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 12.5 inch truss tube dobsonian
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ejbragg
 Monday, July 16 2007 @ 12:10 pm EDT (Read 9594 times)  
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After much study on telescope design and material expenses, I have decided to build my own dobsonian, due to the power per $ being maximized in this model. The Cassegrain, refractors, and other model types have their strengths as well; how ever ease of use, I'm willing to sacrifice a little of, to get my raw power!

From several books I have read, it seems that at AROUND 12", a person starts to maximize the practical (usable) resolution of a telescope, as applied to the maximum usable magnification without a motorized mount. I will not be able to afford such a mount for some time, but I'm sure I will be forever adding adjustments and gadgets to this device, once all is completed.

I have also been greatly inspired by Bob Moody's truss dob, which sealed the fate of the mirror diameter I must have. My design will start with Bob's design in mind; however, I've a few other ideas, as well.

If anyone has any interest (ideas they'd like to share, or questions for me), please flag me down here!

Thanks,
Eric


"Thus the heavens and the earth were completed in all their vast array." - Genesis 2:1, NIV
 
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bobmoody
 Thursday, July 19 2007 @ 12:31 am EDT  
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Say, Eric. Glad to see your post!

I think you've made a great choice in the size of telescope to shoot for. I must tell you, though, that there are few telescope mirror-makers that will make you a mirror of that size, and if you choose to go with the f/7 focal ratio as I did, it will HAVE to be a custom job. There are a couple of amateur telescope maker supply houses and business that offer finished, mass produced 12" or 12.5" mirrors for a reasonable price.

Of course, the other alternative is to grind polish and parabolize the mirror yourself. This is no small task and you will learn a lifetime's worth of patience if you choose to go this route. It's not impossible to make your own high-quality mirror, but its a lot of very tedious and exacting work.

I'm here for you as the project progresses. Ask me whatever you'd like to know and if I don't immediately know it, I'll find the answer for you somewhere. I'd also like to encourage you to make this an article for the website, but it will likely turn into an article with multiple installments over time. I'm sure there are others who might want to consider the same size and type of telescope, possibly many others.

Welcome to the world of Amateur Telescope Making!

Bob


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nspace01
 Saturday, July 21 2007 @ 05:44 pm EDT  
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I have done a lot of research on "parts" for home built dob....On ebay there is a seller that sells "cheap" Chinese mirrors that have pretty good user reviews. You might look into these..............

Here is the link to the supplier.....

http://stores.ebay.com/hubbleoptics

Good Luck..........

nSpace01


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ejbragg
 Monday, July 23 2007 @ 08:10 am EDT  
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Thanks, guys. Bob, I think I may have told you, my primary is already on order. I have already decided I do NOT relish the thought of making my own optics! And Leonard, instead of going with the Chinese market, I decided to stretch my limits with a 12.5" dob by going with an R.F. Royce hand-made mirror - I intend to be able to stretch my high-power magnification as far as it will go. Obviously, without a motorized mount, I won't be looking for ants on Venus, but I'm willing to pay for some good optics.

Royce makes a conical (continuous face) mirror, which has an internal structure, making the mirror very stable and less susceptible to deformation. In other words, I don't have to make a mirror tailgate. In fact, I had ALREADY designed a 9-point mirror holder when I discovered this new mirror type. It's $200 more, but I decided that $200 less of a tailgate construction, basically me paying someone ELSE to weld it for me, plus have less weight to carry, would probably be worth it, especially in the long run.

Furthermore, a typical flat, 2" mirror of this diameter weighs about 19 lbs. The conical version weighs about 13 lbs.

As in all designs, this mirror does have its drawbacks. The outer edge of the mirror is 0.5" thick. The thickness toward the center is actually 1 3/4". When it comes to cooling the mirror, there is the possibility that the outer rim will reach equilibrium before the inside, warping the view, which is something I thought of only AFTER committing the purchase, of course!

For a profile view of this mirror, check it out at the bottom of this web page:

http://www.rfroyce.com/conical/mount CFB.htm

The ventilation process I'm going to test, once the mirror arrives, is the following:
The thickest portion of the back of the mirror is flat, so I intend to coat it with a thin layer of thermal grease. Then I'll place a heatsink against the mirror. The collimation plate will then fit over the heatsink. The fan will blow from the SIDE of the mirror, across its face, as well as through the heatsink at the bottom. The result should (I assume) cool the face and back side simultaneously.

