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#1 rogerry

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Posted 23 February 2009 - 09:10 AM

I am making a tripod and pipe mount for a C6R on the cheap! What leg spread should I have on the tripod if I make it about 4'6" high? Would a 4ft spread be stable or should I go with 4'6"?

#2 rogerry

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Posted 23 February 2009 - 09:21 AM

Or maybe 4ft spread? I want the thing to be stable at 200x so I am using 1 1/2" steel tubing. I do not want to be lying on the ground either when looking at zenith so I was thinking the height should be somewhere around 4'-6".

#3 Spoonsize

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Posted 23 February 2009 - 09:45 AM

Roger...I do not have a specific answer for you.
You can rough figure by dividing the length of the OTA by 2 and adding that to the height you'd want your eyepiece to be when viewing at the zenith.

#4 BlueMoon

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Posted 23 February 2009 - 10:55 AM

Here's a handy little web-based utility for solving your question, which is basically solving the hypotenuse of a right triangle. Just play with the values (in inches seems appropriate) for side d (height) and e (distance from center) to solve f (leg length). That should get you in the ballpark. http://id.mind.net/~...tTriSolver4.htm

When I checked results on my tripod e=28, d=40, then f=48 which was correct to the base of my tripod. I added 12" more to the center of the Tee. The center of my 'scope sits at approx 60" above ground level, perfect for my height and viewing chair. The eyepiece, with the 'scope vertical is at 32" above ground, as low as my observing chair will go. I could add about another 6" to the overall height but the higher you go, the more vibration and stability is effected and I seldom view at more than 80 degrees above the horizon anyway...

Pictures are here: http://www.cloudynig...5/o/all/fpart/1

Cheers.

#5 rogerry

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Posted 23 February 2009 - 11:25 AM

I guess what I am after is what should my leg spread be to be stable? I am guessing that my tripod height would be about 4'-8", then add the 12" pipe mount height. This would get me to the center of the scope, but the C6R is nose heavy so eye height at zenith would be more than 2ft lower than the top of the pie mount, how much I am not sure. Any C6R owners out there that can tell me what the balance point of that scope is?

#6 jgraham

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Posted 23 February 2009 - 12:44 PM

I used to make my tripod legs so the angle they made with the ground was about 60 degrees. The problem with a tall tripod is the spread of the legs can get pretty large, but a 60 degree angle with the legs would give a very stable platform.

#7 BlueMoon

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Posted 23 February 2009 - 12:45 PM

I guess what I am after is what should my leg spread be to be stable?

Ah, sorry Roger. I misunderstood your question...

Cheers.

#8 magic612

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Posted 23 February 2009 - 01:09 PM

Hi Roger -

I faced a similar problem with a 127mm f9.5 refractor I purchased used a few weeks back. If I am remembering my CAD drawing correctly, I think I use about a 65 degree angle for the legs. I'll have to measure the height when I get home - I cut mine down from what I originally built, as I changed the design.

I also employed a counterweight balance system to account for the "lens heavy" aspect of mine - maybe my mount will give you some ideas for yours. The nice thing about the counterweights is it reduces the "radius arm" of the eyepiece side, so that the distance between the horizon and zenith is greatly diminished. That way I'm not on the ground looking at the zenith, even though the eyepiece height is at eye-level when the scope is level.

#9 rogerry

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Posted 23 February 2009 - 01:29 PM

Do you think I could get away with a 4 ft 8 inch high tripod where the legs make a 67 degree angle with the ground? This is about 30" from any leg to the center of the tripod.

#10 magic612

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Posted 23 February 2009 - 01:43 PM

I'll check both my CAD drawing and my mount when I get home this evening, and let you know what the dimensions and angles are. Then you can make a determination based on what I did.

What I can tell you at the moment is that even though my scope seems very high up and appears that it might have a high center of gravity, it is actually very stable, and I did some test "pushes" on it to gauge it's stability. It certainly doesn't have any tendency to want to tip over.

#11 rogerry

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Posted 23 February 2009 - 02:51 PM

Thanks, David. I like your design but I am building the pipe mount based on the article by Mr. Wang. My tripod will not be adjustable so I am trying to come up with a good height whereby I will not be lying on the ground or standing on a step stool. I don't really know where the balance point of the scope is either since I don't have it yet. All of these variables are driving me nuts!

#12 magic612

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Posted 23 February 2009 - 03:11 PM

Roger - you're quite welcome. Though, you needn't build a wood mount like mine to employ some of the similar ideas that I utilized. My tripod is not adjustable either - that's why I used the counterweights to minimize the eyepiece heights differences. In fact, I'm currently finishing up an adjustable height observing chair to account for that.

