Geometry and Leveling of Equatorial Mounts?

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_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

"Davoud" ha scritto nel messaggio
...
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


No, the sole condition to obtain a perfect tracking without any field
rotation is the RA axis pointing to true North, regardless on how you
obtained that.
However, if your scope is mounted on a wedge, you'll be able to measure the
*true* altitude and/or azimut angle of a star only if the plate is
*perfectly* leveled.and, of course, axes true squared.

"Davoud" ha scritto nel messaggio
...
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


No, the sole condition to obtain a perfect tracking without any field
rotation is the RA axis pointing to true North, regardless on how you
obtained that.
However, if your scope is mounted on a wedge, you'll be able to measure the
*true* altitude and/or azimut angle of a star only if the plate is
*perfectly* leveled.and, of course, axes true squared.

On Sun, 11 Apr 2004 17:26:07 GMT, "Antonio Zanardo"
wrote:

No, the sole condition to obtain a perfect tracking without any field
rotation is the RA axis pointing to true North, regardless on how you
obtained that.
However, if your scope is mounted on a wedge, you'll be able to measure the
*true* altitude and/or azimut angle of a star only if the plate is
*perfectly* leveled.and, of course, axes true squared.

Do you mean you'll only be able to measure the true altitude of the polar axis
using some sort of scale? You certainly can measure the true altitude and
azimuth of any star once the scope is polar aligned, regardless of whether the
wedge is leveled and squared. Also, it is worth noting that the standard
procedures for aligning an equatorial mount do not require that the true
altitude or azimuth be measured (indeed, there is no practical instrumentation
available to measure either of these angles with the precision necessary for
good polar alignment).

Both alignment by the drift method, and by the modeling method, require only
relative movements of the altitude and azimuth adjustments of the mount.

_________________________________________________

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

On Sun, 11 Apr 2004 17:26:07 GMT, "Antonio Zanardo"
wrote:

No, the sole condition to obtain a perfect tracking without any field
rotation is the RA axis pointing to true North, regardless on how you
obtained that.
However, if your scope is mounted on a wedge, you'll be able to measure the
*true* altitude and/or azimut angle of a star only if the plate is
*perfectly* leveled.and, of course, axes true squared.

Do you mean you'll only be able to measure the true altitude of the polar axis
using some sort of scale? You certainly can measure the true altitude and
azimuth of any star once the scope is polar aligned, regardless of whether the
wedge is leveled and squared. Also, it is worth noting that the standard
procedures for aligning an equatorial mount do not require that the true
altitude or azimuth be measured (indeed, there is no practical instrumentation
available to measure either of these angles with the precision necessary for
good polar alignment).

Both alignment by the drift method, and by the modeling method, require only
relative movements of the altitude and azimuth adjustments of the mount.

_________________________________________________

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

"Chris L Peterson" ha scritto nel messaggio
...
On Sun, 11 Apr 2004 17:26:07 GMT, "Antonio Zanardo"
wrote:

No, the sole condition to obtain a perfect tracking without any field
rotation is the RA axis pointing to true North, regardless on how you
obtained that.
However, if your scope is mounted on a wedge, you'll be able to measure
the
*true* altitude and/or azimut angle of a star only if the plate is
*perfectly* leveled.and, of course, axes true squared.

Do you mean you'll only be able to measure the true altitude of the polar
axis
using some sort of scale?

I mean that the altitude of a point in the sky is measured above the orizon.
If your mount is not perfectly horizontal, your reference plane would be
tilted and you won't be able to get *by direct measurement* the true
altitude angle.

You certainly can measure the true altitude and
azimuth of any star once the scope is polar aligned, regardless of whether
the
wedge is leveled and squared.

This is obtainable only by complex computations which virtually correct the
placement errors of the mount, but not by the geometrical method I mentioned
above.

Also, it is worth noting that the standard
procedures for aligning an equatorial mount do not require that the true
altitude or azimuth be measured (indeed, there is no practical
instrumentation
available to measure either of these angles with the precision necessary
for
good polar alignment).

Both alignment by the drift method, and by the modeling method, require
only
relative movements of the altitude and azimuth adjustments of the mount.

Of course the methods you mentioned are the most practical and easy ways
used by everyone to align their scope.

However if you have no computer and would like to know the exact altitude
of a star, you must have your azimut plane perfectly horizontal, according
to the definition of "altitude".

Antonio Zanardo

P.S. sorry for skipping my signature in the previous message.



