[M]: Meade Fork Oscillations II
| Subject: | [M]: Meade Fork Oscillations II |
| From: | R. A. Greiner |
| Date: | Sun Mar 09 14:23:18 1997 |
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Apparently Part II never went out.
Somehow Part II got out twice. Very sorry.
Here is part II
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Meade Fork Oscillation II
Data was taken on the 12 and 10 inch Meade LX200 telescopes in the
configurations described in part I.
Equipment was a geophone transducer (low frequency magnetic type).
The output went to an HP spectrum analyzer which could record and
print out amplitude and frequency data. The telescope was excited by
careful tapping in a variety of directions and many locations.
I did not use the more sophisticated impulse hammer since the structure
is too complex and I didnt need transfer function for this approximate
analysis.
The transducer measures in only one direction at a time so it was moved
to several locations (clamped on) on the fork and mirror tube. Almost
all of the motion of the mirror tube turned out to be in a direction
up and down with respect to the fixed base of the telescope. Lateral
jiggling of the fork and tube was minimal. Thus I concentrated
attention on this mode of oscillation.
Measurements were made on spectra to determine amplitude and frequency
components. Time domain measurements on the damped oscillations
yielded the damping factor. The results are described below.
Measurements made of velocity of motion at various frequencies.
These were converted to amplitude of motion of the mirror tube and
finally into angular motion of the mirror tube. It is this angular
motion in arc seconds that is important in seeing and imaging.
The compliance of the fork mount was calculated by measuring the
deflection of the fork with a known weight placed on the fork at the
declination axis.
Many positions of the transducer and location of excitation (tapping)
were tried. The following are my general conclusions and they apply
almost equally to the 10 and 12 inch telescopes.
In all cases the resonant frequencies of the telescopes were in the
range of 8 to 18 Hz. Several frequencies occur at the same time and
vary in amplitude as the oscillation damps out. This is because there
are several modes of oscillation in the tube and fork and they
interact by exchanging energy since they are not orthogonal. (i.e.
the structure, while fairly rigid is of complex shape.)
Damping of the main frequencies was in the order of 8 to 10 seconds.
This is bad news since it means that for short exposures up to a
minute or so one must wait after touching or disturbing the telescope
for at least 10 seconds for the scope to settle down. This is longer
than generally mentioned in the literature.
The immediate force caused by camer mirror flip must be allowed to damp
out before the shutter is released. The forces external to the camera
caused by a shutter releasing are probably not enough to cause
problems. (depends on the camera of course)
Lateral motion of the fork/mirror tube, in response to tapping, was
very small and damped in less than 1 second. Thus it is reasonable
to conclude that unsharp images are caused mainly by up and down motion
of the fork structure no matter what the pointing direction of the
mirror tube in R.A.
Because the fork is very stiff, low compliance, it can be concluded
that essentially all of the compliance is in the bearing mount. To
put that simply, the bearings are slightly weaker than they should be.
The actual numbers for these telescopes are given here. Because they
are so similar it seems to me that they are indicative of what other
telescope of the same type would show. But, remember these figures are
specific to these two instruments.
The main mode of oscillation along with cross coupled modes was at
frequencies of 8 to 18 Hz. Strong modes were at 8, 10, 12, 14 and
18 Hz. Some smaller but measureable oscillations
were at 25 to 40 Hz. With a rather large collecion of small modes
within that range.
Tapping the mirror end of the tube caused the larger oscillations as
did tapping the dew hood.
All North, South, East and West jogs of the scope using the control
pad caused after oscillations. The North, South jogging caused
the worst jiggling. Additionally, there was a distinct but small
jiggling due to the steady running of R. A. motor.
The compliance of the fork was about 1 1/2 X 10E-5 meters/newton.
It was about the same for both telescopes.
This value taken with the known mass of the fork/tube structure
gives a calculated oscillation frequency of 9 Hz which is in agreement
with the measured value. It was slightly higher for the 10 inch
telescope.
The compliance of the fork mount can be attributed mainly to the
bearing and/or its mounting in the base structure. I have not yet
taken either scope apart. The precise number will depend upon the
exact construction details of the bearing and its mounting. I doubt
that attempts to reconstruct the bearing and mounting would be
practical. It is actually quite good for a mount of this type and
cost.
Interestingly, the running of the R.A. drive motor causes a small
bit of vibration of the fork/tube structure. This vibration is at
a very low level (less than .2 arc-sec) and will cause no trouble
since it is so much lower than atmospheric image jiggle.
The amplitude of the oscillation of the main fork mode however is
by no means negligable. A force caused by a weight of 10 grams
causes a deflection of about 1 arc-second. This is a very small
force. Larger forces cause proportionally larger angular fluctuations.
With good seeing, which can be 1 to 2-arc seconds (even in Wisconsin),
forces caused by slight winds are definitely deliterious to good
images.
But now, recognizing the primary source of oscillation, what can be
done about it.
Next section Part III will discuss this issue.
Clear Skys, Doc G
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