|
Hatman,
What deflection calculations are you using to justify all of this ?
With only 2.5kgs of lateral load applied - this will cause a 1m high 5" dia, 5mm wall thickness steel pier to deflect approx 2.3 arc seconds - that is a function of the laws of physics.
Now for someone with an image scale of say 0.4 arc secs per pixel - eg Your example - Meade 14" f8 scope with a KAF8300 chip
Thats 6 pixels in just one direction.
So if we have some quite light breeze gusting on a 14" scope and the pier and mount etc that is going to cause it move in both directions maybe yielding up to twice that movement.
So your small stars just got huge.
You just lost ALL the detail of the pillar nebulosity in the Rosette nebula.
But wind is not the only thing that provides a small lateral load to the top of a pier - cable drag at one end, load imbalance, just the mount with OTA.
Ever undone an EQ5 on a pier and watched it flop to one side - they are naturally unbalanced.
So its a question of what sort of imaging you are doing and what sort of results you are striving to get with what sort of gear.
The one thing you don't want is the pier (a very cheap item in the whole scheme of things) to be the thing that lets you down. Just overengineer it and know that it will not be the factor causing you grief.
This may not matter to you, but it does to some.
Astrophotography pier design calculation is not an engineering exercise in strength - piers could easily support a small building - its an issue of deflection and that requires by all accounts quite massive construction.
Very small lateral loads do occur and thus deflections do occur and they will shift your image.
If it reduces your FWHM by 2-3 arc seconds or more then you may as well pack up and go home.
In answer to all your various points in order.
- You shouldnt discount all the rubbish you read without at least verifiying some of it and ensuring that you havent in fact contributed to the rubbish yourself.
- The scientific evidence was the formula used by every engineer and engineering graduate for the last 100 years listed earlier in the thread.
- The force required to move such a pier enough to seriously degrade an image is actually trivial as calculated herein.
- Maybe the bolts will stretch ! but not as much as the pier will bend. The bolts are already under considerable tension.
- Most piers are held down by numerous bolts all appropriately tightened - so it has been considered otherwise we would be using contact adhesive or staples.
2-10kg lateral load at the OTA will not cause multiple bolts each having tonnes of clamping force to measurely deform enough to affect imagin.
- Astrophotographers tend publish their good results not their bad ones as a rule. In any case its difficult for anyone to be able to accurately identify (with all the parameters involved) exactly which problem caused them grief - they will simply take another sub and reject the bad ones.
If you look at what most of the Pros" and commercial institutions have installed for a pier its usually of a substantial nature and often that is inside a perfect environment like a large dome.
Its a question of improving your odds - your choice by how much.
- It is clear you state this with great confidence - that does not mean you are correct - you are applying incorrect logic to the problem at the very least.
- Since your additional vibration problems statement is so vaugue and generic - its impossible to offer a comment on that. The idea is to make a pier substantial enough that all vibrations are carried to ground and are hence dampened as quickly as possible.
The vibrations you can hear that are dampened by sand have nothing at all to do with pier deflection and the problems that causes. Such frequencies have such small amplitudes that they do not (themselves) affect our imaging.
- Earthquake proof construction is another science altogether (and I dont have any expertise in that field) - we are talking about building a pier for astroimaging in this thread.
We dont want that (pier) to be able to move on a flexible interface and . . if an earthquake occurs then I will reject that particular frame as part of my normal post processing but since it is only likely to last a second or two in a 1-10 minute exposure - chances are it may not even matter.
I don't get that picture at all - I don't want a wobbly pier on a non rigid foundation !
A pier is not designed to absorb vibration at all - it is designed to transmit all it can to the ground.
Its principal design function is to defeat deflection.
A field tripod is not a rigidly designed contruction that is anchored to the ground, it has a different purpose - that being lightweight and portable - these are not the ideal platform choice for astro imaging, but if the conditions are ideal they can still perform, but never as well as a pier under less than perfect conditions and I recall reading a good essay about the comparisons made between a pier and a tripod - but I cant prove that !
Any person that knows concrete knows that a 2.5kg lateral load on reinforced concete is not going to fracture it at all - it will just deflect a little bit, and that little bit is more than a steel pier of equivalent diameter with a suitable wall thickness. But it is cheap - works very well, easily accessible by most amaterurs and is usually solid verses a steel pier
Is concrete really a better conductor of vibation than steel ? - But that matters not - it is not the vibration that is the main issue - if the pier is so flexible that it cant support a a small lateral load without bending, then I guess it will vibrate a lot - but thats because its simply inadequate for the purpose.
Please provide engineering and structural details of this pier and the results of its performance or how it its better or worse than alternatives discussed here - that would be constructively useful to members.
I looked at Pegasus Piers in at Sirius Optics - I didnt see any technical specifications on pier deflection listed
But I can say that 1000mm high 165mm OD 6.5mm wall thickness pier (without gussets) will have a deflection of
just under 1 arc sec with only a 2.5kg lateral load !
Flexure is not eliminated as they say - that is impossible.
The sand will not change the flexure unless its is mixed with cement and water - then it will add a small improvement.
I would estimate that a standard sized Tonka Toy truck used by most 4 year olds without a load of sand on board will in fact deflect a pier by some amount if it was hung off the end of the average OTA assembly causing an imbalance.
Hope that helps address some of the issues you raised.
|