_________________________________________________

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

"Chris L Peterson" ha scritto nel messaggio
...
On Sun, 11 Apr 2004 17:26:07 GMT, "Antonio Zanardo"
wrote:

No, the sole condition to obtain a perfect tracking without any field
rotation is the RA axis pointing to true North, regardless on how you
obtained that.
However, if your scope is mounted on a wedge, you'll be able to measure
the
*true* altitude and/or azimut angle of a star only if the plate is
*perfectly* leveled.and, of course, axes true squared.

Do you mean you'll only be able to measure the true altitude of the polar
axis
using some sort of scale?

I mean that the altitude of a point in the sky is measured above the orizon.
If your mount is not perfectly horizontal, your reference plane would be
tilted and you won't be able to get *by direct measurement* the true
altitude angle.

You certainly can measure the true altitude and
azimuth of any star once the scope is polar aligned, regardless of whether
the
wedge is leveled and squared.

This is obtainable only by complex computations which virtually correct the
placement errors of the mount, but not by the geometrical method I mentioned
above.

Also, it is worth noting that the standard
procedures for aligning an equatorial mount do not require that the true
altitude or azimuth be measured (indeed, there is no practical
instrumentation
available to measure either of these angles with the precision necessary
for
good polar alignment).

Both alignment by the drift method, and by the modeling method, require
only
relative movements of the altitude and azimuth adjustments of the mount.

Of course the methods you mentioned are the most practical and easy ways
used by everyone to align their scope.

However if you have no computer and would like to know the exact altitude
of a star, you must have your azimut plane perfectly horizontal, according
to the definition of "altitude".

Antonio Zanardo

P.S. sorry for skipping my signature in the previous message.



_________________________________________________

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

On Sun, 11 Apr 2004 20:25:22 GMT, "Antonio Zanardo"
wrote:

I mean that the altitude of a point in the sky is measured above the orizon.
If your mount is not perfectly horizontal, your reference plane would be
tilted and you won't be able to get *by direct measurement* the true
altitude angle.

I think you are confusing an altaz mount and an equatorial mount. With an
equatorial mount (which is what is being discussed here), there is really no
concept of a horizontal reference plane (unless you are located on the equator).
There is only the angle of the polar axis- nothing else. If the scope is polar
aligned, you can aim at any star and get an exact declination and hour angle,
and from that it is trivial to compute the true altitude and azimuth (you need
to know your time and location, of course).


This is obtainable only by complex computations which virtually correct the
placement errors of the mount, but not by the geometrical method I mentioned
above.

No equatorial mount can directly provide altitude and azimuth without
calculation. The terms required for that calculation do not involve the angle of
the base of the wedge. Many equatorial mounts don't even use wedges!


However if you have no computer and would like to know the exact altitude
of a star, you must have your azimut plane perfectly horizontal, according
to the definition of "altitude".

How do you directly find the altitude of a star with an equatorial mount? How
does this method involve knowing the angle of the wedge base?

_________________________________________________

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

On Sun, 11 Apr 2004 20:25:22 GMT, "Antonio Zanardo"
wrote:

I mean that the altitude of a point in the sky is measured above the orizon.
If your mount is not perfectly horizontal, your reference plane would be
tilted and you won't be able to get *by direct measurement* the true
altitude angle.

I think you are confusing an altaz mount and an equatorial mount. With an
equatorial mount (which is what is being discussed here), there is really no
concept of a horizontal reference plane (unless you are located on the equator).
There is only the angle of the polar axis- nothing else. If the scope is polar
aligned, you can aim at any star and get an exact declination and hour angle,
and from that it is trivial to compute the true altitude and azimuth (you need
to know your time and location, of course).


This is obtainable only by complex computations which virtually correct the
placement errors of the mount, but not by the geometrical method I mentioned
above.

No equatorial mount can directly provide altitude and azimuth without
calculation. The terms required for that calculation do not involve the angle of
the base of the wedge. Many equatorial mounts don't even use wedges!


However if you have no computer and would like to know the exact altitude
of a star, you must have your azimut plane perfectly horizontal, according
to the definition of "altitude".

How do you directly find the altitude of a star with an equatorial mount? How
does this method involve knowing the angle of the wedge base?

_________________________________________________

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

On Sun, 11 Apr 2004 20:25:22 GMT, "Antonio Zanardo"
wrote:

However if you have no computer and would like to know the exact altitude
of a star, you must have your azimut plane perfectly horizontal, according
to the definition of "altitude".

I beg to differ. If you have a star chart that gives the RA of the
star in question, and a decent timepiece, the elevation is easily
calculated.

Wayne Hoffman
33° 49" 17' N 117° 56" 41' W
"Don't Look Down"

http://users.adelphia.net/~w6wlr/