Telescope building - my dream telescope

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Concept

The story goes on

Construction

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Concept (February 5, 2001)

Often when going to the remote campsite I wish I had big enough automobile I could take along the telescope. Telescope big enough so I could really enjoy the night.
The other concern is that remote sites are not always suitable for observing.
That would mean I would have to take a walk with the telescope to a good spot - an open patch far away from campfires and the lights.
Finally I would like to be able setup the telescope and do not worry about collimation.

Above notes set the parameters for the dream scope.

Aperture should be minimum 12.5".
The telescope in the folded form should be the manageable - in the form of some kind of backpack.
The whole assembly including eyepieces, laptop, and other gadgets should weight no more than 45 lb, so I could hike with it - for lets say a mile.
Precise setup - after long research I have concluded that only 'strings scope' is precise enough. Other plus of this construction is the fact that is has little loose parts and setup time is quick.

During my research, I have found many interesting WEB sites. Many of them contain very good information. Many of them are just not detailed enough to draw any ideas.
I have listed the ones that I found useful in the links section below. There is only handful of them because I do not want to spend much time on maintaining the links.

As time progresses and I will start work on the construction of my dream scope, I will post here photos, and updates to the plans.

The detailed plans are not finished yet. I have stumbled over the problem of attaching the altitude bearings to the mirror cage.
In addition, I need to make mass analysis. I do not want to cover for rushed design with heavy counterweights.
I am using AutoCAD 2000 - my favorite CAD program - for developing the drawings.
It is very powerful, but as other CAD programs it calculates center of gravity allowing only uniform mass.
So after resolving all details I need to produce separate drawing in which solids are scaled proportionally to their mass.
The other problem is how to publish the plans. Not everyone has and knows how to use AutoCAD. The raster images are not good enough, because too much detail would be lost. However, I am sure that the idea will come soon.

As for materials, most of the construction would be composite fiber-carbon and Kevlar.
The mirror should be about 1" thick, so it would weight under 13 lb.

For now, you can enjoy the images of my (not finished) dream scope that were prepared with 3D Studio Max, my favorite modeling program.
The scope is high in the clouds above ground, because it is a dream for now :).
The blue sphere above mirror is to be able to see where the mirror is.

The background images on this WEB site are screen shots from 3D Studio Max.

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The story goes on. (March 18, 2002)

While being unemployed I have gone back to design.
Between clicking send/receive button on Microsoft Outlook, cleaning the house and other duties, I had plenty of time.
The design has not changed much, but I have finished the base and resolved most of the important details.

Even if I cannot execute this design, I have learned much and I have had a lot of fun.

The purpose of this design is to:

Get the height of eyepiece at Zenith, which should be no more than 67 inches.
I do not want to hike with stepladder.
I want the telescope look decently. Resolving critical details during design phase results clean design. Improvisation does work well for simple projects, but when the project is more complex, unresolved details may spoil all idea.
I want to be able to pack the telescope in to the backpack. All assembly must break into manageable pieces. Again, the design helps to solve this problem.
I do not want to hike with big counterweights. It is quite a work but doable. With solid modeling, I was able to get center of mass for groups made from the same material. Then each group had to be scaled up, accordingly to the density of the material. But this is just the theory. In practice I will have to use some counterweights, I just want to reduce them to the minimum.
I plan to use rechargeable batteries mounted under the mirror cell. The batteries are better than the counterweights in my opinion. There is always need for some electricity.
Finally but not least, I need to generate DXF (Data Exchange Format) files for machine shop. The mirror cell frame requires some precision. I was thinking about connecting straight pieces with screws, but the problem with straight pieces is that they need to be cut precisely, and any discrepancies will add at the end.
Than I thought about welding, but the problem with welding is that aluminum is changing the shape after is has been welded.
My friend suggested milling because it is much easier (provided you have access to this machine).

Again, I have done solid modeling (design) with AutoCAD 2000.
Total time 198 hours 22 minutes.
Then I have exported the file to 3D Studio Max, mangled it a little, and then exported as VRML (Virtual Reality Modeling Language). I could manipulate the design, check the clearances, and feast me eyes.
Total time 72 hours, approximately.

