Vibration - looking for a solution

[Stub Mandrel]

This is an odd one. I think I have worked out why I'm seeing a larger proportion of my subs showing trailing, almost independent of exposure time.

I think something I have done is making my mount more prone to vibration - but not all the time. Last night it was very clear, even though i was only using a 500mm lens, roughly every second the stars I was lined up on (Mizar and Alcor) 'vibrated' - it was in time with my stepper (which despite microstepping pulses slightly once for each full step). This isn't unusual and it should still be a bit smoother than just using ordinary steps.

Even so, the 60-second subs of M51 I got looked like this, all 50 of them:

As you can see there is one slightly brighter spot for each star, and another above it, with a blur in between, lined up with the RA direction.

This is with a very light lens and a camera; the heavier the gear the LESS the problem so it isn't overloading the mount or the tripod flexing. With long focal length, the blur covers a similar number of pixels so the size of the vibration is less, which makes sense.

Importantly the setup didn't do this as badly when I started, but the number of subs I am losing has increased.

Soo... I think I need to play with the setup of my mount - I suspect I may have actually freed things up too much. And I must look at ways in which I might be able to damp this brief (about 0.5 second) and very small vibration.

It's a pain that clear nights are so rare that giving one up just to testing things is hard.

[spaceman_spiff]

Have you tried slightly de-balancing the RA axis so that the gears are always meshing together?

Dan.

[Peter Drew]

Good call Dan.

[spaceman_spiff]

I once spent ages redoing my polar alignment after I slewed to a star in the east and got elongated stars. The counterweight was at its lowest point and even though it was slightly de-balanced, it was moving slightly because of play in the gears. 1 hour wasted!

BTW Peter Drew, many thanks for showing me round The Astronomy Centre on Sunday, Sarah and I are still talking about it! 

Dan.

[Stub Mandrel]

48 minutes ago, spaceman_spiff said:

Have you tried slightly de-balancing the RA axis so that the gears are always meshing together?

Dan.

Yes, it does normally make a difference, but it reduces 'random' errors. I tried it last night and it made no difference.

But I've got a nagging doubt that I may not have clamped up the speader plate on the tripod after doing my PA...

Over the weekend I'm going to set up in daylight and work out the balance points for each of my setups. I might try using a dial indicator on the end of the counterweight bar (for consistency) to see if I can measure the movement/vibration.

The annoying thing is I was losing about 30% of 1:30 subs - I was hoping to get up to 2 minutes, but now 1 minute is a disaster and at 1:50 I'm losing at least 50%. Something has changed - perhaps it's the cold?

[spaceman_spiff]

4 hours ago, Stub Mandrel said:

But I've got a nagging doubt that I may not have clamped up the speader plate on the tripod after doing my PA

hmmm...would that make it prone to vibrations? You could easily knock it out of PA but I'm not sure about vibrations. Maybe someone can offer another opinion here .

Could it be that when using a lighter scope on an eq mount the centre of gravity is offset (towards the counterweights). This could increase vibrations because when lateral forces (wind, focusing movements...etc) are applied to the mount in the direction of the imbalance there is less opposing force from the mount to stabilise the return back to equilibrium.

I may of course be completely wrong here .

[Dave In Vermont]

I keep my scope/mount set up with the mounts balanced - which helps more than many people think - and I use Sorbothane™pads under the legs. These, I've found, work better than the ones offered my Celestron, Orion-USA, and others - and a lot less £€$. I also try to keep a wind-break between my scope and the wind-direction, which is easy to predict from where I live & observe. And as my other proactive approach to vibration - I keep everything I have that moves be properly lubricated (without any excess) with a good lubricating grease w/Teflon® - if possible.

I think that's about all one can do, I think. Aside from my yearly tossing of a virgin into the local volcano to appease the Earthquake-Gods.

Dave

[Stub Mandrel]

9 hours ago, spaceman_spiff said:

hmmm...would that make it prone to vibrations? You could easily knock it out of PA but I'm not sure about vibrations. Maybe someone can offer another opinion here 

On my tripod the top knob locks the PA, then you have to re-tighten (or at least check) the lower knob that pulls up the spreader.

10 hours ago, spaceman_spiff said:

Could it be that when using a lighter scope on an eq mount the centre of gravity is offset (towards the counterweights).

There has to be a geometry effect - it's always worst when pointing 'up', although the movement is in RA.

Last night I did an experiment, listening to the note in the legs when I tapped the end of the counterweight arm. A sash weight hung off the spreader had zero effect.

Tightening the spreader beyond normal made it a bit higher, but gripping the tube near the spreader dampened the vibration (unsurprisingly). A marginal reduction was achieved when I used split 3/4" rubber hose as pads between spreader and tripod legs so I will try using that 'in action'.

I might make a low down spreader but with wooden arms in the long term.

Yes - I've lubed the worms with teflon grease but a full strip down and lube is needed. My hope is that re-greasing will make the movement smoother.

 

Neil