I just thought it might help you with ideas for your particular design issues. If it does - cool, I'm glad to help. If not, I hope you're able to design and build what you need.

#13 BlueMoon

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Posted 23 February 2009 - 04:09 PM

Roger, just how heavy is your C6-R OTA with finder and EP? I've looked about the 'net and there's seems to be wide variation in the reported weights.

#14 rogerry

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Posted 23 February 2009 - 04:42 PM

I don't know, it won't arrive until Thursday or Friday. I was just going by other CNers comments that the balance point was not center of the tube but closer to the lense end. How much closer I do not know. I was hoping someone with a C6R would chime in.

#15 mathteacher

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Posted 23 February 2009 - 06:44 PM

Hi, Roger. I believe I read the C6R OTA was 14 lbs bare. Add a finder, diagonal, EP, I think it gets ~16-17 lbs. This is consistant with my experience with my CR150HD (an earlier version of C6R with collimatable cell). The removeable dew shield is very heavy. Some folks replace it with an astrozap flexible dew shield to make the scope less nose heavy (I use an Orion dew shield that is $24). The problem with using a flex shield is it's harder to mask down the aperture to reduce CA. However, it helps to shift point of balance closer to the middle. Some go as as far as wrapping ankle weights near the focuser. This is all to move the EP closer to the Altitiude axis to reduce the change in EP height from horizon to zenith.

Here is a photo of my CR150HD on my pipe mount with lightweight flex shield in place. You can see the point of balance. PHOTO

#16 magic612

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Posted 23 February 2009 - 07:08 PM

Okay Roger, I did some measurements, but realize that my mount style is different than yours, and my scope is also not the same. But perhaps you can extrapolate some useful information from it:

Angle of tripod legs: 65 degrees, from ground to leg
Height of actual tripod: 39 inches (this is the top of the azimuth bearing surface)
Height at altitude axis fulcrum: 64 inches (this is the top height of the "U" shaped cradle)
Radius of tripod legs: 23 inches from center to tripod "foot"

By the way, the total "eyepiece swing" of my telescope is about 20 to 22 inches, depending on the eyepiece used, from horizon to zenith.

Hope that helps!

#17 mathteacher

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Posted 23 February 2009 - 10:27 PM

Roger, I measured the distance from the center of the Altitude axis to the end of my visual back (using the setup in the picture). It is 25". Figure in a diagonal, focus travel, and your EP is about 28" from the center of the Alt. axis.

You asked about the dimensions of your tripod. I made a little sketch (top down view). The points of the triangle are the feet of the tripod. As long as the scope + mount's center of gravity (CG) is within the triangle, you will not tip over. Now, when your scope is mounted with counterweight (CW), center of gravity should be pretty close to the center of the triangle. Only when loading and unloading is the load off-center. The diagram depicts the location of OTA and CW when I load up. I usually put my 12.5 lbs of CW on first. The OTA is held vertically (between the legs for clearance) and attached next. When fully loaded, there is over 30lbs of scope and CW centered on the tripod. Although it is top-heavy, it would take a helluva breeze to blow it over (I would not be observing then).

Worst case scenario for tip-over would be if the OTA was attached and no CW was attached. I measured the distance from the center of the tripod to the center of the OTA to be 9". Figuring in the weight of the tripod, I'd guess the CG to be 6" off center. The distance from the center of the tripod to the midpoint of a side on the triangle is half the distance of the center to the foot of the tripod. For example, you mentioned 30" from the center to the foot. The midpoint of the side of the triangle would be 15" from center. With a 6" off center CG, you have a safety factor of 15/6 or 2.5. I'd say that is pretty stable. If you had a safety factor of 2, or 24" distance from center to foot, I'd say that is still okay.

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#18 rogerry

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Posted 23 February 2009 - 10:36 PM

Thanks Mr. Wang, the sketch is a big help...exactly what I was looking for. By the way, I was at Lowe's today looking at galvanized pipe fittings. They didn't have 2" pipe, only 1- 1/2" pipe which is about 1-3/4" OD. Was it this that you used?
Thanks again,
Roger

#19 mathteacher

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Posted 23 February 2009 - 10:42 PM

Roger, FYI, the sketch is NOT to scale! It is just a sketch.

I used 2" pipe parts from Home Depot. EDIT: If all you can get your hands on is 1 1/2", I would use it as a last resort.

