263 replies to this topic

[tommm]

https://stellafane.o...ltbearings.html
 

stellafane says the optimal size is 1.8X the mirror diameter. Going up to 2X seems a logical step.

There is no "optimum" size.  The radius of the Alt bearings determines the position of the Alt axis of rotation and the OTA balance as a result.  You optimize it for your scope, depending on relative masses of the UOA and LOA (upper and lower optical assemblies, aka mirror box and secondary cage). 

 

Because the Alt bearings are just a segment of a disk, they move the center of mass of the OTA off of the optical axis, to "forward", or "below" it, depending on angle of the OTA.  Other off-axis things such as focuser/ep assembly also effect that.

[Starman1]

There is no "optimum" size.  The radius of the Alt bearings determines the position of the Alt axis of rotation and the OTA balance as a result.  You optimize it for your scope, depending on relative masses of the UOA and LOA (upper and lower optical assemblies, aka mirror box and secondary cage). 

 

Because the Alt bearings are just a segment of a disk, they move the center of mass of the OTA off of the optical axis, to "forward", or "below" it, depending on angle of the OTA.  Other off-axis things such as focuser/ep assembly also effect that.

Yes, but the same altitude axis in the OTA can result from many different diameters of altitude trunnion.

The radius of the trunnion does not determine the altitude axis of the scope, it's the other way around, but there are choices in trunnion diameters for the same alt.axis.

 

The diameter of the altitude trunnion determines the height of the rocker box and the force needed to move the scope in altitude.

There can be altitude trunnions that are too small and require too little force to move the scope, or too large and require too much force.

You are right there is no "optimum" size, but there is a range of sizes that could be considered optimum, i.e. not too small and not too large.

Edited by Starman1, 02 May 2022 - 01:02 PM.

[tommm]

Yes, but the same altitude axis in the OTA can result from many different diameters of altitude trunnion.

The radius of the trunnion does not determine the altitude axis of the scope, it's the other way around, but there are choices in trunnion diameters for the same alt.axis.

 

The diameter of the altitude trunnion determines the height of the rocker box and the force needed to move the scope in altitude.

There can be altitude trunnions that are too small and require too little force to move the scope, or too large and require too much force.

You are right there is no "optimum" size, but there is a range of sizes that could be considered optimum, i.e. not too small and not too large.

I should have said "distance of the Alt axis from the primary mirror".  Yes, other things change it's height relative to the ground which I think is the way you are thinking of it.  I was thinking of large Dob's where the primary mirror is already as close to the ground as you can get it.  In that case the only way to change the balance without changing mass distribution in the OTA is by changing trunnion size, and trunnion radius determines the position of the Alt axis, which is at its radius of curvature.

[Starman1]

I should have said "distance of the Alt axis from the primary mirror".  Yes, other things change it's height relative to the ground which I think is the way you are thinking of it.  I was thinking of large Dob's where the primary mirror is already as close to the ground as you can get it.  In that case the only way to change the balance without changing mass distribution in the OTA is by changing trunnion size, and trunnion radius determines the position of the Alt axis, which is at its radius of curvature.

I presume you are including the mass of the altitude trunnion itself in your point.

In most scopes, it is not a large % of the total mass of the OTA.

Especially on something big, like a 25" Obsession.

 

Again, trunnion radius does not determine the position of the alt axis unless the trunnions are excessively heavy, like steel or some such.

Then, the trunnion weight would lower the altitude axis some.

 

But normally, it is the position of the altitude axis that determines the location of the altitude trunnions, after a diameter is chosen.

My 12.5" dob has 19.5" altitude trunnions, but they could have been anywhere from 1.0x to 2.0x the diameter of the mirror and had only a minor effect on the COG.

That diameter would determine the height of the mirror box and rocker box sides and determine the force needed to move the scope.

It would determine where on the mirror box the trunnions were attached as well.

They do have some mass, but, as on many dobs, some of that mass is above the altitude axis when the scope points high so the weight of the trunnions themselves have less than a profound effect on the position of the altitude axis.  Some, but not enough to determine the diameter of the trunnion.

[Oberon]

Each is dependent on the other, and thus defined by design constraints.

In Merope’s case the position of the altitude axis determined the radius of the altitude trunnions because the mounting position of the trunnion is fixed to the mirror cell. In Merope’s first iteration the trunnion radius was too small, and required me to lower the balance point by adding more than 10kg of weights below the mirror (iirc).

To remove these weights I enlarged the trunnions to increase their radius to the point where their rotational center matched the unweighted balance point of the telescope. Relocating the trunnions was not an option due to Merope’s cell (as distinct from a classic Obsession style box), therefore the trunnion axis was determined by the trunnion size.

[tommm]

I presume you are including the mass of the altitude trunnion itself in your point.

In most scopes, it is not a large % of the total mass of the OTA.

Especially on something big, like a 25" Obsession.

 

Again, trunnion radius does not determine the position of the alt axis unless the trunnions are excessively heavy, like steel or some such.

Then, the trunnion weight would lower the altitude axis some.

 

But normally, it is the position of the altitude axis that determines the location of the altitude trunnions, after a diameter is chosen...

We seem to be talking past each other.  The OTA rotates about the center of curvature of the trunnions. They determine the point it rotates about.  Where the Alt axis is in space also depends on the position of  the trunnions on the mirror box, the rocker box height, and the height of the ground board pads above ground.   My scope has shallow mirror box and rocker box so there is little room to move the trunnions up/down on the mirror box.  The only way to change the position of the altitude axis is then by changing trunnion radius.  That is common on low profile mirror boxes - like Merope.  It has nothing to do with the mass of the trunnions.  However, my trunnions have a radius of 20 1/4" and do add significant mass to the OTA, moving it's center of mass forward of the OTA optical axis since they are only sections of (less than half) a disk.