I intend to make the mirror box fully enclosed, except at the top. The collimation bolts will be the only items protruding through the bottom. One fan will suck in air through the side. Another on the opposite side will carry the air back out.

I can post a diagram, if you're interested in seeing this hair-brained idea..

Eric
P.S. - Leonard, I'm sure I met you at my first star party out at Coleman, before all the rains, this past spring. Aren't you the Army guy?


"Thus the heavens and the earth were completed in all their vast array." - Genesis 2:1, NIV
 
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ejbragg
 Monday, July 23 2007 @ 08:13 am EDT  
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One more thing, Bob. The focal ratio of this mirror is an F/5. The one and only thing I didn't like about yours is that I have to use a step ladder to look through the eyepiece when approaching zenith. I realize that it will make the scope more difficult to collimate. So I suppose I'll be getting a laser collimator to help with that.


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bobmoody
 Monday, July 23 2007 @ 06:01 pm EDT  
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Hi, Eric.

Some very interesting ideas. You're really doing a lot of homework on this, which means you're gaining a lot of valuable experience in optics and mechanical engineering subjects. Kinda fun, huh?

I did want to interject a couple of things. The mirror design, I've heard of and we actually have a couple of this type mirrors in the 8" f/1.5 Schmidt camera, and also in every Schmidt-Cassegrain telescope as well. The recent addition of cooling fans to many telescopes has increased the quality of viewing by their faster equilibrium of the mirror temperature. With the mirror shaped as you describe, I think it would be best to blow air from the back directly into the center of the mirror. The heat sink is a good idea, but whether you have that or not, cooling the thickest portion of the mirror faster than the edges should get the equilibrium over the entire surface at about the same time. This is probably why the SCT's that have the fans mount them in the center of the back of the cell. Blowing across the face will cool the mirror more on the one side than the other, while not concentrating enough of the cooling power right where it needs to be, in the CENTER where the glass is thicker.

In an old article on mirror cooling systems and their pros and cons in Astronomy, the amount of time saved by cooling versus not cooling the mirror with fans was about 30% (I believe) faster with the fans. The time involved was about 30-45 minutes for a non-cooled mirror and about 20-25 minutes with fans. All this worry is minimal when you make sure you setup and collimate at least 45 minutes before observing begins, which we nearly always do anyway. Heat currents coming up through a solid tube also was discussed and there is another plus for you with that truss-tube design.

When its time to have your mirror coated, don't skimp! Go with the highest light transmission coating there is, with either a 98% or 99% reflectance on the primary as well as on the secondary. Your secondary will be bigger than mine due to the shorter f.l., but carefully figuring the exact specifications of the optical path and the placement of the components for MAXIMUM usage of every sq.cm. of the primary surface will get you a great image of every celestial target.

You're set to wind up with a very nice scope when all this is done, and I look forward to the night when I can see how well it stacks up side-by-side against ole' RedShift.

Bob


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ejbragg
 Wednesday, July 25 2007 @ 12:30 am EDT  
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Bob,

I do understand the ventilation of the center of the mirror, for just the reason you mentioned. In fact, I'm toying with some alternate ideas, for the sake of breaking the boundary layer and cooling the mirror at the same time. Your idea is certainly the tried and true version. Mine is .... well, up to now, I don't know anyone who's tried it. But rest assured I'm not going to use it until I test it. Please see the following diagram:



From where you are seated (pretending you are in front of the mirror in the diagram as shown), if you blow air toward the mirror, the surface and bottom should be cooled relatively efficiently, wouldn't you think? I'm thinking of using two fans side by side. Using a 4" fan, at 1" higher than the mirror, the surface boundary is eliminated. The remaining 3 inches with be directed under and around the mirror, but the cone shape of the mirror should scoop much of the air right into the heatsink. I could assist by creating a couple of air stops on either side of where the fan is blowing, scooping even more of the air into the sink.

