminum would be better because it is lighter.
There is one very important consideration for the mounting of the secondary mirror and the spider. I am by no means an expert, if not for Shane I would not have gotten this. When attaching the secondary mirror to the holder with silicone it can be put on two ways, one of which blocks more light than the other. It is difficult to explain but if you will look at the back of your secondary at a 45 deg. angle as if it was installed, one way you will see the edges of the mirror which would block light needlessly and the other way you will not.
When it's time to drill the holes for the spider and mount the secondary holder you must have already calculated the location of the focuser hole in relation to the primary mirror surface to obtain the correct focal length. I used the Newt software program for this. I made a mistake and measured to the center of the wooden secondary mirror holder to calculate where the spider holes would go.Arrggh.... my secondary mirror was too deep in the tube to align with the focuser because of the thickness of the secondary mirror and its' mount position on the holder. You must measure from the center of the face of the mirror to the spiders to determine how far up from the center of the focuser hole to drill for your spiders. I had to re-drill and now have extra holes in my OTA.
Building the Rocker Box
The rocker box is made of laminated knotty pine, chosen to follow the laminated theme set by the coopered tube. My daughter chose the direction of the lamination grain in relation to the tube. The front and rear panels were cut so that top panels would sandwich the split rings providing support for the portion that clamps to the tube. Brass piano hinge was used on the bottom and brass latches on the top. The box was assembled to a cube using 90 degree corner clamps, glue and brass screws flushed. After assembly the box was sanded on all sides until seams were all flush. The tube was used to draw the circles for the holes and then cut tight with a jigsaw. The holes were then sanded smooth with a drum sander, which opened them up until the tube just slid through. At this point I marked the box and cut it on the table saw, leaving a small portion uncut on either side of both holes. The hinges and latches were mounted and the cuts completed with a jig saw. This made for a perfect seam gap when closed up.
Keep in mind the cut when the screws are installed. I placed screws in the center of the top boards and wound up having to cut my box slightly off center to miss them but turned out to not be noticeable. All outside edges were rounded with a router prior to finishing.
I looked at lots of methods for making the elevation bearings, even bought some Teflon pads for this and then had an idea. PVC pipe and felt. 3" PVC pipe was cut 1 1/2" long on the radial arm saw and installed sandwiched between the rocker box and the 8" wheels cut out of 3/4" oak plywood.
To build the wheels I drew the circle with a compass, cut them out with a jigsaw and then sanded to the line on a disk sander. Not just decorative, these wheels sandwich the PVC pipe elevation bearings. They are attached with a single 5/16" x 2 1/2" oval head counter sunk machine screw through the center of the wheels. I used a nylon lock nut on the inside of the box to keep them from backing out.
Building the Dobsonian Base