George's Telescope - Consolidated Threads

[David DeCristoforo]

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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.

[John K Jordan]

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

[Thom Sturgill]

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!

[Robert Henrickson]

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.

[Roland Martin]

Very impressive, David. Am looking forward to following the progress.

[David DeCristoforo]

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

[David DeCristoforo]

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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.

[Sparky Paessler]

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".

[David DeCristoforo]

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.

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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.

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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!

[David DeCristoforo]

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,
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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.

[John Keeton]

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!

[William C Rogers]

Very nice work. I like the detail work and choice ow woods.

[Allan Ferguson]

Real quality work as always.

[Steve Arnold]

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

[Clint Bach]

What? No lense caps? Astronomers NEED lens caps!

Just kidding. Great finder scope.

clint

[David DeCristoforo]

"What? No lense caps? Astronomers NEED lens caps!"

Working on that...

[David DeCristoforo]

Lens caps...

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[Clint Bach]

Excellent! Great in every way! How do they stay on the scope? Friction fit... Screw on... O ring in a groove... Something else?

c

[David DeCristoforo]

... How do they stay on the scope? Friction fit... Screw on... O ring in a groove... Something else?

c

Friction fit with leather liners that were moulded over a form and then cemented into the wood caps.

[Eric Gourieux]

Wow, DD. You're going all out on this! I, too, am glad to see you posting again. Your work is exemplary, as usual.

[Doug Herzberg]

Great to see you posting again. Love the scope.

[David DeCristoforo]

FWIW... I took the whole thing apart yesterday and throughly cleaned the lenses and prism. Then took it outside to test it out. Seems to work just fine!

[David DeCristoforo]

I have had little time lately for turning or posting. But I have managed to creep forward a bit on the telescope project. This is the polar shaft for the mount, the basic plan for which can be seen here: http://daviddecristoforo.com/Misc/mount.pdf

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This is six inches in diameter and twenty inches in length. The "axle" is a 2" diameter hard maple shaft. The housing was made from a laminated tube purchased from Anderson International. I was origionally planning on laminating my own tubes but the price on these is very fair and I was not up for doing this on my patio! Plus, my son's family bought these as a Father's day gift so I could hardly refuse....

The knurled "washer" on the end is threaded and serves as a tensioning ring. The "finial" is also threaded and locks down against the "washer"

These are the bearing housings, turned from hard maple and "ebonized", one for each end of the two shaft housings:

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I also added a rotation lock. I was not completely sure how this would be made but ended up with a simple threaded dowel. It was the first time in many years that I have had an oppurtunity to use my thread boxes.

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More to come.......

[John Keeton]

Precision work from the master!!! Good stuff, David.

Here is the previous thread on David's project - http://www.sawmillcreek.org/showthread.php?254078-George-s-Telescope%85-The-Finder-Scope

[Allan Ferguson]

A second to John Keeton' comment. It is always good to see your works.

[John K Jordan]

Once again your work is outstanding. I'm happy to read whenever you can post. I want to buy the book when you are finished. :)

JKJ

[David DeCristoforo]

Moving along, albeit at a snail's pace! (Sorry about the crappy "photos")

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[John K Jordan]

I look forward to seeing new pictures on this! In fact, I'd be tempted to take a trip to see it with my own eyes some day! Beautiful work - I can't imagine the number of hours you have in this so far.

(To me, the photos are perfect. Time for studio quality later!)

I carried two of my (store bought) telescopes to a fellow woodturner's place (John Lucas) for the recent eclipse. Amazing. I don't know if you saw this picture earlier but he made this hand mirror for me to remember the occasion:

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http://www.sawmillcreek.org/showthread.php?257443-Eclipse-mirror-by-John-Lucas

JKJ

[Steve Schlumpf]

David - for most of us, learning how to turn is a lot of fun but extremely challenging when trying to turn two things that look alike. The precision you show in every aspect of your telescope is amazing but I am afraid leaves most of us searching for words. I look forward to each update and really want to see the photos when your grandson actually gets the telescope! Thanks for sharing the journey!!

