I've started this thread up so as not to hijaak the other thread on pier design.
I don't believe in putting an adjustable plate/cage on top of a pier, purely because I see it as unnecessary as you should get the pier levelled and aligned during the construction phase, and the mount-head should have enough adjustment within it to pick up from there.
I think adding extra materials, complexity and $$$ rarely makes sense if there is no real payoff, or can be avoided.
I am interested from a purely knowledge gaining point of view though whether they weaken the design in enough of a way to affect imaging, or or example in lengthening the damping down time if the mount it slewed quickly (like when traking satellites) or focusing etc. (Just like I was interested in actual Losmandy vs Vixen bar compares)
Or are piers just generally over-engineered, and way exceed the specs required?
Has anyone got any empirical evidence of a cage on top of a pier causing problems with imaging?
And just to add to this thread. If you have read something somewhere that somebody had read something of something else that their fathers brothers sisters twice removed grandmas nieces best friends dad said. Please keep it to yourself as it means squat.
Keep it to empirical evidence. Your own actual results and a method that is scientific and can be peer reviewed. If you have actual formal training and experience say so. if you have no experience in the area be prepared to be challenged!
That aside I look forward to some cold hard steeely evidence!
P.S. The youtube vid that relates to "the rat cage" the guy is a fabricator not an engineer, there is a vast difference and you can tell by the language he uses as engineers find it really hard not to use engineering expressions and explanations. He is hyping up his product, does it work yes how add more steel! I could go into an analysis why some of the things he does are right and most of them are just hype but not today!
These rat cage things are totally unnecessary for EQ mount users, so why use them? Especially since they weaken the entire footer-to-optics system. Why willingly introduce that weak link? Because others misguidedly do so? I hear stung egos at work.
No, I'm not a mechanical engineer, but as has been said, EQ mounts don't require it, so why do it? Disregarding whether it's as stiff or not, it adds more work, more cost, and more fiddlyness to the whole thing.
Sure, if you have an alt-az mount on a wedge then you might need it, but for an EQ, you don't.
Think of a pier with a top that's not perfectly level. Now transpose this plate to somewhere else on Earth not too far away, and it will be perfectly level. All the adjustment to get perfect polar alignment is there to use, everything below the polar axis is irrelevant.
I'd like to focus on do they compromise the mount rather than are they required.
I don't see the need for one, but maybe some mounts have connections that require a raised plate, or you are a wedge user or already have a pier with a top plate.
Question is, does it cause a measurable/visible detriment to imaging, and why?
Zaps you are making this very easy for me to prove that you do not know what your talking about and furthermore to the situation are inadept at comprehension and reading. For your sake please go back to my posts and read what i said without letting your brain cloud what it is that you think you are reading.
You hear stung egos at work. No. I have mearly proven to you in a mathamatical form (yes nearly all science is mathmatically based) that the flexure and "Vibration" that you are supposedly quoting as gospil does not exist. This you cannot accept and persue to see if im actually telling you truth.
You are just quoting from what you think is truth, be it read on a forum somewhere, or a magazine. I have known my university lectuers to get it very very wrong!
Basics 101 of research do not pin all your tails on the same donkey! Its bound to buck you off at the most in oppertune moment.
We are not saying that the top adjustment plate does not increase the complexity of the mounting system because it does. What Peter and I are asking is does it actually make a difference to the end result. To which my reply is if done correctly NO IT DOES NOT, I stand by this 100% untill somebody can show me other wise. If you put a few piss ant bolts there yeah it will matter. If you don't know what your doing, yes it will matter.
So please Zaps before you post again stop breath think and formulate a logical response with some proof, theory or something other than i told you so.
This thread is about proofs and peer review to get the truth not just another arm chair expert/engineer i think this is what it means, if you want one of them go elsewhere!
B. Mitchell
Quote:
Originally Posted by Zaps
These rat cage things are totally unnecessary for EQ mount users, so why use them? Especially since they weaken the entire footer-to-optics system. Why willingly introduce that weak link? Because others misguidedly do so? I hear stung egos at work.
I think Zaps is right in that most people put one in there because they see them on other commercial designs, and they would be embarrassed to pony up and say "I put it on there because I didn't know any better".
So far everyone seems to be in agreement that it would be silly to add one to a pier unless there was no other option.
However, many piers have them, so are they compromising the mount?
Now I hear about people 'ring testing' the mount by hitting it on the side with a hammer.
Brendan points out that we rarely do this while imaging, but that doesn't mean it is a totally pointless exercise (though it may be). We often test things in extreme ways to find out their potential strengths and weaknesses.
So what does the hammer test tell us?
I know if I walk near my EQ6 on tripod that I get vibrations that affect the image, I can quantify that quite easily.
Has anyone tried imaging with and without a top-plate and gotten a different result? Or has anyone managed to induce vibration in one with a top-plate that has been fixed by removing it?
Or has someone got the math that would show some potential problem with the top-plate.
Again, I think they are a stupid idea (though I could be wrong), but I'd like to know how much they affect things, and under what circumstances?
(I had similar questions long ago about vixen vs losmandy bars, everyone told me losmandy was better and stronger, many people spent good money upgrading, but amazingly I couldn't find a single piece of empirical evidence of its superiority in use).
So do the rat-cages screw things up or not? If you have a 3m long scope and windy conditions, could it make a difference?
Brendan, you have said that they don't if you know what you are doing, can you elaborate on what knowing what you're doing entails from a design standpoint?
