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| At left, rocker box with tube box. The silver altitude bearing mounted on the tube box is a painted film can I purchased at a rummage sale for 25 cents. A strip of Ebony Star laminate is glued around its circumference. It rides on two quarter-inch thick pieces of teflon. At right, the view looking down into rocker box. It swivels freely on the ground board thanks to Ebony Star laminate and teflon. The lag screw in the center of the rocker box secures it to to the ground board. |
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| Counterweight |
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| At left, bottom of the ground board with plywood feet, which which work fine. Others have used hockey pucks to absorb vibration, wheels for portability, or stakes for stability. On the right is a view of the installed tail-gate with the three collimation bolts. The reflection coming from inside the optical tube assembly (OTA) is the secondary mirror. |
| Tube Extension |
| Cut the tube with plenty to spare. I wish I had. However, I made the best out of a bad situation and like what I ended up with. I drilled the first hole for the eye-piece in the wrong place, which is a common enough error I'm told. When I finally tired of using an extra-long draw tube, I decided to move the eye-piece hole to its proper distance from the primary, where it is above. I did save the cut-out from the first hole and could have glued that back in place, but decided instead to improvise a little and build a red map-light to occupy the space of the old focuser hole. The map light gives me a free hand while referring to star charts and recording observations during viewing sessions. It's just a $2 dollar flash-light with the light/head assembly removed and attached to a cardboard roll (scrap left over from the telescope focuser). I rewired the bulb to a new on/off button. To make the light red, I removed the lens and sandwiched some red cellophane between two pieces of acrylic that I cut to fit. Other options would have been to use a red bulb or LED or simply to paint the flashlight lens red with fingernail polish. I found out about the latter after completing this project, but did try the fingernail polish on another flash light and it seems to do the job. Next to the light assembly is the battery pack, held in place by a velcro strap. The white bracket attaches to my home-made finder. |
| Moving the draw-tube closer to the end of the optical tube assembly had the noticable effect of letting in too much stray light. To remedy the problem I made a tube extension out of 1/2 inch polyethelyne camping pad available at sporting goods stores. The material cuts easily with scissors and the ends can be glued together with Shoe-goo or Plumber's Goop. I covered mine with monokote. A flatter color would keep glare down near the eye-piec, but I opted for the chrome for asthetic reasons. The idea for using the camping mat came from the ATM list. I would like to give credit to the individual but can't remember who it was. |
| The counter-weight, which is my own design, became necessary when I added the finder to the OTA. It is made from a one-serving size Pringles can with nine rolls of pennies inside to get just the right balance. It attaches to the telescope's OTA with a universal mounting bracket from Radio Shack designed to mount small speakers. I mounted the counter-weight on top so that the OTA has a full range of movement with no clearance problems that an underside mounting would present. By positioning the counterweight so that the removeable plastic lid is facing up, the penny rolls never slide out and I find a can use the lid to trace field-of-view circles in my sketch book of observation drawings. |
| My 8 inch F7 Dobsonian telescope complete with finder, counterweight and tube extension. This project is a testament to the fact that even those of us with no prior experience can produce a fine quality telescope, superior to department store models and comparable to those produced for the serious amateur market. The basic instructions which I used to build my telescope are available on-line from the San Francisco Sidewalk Astronomers. I also made my own finder whch you can see on my Finder Page. More photos of my telescope accompanied by some construction tips and observations follow below. |
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| I purchased my mirror, but you can also gind your own if you want to do the whole thing yourself. If you're interested in making a mirror from a glass blank as well as other parts for the telescope, check out my links below. With the exception of the mirror I got virtually all my parts for free. Wood, ebony star, and teflon were all obtained for free as scrap. I have basic tools. The only power tool I used was a drill. I found contracters in the area who were willing to cut my wood to size and cut pipe for the draw-tube. |
| Links to Some ATM and Astronomy Sites |
| StarAstronomy - The master list of ATM links for a host of do-it-yourself astronomy projects Mel Bartels' Page - The ATM guru ATM Archives - A great place to find answers to your ATM questions Build Your Own Dobsonian - Another DIY site Absolute Beginner's Astronomy Page - The title says it all Learn What's Up - A current calendar of the night sky |