[andy bessette]

...(Sorry about the crappy "photos")...

Your photos can be immediately improved by substituting a dark, non-reflective background material. The light material is a poor choice as it fools the camera's exposure meter into underexposing the subject.

[Len Mullin]

David, you are quite the craftsman, this piece is amazing already. It'd hard to imagine that your not finished, with the way it already looks. If it was me creating it, I would stop now before buggering it up.
Len

[Frederick Skelly]

David - for most of us, learning how to turn is a lot of fun but extremely challenging when trying to turn two things that look alike. The precision you show in every aspect of your telescope is amazing but I am afraid leaves most of us searching for words. I look forward to each update and really want to see the photos when your grandson actually gets the telescope! Thanks for sharing the journey!!

+1. And it will be a gift he will cherish his entire lifetime.
What a beautiful job!

Fred

[David DeCristoforo]

Thanx the forum moderators for putting this all in one place and to John Keeton for suggesting it. I have a couple of other parts done and I'll post those here as well.

[Steve Schlumpf]

Looking forward to seeing the parts David! Any idea on a projected completion date? Just curious as I realize it may be another month or two before George is ready for the telescope.

[Len Mullin]

David, what type of lathe are you using to do this turning with, a metal turning lathe, or a wood turning lathe?
Len

[David DeCristoforo]

My lathe is the one pictured in my avatar, a 50's Rockwell wood turning lathe I inherited from my old man. Stock unit except for the paint job...

[Rick McQuay]

This is fascinating. I look forward to seeing more.

I'm an occasional stargazer and am wondering if the wood will have any effect on collimation? If you will have to realign more often or maybe less often since wood is not affected by temperature.

[Clint Bach]

My experience with telescopes is the material the scope is made of is less of a factor in stable collimatoin than the design. I have built or helped build or designed scopes from 25 inches to 1 1/4 inch diameter. The usual suspect in collimatoin issues is the mirror cell. Followed by spider/diagonal flex or slop... The sneakiest issue is focuser tilt. Most focusers don't have adjustments for tilt. Mine do... Occasionally a loose lense in the eyepiece itself appears as a collimation problem. Tube flexture sometimes makes a telescope impossible to collimate reliably.

some of my telescopes have not needed recollimation for years. Truss tube scopes can need recollimation after each time they are touched. My tensioned truss tube scopes generally stay collimated even after being disassembled and reassembled. Wood is good as a scope material.

clint

[David DeCristoforo]

These are a couple of additional parts that have been completed.

This is a "Telrad" finder sight. It projects a target onto the angled glass. I deconstructed one and remade it out of padouk and ebony...

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The "stock" plastic version...
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This is the primary mirror cell. The design was generated by a program called "GUI Plop" which plots out the best support points for the mirror. When my grandson heard me talking about this program, he said "Eeew... gooey plop" and went around saying "gooey plop" for the rest of the day.

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The cell will live in the bottom of the main optical tube and, most likely, never see the light of day again once installed.

[Clint Bach]

David,

planned a dew shield for that telrad? They tend to dew up and become almost useless in a short time out under a clear night sky. Just my experience...

Nice looking mirror cell...

clint

[David DeCristoforo]

David,

planned a dew shield for that tetrad...

clint

I have seen several posts on amateur telescope making forums that have to do with adding a dew heater to a Telrad finder. I may get involved with that at some point but first, I want to get the thing built!

[David DeCristoforo]

Here is my plan for the “fine adjustment” mechanism for the mount. Basically, it’s a captured nut and a “gimbal” (dark gray) that pivots in an opening in the center brace. I’m not sure the exact geometry is correct so I will mock this up before making any of the pieces. Any thoughts will be most appreciated. I have already been given a rather brilliant idea in the inclusion of two locations for the locking nut which allows the curved slot to be much shorter that in my original plan while, at the same time allowing a greater range of movement. Needless to say, this will be made of wood with maybe some metal for the pivot pins and possibly bronze bushings for the gimbal pivot.