Of course, I'm using computer technology to cool of a plate of glass, but the physics are the same, concerning metal against any object of higher heat enthalpy, and moving air. With the new conical mirror design, the mounting possibilities are rather open range, if you ask me!!

What you thinksies?


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bobmoody
 Friday, July 27 2007 @ 12:41 am EDT  
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Hi, Eric.

Try everything! You're getting a lot of exposure to astronomy and amateur telescope making and that's what I consider the best thing about your entire project. BTW, I have another booklet I want to loan you next week when we have our Aug 4th star party, that is, assuming you come up. Its the booklet that was supplied with the tube assembly kit I bought for Redshift. All kinds of formulas for EVERYTHING you're working with. You'll just have to plug-in the numbers for your own project. You'll find it invaluable in this project since you want a truss-tube design.

We have our next meeting at UAFS next Friday evening at 7pm in rm 211 of the MS-UC bldg. Come join us if you get the opportunity. I'm sure you'd enjoy meeting more of our members. In case you can't come up Saturday, but you can come to the meeting or at least meet me there on Friday, then let me know and I'll bring the booklet with me.

Keep plugging away........OH!, and take pics of your progress, too. It'd make a great article someday.
Bob


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ejbragg
 Friday, July 27 2007 @ 07:25 pm EDT  
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Thanks, Bob,
I will try to make the meeting. If you could e-mail me some directions, that would help (I live in Ft. Smith & don't really know my way around). And certainly, I'd be very glad to read whatever material you've got!

For these early stages, I have purchased the following pieces:
---------------------------------------------------------------
1) 12.5" Royce Primary w/ 96% enhanced reflectivity
2) 4" x 4" aluminum heatsink, 3/4 inch thick (fin type)
3) 1 tube Arctic Silver thermal grease
---------------------------------------------------------------
The mirror has yet to arrive.

And I am about to order:
---------------------------------------------------------------
1) Digital thermocouple thermometer (as soon as I can find a decently priced set-up!)
2) 4 Vantec double ball bearing "silent" cooling fans
3) 2.6" Protostar Quartz secondary
---------------------------------------------------------------

Here's my method / layout of the heatsink test I propose (any help offered is appreciated).

There will be two stages of testing. Stage 1 will test the temperature change of the mirror, from the bottom up. This will tell me how long it takes the mirror to stabilize to room temperature when cooled STRICTLY from the bottom, while the top (or face) of the mirror is protected and insulated from the ambient air change. This will reveal how long it takes the internal glass in its bulk to come to equilibrium. By cutting a hole in a large piece of cardboard, and placing the hole around the base of the mirror about halfway up, and caulking it into place, I effectively insulate the airflow across the bottom of the mirror from the top side. The mirror will go into my trusty oven, set to 100F for 2 hours for a good, stable adjustment! A thermocouple (carefully) stuck to the center of the mirror will help me with the temperature of the glass. A thermometer in the oven will also assist, of course! I will use a towel / fiberglas sandwich (towel, fiberglas bat, towel) laid over the mirror face to insulate it from the abmient air, once it is removed from the oven. I will test the mirror both with and without the heatsink attached. Of course, during the heatsink test, the heatsink will go into the oven already attached to the mirror. And the insulation batt will share space in the oven during heat time, too.

So the process will be:

1. Heat up mirror & towels to 100 deg for two hrs
2. pull from oven into 75 degree room, insulating face
3. blow fans across bottom of mirror (only)
4. Keep track of time and temperature data gathered from thermocouple
5. Once equilibrium is reached, place items in oven & repeat

3 tests will be conducted without heatsink; 3 tests with heatsink.

On might argue that the true representation of mirror cooling is performed by cooling the mirror without any insulation present. Though this is true, I suspect the surface temperature COULD reach ambient equilibrium even while the mirror's interior is still several degrees above ambient temperature. A mirror under such hidden stress could still contain deformities. Therefore this is the more important of the two test stages, in my mind.

The second stage will focus more on how long I can expect the mirror to cool down in a real application. The caulk and cardboard removed, no insulation on top, the test will be repeated with and without the heat sink, (or strictly without, if the above findings signify that the heatsink adds negligible benefits).

