In reference to my earlier missive, Doc Greiner wrote:

> You are very quick and alert about this
(snip)

Now, isn't the above a mapug gem? Seems so to me, anyway.


> I would like to see some people experiment with this scheme. and
> report their results in a systematic way. A weight on soft rubber
> bands or on a soft bungee cord might be a good starting point. I
> might give it a try when time allows.
>

Miracle of miracles, on a holiday weekend that was predicted to be
constant rain and gloom the sky cleared last night around midnight.
I gathered my bungees and went to work on the 16", although the
transparency and coolness of the air correctly predicted mediocre
seeing.

Halfway-systematic fiddling showed that tethering the end of the OTA
either to the base of the pier or to a 1.5 kilogram mass hanging freely
at the end of a string of mini-bungees reduced the amplitude of vibration
of the OTA enough to make Mars beautifully steady in an 18mm Nagler 4
(220X). It was even steady enough to view in a Meade 8.8mm UWA (450X),
although the seeing didn't even support the 18mm. This was an incredibly
good result. Hitherto we had been limited to viewing at 100X or less due
to the vibrational motion. By the way, apparently there's a threshold in
the rate of motion of an object in the field such that rates above threshold
make observation impossible, while rates below threshold are compensated
successfully by an almost subconscious visual tracking. This is one of
the glories of visual (rather than photographic) observation: the eye-
brain combo can effectively eliminate motions that would be lethal with
film or CCD. On the other hand, I wouldn't expect bungees to compete
well against sand-filling the pier or other such methods, if photography is
the aim.

2.5 kilograms on the bungee was too much - the vibration returned to an
objectionable level. Also, it seemed that the thicker bungees were too
stiff to do the job, although I admit I didn't try very many mass/length
combos with the thick ones.

The winning bungee was somewhat less than a meter long, with one kilogram
at its end. That was for an OTA aimed at Mars, 23 degrees above the
horizon. Maybe an OTA aimed higher would need more or less weight -
I didn't take time to look at that. The bungee extension was about 0.5
meters (it almost touched the floor of the observatory) with the 1.5 kg
(i.e., 15 newton) weight, for a spring constant of about 30 NT/m, equivalent
to a vibration frequency of about 1 Hz. That's about ten-fold less than
Doc G's forks resonance probably because it's the whole Meade pier bolted
to a cantilevered balcony floor, not just the forks.

Why a 1-dimensional bob would work in a 3-D situation is beyond me.
Maybe Doc can explain that. Anyway, I'll submit a photo of the
device installed on the 16" (complete with optional spring-balance
analog readout) to Ed Stewart for the archives.

You can call it "Quick and Alert Rob's Bob" if you want to, although I
won't insist on it.

Rob P.
Pittsburgh PA
40N 80W

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