[M]: Meade Fork Oscillations I
| Subject: | [M]: Meade Fork Oscillations I |
| From: | R. A. Greiner |
| Date: | Fri Mar 07 21:24:04 1997 |
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I tried to attach the file and it seems to have vanished in webland.
This is a second try.
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As well it should be, the issue of vibration or oscillation of a fork
mounted telescope is of high interest to users of LX200 type telescopes.
Because of the considerable interest shown by MAPUGGERS, I have decided
to share my investigations of this problem. I have extensive data on
two telescopes, a 10 inch and a 12 inch Meade LX200. The 12 inch is
mounted on a superwedge on a 4.8 ton concrete pier imbedded in bedrock.
The 10 inch is mounted on a superwedge on a giant field tripod sitting
on a concrete driveway.
Both wedges are set for 43 degrees which is my latitude and all data
and conclusions drawn are for this setup. Some conclusions can easily
be extrapolated to other setups but be warned that this topic is full
of surprises. Most of the conclusions can also be extrapolated with
a bit of thought to German mounts since they also exhibit serious
oscillations.
Because I did an extensive amount of work on gathering data and
working out the results and conclusions, I will take time and care to
explain my techniques and results at some length. Thus be warned that
there will be several of these memos following at a day or two interval
and that they will be fairly long. I am trusting that the MAPUG
control will allow these discussions on MAPUG site as ON TOPIC.
I am sure there will be others with experience and excellent thoughts
on this topic so I encourage others to participate in this discussion.
I will try to answer questions as they come up. But, if the task gets
too great, there might be delays.
I will use the Subject line, as suggested by control, to label each
successive section. I expect there to be at lease three and possibly
five sections. Section I follows:
----------------------------------------------------
Meade Fork Mounts I
All telescopes and their mounts, consisting of forks, bearings, tubes,
shafts and the like can be thought of as masses connected by rods that
have compliance in some fairly complex configuration. Notice I use the
term compliance. This is the inverse of springiness. A soft spring
has high compliance and a stiff spring has lower compliance. The
masses move in unwanted ways because of the necessary forces applied
by the drive motors, wind, or other driving forces. Some of these
driving forces are necessary such as the drive motors to sustain
correct R.A.
There are unwanted forces like the wind or movement of the mount
structure that are unwanted and cause the telescope to oscillate or
jiggle. Any unwanted motion of the telescope tube causes visual
jiggling or unsharp imaging.
All forces on the telescope from whatever source, including irregular
drive speed which are not perfectly steady will excite oscillations in
the main tube of the telescope. These oscillations are entirely
measurable and predictable within the accuracy of the model used for
the telescope parts. A rather simple, first order model in this
analysis.
The trick needed to make sense of the problem stated is to make a model
that is trackable but still one that gives some useful results for the
most important deviant motions of the observing/imaging tube.
I have found that the simplest model, fortunately, accounts for the
main oscillation of the tube that effects viewing and imaging. While
there are numerous secondary jiggles and wiggles they are of second
order. If the main oscillations can be reduced or damped, greatly
improved imaging will result.
The simplest model consists of three parts. The first is a base
consisting of the pier/tripod and the wedge. With the pier/tripod
and super wedge used, this base can in first order analysis be
considered stationary. To be sure, there are inadequate piers and
tripods sitting on unsatisfactory ground but for my setup the wedge
surface upon which the telescope base is mounted can be considered
rigid and unmoving.
The second part is the fork, in the case of the LX200, and its bearing
structure. Within this part, the fork is quite rigid but the bearing
structure is quite compliant. Thus close attention must be paid to
the bearing structure as the "weak" point and the largest source of
oscillation.
The Final element is of course the main telescope tube and the toys
attached to it. This structure may in the first order be considered
a single rigid mass. This mass moves on the declination bearings
but these bearings are so strong and stiff that the tube mass and the
fork mass may be considered one.
Thus for a first order system we have a spring of some compliance
sticking out at an angel of 45 degrees and a large mass consisting
of the main tube, its attachments and an appropriate part of the fork
mass. Fortunately this is the simplest mechanical vibratory system
possible. It will have two important properties. Frequency of
oscillation and damping. The frequency is simple to understand and
is measured in Hz. (used to be called cycles per second, cps)
The second factor is a bit harder to understand. I will use the term
damping (or its inverse, Q, or quality factor) in this discussion. It
is only necessary to understand that a weakly damped system will
oscillate for a long time and a highly damped system will not oscillate
very long at all. The inverse, Q, means that a high Q system
oscillates for a long time and a low Q system does not oscillate very
long.
Clearly for the telescope we want high damping and low Q. Such a system
will stop oscillating quickly when it is excited by unwanted forces.
Again, a reminder. Unwanted forces are wind, bumping, pier/tripod
movement of any sort and any irregularity in the forces from the drive
motors. Additionally, any irregularities in the moving bearings or
surfaces in sliding contact an be interpreted as irregular forces.
My study of the two telescopes in question consists of two parts. The
first was to take some data and the second to try to figure out what
in the world was going on.
I believe I have deciphered some of the problems and have in the final
part of this discussion made some suggestions to minimize the oscillatory
problems.
End Part I
Since this is being written off the top of the head, as it were, I
apologize in advance for any unclear statements or inadvertent typos.
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