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[Clint Bach]

David,

that will work. But... Having the elevating screw perpendicular to the lifted axis will be more robust and put less stress on the screw and mount in general. As an example, at very low latitudes, or the equator the elevation screw would not function at all or poorly. That scope may never be set up at or near the equator so it probably is a non issue. Keep in mid that a scope can last hundreds of years and the future is unknown.

a fine adjustment to align the mount east and west comes in very handy. Imagine the hassle of aligning the elevation and the having to pick up the mount to rotate it east or west... That screws up the elevation. repeat. Repeat. Repeat. Not quick or fun.

great looking scope!

clint

[David DeCristoforo]

David,

that will work. But... Having the elevating screw perpendicular to the lifted axis will be more robust and put less stress on the screw and mount in general. As an example, at very low latitudes, or the equator the elevation screw would not function at all or poorly. That scope may never be set up at or near the equator so it probably is a non issue. Keep in mid that a scope can last hundreds of years and the future is unknown.

a fine adjustment to align the mount east and west comes in very handy. Imagine the hassle of aligning the elevation and the having to pick up the mount to rotate it east or west... That screws up the elevation. repeat. Repeat. Repeat. Not quick or fun.

great looking scope!

clint

The intent here is to have tilt range of 25 to 50 degrees which will make the mount useable anywhere in the continental US. It will initially reside in Davis, CA which is at 38 degrees longitude. The gray rectangle, which represents the carriage that the polar shaft will mount on, is drawn at that angle. I have revised this drawing at least a dozen times. Here is the latest revision.
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It is taller by four inches (now a total height of 16") to allow for a more perpendicular angle between the carriage and the adjusting screw. The two small circles in the curved slot represent threaded holes for the pin and locking knob (not drawn). Having two locations allows for a greater range of movement without having to make the slot longer.

Interesting to note: Most of the custom made GEM mounts I have looked at are built to be more or less permanently installed at a specific location (longitude) and therefore have a very limited adjustment range. I wanted this one to be a bit more flexible.

PS The two pieces of the base are on a bearing (think "lazy susan") so rotation will not be an issue. There will also be an inlaid compass and leveling bubble as well as a locking knob which are not shown in the drawing. This will all sit on a pedestal (design yet to be finalized) which will have adjustable casters. This will facilitate easy movement of the scope as well as leveling once it is in position.

[Clint Bach]

David,

Much better! Your scope will appreciate the extra stability. Some scopes use screw downs to lift the casters off the ground. Even heavy duty casters can be a source of slop while observing. At high powers small flexures add up quickly. The quicker a scope settles down after a move the more enjoyable it can be. I have observed through scopes the "ring". That means they shake after any small movement such as focusing or small adjustments. Your scope seems to be well on its way to being stable and not ringing. Leg to pedestal connections can be a weak point.

Clint

[David DeCristoforo]

I plan on using these:
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I have used these on some very heavy machinery. They are rock solid and stable. The scope will have to be easily moved so wheels are crucial. I have tried to design everything to minimize unwanted movement and backlash as much as possible without resorting to materials other than wood. Metal has been used wherever absolutely necessary, collimation bolts, focuser shaft, bearings, etc. The whole idea here is to have a properly functioning scope made mostly of wood. But the idea of it being 100% wood must be compromised at times to insure functionality.