These rat cage things are totally unnecessary for EQ mount users, so why use them? Especially since they weaken the entire footer-to-optics system. Why willingly introduce that weak link?
Vibration in mounts and their optical systems are quite a real phenomena.
I was exposed to a significant vibration problem with a large telescope mounted on a P-ME.
In order to counterbalance the scope it was considered easier to make one large counterweight and add that to the end of the counterweight bar rather than many extra weights all the way along the bar where they served less effectively - which would have meant extra mass was needed and that was the purpose of the exercise - to keep the total mass contained.
The single large weight introduced a significant vibration (probably a resonance) into the system that was unable to be controlled even by AO.
The mounts motors are effectively being pulsed or frequency controlled and other components such as AO and fans introduce vibration into the system, if a component or arrangement of components end up in resonance then its very difficult to dampen the vibration in some cases - noticeably that one.
Wind gusting and Wind induced vortex shedding vibrations are also a cause of vibration in telescope systems.
I think we have already answered that, (i.e. don't use one if you don't need one) however it isn't the only question worth asking.
For instance if you already have a cage on your pier, it is worth knowing whether any advantage exists by removing it. So knowing if an adjustable plate actually can cause problems makes it a question worth asking.
I'd like to focus on do they compromise the mount rather than are they required.
I don't see the need for one, but maybe some mounts have connections that require a raised plate, or you are a wedge user or already have a pier with a top plate.
Question is, does it cause a measurable/visible detriment to imaging, and why?
Answer = Absolutely "NO"
As there is "NO" measurable adaptor plate movement there can be "NO" visible or measurable detriment to imaging
p.s. I had to add the adaptor plate as the Pier top buckled when welding it on, therefore there was "NO" flat surface for mounting a scope.....
Using M16 bolts and holes that are 18mm is pointless, there is ... to much air
Using M20-24 bolts that have minimal clearance and not setting the top plate too far away (like i do see on on some piers). My opinion is i wouldn't go further than 100mm difference between the plates.
if you spread the bolts sufficiently eg 300mm+ and use a decent plate 12mm or so the top plates are sufficient enough to withstand any forces applied to them and transfer these forces out to the bolts.
Its the same premise why truss tube telescopes work so well. Intentional design putting elements into their strong points eg. Tension and compression. Geometry plays a key point here in putting mass where it is required and taking mass out where it is not.
Like i have said before a pier I designed, fabricated and installed clears the ground by 150mm or so... there are no issues running at 2000mm FL.
On your point of testing your rig to extremes. If you really wanted to do a test that actually has merit not just oh i hit it with a hammer which will never ever ever occure (your test has to be meaningful).
Get a spring weight and hook it onto the top of your pier. Maximdl reports in seconds of arc so you can grab the pier and quickly put 50kgs worth of force at the top of the pier.
Take a reading of the arc second error the guider throws up. upon release measure it again. do that 30 times to gain a good statistical average. That is an extreme case and is measuring your static deflection which is the maximum the pier can move at a said force. This is how I theoretically determine my "vibration" in my normal day to day work by looking at what force produces what deflection.
As for dynamic vibration thats a whole new ball park. It takes into account Natural frequency of the structure, Frequency of the dynamic force, Dampening and a whole host of other things.
Think of a combustion engine sitting on a flat floor with an out of balance crank shaft or even your car the vibration you feel is dynamic it changes with respect to time.
So thats my view point on the dynamics of it and a design that will work.
For very large piers leveling adjustment at the top can be useful. It does need to be very robust to not introduce a source of flexure under varying loads. This unfortunately can limit the amount of travel past the meridian before a flip is needed.
There is no correct answer.
This is my solution and it works for me. I never need to do a meridional flip as I can go more than two hours past the meridian. My western sky is more light polluted than my eastern sky. I can adjust the level by the eight bolts at the bottom of the pier.
Resonance is the major problem if you are not careful. The tapered gussets on my pier give the pier a low 'Q' for resonance.
As for dynamic vibration thats a whole new ball park. It takes into account Natural frequency of the structure, Frequency of the dynamic force, Dampening and a whole host of other things.
B. Mitchell
This is the part that interests me the most, I trialled a very large mount with a lovely smooth PE, but it had a vibration in the images that wasn't there with my EQ6, and I didn't have it long enough to track it down.
I think mounts and piers generally are way, way stiff enough to handle issues of flex. But with autoguiding and PE correction etc. I wonder if there can be issue with resonance (not sure if I am using the right term) in the pier, and if a top plate can make that worse, and if it can, what design considerations would you do to combat it?
I know people use tapered gussests to minimise it on the pier itself, but what about on the plate?
So your issue was high frequency vibrations forming a dynamic resonance. Is this a issue of the pier or is this a issue of the mount?
To me it would be more a case of in the pier, with adjustments and tolorances. Possibly due to the telescope being sufficiently big enough coming upto the PME's rated capacity and hence any dynamic vibrations start to affect.
If this where a real case and it happend all the time wouldn't mount design take this phenomena into account?
I also think that placing the CW further out would make the issue worse not better. Yes the leaver arm is bigger hence less weight but the ability for a weight further out where your natrual frequency decreases and hence has more chance of coming into resonance.
It is a tricky one high frequencies because they are hard to measure unless you have some really expensive tools.
The new peir under construction for another scope is reinforced 500mm round concrete with 4x22mm SS rod embedded 3ft. and a plate on top.It will be super!
I look forward to some cold hard steeely evidence!
