Geometry and Leveling of Equatorial Mounts?

And Oh, by the way, leveling the base of a tripod is really done more to
allow the wedge to not be messed with a lot between setups. The more
perfect that level is, the less possibility of having to reset the angle of
the wedge. Some people do get a bit anal about the leveling issue tho.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!

I'm not an expert on telescope mounting, just a silly engineer, but if
I were to build something like this, I would NOT mount it in the
manner you describe (slightly raised and using steel bolts to both
hold the pier and to level it). I would mount it directly to the most
solid surface I could.

I've got several reasons for that. Steel is "springier" than
concrete. You should get more vibration the way you describe. Also,
the coeffecient of expansion of concrete is essentially zero. Steel
will expand/contract with temprature. What that means to you is that
your degree of "level" will change if the adjustment on all of the
bolts isn't exactly the same. The amount that this will change the
level may be outside of your tolerance for caring about, but it does
tend to negate what you are trying to accomplish.

Finally, since you are making a permanant mounting system, I would try
very hard to dispense with the wedge altogether, probably by trying to
design a head for the pier that would already be in the correct
position for polar alignment.

Just my thoughts.

I'm not an expert on telescope mounting, just a silly engineer, but if
I were to build something like this, I would NOT mount it in the
manner you describe (slightly raised and using steel bolts to both
hold the pier and to level it). I would mount it directly to the most
solid surface I could.

I've got several reasons for that. Steel is "springier" than
concrete. You should get more vibration the way you describe. Also,
the coeffecient of expansion of concrete is essentially zero. Steel
will expand/contract with temprature. What that means to you is that
your degree of "level" will change if the adjustment on all of the
bolts isn't exactly the same. The amount that this will change the
level may be outside of your tolerance for caring about, but it does
tend to negate what you are trying to accomplish.

Finally, since you are making a permanant mounting system, I would try
very hard to dispense with the wedge altogether, probably by trying to
design a head for the pier that would already be in the correct
position for polar alignment.

Just my thoughts.

Davoud wrote:
The expert said that it is unnecessary to mount the pier in this
manner;

The expert is correct.

He said that it does not matter whether the pier is exactly
perpendicular to the base; a couple of degrees in any direction will
not affect telescope tracking.

That is entirely correct. As Chris has stated, there is exactly one
condition that needs to be met, i.e. that your polar axis is parallel to
Earth's spin axis. Obviously, from a mechanical point of view you don't
want the pier cantilevered at some crazy angle, but even if that was the
case, the base would still be parallel to a perfectly horizontal base
*somewhere* in the world. Or, put another way, those who insist that a
base needs to be perfectly horizontal are also insisting (by
implication) that their location is the only one on the planet from
which polar alignment is possible. This is clearly ridiculous!

I've always assumed that that is why they put bubble levels on wedges.

They help if you are setting the wedge purely according to its altitude
scale, as opposed to by some optical method of polar alignment. They
also help to ensure that your tripod is not so canted that it might fall
over or be otherwise unstable in some orientations of its load.


Best,
Stephen

Remove footfrommouth to reply

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Davoud wrote:
The expert said that it is unnecessary to mount the pier in this
manner;

The expert is correct.

He said that it does not matter whether the pier is exactly
perpendicular to the base; a couple of degrees in any direction will
not affect telescope tracking.

That is entirely correct. As Chris has stated, there is exactly one
condition that needs to be met, i.e. that your polar axis is parallel to
Earth's spin axis. Obviously, from a mechanical point of view you don't
want the pier cantilevered at some crazy angle, but even if that was the
case, the base would still be parallel to a perfectly horizontal base
*somewhere* in the world. Or, put another way, those who insist that a
base needs to be perfectly horizontal are also insisting (by
implication) that their location is the only one on the planet from
which polar alignment is possible. This is clearly ridiculous!

I've always assumed that that is why they put bubble levels on wedges.

They help if you are setting the wedge purely according to its altitude
scale, as opposed to by some optical method of polar alignment. They
also help to ensure that your tripod is not so canted that it might fall
over or be otherwise unstable in some orientations of its load.


Best,
Stephen

Remove footfrommouth to reply

--
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ Stephen Tonkin | ATM Resources; Astro-Tutorials; Astro Books +
+ (N51.162 E0.995) | http://astunit.com +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

I've got several reasons for that. Steel is "springier" than
concrete. You should get more vibration the way you describe. Also,
the coeffecient of expansion of concrete is essentially zero. Steel
will expand/contract with temprature.

A couple of things to consider.

1. As a material, steel is less "springy" (stiffer) than concrete. Young's
modulus of Steel is around 30,000,000 psi, concrete is around 5,000,000 psi.