[David DeCristoforo]

Finished the altitude adjustment mechanism.
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I wanted to have a stainless steel ring for the gimbal but I could not find anything that was “just right”. I ended up cutting down a part from a 2” iron pipe union. It worked out pretty well. The resulting ring was drilled and tapped for two short axels which fit into the two bushings. The bushings will be captured on the mount structure.
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The gimbal was fitted between two halves of the adjusting knob. The knob it a bit bigger than I had originally drawn it but I wanted a good sized grip and rather than end up with a big knurled ring, I just beefed up the whole thing.
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[Frederick Skelly]

This is such a neat project. A real labor of love. Can't wait to see it when it's all done!
Fred

[David DeCristoforo]

I thought it would be cool to have a nut and washer on the end of the pivot pin but I just didn’t like how it looked. So I swapped out for a matching knob. I think this what is called “sweating the small stuff” and “they” say that is not good. But I figured, like Doc Brown said… “What the hell…”
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[Mike Turner]

Wow these are nice telescopes.You guys are raising the bar but my mind cant think that high! Awesome work guys!!!!!

[David DeCristoforo]

More forward movement on the telescope mount. For some reason, this has been the most difficult part to complete so far. What amounts to a fairly straight forward piece of (mostly) flatwork ended up being unbelievably fussy. Three steps forward and two steps backwards was the order of the day. It seemed like every piece had to be rethought or modified in some way. Funny thing , this was the most carefully planned of all the parts made to date. I went in thinking this was going to be pretty much a cakewalk. Not! But I got it done nonetheless. Still needs some touching up and I still need to build the pedestal and the mounting platform for the scope. But this represents real progress and it functions well.

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[David DeCristoforo]

For a number of years now, I have been considering building a bed extension for my old Rockwell lathe. But the lack of real need has pushed the project onto the back burner. But now I need to turn the declination shaft for the telescope mount and that needs to be longer than the 37” capacity of my lathe. So the time finally arrived to take the plunge and make the bed extension.


Initially, I wanted to make it out of cast iron. But with all of crap I was getting from the city over needing to bring in a six inch gas main to fuel the forge and freaking out about the pigs of iron swinging around the yard, I was forced to relinquish that idea.


I next thought about steel. Steel would have been good but, after looking at some of the ‘diy’ extensions posted on the internet, I decided that I would go with what i knew best and make it out of wood. Not only easier but also much more “modification friendly”.


I realize that this might be beyond the abilities of the average amateur but if anyone ever wants to build one and is up for the challenge, I would be happy to make my design, stress load calculations and measured drawings available.

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[John Keeton]

Wow, DD!! You went high tech with this project. Good you have gathered all that experience with the telescope as the adherence to close tolerances has come in handy. Space age materials, too. Any patents pending??

[Bob Bergstrom]

Wow, DD!! You went high tech with this project. Good you have gathered all that experience with the telescope as the adherence to close tolerances has come in handy. Space age materials, too. Any patents pending??
John is ready to launch a another new career as a “Patent Law Consultant”?

[John K Jordan]

There may be prior art. I knew a guy who built one of those decades ago. He mounted it on a workbench so he could turn a really long pole. It was probably as much work to make a long wooden tool rest.

[David DeCristoforo]

Foolish as it may seem, I have no inclination to attempt to profit from this design. I have always been an “open source” kind of guy, dedicated to furthering the greater good. As I stated in my original post, I would be happy to make the necessary technical information available to anyone wanting to attempt a project of this magnitude. Besides, I have already been offered suggestions that would improve on my concept, using duct tape in lieu of drywall screws for example, which would not only cut the fabrication time by at least half but would also bring the overall cost down significantly. This is the beauty of the open source concept. Many people contributing their input to improve an already brilliant idea.

[Ron Stadler]

The only thing i would change is the color, doesn't match the rest of your lathe.

[Sid Matheny]

Just add a good hinge and you have a first rate swing away. :rolleyes:

[Jim Colombo]

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I made one too but I'm thinking of charging a biiiiig fee for the plans.

Jim

[David DeCristoforo]

The funky lathe bed extension worked out just fine.