 

I agree with you that on a scope where you have a range through which you can locate the trunnions on the mirror box, or can change the rocker box height, you can take the approach you outline, fixing the Alt axis position, then adjusting trunnion radius. On a tube OTA you could choose your rocker box height.
 

Edited by tommm, 02 May 2022 - 11:52 PM.

[Starman1]

The alt axis is fixed, even without trunnions present.

If you attach trunnions, the center of the curve must be at the altitude axis.

The altitude axis determines the trunnion.

If the small mirror box doesn't allow movement of the trunnion, then a very limited diameter of trunnion is allowed.

That isn't common on dobs, but on a scope like a Merope or a Waite Renegade, it is a factor.

 

Add trunnions and the center of mass might change, but so does the altitude axis.  In the ideal dob, if it is well designed, the COG is on the altitude axis.  It defines it.

 

Changing trunnion radius has no effect on the position of the altitude axis.  Changing trunnion MASS changes the position of the altitude axis.

Enlarging your trunnions don't move the center of mass forward of the altitude axis, they move the altitude axis forward with it, as long as the scope is balanced.

I don't think we disagree, but what I'm saying is I think your descriptive language is backwards.

The center of gravity doesn't define the altitude trunnion diameter except in a special circumstance (like the Merope).

Normally, there is a whole range of altitude trunnion diameters that could work with the same scope.

That is certainly true of nearly all commercial dobs.

Edited by Starman1, 03 May 2022 - 12:33 AM.

[jtsenghas]

Perhaps out of courtesy to Jonathan we shouldn't discuss at length in this build thread these competing general design issues on dobs.

That being said, My 13.1" dob, which is similar enough to Merope that it is nearly a 3/4 scale model, has 16" radius trunnions, which is nearly a diameter three times the primary. The only weight I needed to add to the lower end was five pounds of lead strategically placed to address the asymmetry of balance of the trunnions themselves,  which were only partial offset by UTA asymmetries in balance. .

Despite accommodating several pounds of focuser, finders, Paracorr 2, and heavy wide angle eyepieces, this scope balances well with a 1.021" thick primary.

Despite having the primary very low between the trunnions, the center of gravity is only a few inches below the midpoint of the trusses. That last point makes it pretty much moot that my aluminum poles are unnecessarily thick at 0.062" wall thickness too.

Edited by jtsenghas, 03 May 2022 - 10:48 AM.

[tommm]

The alt axis is fixed, even without trunnions present...

The altitude balance point is fixed without trunnions.  The altitude axis is determined by the radius and location of the trunnions.  Without them there is no altitude axis because there is no way to rotate the OTA.  The designer selects proper radius and location of the trunnions so their radius of curvature is at the balance point of the OTA so it balances on them.
 

[Adam Long]

The altitude balance point is only fixed if you never plan to change EPs or accessories. So locating the trunnion axis 'near to the balance point' is a more accurate statement than 'at'. 

 

Add the fact that the balance point is rarely on the tube centre-line, and most of us want to err on the side of bottom heavy, 'just above and slightly behind, most of the time' is probably closer to the truth for most of us.

Edited by Adam Long, 04 May 2022 - 08:34 AM.

[Oberon]

The altitude balance point is only fixed if you never plan to change EPs or accessories.

Which (to get back to the question in #246) is an important justification for choosing large diameter trunnions. Larger trunnions are more tolerant of imbalances, and permit changing of eyepieces without loss of position. In Merope’s case I have no trouble going from no eyepieces at all to loading up with Paracorr and Nagler 31 making it very painless to use. Same with the baffle, the finder and the shroud. I can make any change from lightest to heaviest combination without losing balance, and it is still easy (very sweet) to move by hand.

A better rule of thumb for trunnion diameter is to make it similar to the azimuth platform, the object being for both axis to have a similar feel, so that the force required to move in any direction is similar.

[Oberon]

One feature I've never really drawn attention to in much detail is the pair of latches at the rear of the trunnions that lock the OTA to the Azimuth section, enabling me (together with other latches) to wheel the telescope around as one piece, and saving my back.

 

The reason being that I was never entirely happy with the solution and felt that one day I would do something better.

When the latches slipped open recently and the telescope shifted unexpectedly with potentially serious consequences I decided that the day had come, I needed a better solution.

Time to fit a pair of sliding bolt latches...

 

 

After drilling, the teflon side plate is removed and a shallow recess cut with the slide saw to enable the lock to fit flush...

 

Edited by Oberon, 10 May 2022 - 08:42 AM.

[Oberon]

The latch fits snugly into place, and the keyhole is drilled and key fitted. When the bolt is withdrawn it sits flush and doesn't interfere with the trunnion.

 

Turn the key and the bolt protrudes into the trunnion, locking the telescope firmly into place.

 

Note the sharp point on the end of the bolt, it acts as a marker and **** punch for accurate drilling.

A stepped cone drill ensures the pilot hole does not wander when drilling the trunnion, as it is important that the bolt-hole is accurately aligned. Finish off with a plain ordinary drill marked with tape to control depth.



Rinse and repeat for both sides and put telescope back together.



The key is almost invisible, holds firmly in place (is not loose) but is easily removed should I want to take advantage of the flat surface for compact packing of the scope as originally designed.

And it works like a treat! Very happy to share these details as a robust solid solution.

 

Edited by Oberon, 10 May 2022 - 08:57 AM.

[tommm]

...A better rule of thumb for trunnion diameter is to make it similar to the azimuth platform, the object being for both axis to have a similar feel, so that the force required to move in any direction is similar.

Of course that required force for the azimuth axis will change with altitude, since the user often applies that force to the UTA (or upper tube end) and it's moment arm for applying torque to the azimuth axis is a function of altitude.