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Thread: George's Telescope - Consolidated Threads

  1. #1

    George's Telescope - Consolidated Threads

    focuser_1.jpg focuser_2.jpg focuser_3.jpg

    So it begins. My plan is to build an 8" Newtonian reflector based on a simplified design first built by John Dobson, AKA a"Dobsonian" or more simply, a "Dob". I want to make as much of the scope as possible. I decided to start with building a focuser. This is probably the most critical component in terms of precise fitting and movement. After careful study, I decided on a Crayford type focuser. These use a friction wheel to move the focusing tube instead of the more common rack and pinion. Advantages are a much smoother "stepless" mechanism that is also more suited to the kind of fabrication I am capable of. This would probably be much more suited to making on a machinist's lathe but I don't have one of those. As work progressed, I found myself thinking about how things were made during the pre industrial revolution period when each part had to be carefully hand fitted to achieve the desired result. The hardest part was getting the focusing tube to fit into the base properly. This has to be an easy slip fit but with no wobble. With repeated test fitting and sanding with 220, I was finally able to get a pretty sweet action.

    The base has a PFTE bushing and a key to allow the tube to have as little resistance to movement as possible. The key also serves as a focus lock via the thumb screw visible on the side of the focuser body. Tensioning screws were added to the adjusting rod tube. These are necessary because you need to be able to compensate for the varying weights of different eyepieces. The focuser will accept a standard 2" eyepiece and, via the adaptor, the more common 1 1/4" eyepiece.

    Of course, it would have been much easier to simply buy a focuser. They are many good quality units available for very reasonable prices. But then, it would just be something I bought and bolted onto the tube and that was not in keeping with my intent which is to actually make the thing and not just assemble a bunch of parts.

    The other thing I discovered was that there are "dual range" focusers which are much preferable to single range units. Dual range focusers have fine and course adjustment dials and incorporate a planetary, typically with a ten to one reduction, using ball bearings instead of gears. While trying to find a plan to build the planetary, I came across an adjuster that is designed to retrofit an existing manufactured focuser. It looked like it would be pretty simple to adapt this and it was very reasonably priced so I ordered one and set about working it into my "design". Another critical element was getting the shaft positioned exactly right to bring the rubber friction roller into contact with the focusing tube. Too tight and the whole thing would bind up. Too loose and it would not work at all. Again, this would have been much easier had there been a machine shop available. As it was, I had to "sneak up on it" until the fit was correct.

    So here we have it. A fully functional dual range Crayford type focuser made mostly out of wood. This is a truly "form follows function" object. Few embellishments were added outside of what was needed to make it work. We are often discussing the various ways that this kind of work provides satisfaction. The thrill I felt when I put all of these parts together and realized that it was actually going to work, well there are few things in life more satisfying!

    The mounting plate still has to be fine fitted to the main scope tube and there will be a few other minor adjustments necessary but these will not be done until the scope is basically completed and the focuser is mounted. A bit down the road at this point.

    Materials: Cocobolo, Honduras rosewood burl, African blackwood, brass, stainless steel, Teflon.
    Last edited by David DeCristoforo; 05-14-2016 at 2:09 PM.
    David DeCristoforo

  2. #2
    Join Date
    Feb 2008
    Location
    E TN, near Knoxville
    Posts
    6,212
    Wow, I'm impressed! I'm a telescope nut but never thought about making one like you are. I'm anxious to see the pictures of the progress. It looks like you are going to end up with a very fine instrument!

    I have a friend who ground a 12" mirror once by hand and built a dobsonian. What a chore! I wimped out and became friends with Meade. I have an 8" schmidt-cassigrain and some smaller scopes.

    JKJ

  3. #3
    Join Date
    Nov 2007
    Location
    Fort Pierce, Florida
    Posts
    3,492
    My father once built a 10" reflector. I do not know enough about telescopes to say more than that. I became interested in my teens after he had long since lost interest and wanted to grind my own mirror. He hit the roof and said he would NOT help and it was beyond me. So died my early interest. At this point I appreciate the challenge and take my hat off to you sir!
    Retired - when every day is Saturday (unless it's Sunday).

  4. #4
    I'm certainly impressed, by the quality of workmanship but also the decision to do it. Back in my undergrad days, when I thought I would be an astronomy major, I ground two 6" mirrors (f/8 and f/4). Rather than shipping them off to have them aluminized for the reflecting surface, I took the old-school approach and silvered them myself.