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[David DeCristoforo]

Here is the mount so far. The declination shaft is installed in it's housing. I ended up cutting the housing down by about eight inches in length after realizing that it did not need to be nearly as long as I had made it.
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Next up is the pedestal and the scope mounting platform and rings.

PS Sorry about the terrible image. I have exceeded the capacity of my "photo booth"!

[John K Jordan]

Here is the mount so far. The declination shaft is installed in it's housing. I ended up cutting the housing down by about eight inches in length after realizing that it did not need to be nearly as long as I had made it.

Next up is the pedestal and the scope mounting platform and rings.

PS Sorry about the terrible image. I have exceeded the capacity of my "photo booth"!

David, your work really deserves a new photo booth! I used some PVC pipe, some thin white cloth, and a piece of curved mat board to make a booth for turnings. But for the whole telescope you might need a really big booth!

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I'd love to read the book you publish on making this - sketches, shop shots, finished scope out under the stars! :)

JKJ

[David DeCristoforo]

I made a photo booth like that years ago. But this thing is too big for it. So I either have to make a bigger version or fake it. As it progresses from here, it’s only going to get bigger!

[David DeCristoforo]

Scope rings done and mounted. I left a bit of “wiggle room” on the I.D. to allow for still to be applied leather liners. Next up is the pedestal.

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[David DeCristoforo]

For almost two years now, I have devoted every spare minute and every cent I could get my hands on to my telescope project. All other ideas about things I might want to make have been pushed to the back burner. When I started out, my intent was to build a Dobsonian style mount which is a fairly simple rocker box design, mostly simple “flatwork” using plywood. But once I discovered the GEM mount I could not shake loose the idea of building one. The decision probably cost me a year and required much more expensive material, time and fussy detail work than I ever intended to invest in this. Getting it (mostly) done was a great relief and my focus could shift back to the scope itself.


I have documented every step of this project and I needed a pic of the mount in it’s completed state. Having no really good setup for this, I set the mount on top of a stool with a fabric backdrop. That worked out OK and I goy my pics. Then I had to get to work so I thought I would move the mount back into the house the next morning. You might well imagine my discouragement when I entered my shop to see the mount lying on the floor amidst a tangle of lathe tools, extension cords, pieces of wood and other stuff. Apparently, since there is, at this point, no counterweight to offset the weight of the scope rings, the upper (declination) shaft had rotated, throwing the whole thing out of balance and causing it to topple over.


My first action was to simply turn around and go back into the house. It took me almost an hour to calm myself enough to go back out to asses the damage. After detangling the cords and tools and wood enough to pick up the mount, I got it back up on the bench. The whole time I was looking for the inevitable broken parts and ruined components. I could not imagine this happening without major damage and I feared that I might not have the heart to do it over even if I could salvage many of the pieces.


But to my utter amazement, there was almost no damage. A few chips in the paint, easily touched up and one slightly more significant chip on the upper edge of one of the rings that required a bit of bondo to repair. But, outside of that, the mount was completely intact and undamaged! My sense gratitude is of galactic proportions. I might take a break from this for a few days….


FWIW, The mount is now safely back in the house, no worse for the wear and all is well...

[Mel Fulks]

I was fearful ,though safe enough, as I read. Thankful all is well, thanks for keeping informed.

[John K Jordan]

Yikes, I probably would have fainted!

[David DeCristoforo]

After the near death experience with the mount toppling over, I decided that I’d better come up with a counterweight right away. I will have to add weight to this once the scope is mounted but until that point I cannot determine exactly how much weight will be needed. The intention is to drill into the ends of the counterweight, fill the holes with lead shot and then plug them. That will be easy enough to do when the time comes but for now, this is sufficient to offset the weight of the mounting platform and rings.

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[John K Jordan]

...The intention is to drill into the ends of the counterweight, fill the holes with lead shot and then plug them.

I've also used lead shot for balancing in wood. After determining the exact weight needed I mixed the shot with a little epoxy to keep it in place. My use was to balance dressage whips for equestrian use. For this use it kept the loose shot from moving.

