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etalon
member

Reged: 01/07/13

Loc: Germany
#5661587 - 02/04/13 11:35 AM

Danny, Iīm sorry, but you are wrong. Dick explained it correctly. You have an increasing torque, when the tube move in a horizontal orientation. Thatīs the reason, why I used springs with different power moments and different fix points on my travel Dob. The springs have a nonlinear power moment, and normally you need more than one spring to compensate some hysteresis effects...

@all: Very nice scopes are shown here!!!

cs
Markus

Edited by etalon (02/04/13 12:16 PM)

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Pinbout
Postmaster

Reged: 02/22/10

Loc: nj
#5661613 - 02/04/13 11:55 AM

I'd appreciate if you spelled my name correctly.

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etalon
member

Reged: 01/07/13

Loc: Germany
#5661661 - 02/04/13 12:27 PM

Sorry, I have corrected it.

My (technical) English is much to bad for explaining such things, Iīm sorry, but you can read it in every book about primary physics or technical engineering...

Maybe there would be some other guys, who speak English much better than I, and who can explain it. I also donīt know, whether, and when how, I can write math formulas here on cn...

Markus

Edited by etalon (02/04/13 01:00 PM)

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Pinbout
Postmaster

Reged: 02/22/10

Loc: nj
#5661793 - 02/04/13 01:35 PM

the more the expansions springs are expanded the less effective they are.

the lever arm on the scope I drew is about 6in away from the center of rotation, and the 6in mirror is approx. 32in away from the center of rotation.

so if I rough in the torque balance eq.

6in*(x)lbs = 32in*(y)lbs

so in this equation where do you implement the angle of momentum.

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etalon
member

Reged: 01/07/13

Loc: Germany
#5661882 - 02/04/13 02:18 PM

The spring would not be less effective when it work in the range of elasticity. If you over expand the spring, it deform plastically, would became less effective and would be damaged.
Here is a link to calculate springs (compression-, extension- and torsion-springs):

http://www.federnshop.com/Berechnung/Schenkelfeder/SchenkelfederBerechnung.aspx

Maybe it would help.

In case of your Dobson, you get a torque moment depending on the length of the leverarm, the angel of the lever arm to the gravity force vector and the wight force at an arbitrary point on the lever arm.

Markus

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Dick Jacobson
scholastic sledgehammer

Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
#5662111 - 02/04/13 04:51 PM

Maybe it would help to imagine holding a heavy dumbbell, simulating the weight of the mirror. When your arm is about vertical, it takes only a little force to start raising it, but when it reaches horizontal it takes a lot more force on your shoulder. Mathematically, the torque due to the mirror is proportional to the sine of the angle of the tube away from vertical.

I hope we haven't totally derailed this topic. There are some wonderful ideas here. Markus, your travel scope at the top of page 6 is magnificent!

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Pinbout
Postmaster

Reged: 02/22/10

Loc: nj

Quote:

Mathematically, the torque due to the mirror is proportional to the sine of the angle of the tube away from vertical.

just one more thing.

on the mirror side,from angles 0-45, torque=Length*lbs*cos angle, from angles 45-90, torque= length*lbs*sine angle.

so at 45* the torque is the least.

Still working on my travel dob

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droid
rocketman

Reged: 08/29/04

Loc: Conneaut, Ohio
#5662997 - 02/05/13 07:48 AM

That gonna be a carry on scope? looks almost ready to go. Nice workmanship

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Pinbout
Postmaster

Reged: 02/22/10

Loc: nj
#5663094 - 02/05/13 09:01 AM

yep,

but I'm making different truss mounts,

like these

and my mirror is still hyperbolic

[I got a blank, but found it was polished figured and very,very hyperbolic now its only slightly hyperbolic]

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droid
rocketman

Reged: 08/29/04

Loc: Conneaut, Ohio
#5663154 - 02/05/13 09:33 AM

Cool video......my only concern would be Id get to the dark sky site and realize I didnt load the Allen wrenches, lol.
Could that be done with bolts with knobs??or would they get in the way of when packaging up for travel?

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Tom Clark
professor emeritus

Reged: 11/14/07

Loc: North of Deming, NM

Hows this for ugly? It is a 42" f/4. The reason for the square tube is to raise the balance point up towards the center of the tube. This allows for the scope to fit into a much smaller dome than the typical dob would - with a balance point closer to the mirror end. It also allows for the scope to be completely sealed up to keep dirt out when not in use. Interior doors close just over the mirror to keep the mirror clean.

The box near the focuser holds five of the most used eyepieces. The finder is a 5" f/9 to allow for wide-field views.

Edited by Tom Clark (02/05/13 09:47 AM)

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tag1260
scholastic sledgehammer

Reged: 10/07/12

Loc: Ohio, USA
#5663191 - 02/05/13 09:49 AM

Looks like a fine piece of furniture to me!!! Beautiful wood working there.