The preliminary calculation of the total weight of the telescope excluding counterweight is approximately 25lb. It is much less that I thought it would weight.
I am mixing the materials. My main concern is weight. Therefore, whenever the space is not a concern I am using something lighter than aluminum.

On March 24, 2002 I have finished weight and CG (Center of Gravity) calculations.
The images on the left and right are screen shots from AutoCAD to illustrate how complex this task was. It is almost parametric model. . I had to write VBA application in order to do it faster.
I could say that I am lucky (in theory):

Weight = 25.6735 lb
CG, relative to mid-point between centers of altitude bearings
X = -0.4 in.
Y = -0.5 in.
Z = -0.25 in.

It is interesting what practice will show.

The base and vertical bearings will be made from Ľ in. plywood. The cross section is an 'I' beam. The top and bottom sections are made from plywood laminated with carbon fiber. I like combination of wood grain and pitch-black carbon.
The base breaks into three sections. I have not yet drawn the locks at the corners. They will be very similar to the ones used in the ski boots. The wooden legs are locked between the three sections. When locks are unlocked, the legs can slide up or down in order to level the base.

The bottom ring (horizontal bearing) is to be made from ³/₈ in. plywood.

Top and bottom cage are to be made from Styrofoam laminated with carbon fiber, 1"×3" section. The bottom cage has no other function than to protect the mirror, also I thing it looks nice. The top cage is a structural part. It is hard to determine flexing. In the worst case, I will have to go to the idea of three spreaders.
I have read a discussion on newsgroups about stability of string scope with two, three and even four spreaders. Many people argued very convincingly for three spreaders, because if they would support the top cage at the points where three pairs are attached to it, the bending forces would be zero.
Well may be so, but they talking about springs rated for 180lb, that would mean they assume loads 4.5 time more than mine.
I hope I am not wrong with my calculations.

For the mirror cell, I am going to use 6061 aluminum, with exception of the triangular frame. I have large piece of 5/8 7075 aluminum for this part.
This piece is not only a cell support but also a backbone for all 'tube' assembly.

Some may notice lack of collimation screws.

Adjusting the strings in pairs at the bottom and top does the collimation.
I have not seen anyone do it this way, but it works. I have done model and simulation in 3D Studio Max. The collimation screws at the bottom and the top will result the same behavior as if they would be located under mirror cell and in the secondary mirror holder.
Instead adjusting the mirror position within the tube, I just adjust the tube.
This saved precious height of the mirror cell and allowed to get mirror lower to the ground. Consequently, I could have mirror with longer focal ratio - 5.8.

The spreaders and bracing are standard carbon tubes, 12.7 mm ID, and 1.4 mm wall thickness. I have found the manufacturer in UK. The tubes are in sailboats.

Someone could wander about the sections. They seem to be small.
I have done structural calculations, and section analysis.
The reactions at the bottom of the two spreaders should be approximately 19 lb, when the scope is in horizontal position. That is not so much.
The joint displacements (flexing of the 'tube' structure) are in the thousands of an inch. This should not effect the collimation too much. Besides - this is a scope for visual observations, not for the photography.

The strings are braided Kevlar with unbraided core. I have found this cord at the aircraft supply store.

One detail has taken great chunk of my time - tensioning the spreaders.

In Cortona VRML viewer, you can set renderer to wireframe mode. You will see lots of detail, but the application will consume much more system resources.

In Jane's 16 inch string scope (see Links below), Dan Gray uses springs.
I am limited here by space.
The spring constant (while you depress the spring its force increases) was my concern.
I was thinking and reading and looking on the web and finally have found 'gas springs'.
Their constant is almost nothing comparing to the regular springs. They come in large variety of sizes and loads. I have found a mini gas springs used in furniture to hold small lids and doors. They come rated for extension force from 5 to 30 lb in 5 lb increments, and their OD is 0.47" so thy will fit inside carbon tube. Huh - I hope they will do the job.