After the close encounter with splintering I think I might also do something else temporarily, such as tying a rope to a screw eye in the ceiling!

[David DeCristoforo]

I've also used lead shot for balancing in wood. After determining the exact weight needed I mixed the shot with a little epoxy to keep it in place. My use was to balance dressage whips for equestrian use. For this use it kept the loose shot from moving.

After the close encounter with splintering I think I might also do something else temporarily, such as tying a rope to a screw eye in the ceiling!

We used to bed the shot in silicone to weight chess pieces. That or epoxy will be used here. I actually thought about considered the whole tie down thing but sitting on a solid table with the counter weight, it’s actually quite stable.

[David DeCristoforo]

In order for a GEM mount to function properly, several adjustments need to be made. Probably the most important is that the polar shaft angle needs to be set to the lattitude at which the scope is located. Some form of indicator is needed. I decided to mount a simple pendulum type inclinometer on the polar shaft. There are many available but all suffer from the same drawback in that they are made of plastic. Not good so I decided to try making one out of wood. This is not, I am sure, up to NASA specs but it gets the job done well enough. The pointer was turned using a split paper joint. The lower bulb is drilled out on the back and a 1/4” steel ball was glued into the hole to add some weight at the bottom.

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Here is is mounted on the polar shaft...

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[David DeCristoforo]

So it's finally standing on it's own two... or three, yes, three... feet...er... wheels... I mean wheels. Whatever... the mount is done. Well, it's as done as it can be at this point. I may change out the casters for something a bit more elegant before I'm finished with this and I'm still contemplating adding setting circles. But for now, it's finished and I can move on to completing the scope itself.

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The upper part of the mount rides on a heavy duty (350 lb load rating) turntable bearing and rotates a full 360 degrees. It is pretty free spinning, maybe more than one might want. I had anticipated this and built in a tensioning nut. You can see it under the mount tubes. There have been many "design opportunities" in the building of this thing and one of them was making the opening for the tensioning nut too small to get your hand in to grip it. So I drilled six holes around the rim of the nut and made a spanner to tighten it.


All in all, this has been a major diversion from my original plan which was to build a Dobsonian type scope which uses a much simpler rocker box mount. I'm glad now that I went with building the GEM mount but I also glad it's done and I can move on now..

[daryl moses]

All I can say is WOW!!!!!!!
What an awesome piece. I'm sure it will be a family heirloom.

[Tom M King]

Sorry I missed this from the beginning. I'm just now seeing it.

My best friend, and I, started building telescopes when we were 14 and 15, in 1964. Getting 60mm refractors for Christmas one year, and being disapointed in them, we decided to start building larger ones. Complete telescopes then cost about the same amount of dollars as they do now, so that made them REALLY expensive back then.

Edmund Scientific sold telescope making kits, and books. That's where we started, and bought the Amateur Telescope Making series of books. The kits were glass blanks, and tool, for grinding mirrors of different sizes.

We built a number of them, with the largest being a 12-1/2" f/6. For the 12-1/2", we built a 12x12 observatory with a roof that rolled off, when we were Junior, and Senior in High School, by truck headlights at night, in the middle of one of the cow pastures where I lived. Back then, we were fortunate to have very dark skies where we lived-different than that now.

Ours, of course, looked nothing like yours. Ours were simply for being used.

Long story shortened, my friend now has one of the offices on the end of a hall in the Science building at NASA. He's won all sorts of awards. He wasn't in on the building of the Hubble, but when they needed to fix it, he took our 12-1/2" mirror in to be checked, and it was something like a factor of 3 better than anything they had there. He was put on the team to come up with a fix for the Hubble. We had used a micrometer, for the Focault test, made from a 1/4-20 screw, and the mirror was ground in my bedroom.

I have very much enjoyed the building of your telescope.