  5. #5
    Join Date
    Jan 2010
    Location
    Lewiston, Maine
    Posts
    1,502
    Very impressive, David. Am looking forward to following the progress.

  6. #6
    Well, I'm not grinding any mirrors! Thats one of the few things I will be buying.
    David DeCristoforo

  7. #7

    George's Telescope Phase 2 The Spider

    spider_1.jpg spider_2.jpg


    Reflecting telescopes have two mirrors, the primary mirror which is mounted at the bottom of the main tube and a smaller secondary mirror which is positioned at an exact point between the primary mirror and the focuser. It sits at a 45 degree angle and reflects the light gathered by the primary mirror into the focuser/eyepiece.


    The secondary mirror is held in place by a device called a "spider" This particular design is called a "curved vane" spider for obvious reasons. The vanes are made of spring steel and are intended to be as thin as possible so as to reduce the obstruction of light into the main tube.


    The mirror holder itself has to have two adjustments. The first is the fine tuning of the distance between the centers of the secondary and primary mirrors. this is accomplished by the center bolt. The three smaller bolts are called "collimation" bolts. The process of getting the mirrors and the eyepiece in precise alignment is called collimation. Why? well, you gotta ask someone else about that. I would have just called it "adjusting" or "aligning". But I guess collimation sounds much more scientific. Either way, the adjustment is necessary.


    The materials used here are cocobolo, African blackwood and stainless steel.
    David DeCristoforo

  8. #8
    Join Date
    Feb 2003
    Location
    Johnson City, TN
    Posts
    652
    Wow that looks great David. The finished product will be impressive. my late brother use to build telescopes and left me a 12 1/2" Dobsonian. It does have a purchased Focuser on it thought. The largest he did was a 16" that was a monster! He did hand grind the mirrors. I also have an 8" he made the tube on and I made the equatorial mount for. Here's a picture of the 12".
    Attached Images Attached Images
    Sparky Paessler

  9. #9
    That's a great looking scope. I hope mine turns out as well!

    PS The focuser was dismantled and reworked a bit. Most of the teflon bushing was removed and replaced with two aluminum blocks, each fitted with a pair of small ball bearings. It's a bit funky as my machining capability is limited to small bench top drill press, a bench grinder, a Dremel tool and a box of files. The work is rude and crude but everything works.

    bearings.jpg

    There are two shallow holes drilled in the back of each block that engage the small set screws now visible on the outside of the base.

    bearings_2.jpg

    These keep the bearing blocks in place and also allow for adjustment of the pressure of the bearings against the drop tube. All in all, I'm very glad I did this. The action of the focuser was pretty good before but now it is soooo sweet. The tube glides now whereas before it had more of a sliding feel. Now it feels like a real focuser!
    Last edited by David DeCristoforo; 06-14-2016 at 9:46 AM.
    David DeCristoforo

  10. #10

    George's Telescope… The Finder Scope

    After a somewhat extended and unintentional sabbatical, I am back to working on my grandson's telescope project. I'm still refining my plans for the mount but in the meantime I wanted to make some progress. This round was making the finder scope, or rather, re-making it. After much (more) research, I decided on a 9X50 Right Angle Correct Image finder.


    I decided that the easiest way to go would to buy a relatively inexpensive finder, deconstruct it and remake it with wood components. I bought an Orion 9X5 Right Angle Scope,
    scope_3.jpgscope_2.jpgorion_9x50_finder_scope.jpg
    took it apart and remade most of it out of wood. I ended up modifying and using some of the threaded parts from the original scope as the threads are far too fine to cut into wood.
    David DeCristoforo

  11. #11
    DD is alive!!! Good to see you still have that special touch. I can't assess the functionality, but aesthetically, you certainly have improved the telescope. I like the checkering - is this the first time you have done that? Really well done, David!

    Left click my name for homepage link.

  12. #12
    Join Date
    Feb 2012
    Location
    Haubstadt (Evansville), Indiana
    Posts
    1,089
    Very nice work. I like the detail work and choice ow woods.
    When working I had more money than time. In retirement I have more time than money. Love the time, miss the money.

  13. #13
    Join Date
    Dec 2010
    Location
    Republic, Wash. State
    Posts
    1,149
    Real quality work as always.
    C&C WELCOME

  14. #14
    Wow, that looks much too nice to mount on a telescope.

  15. #15
    What? No lense caps? Astronomers NEED lens caps!

    Just kidding. Great finder scope.

    clint
    Last edited by Clint Bach; 04-21-2017 at 12:18 PM. Reason: Typo

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