Geometry and Leveling of Equatorial Mounts?

I got into a little argument today with an expert on telescopes and
just about everything to do with them (not a self-styled expert,
either, but a recognized expert). I showed him my drawings for a pier
that I'm going to have built.

My base mounts in the same manner as an AstroPier
http://www.astropier.com/installation.html, which means that it will
be mounted slightly above the concrete foundation by the use of hex
nuts on the J-bolts above and below the pier's base plate to allow for
precise leveling of the top surface to which my Milburn wedge will
mount.

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.

Otherwise, I reason, as the telescope follows a fixed star, adjustments
in declination will be required as well as movement in R.A. The result
of that, I believe, would be slight field rotation over time.

So, who's right?

Davoud

http://www.davidillig.com/observatory.shtml

--
usenet *at* davidillig dawt com

If the wedge is adjustable i'd think you could correct for a pier that's =
not true. But i'd mount the pier the way you plan (and like the Astro =
Pier site shows) if it was mine.

-Florian

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

I got into a little argument today with an expert on telescopes and
just about everything to do with them (not a self-styled expert,
either, but a recognized expert). I showed him my drawings for a pier
that I'm going to have built.

My base mounts in the same manner as an AstroPier
http://www.astropier.com/installation.html, which means that it will
be mounted slightly above the concrete foundation by the use of hex
nuts on the J-bolts above and below the pier's base plate to allow for
precise leveling of the top surface to which my Milburn wedge will
mount.

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.

In general, I agree. The best pier is one that is set solidly in concrete. It
really doesn't matter if it is slightly cocked. You perch the pier up on bolts,
and you just increase the possibility of nasty resonant modes.


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.

Otherwise, I reason, as the telescope follows a fixed star, adjustments
in declination will be required as well as movement in R.A. The result
of that, I believe, would be slight field rotation over time.

In an equatorial configuration, all that matters is that the polar axis of the
scope is pointing at the pole. This is not determined by how level the bottom of
the wedge is, but by the position of the top of the wedge. The only reason to
have the bottom of the wedge level is so that the altitude and azimuth controls
used for polar alignment really are altitude and azimuth. If the base isn't
level, there will be some interaction between the two. But a few degrees out of
level is going to result in negligible interaction, and if you are permanently
mounted, it doesn't matter because you will only polar align once.

I have an LX200 mounted on a Superwedge, which is on top of a pier set in
concrete. I made no heroic efforts to make sure the pier is perpendicular to the
ground, or that the top of the pier is parallel to the ground. I'm sure it is
within a degree or two, that's all. My tracking and goto accuracy are superb.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Hi Davoud,

Will you be able to rotate the wedge about the pier? It doesn't need a lot,
just a little bit. In other words, will you have the ability to adjust the
wedge base so it points true north? If so, you don't need any other
adjustment outside of the wedge's ability to adjust for latitude.

It may sound wrong, but picture what is happening. All of the adjustments
have nothing to do with being level. The only key adjustment is making sure
the polar axis is aligned with the earth's axis of rotation. The polar axis
will be set at a right angle to the top of the wedge. If you can adjust the
wedge base around (az) and the wedge is adjustable (alt), you have a simple
alt-az mount for pointing the RA axis. If instead of your big scope, you
mounted a plate with a small polar pointing scope welded in place, at a 90
degree angle to the plate, you would be pointing that scope at Polaris, with
the offset dialed in.

It is the same as a GEM. Unless the goto requires it, a GEM does not need to
be level. In fact, you could bolt it to the side of your house so it stuck
out parallel to the ground (or at any angle you wanted). Then just use the
az adjustment and the latitude adjustment to bring the polar axis in line
with the earth's axis. Once that is done, the scope will track perfectly.

However the part about being a few degrees off not hurting anything is
wrong. That is not why you don't need the extra provisions for adjustment.
Being off a few degrees would hurt your images. This is why people take time
for drift alignment. The reason you don't need the extra adjustment screws
is because by rotating the wedge in az and adjusting it up and down, you can
set it so you aren't off by a couple of degrees. Your accuracy is only
limited by the time you put into it.

Hope this helps.

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
Lunar Picture of the Day http://www.lpod.org/
************************************


"Davoud" wrote in message
...
I got into a little argument today with an expert on telescopes and
just about everything to do with them (not a self-styled expert,
either, but a recognized expert). I showed him my drawings for a pier
that I'm going to have built.

My base mounts in the same manner as an AstroPier
http://www.astropier.com/installation.html, which means that it will
be mounted slightly above the concrete foundation by the use of hex
nuts on the J-bolts above and below the pier's base plate to allow for
precise leveling of the top surface to which my Milburn wedge will
mount.

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.

Otherwise, I reason, as the telescope follows a fixed star, adjustments
in declination will be required as well as movement in R.A. The result
of that, I believe, would be slight field rotation over time.

So, who's right?

Davoud

http://www.davidillig.com/observatory.shtml

--
usenet *at* davidillig dawt com

Basically, you don't want to use a wedge on a permanent pier. Rather you
should endavour to mount the scope to the angled pier itself and that means
that the pier surface won't be level but rather related to the polar axis.
Basically, why include a wobbly wedge when you can have a perfectly solid
pier itself to mount to?

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

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'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

(Jon Isaacs) wrote:

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.

While steel's resistance to bending is much greater than concrete's,
the concrete has a higher damping factor. This means that a steel pier
will vibrate for longer than a concrete one. Also, the proposed
design used standoff bolts, which made it structurally less rigid than
a solid concrete pier.

[...]
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.

A concrete-filled steel pipe would give you the stiffness of steel and
the damping of concrete, but concrete alone should be quite adequate
for this application, IMO.

[...]
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.

Practically speaking, you will need a few degrees of fine adjustment
in both altitude and azimuth to achieve good polar alignment. That
could be done with a fairly simple tilting plate, but even so, many of
us would find a solid commercial wedge more time- and cost-efficient
than designing and fabricating such a mechanism.

David Brodeur

On 11 Apr 2004 09:27:06 GMT, (Jon Isaacs) wrote:

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.

Hmmm...I'm not used to steels like this, but you are probably right,
this is more of an average type stell than I'm used to.

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

Unfortunately, I don't have my references at home (and I won't be at
work for another 10 days), but I didn't realize there were concretes
with expansion coeficients that high anyway so I'll go ahead and
concede.

My point is still valid, however. If you have bolts of different
lengths with a couple of lever arms, the differences in total
expansion of the individual bolts when multiplied by the lever arms
may be significant at the telescope.