#20 BlueMoon

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Posted 23 February 2009 - 10:42 PM

I can pretty much confirm Mr.Wang's measurements from my own mount the difference being being 12" instead of 9".

The safety factor I think is pretty close too. With my OTA center at 12" from center of the Tee and a measured center to leg pad distance of 28", I can take the 25# of counterweights off and the assembly is still very stable. My OTA assy weights approx 25.5#...

I would not sweat it and just use it.

I must respectfully disagree with Mr. Wang on this point. Locating 2" fittings really would pay off in the end. The larger the bearing surface you can provide, the smoother the movement and more dispersed the loading on the threads. A smaller bearing area will concentrate the loading force and be subject to "binding" more easily. Also, to achieve better balance with a smaller load bearing area your counterweights would need to be closer in weight to your OTA assy and placed equidistant from the center to equalize the leverage.

I know, it's a low tech solution, but physical forces still apply.

Cheers.

#21 mathteacher

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Posted 23 February 2009 - 11:18 PM

Also, to achieve better balance with a smaller load bearing area your counterweights would need to be closer in weight to your OTA assy and placed equidistant from the center to equalize the leverage.


Jeff, I don't understand this part. I thought as long as you have the same Moment (weight x distance), the load will be centered on the azimuth axis. So, you can use the same weight at the same distance from center, or half the weight at double the distance. I chose to use less weight at a longer distance because I dislike carrying stuff. Also, I think it has an added benefit of reducing overall load on the Azimuth axis.

I agree with you on the smaller fitting having more stiction. 2" fittings would be best, no doubt. However, I think it may not be a deal-breaker. Roger may find it hard to use at high power (150x and up), but I think it may be still useable at 200x. The area of a cylinder (the threaded surface on a pipe) is 2*pi*radius*length. Assuming the smaller pipe can engage the same length of thread (maybe questionable), the area is directly proportional with the radius. A 2" pipe will have 1.33x the area of a 1.5" pipe. I'm assuming friction is directly proportional to area here (correct me if I'm wrong). So, a mount with 1.5" pipe fittings will have 33% more friction than one made with 2" parts. While less desirable, I think it is still usable.

EDIT: I think you should try to get 2" parts if you can. Use 1.5" parts only as a last resort.

#22 BlueMoon

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Posted 24 February 2009 - 12:41 AM

Jeff, I don't understand this part. I thought as long as you have the same Moment (weight x distance), the load will be centered on the azimuth axis. So, you can use the same weight at the same distance from center, or half the weight at double the distance.

I apologize, you are correct. I was confusing torque with the moment arm. The longer arm would produce more torque upon the Tee (the pivot) unless the weights were adjusted to balance the moment arms.

A 2" pipe will have 1.33x the area of a 1.5" pipe. I'm assuming friction is directly proportional to area here (correct me if I'm wrong). So, a mount with 1.5" pipe fittings will have 33% more friction than one made with 2" parts. While less desirable, I think it is still usable.

For the very low rotational speed and loading, I'm sure your right about the friction being proportional (or close enough!). The smaller diameter pipe will have a higher coefficient of friction than the larger. Carefully balancing the assembly and lapping the threads well would give good compensation for the increased thread loading and friction.

Wow! There's more science in "low tech" than one thinks!

Cheers!

#23 mathteacher

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Posted 24 February 2009 - 12:51 AM

Hi Jeff, I'm glad you brought up the 2" vs. 1.5" issue. You are absolutely right on that. I edited my posts after revisiting Jeff Duntemann's website. He only had 6" or 8" tubes on 2" pipe mounts. These were homemade OTAs, and I assume they were heavier than today's tubes (maybe 25 lbs. for the 8"?) So, there is a weight limit to maintain smooth motion, and 2" would worth the effort to obtain.

Roger, do you have a home depot near you? You can also check a plumbing supplier, though my local plumbing store charged a lot more for the same parts.

#24 rogerry

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Posted 24 February 2009 - 06:23 AM

Thanks, Mr. Wang. I am sure with a little bit of hunting I can locate 2" fittings. As a last resort I can always order stuff online but the 2" stuff is heavy!
Wow, you are not kidding, Jeff! There is more science than I originally thought into this mount. This would make a nice experiment for some physics class!
Thanks for all the help guys, I will post photos when I am done.
Roger

#25 BlueMoon

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Posted 24 February 2009 - 09:21 AM

Thanks for all the help guys, I will post photos when I am done.

You bet! Be looking forward to seeing your handiwork!

Cheers!






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