Pier Design

[wasyoungonce (Brendan)]

Originally Posted by Kunama

......With all this talk of resonances etc, I wonder how many of the imagers actually tap, kick, rap etc their piers during imaging ....................

Of course not...but basic physics and engineering dictates we can minimise influences cheaply and easily during design that all combat successful imaging. Influences that are harder to fix once built.

Worst case...a pier that has a natural frequency harmonic in sync with applied average wind force...in other words this causes it to hum or resonate...which in turn adds amplitude to the natural freq....just sayin!

Motor resonances...no these are not an issue at play here.

[el_draco (Rom)]

Originally Posted by bert

Another factor in designing a pier:

If you use a lot of mass to control damping, you will get a lot of thermal inertia, and that means local seeing effects as the pier stores heat and releases said heat through the night.

This pretty much precludes using concrete for a pier.

Like logie, I am a big fan of square section hollow piers, they are just easier to work with.

The thermal mass of a pier is a significant issue. Concrete always bothers me and I was recently advised to fill my new pier with sand. Soooo much extra mass!

The system I am currently assembling has a 33cm diameter steel pier and to dampen this down I'll be using expansion foam... Next to nix mass/thermal retention, but past experience has shown it to be excellent in dampening vibration.

Cow of a problem all round really. Hot concrete slabs, massive equipment and then we wrap it all up in a metal dome/oven?

Originally Posted by Kunama

With all this talk of resonances etc, I wonder how many of the imagers actually tap, kick, rap etc their piers during imaging ....................

I stubbed my toe on mine ...... does that count?

[avandonk]

My reason for posting these vids was to show how many fallacies become gospel in the amateur astro community. These vids do go some of the way to dispel the myths about pier design.

My background is in Applied Physics and we prefer to analyse any system from first principles with a minimum of assumptions. We then take the simple step of measuring this putative system to see if our ideas are valid.

I only wanted people to think about what was happening with their system.

Is your continuous poor astro photographic 'seeing' due to pier and/or mount oscillations?

It was never about denigrating any system.

I tightened the bolts holding my pier to the concrete block far higher than I did initially. The rms errors in guiding have nearly halved to better than 0.5" ! It could be that I have suddenly been blessed with better seeing. My exposure for guiding was two seconds so this should eliminate seeing. Only time will tell.

Bert

[avandonk]

Originally Posted by RobF

I think Steve has said what I was thinking. Many of us with EQ6s sitting on a pier probably have a hell of a lot more trouble with backlash and PE than movement from the pier. I suspect even a stable tripod has minimal influence on the final result when you still have mount issues to guide out.

For those with permanent observatories and PME/AP mounts, MIGHT be worth considering, but really, if you rpier is isolated from floor vibrations do you really think vibration from the mount motors is going to resonate that badly?

If I touch any part of my optic train even very gently while watching the guiding in MaximDL the corrections become larger and oscillate. Wind has the same effect. It is only about a few seconds of arc.

Drive motors will start bad oscillations when there is any tiny resonance in the system between the motor and system.

Remember any unwanted vibrations/oscillations ADD to the seeing and inherent mount drive variation.

The worst case of wobbles I had one night was when a possum decided to jump onto the telescope from the observatory wall. It was a small ringtail possum. I had to go out and coax it to get back in its tree. When the RoR roof is open possums use the exposed tops of the observatory walls as highways.

Bert

[RobF (Rob)]

Originally Posted by avandonk

If I touch any part of my optic train even very gently while watching the guiding in MaximDL the corrections become larger and oscillate. Wind has the same effect. It is only about a few seconds of arc.

Drive motors will start bad oscillations when there is any tiny resonance in the system between the motor and system.

Remember any unwanted vibrations/oscillations ADD to the seeing and inherent mount drive variation.

The worst case of wobbles I had one night was when a possum decided to jump onto the telescope from the observatory wall. It was a small ringtail possum. I had to go out and coax it to get back in its tree. When the RoR roof is open possums use the exposed tops of the observatory walls as highways.

Bert

Thanks Bert. Interesting to hear how top end gear behaves at the extremes of performance. And mental note to self for future too - steer clear of possums