This second test will also be carried out 3 times for each scenario. I promise to tabulate / graph the results, with MANY pictures for Bob!! Cool


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ejbragg
 Tuesday, September 04 2007 @ 01:43 pm EDT  
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The mirror arrived last week. Please see the thread "Mirror Heat Dissipation Testing" for the activities concerning how the mirror should be cooled. Presently, I'm going to add some pictures for the little work I've had to do in the first construction phase...

A picture of the mirror with spec sheet.

And here's a picture of my first wood cut-out from my 2 sheets of Russian Baltic Birch (about $30 / sheet from Plunkette Distributing). This stuff is tough, but lightweight and easy to cut! This is to become 2 parts: the mirror back plate, + a round interior shelf for the mirror box.

Using my new router ($70 from Sears), I begin my first circular cut!

Once complete, I attach it to the primary mirror (notice I cut an additional groove, just for gripping power). - This is the new plate, by which I will handle the mirror, from now on. I will also use it for mounting and collimating the mirror, later.

Note also that the hardware that comes with this mirror is a brass 1/2" diameter bolt, a couple of nuts, washers, and two spring washers. In this first view , I haven't yet tightened the nut (thus no pressure applied to the mirror).

In the second view, I tighten the nut until some (but not all) the play in the spring washer has been used. With this method (recommended by the manufacturer), sufficient pressure holds the back plate to the mirror, and should the telescope be exposed to a wide range of temperatures, there is no danger of mirror warping due to too much back plate pressure, or loosening of the back plate from the mirror due to shrinkage.


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ejbragg
 Friday, April 11 2008 @ 09:57 pm EDT  
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It's been a while since I last posted. I have almost completed the assembly and it's time to catch up with posting the pictures. This first link is to a photo showing almost all the pieces after the staining and during varnishing. I decided to stain some, and leave other pieces blonde, for a two-tone effect.

http://www.FountainSquareHouse.com/assets/dobsonian/01-Varnishing%20Parts%202.jpg


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ejbragg
 Friday, April 11 2008 @ 10:17 pm EDT  
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For the ground board, two pieces of 1/2" plywood were glued together, then cut out. My bolt for the azimuth pivot assembly was a bit long, so I glued an additional piece to the center. I also cut out several holes, stategically placed, so as to keep the structural integrity of the ground board, yet remove some of its weight. Three feet were then added - from a refrigerator kit at a hardware store. The feet are adjustable. This is the bottom side.

http://www.fountainsquarehouse.com/assets/dobsonian/02-Foot%20Board%20a.jpg

On the top side, I glued on a laminated degree circle (printed at Kinkos). In the center, around the pivot bolt, is teflon. There are also three more teflon points for the soon-to-come rocker assembly above it. Note that the teflon pieces contain a divit for the screw heads, such that the screw heads will not rub the surface above. This structure is heavy duty and light weight, and sits solidly on the ground, and can be made flat, even if the ground is somewhat sloped.

www.FountainSquareHouse.com/assets/dobsonian/03-Foot%20Board%20b.jpg


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ejbragg
 Friday, April 11 2008 @ 10:37 pm EDT  
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The rocker assembly absorbs a lot of strain from the pushing and pulling above. Consequently, this piece is also double thick for strength, durability, and most importantly, rigidity. Having a flimsy rocker box or mirror box will cause your more distant objects to be very difficult to find or maintain in the eyepiece on zoomed-in views. Again, there are many holes to lighten the telescope part. Shown here, the rocker box is placed on the ground board. Not shown is the formica ring glued to the underside of the rocker box, where the rocker box makes contact with the teflon points. Teflon and a particular style of formica are famous for their ideal friction when in contact with each other. All teflon points are in contact with this formica type.

www.FountainSquareHouse.com/assets/dobsonian/07-Rocker%20Assembly.jpg


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ejbragg
 Sunday, April 13 2008 @ 12:18 pm EDT  
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The rocker box spins around on the ground board. The curved cut-outs on the upper two sides of the rocker box are made to hold the elevation bearings, which are basically half circles. The next picture is a shot of one of the side bearings, which will be affixed (rigidly) to the sides of the mirror box.