I am on the tight budget, so the mirror has to wait. In the mean time, I was able to get my hands on the aluminum plates. They came from junkyard.
I have almost all aluminum I need for this project.

I would like to invite you to check this page from time to time. As the time progresses I will be updating this page. I am welcoming any comments: michaeldaj@comcast.net

I still have to resolve few details:

Binding the three pieces of the base together must allow for quick release and slide the legs up or down.
Focuser - I may decide to do my own. I do not intend to go beyond 200X magnification. I am going to make an extension tube if needed.
Actually, in clubs 13" Coulter I keep tube half loose and focus by simply sliding the tube in and out. It is much faster than turning the helical - never know which way :). So I do not care much about the focuser travel. One inch should be more than enough.
I need to decide for finishing on vertical and horizontal bearings. If I am going to motorize the telescope it should be aluminum or steel.
Motorizing would be another big project by itself.
Telrad - I plan to take it apart and burry it in the secondary cage. I hope that 1 inch of tube wall is enough. The glass plate would be snap on. Batteries will be located below mirror cell.
Carrying case - this will be something soft and padded and with possibly backpack frame.
Shroud - many do not make one. Honestly, I do not know at this point, what I am going to do about this item.

The images in this section are renderings from 3Dstudio Max or screen captures from VRML viewer. Clicking on the thumb will make the image larger.

VRML (Virtual Reality Modeling Language) has been around for quite some time.
It has been revived recently when it become part of MPEG4.
It is one of my favorites. The files on this page are in VRML97' format.
The VRML files are very large (over 1MB when expanded). Therefore, I have decided not to embed them in the page. Instead, I have put them in the ZIP files, and you can download them by clicking on the appropriate thumb image.

I use 'Cortona VRML client' from ParallelGraphics, http://www.parallelgraphics.com.
It works very well with MS Explorer and Netscape, and it is free.
You will need some power from your computer to manipulate these VRML files.
I have PIII 500MHz and they are manageable, but I wish I had more power.
Lots of memory and nVidia based video adapter - that is what is needed.
If the motion is very jerky, make the window smaller.
This will ease on the CPU and the Video Card.
I did not optimize the files for the size and speed, and I do not intend to.
These files are translation from solid modeling done in AutoCAD and they have lot of detail. If you switch to wireframe mode you may see gas springs inside the graphite roads, aluminum fittings, screws and much more.
The mirror cell is a separate file. It has lots of detail and would double the size of the other file.
I am not describing here every part. I will do it while making the scope.
I have found all parts at McMaster-Carr, including gas springs.
There are some parts of the telescope that when you click on will trigger the animation.

Here are the VRML files:

Ani04.wrl - 1877KB

For now clicking on following objects will result animation:

Vertical bearings - tube rotation from Zenith to horizontal or vice versa.
Horizontal bearing - 360 deg rotation, once.
Base pieces - collapse and erect the scope.
This animation is not the best but it is just for your amusement.

Download ani04.zip 310KB

Cell.wrl - 852KB

You can click on one of the support triangles.
It will explode the assembly.

Download cell.zip 182KB

Have fun.

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Construction (May 10, 2005)

On Sunday, March 20, 2005 I was depressed, I have not done much since last year except work hard and helping others.

I thought about the telescope and realized that once it was my priority and now it is not.
I had some cash and could buy the mirror and wanted to spend some money just on myself.
Occasionally I was contacting mirror makers - they all turned me down.
"It will not work because it is too thin" was the phrase I have heard the most, even from most famous.
I went once more time through websites and stopped at Swayze Optical.
It was Sunday, late afternoon; I picked the phone and dialed the number from the web page.
I was so surprised when Steve answered that I almost forgot how to speak.
We talked a little and I asked him to not to turn me down, because he is my last hope.
He did not and within half an hour I was typing specifications and writing the check.

I guess this is beginning of construction phase for my dream scope.

The mirror will be 12.5" diameter, F5.8, and 1" thick.

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