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droid
rocketman

Reged: 08/29/04

Loc: Conneaut, Ohio
#5663232 - 02/05/13 10:07 AM

As an old ATMer told me years ago, after I fielding a dob uglier than the south side of a mule, " aint no such thing as an ugly telescope in the dark "

nough said, totally true

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Pinbout
Postmaster

Reged: 02/22/10

Loc: nj
#5663259 - 02/05/13 10:17 AM

that's what I'm working on.

since it's made out of 3/4 ply, I need to raise it about a 1/4 in, so I made plates to raise it up. I have press fit knobs for the allen head screws. i'm using them here on the alt bearings

also its better to come in from the aperature hole side with the screw, the alt bearings kind of get in the way, on the backside it fine. compare the previous pic with this.

so goes my life make it twice before I get it right.

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Pinbout
Postmaster

Reged: 02/22/10

Loc: nj
#5663267 - 02/05/13 10:21 AM

Quote:

Hows this for ugly? It is a 42" f/4.

I would have to agree, that is very ugly indeed.

now if you had a dragon [for heavan] or a tiger [for earth] burnt into the wood somewhere it would add the coolness back.

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etalon
member

Reged: 01/07/13

Loc: Germany
#5663455 - 02/05/13 11:52 AM

Quote:

There are some wonderful ideas here. Markus, your travel scope at the top of page 6 is magnificent!

Thanks a lot, Dick.

Danny, at 0° you will have the least torque moment.
Letīs have a look to a bearing and a mass (kg) which is fixed to the bearing with a stiff rod. Now, the gravitation will accelerating the mass by 9.81 m/s^2. This mean, that a wight force will pressure to the bearing in N ((kg*m)/s^2). If the force vector is going vertical through the bearing, your mass will hanging vertical, and the torque moment in the bearing is 0 (like the sin0°=0). Now we will deflect the mass (i.e. the scope) for some degree. There is now a distance (measures in m) between the wight force vector and the mass. This distance is your lever arm (standing rectangular on the wight force vector). Because of this lever arm, you get a torque moment ((kg*m^2)/s^2 or Nm) in your bearing. This will increase, when the deflection angle will increase, because the distance between the wight force vector and the mass (your lever arm) will increase, too. The torque moment will have the maximum at 90° (like sin90°=1), because if the angel further increase or decrease, your lever arm will always decrease, and there fore the torque moment will always decrease, too.

Now letīs look to the other side:

You have a counter mass hanging vertical and produce a wight force on the bearing (N). If you deflect the telescope, the mass will move along the wight force vector, so the wight force would always be the same. Even if you give a lever arm to that counter mass, the resulting negative torque moment at the bearing will be always the same, too. Now you can adjust the counter mass to equal one torque moment at the bearing, which is equal to one deflection angel of the telescope. If you deflect the telescope over that angel, it will sink back and in the other direction, the counter mass will rise up the scope to the point of torque moment equation.

I hope, you can understand, what I would say...

@Tom: Wow! Thatīs a huge scope! If you will make a more beautiful telescope for you, I will dispose the old one for you in the "garbage can" in my backyard...

cs
Markus

Edited by etalon (02/05/13 12:07 PM)

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Pinbout
Postmaster

Reged: 02/22/10

Loc: nj
#5663568 - 02/05/13 01:07 PM

Quote:

You have a counter mass hanging vertical and produce a wight force on the bearing (N). If you deflect the telescope, the mass will move along the wight force vector, so the wight force would always be the same. Even if you give a lever arm to that counter mass, the resulting negative torque moment at the bearing will be always the same, too. Now you can adjust the counter mass to equal one torque moment at the bearing, which is equal to one deflection angel of the telescope. If you deflect the telescope over that angel, it will sink back and in the other direction, the counter mass will rise up the scope to the point of torque moment equation.

I hope, you can understand, what I would say...

I understand all that but don't you think there would be capacity because of the radius of the bearing to minimize the differences in the angled vector force of the mirror and on the opposite side, the counter weight that has no changing angled vector, if you could understand my english?

one reason why I asked this is in a dob I rehab'd it wasn't balance very well. so when it got too close to zenith it tilted back, when it got below 30* it would nose dive. like the forces weren't linear and formed a curve base on what direction I was from 45*. I since have balanced it better and made bigger bearings.

just starting to rehab it.

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etalon
member

Reged: 01/07/13

Loc: Germany
#5663626 - 02/05/13 02:00 PM

Yes Sir, hardly, but I can understand it...

Your problem looks to me, that the bearing was mounted in a way, which placed it outside the balance point in the long axis as well as outside the balance point in the short axis of the scope. in that case, the two torque moments are be superposed and effect the scope like you told it. Making the bearings bigger means only to increase the static friction because of a larger face. This can compensate a little imbalance...

cs
Markus

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killdabuddha
Pooh-Bah

Reged: 08/26/11

Since the talk has turned to ugly scopes, I feel i can post a pic of ours in the basement where it was built. Do we win anything?

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Pinbout
Postmaster

Reged: 02/22/10

Loc: nj
#5663668 - 02/05/13 02:25 PM

Quote:

Since the talk has turned to ugly scopes, I feel i can post a pic of ours in the basement where it was built. Do we win anything?

double the pleasure, double the fun.

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