[David DeCristoforo]

I guess everyone got tired of waiting for me to get back on this project! My sons got together and bought a tube from Anderson and gave it to me at Christmas. So I now have the final major component needed to complete the scope. I’m having to make a few minor adjustments to some of the previously completed parts as the dimensions of the tube are slightly off what I had planned for. But nothing that some small mods won’t accommodate. Here’s a pic... I’ll be laying out the OTA (“Optical Tube Assembly’...astronomers lingo) and getting it ready for finish over the next few weeks. Then it will be time for putting the whole thing together at long last!

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[Roger Chandler]

When finished this will be a treasured heirloom unlike almost any other. I applaud your skill and your perseverance to bring this to completion. I sure hope George understands the amount of effort, skill and love that has gone into this special piece of equipment. I look forward to its unveiling when completed. A proud grandpa you must be!

[David DeCristoforo]

After searching everywhere for a decent price on some nice veneer for the tube, up pops this 48” X 120” quarter sawn cherry for.....ten dollars a sheet?!?! Ok. Something’s wrong here. The shipping is more than the veneer. Gotta be a mistake. I ordered a sheet and then waited for the “oh no... wait... that’s an error... it’s supposed to be $110, not $10... sorry. But the order goes through with no issues and three days later I get a shipping notice. On its way. Now I’m thinking this is probably going to turn out to be one baggy piece of veneer. But the other day it arrived and it’s gorgeous!!! Beautiful bookmatched straight grained cherry with a really nice curl all across the entire length of the sheet. I have no idea why this was so cheap but it’s not because it was flawed in any way.

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Next weekend, my son and I will laminate this to the maple tube using the "iron on" method with Titebond 2. Then it will be stained to a close match with the mount tubes.

[David DeCristoforo]

Update... My son came over today and we got the veneer laid up. In case you are wondering why this is being posted on a turning site, if you look carefully, you can clearly see the vital role the lathe played in this phase of the project.

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[David DeCristoforo]

Almost three years to the day I started this, the telescope is finally finished. I’ve had to make a lot of changes along the way. Refinements, tweaks and modifications abound. I really never intended to do anything this complicated but the thing just kind of took me over. My sense of relief at having finished it is colossal.

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I am aware that I’m going to get totally raked over the coals about these images. At some point I will have to get better photos. But the weather is crappy here and I have no “studio” to photograph in so I had to kind of fake it.

I still need to adjust the scope (collimate is the correct term) but that will have to wait for some decent weather as it has to be done outside and takes a bit of time. But the thing balances well (that was a fussy process in itself) and the movement is smooth and stable in all directions. I needed a lot more counterweight than I originally thought so I had to whack my counterweight and fill in with some cast iron weight plates. I needed twelve pounds to counterbalance the scope and there was no way to cram that much shot into my wood weight.

So, anyway… for what it is worth, here it is and now I can get on with the other part of my life!

[David DeCristoforo]

Amateur telescope makers say a scope in never “done”. Apparently, the urge to tweak, refine and otherwise modify is too great to resist. But sometimes, changes have to be made out of necessity. In this case, getting the scope to balance was proving to be a bit tricky. I could get “close” but the tendency of the scope to slew was apparent. I finally realized that the action of the mount axes was such that any slight offset would throw it out of balance enough to cause movement. So I needed some way to counter this. My solution was to add another small counterweight opposite the focuser and finder to offset their combined weight. The post is a seven inch long piece of 1/2” all-thread with a wood sheath covering it. The weight is drilled out on the back side, holes packed with two pounds of shot and plugged. This has totally eliminated any tendency to slew and the scope now balances perfectly and holds it’s position nicely.

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I had also added another small counterweight at the bottom of the tube. This is to provide a way to offset the weight of the eyepieces, some of which can weigh up to five pounds. The only other way to counterbalance them is to shift the position of the tube in the rings, a cumbersome solution at best.

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