2. The coefficient of thermal expansion for concrete is quite similar to that
of steel according this website:

http://www.tfhrc.gov/pavement/pccp/thermal.htm

"The CTE of Portland cement concrete (PCC) ranges from about 8 to 12
microstrains/°C"

Steel is around 10 microstrain/°C

----------

Whether the stucture that is built will be as stiff depends on the design,but
certainly a steep pier can be built that is sufficiently still, the only mode
that is of concern in bending and that a steep column can be plenty stiff. A
properly designed steel column ought to be signifcantly more solid that a
moveable tripod.

As far as the need to have the top of the mount level, as I understand it, this
is not necessary for proper tracking.

From a design point of view, I think it would be wise to do any leveling at the
top of the pier rather than the bottom, though if things were robust it would
not matter. The leveling mechanism could reduce the stiffness of the system.

As far as using a "wedge", I agree that one would be better off with a simple
angled plate or some such thing. Wedges are designed to allow wide ranges of
adjustment which in your case is unneeded.

Jon Isaacs

I've got several reasons for that. Steel is "springier" than
concrete. You should get more vibration the way you describe. Also,
the coeffecient of expansion of concrete is essentially zero. Steel
will expand/contract with temprature.

A couple of things to consider.

1. As a material, steel is less "springy" (stiffer) than concrete. Young's
modulus of Steel is around 30,000,000 psi, concrete is around 5,000,000 psi.

2. The coefficient of thermal expansion for concrete is quite similar to that
of steel according this website:

http://www.tfhrc.gov/pavement/pccp/thermal.htm

"The CTE of Portland cement concrete (PCC) ranges from about 8 to 12
microstrains/°C"

Steel is around 10 microstrain/°C

----------

Whether the stucture that is built will be as stiff depends on the design,but
certainly a steep pier can be built that is sufficiently still, the only mode
that is of concern in bending and that a steep column can be plenty stiff. A
properly designed steel column ought to be signifcantly more solid that a
moveable tripod.

As far as the need to have the top of the mount level, as I understand it, this
is not necessary for proper tracking.

From a design point of view, I think it would be wise to do any leveling at the
top of the pier rather than the bottom, though if things were robust it would
not matter. The leveling mechanism could reduce the stiffness of the system.

As far as using a "wedge", I agree that one would be better off with a simple
angled plate or some such thing. Wedges are designed to allow wide ranges of
adjustment which in your case is unneeded.

Jon Isaacs

On Sat, 10 Apr 2004 20:43:54 -0400, Davoud wrote:

The expert said that it is unnecessary to mount the pier in this
manner; it should bolt directly to the concrete footing for better
stability. He said that it does not matter whether the pier is exactly
perpendicular to the base; a couple of degrees in any direction will
not affect telescope tracking.

I argued that the base of the wedge (and, if shims are to be avoided,
the surface on which it mounts) must be as level as possible;
perpendicular to a line dropped from the bottom of the wedge to the
center of the Earth, if you will. I've always assumed that that is why
they put bubble levels on wedges.

If your wedge is adjustable in azimuth your expert is correct and you
can adjust out any pier tilt within the range of adjustment on your
wedge. The bubble levels they put on telescope mounts aren't accurate
enough for polar alignment so even with a bubble level you would need
adjustment in azimuth as well as altitude to get a good alignment. I
think they put levels on wedges because they can get people to pay
extra for them.

If you were to weld together your own wedge you would need to have a
means of adjustment that is better than hitting it with a big hammer,
so bolts and shims are wanted. They belong at the top of the pier
instead of the bottom.

On Sat, 10 Apr 2004 20:43:54 -0400, Davoud wrote:

The expert said that it is unnecessary to mount the pier in this
manner; it should bolt directly to the concrete footing for better
stability. He said that it does not matter whether the pier is exactly
perpendicular to the base; a couple of degrees in any direction will
not affect telescope tracking.

I argued that the base of the wedge (and, if shims are to be avoided,
the surface on which it mounts) must be as level as possible;
perpendicular to a line dropped from the bottom of the wedge to the
center of the Earth, if you will. I've always assumed that that is why
they put bubble levels on wedges.

If your wedge is adjustable in azimuth your expert is correct and you
can adjust out any pier tilt within the range of adjustment on your
wedge. The bubble levels they put on telescope mounts aren't accurate
enough for polar alignment so even with a bubble level you would need
adjustment in azimuth as well as altitude to get a good alignment. I
think they put levels on wedges because they can get people to pay
extra for them.

If you were to weld together your own wedge you would need to have a
means of adjustment that is better than hitting it with a big hammer,
so bolts and shims are wanted. They belong at the top of the pier
instead of the bottom.

I believe, would be slight field rotation over time.

So, who's right?

Hi Davoud:

Leveling is not at all necessary for an accurate polar alignment. It will help
with a goto scope during the initial acquisition of alignment stars, but even
there, once the scope is aligned it does not make any difference. Your goal
should be to design a pier that's as vibration free as possible.


Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
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