A Beginners ATM Project – 6” Newtonian Reflector by Mike Moffatt
Thursday, December 22 2005 @ 10:01 PM CST
Contributed by: bobmoody
|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.|
My wife says I'm insane when I get an idea to do something and it shows in this project. 4 weeks start to finish (is anything ever really finished??). Wish I had actually counted the hours, but suffice it to say there were hundreds. I have three day weekends that were totally devoted to this project, spending approx. 18 hours each Fri. Sat. and Sunday and every day after work during the week from 6:00 PM to sometimes 3:00 AM. The primary and secondary mirrors as well as the focuser were purchased; all other parts were built in my shop.
• Would I have started it if I had known how much work? Probably not.
• 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.
Why build a coopered tube for the optical tube assembly? I looks cool and blew me away when I saw Shane's. To see some really outstanding scopes built this way by a real craftsman click on Chuck Fellows website link above. Follow the custom telescope link and check out his work. He has complete instructions on his site for calculating the tube.
NOTE: you must have already calculated what you need your tube diameter and length to be before beginning this step. I used Newt for Windows and a lot of help from Shane to figure this out.
You will quickly discover when playing with the math to figure the angles to cut on your slats that it is difficult to come up with a slat width that give exact angles to equal 360 degrees. I did not get perfect but the glue filled the gaps and the tube is very strong. HOWEVER, I did not sand the tube completely round because of feat that if the wood did not fit tight the full depth of the angle it would expose glue at the seams and glue does not accept stain. Turns out I really like the tube with its' slightly ribbed feel, it adds character.
I used oak and poplar from Lowe's for the 27 wood slats glued together to make the OTA. Purchased in 1/4" x 6" x 48" stock, I divided the width of the stock by the number of pieces I could get out of it and then cut it into separate strips with the first angle cut. Be sure and leave enough waste width on your cut strips to make your second pass putting the final angle on the second edge with enough drop to be able to control your piece while cutting. I chose to do it this way rather than reset my antique table saw for each cut. I since have purchased a new table saw with a very stable fence system.
Be sure and cut all of your base stock very carefully to the same length before you start ripping them. They vary in length from the lumberyard and as you can see in a later picture it will give you differing length ends on your tube. I sanded the edges relatively even with a drum sander using a combination square for reference and then covered them with rings on each end of the OTA.
The individual slats for the tube were laid angle side down on a very flat table inside an angle iron jig set to an exact 90 deg. angle. The slats were taped together edge to edge with masking tape, making sure the edges were tight to one another and angles facing down. The taped together slats are then turned over and the groves filled with polyurethane glue, rolled into a tube and clamped together. I found from the mess later it takes less glue to fill any errors in the angles that one would think. I at first had trouble clamping the tube in a perfect circle but discovered that moving the clamp screws so they were at different places around the circumference distributed the pressure and the oval problem went away.
Before leaving the glue to dry I used a plastic body putty knife to smooth the polyurethane glue in each end of the tube as far as I could reach. This turned out to be a very good idea as it made installation of the mirror cell and the spider much easier than it would have been otherwise.
I made light baffles using the Newt software to calculate and constructed them using the split foam pipe insulation purchased from Lowe’s, ripping them to size on my table saw. However, I am planning on removing the baffles after reading a few articles on tube currents. I think the baffles are actually causing the currents to flow into and out of the light path as they move up the OTA and would be less of a problem if allowed to flow straight up the tube walls.
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
The cross pieces, made from 3/4" oak plywood, are joined to the uprights with brass screws and finish washers. A very elegant look when it was finished.
The uprights were joined to the top round base with countersunk screws from underneath. I chose not to glue these pieces to the base in case the base needed to be changed for some reason.
Felt was added to the rocker portions of the uprights for the PVC to glide on. I tried it without the felt and found it to not be completely smooth. The felt made it like oiled glass.
I have gone though many gyrations of elevation bearings searching for a perfect solution. The first generation of elevation bearings was PVC pipe riding on felt in the cradle. This worked very well for a time but the felt began to take a “set” and/or as my observing skills grew I became more critical and the slight jump in the eyepiece when first moving the OTA in elevation became annoying. I have since cut bearing surfaces to replace the felt out of virgin Teflon pipe and installed them with counter sunk screws. Now the elevation is too smooth and requires no pressure at all to move which is itself annoying. Stability has also suffered due to this change. I am going to try sanding the surface of the PVC and filling a flat spot on top of the Teflon pipe quarters to hopefully add just a tad bit of friction.
Round felt pads were installed inside the uprights to provide protection when placing the rocker box on the base and more importantly to provide a smooth bearing surface when moving the scope in elevation. I found that azimuth slewing could move the box all the way to one side or the other causing binding. The felt pads did away with that problem completely.
I used angled leg bases from Lowe's, placed at 120 degrees around the base, and oak legs with corresponding angles cut to make the legs. I purchased the shortest legs I could find and then cut them to the shortest effective length to keep the scope as low as possible.
Be sure and screw the legs onto their bases and then mark them so the angle you grind, sand or cut is on the correct side of the legs. I marked each base with a number corresponding to its' leg so that when they are screwed in for the final time the angle is where it should be.
The tabletops were stacked with center marked very accurately on the top circle. A pilot hole was drilled in the top one and then both were aligned, clamped together and drilled at the same time. I figured this was key to keeping the alignment between the two discs the same and making the azimuth slewing smoother.
A record was used for the azimuth-bearing surface and Teflon furniture leg pads were screwed into the bottom of the top disc. The photo on my website shows the first ill-conceived attempt, I figured that if 3 pads were enough 8 would be better. WRONG! Stick with three at 120-degree intervals, three points define a plane and any more induces rocking. A little WD40 and this things slews like oiled glass.
As of this writing, 18 Dec 05, I have enjoyed this little 6” for just over a year. It has captivated me in ways I can’t describe and taught me how to relax. An important thing that was lost somehow over the years raising kids and making a living. It has provided countless hours of missed sleep while staring awe struck at things I can hardly believe exist. Presently I have captured 43 of the Messier objects towards every newbies goal of their first certificate.
My buddy Shane just called and said the skies are clearing so I’m going to end this on a joyous note and run outside for first light on Mars with my new 4.8mm Nagler, an early Christmas present from my wonderful wife.
Many more photos of this telescope project and other woodworking and astronomy projects can be seen on my website at: www.tulsawalk.com