The diameter of the bearings is recommended to be 50% greater than the diameter of your mirror. This large diameter serves to prevent your finished scope from falling down or swinging upward on its own, if for example, you have a heavy eyepiece in the focuser, or anything else you might add which serves to throw your scope out of balance. A large diameter bearing tends to make slight imbalances more forgiving, and allows for a more stable telescope. Since the mirror in this case is 12.5" in diameter, a circle of wood at 19" diameter was cut out. I then sliced this down the middle for my bearings. Because the rocker box is double thick, I glued an additional 19" ring along the edge. This widens the bearing surface. The surface of the shown is to be covered with a strip of formica. Note more holes have been cut!

By the way, the "perfect" surface of formica was set by the standard made by the telescope-building powers-that-be: Use "WilsonArt" brand formica; the model is "EbonyStar # 4552-50". And if anyone decides to build one and needs some formica, I have a large leftover sheet in my garage. I'll sell pieces of it at cost, which is $2.22 per square ft. (Contact me at ejbragg.forwardedmessages06@zoemail.net)

http://www.FountainSquareHouse.com/assets/dobsonian/08-Side%20Bearing.jpg


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ejbragg
 Sunday, April 27 2008 @ 12:31 am EDT  
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The mirror box required more planning. In fact, there are a few things Id do differently, if I were to start again. However, what I wound up with works great. Note that the way dobsonians are designed, one corner which pivots near the top surface of the rocker box, needs to be cut off to prevent rubbing the rocker. This also introduced a bit of a problem, since I was earlier planning on positioning my cooling fans in that vicinity. Behold, I worked around the problem and everything turned out fine. In the first picture, you can see the bulk of the box going together (shown upside down).

http://www.FountainSquareHouse.com/assets/dobsonian/11-Mirror%20Box.jpg

I built four small shelves to go into the corners, designed to stay just above the mirror surface, about an inch. A mirror protector lid will fit on the shelf. Two are seen in this picture. Also note the two inlet fans. The upper 1/3 of the fans cool the surface of the mirror. The lower 2/3 pass underneath the mirror (through the heatsink shown in the following section).

http://www.FountainSquareHouse.com/assets/dobsonian/12-MB_intake_fans.jpg

The next picture shows the same box from the opposite side. Due to the 45 degree corners, the exhaust fans had to be moved to the outside to prevent bumping the primary mirror.

http://www.FountainSquareHouse.com/assets/dobsonian/13-MB_exit_fans.jpg

Another look at how the fans were installed. I cut out a rubber gasket to place between each fan and the mirror box to reduce the vibration caused by the fans. I haven't tested the telescope without the gaskets in place, so I cannot verify that the gaskets are needed, but I will say this: the telescope later proved to be very clear in viewing, and turning the fans on or off has not yet revealed any noticeable difference.

http://www.FountainSquareHouse.com/assets/dobsonian/09-Mirror%20Box%20Fans.jpg

http://www.FountainSquareHouse.com/assets/dobsonian/10-Mirror%20Box%20Fans.jpg


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ejbragg
 Tuesday, April 29 2008 @ 02:41 pm EDT  
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The back plate for the mirror provides 3 uses:
1. A stiff handling mechanism for the primary mirror
2. A surface by which to collimate the primary
3. A surface by which to funnel air into the heatsink.

Here, two, thin wood triangles were installed (with gorilla glue) to help scoop the air from the inlet fans through the heatsink. Once the backplate is installed, you should be able to see right through the heatsink.

http://www.FountainSquareHouse.com/assets/dobsonian/14-Mirror_board.jpg

http://www.FountainSquareHouse.com/assets/dobsonian/15-Mirror_board2.jpg

http://www.FountainSquareHouse.com/assets/dobsonian/16-Mirror_assmbly.jpg

Then the mirror is installed in the mirror box. I attached the mirror to the bottom plate of the mirror box by connecting its back plate to the mirror bottom plate with collimation bolts. In these pictures, the reference between the mirror and the fans can be seen. The mirror lid goes inside the box. An outer lid is to be made later.

http://www.FountainSquareHouse.com/assets/dobsonian/17-Installed_Mirror.jpg

http://www.FountainSquareHouse.com/assets/dobsonian/18-Installed_mirror2.jpg

http://www.FountainSquareHouse.com/assets/dobsonian/19-Mirror_Lid.jpg


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