Losmandy vs Vixen - Difference in imaging

[naskies (Dave)]

Originally Posted by bmitchell82

I have assumed a 20kg telescope which is about the limit that most people impose on their equipment, if you wanted to double the weight as it has been said before assuming that you are opperating within the elastic limits of the material (which you would be as nothing you have will go towards plastic limits) then you could easily say that a 40kg scope on a 100mm cantileaver would flex .0008 mm... or .8 micron as materials operate along a linear line in the Stress Vs Strain or Youngs modulus.

Thanks for the analysis, Brendan. It's been a while since I took introductory mech eng subjects during undergrad (10 years, in fact!) but it makes sense to me.

One minor query... since you asked I noticed that you highlighted the phrase 157% stronger in green... did you by any chance happen to lose a 0 as a typo/writo? Your working suggests 1570% stronger. Or 1463% stronger if we're being pedantic

[bmitchell82 (Brendan)]

Originally Posted by naskies

Thanks for the analysis, Brendan. It's been a while since I took introductory mech eng subjects during undergrad (10 years, in fact!) but it makes sense to me.

One minor query... since you asked I noticed that you highlighted the phrase 157% stronger in green... did you by any chance happen to lose a 0 as a typo/writo? Your working suggests 1570% stronger. Or 1463% stronger if we're being pedantic

Indeed you are right. Missed the zero I'll put it to Friday itis.

[g__day (Matthew)]

To throw another variable in - telescope itself isn't a infinitely rigid structure; tilt it from vertical to horizontal and it will discernibly flex - hopefully well within limits that frustrate a user!

However if you securely mount that telescope onto a solid bar (with a web design) - the bar will also help make the OTA more rigid itself.

A cylinder secured at one end on a cantilevelr design (as opossed to a craddle) will flex every so slightly more as you move away from the point of attachment. When I first picked up a Williams Optics refractor I was stunned by how robust it felt. I expect the tube it that solid to really limit differential flexure.

Flexure errors are generally all additive - so more rigidity in simple terms is really great! When you're dealing with fractional wavelengths of light - microns count I guess!

[Poita (Peter)]

So if I read it right, there is no actual advantage in going from a vixen to a losmandy dovetail setup for a scope under 10kg, you'd just be adding more unnecessary weight to the mount, or do I have it backwards? I'm reading it in bed on my phone in the dark...

[Tandum (Robin)]

Poke the scope with your finger and see if it wobbles.
The rings to suit an L bar have a 100mm flat mount point on the bottom with 3 bolts to hold down each ring. The V bar has one bolt and a 10mm mount point.
The bigger the scope the more secure it will be with an L mount.

[Poita (Peter)]

My vixen dovetail bar has the 3 mount points. No real wobble, seems solid.

[Tandum (Robin)]

You mean it has one mount point and 2 push offset points. And all are within a 40mm area not 10mm as I mentioned before in error.
If it doesn't wobble then the scope is probably small enough to use it.
I know a 10" newt is too big for vixen style rings even on an L bar never mind a V bar.

[bmitchell82 (Brendan)]

its more about the mounting point to the rings so the actual tube rings have a good footing and don't flex off that point eg, 100mm bearing makes the moment couple in terms of a telescope quite large

Like i said before .0008 mm you will never ever see and if you can what kind of tools are you using to measure it because most digital verniers go to 1.00 dp.

You will have far greater issues to attend to in the world of flexure, trust me i know as the owner of a medium size newtonian a guide scope could no longer help me, i had to turn to OAG to make it work the way it does now. So no matter the flexure there is always more than 1 way to skin a cat!

I think everybody gets too caught up in microns this and microns that when really and im sorry to harp on about it nine times out of ten seeing and mount quality play far bigger roles in your errors. I also mentioned before that even if you do hold on to the torque that is applied by the telescope something has to actually provide the holding action and that is 100% the motors in your mount if that moment is exceeded, then your motors slip probably in the order of mm.

So a definitive answer to your question posed Peter is, if you are running light gear then the vixen bar is ample sufficient in every way shape and form, if you start running bigger gear, the larger top plate will help mount your OTA better.

The Losmandy style D saddle will resist torque or twisting in it strong axis better than the Vixen will. In the weak axis or with the dovetail laying flat both will proform about the same, the limiting factor is the length of the un supported dove tail so make sure you have enough spacing on the ota rings to support the OTA well eg spread the load, but try to limit the over hang of the Dovetail to Saddle connection or in other words keep it central/concentric to the mount.

Brendan

[OICURMT]

Originally Posted by bmitchell82

I hate speculation, hear say and mis information

I feel your pain....

[bmitchell82 (Brendan)]

I think for giggles im going to model a dove tail and mount head in strand 7 which is a FEA program that will show stress in the materials and deflection by rigorous analysis.

[OICURMT]

Originally Posted by bmitchell82

I think for giggles im going to model a dove tail and mount head in strand 7 which is a FEA program that will show stress in the materials and deflection by rigorous analysis.

pfftt.... Young'ens and their high speed computers...

3D FEM in Ansys way back (is Ansys still around ) would have taken the better part of the morning to get answers for a simple strain analysis on a flat plate, computing nowadays allow you to play a million what-if games while getting coffee.

The engineers and geoscientists working for me can hardly believe it when I tell them we were doing "huge" finite difference 3D models with 10000 cells that would cost us day(s)... now they model 2-5 million cells in the half the time...

I guess that's a good thing... everyone does need time for coffee


OIC!

[bmitchell82 (Brendan)]

hahaha don't hate the technology, my average analysis nowdays is somewhere in the order of 20'000 degrees of freedom that takes somewhere about 15 seconds so no i cant get my coffee!!!!!

Some of the other analysis programs we use we have to run over night! so that gives you a quick idea of how massive they are but its full dynamic analsys in 3D Its amazing to think though that they designed the World trade center in 2 dimensions eeek. So ill just fiddle with the small model and see if i cant get some good information thats really on the ball!

Brendan

[Poita (Peter)]

Yep Ansys is still around, we use it. It is now CUDA accelerated and can handle massive models, I thought about doing a FEA on it, it would be fun to view the results.

Thanks for confirming Brendan, it backs up what I suspected, that in most cases, the money would be better spent elsewhere in the chain.

[bmitchell82 (Brendan)]

Not a problem peter!

[TrevorW]

Lot of over engineering going on here KISS but logically the more robust the plate the more rigid it would be IMO but then again I'm no engineer

My RC has a full length Vixen plate which I fit into an ADM Vixen to Losmandy adapter the Vixen dovetail is supported along twice the distance as it would be on an EQ6 saddle and it works fine AFAIC.

With the type of imaging I do (no permanent setup), I don't even consider flex an issue, as Brendan said seeing etc will probably impact more on the quality of the image I take.

[UniPol]

Originally Posted by TrevorW

Lot of over engineering going on here KISS but logically the more robust the plate the more rigid it would be IMO but then again I'm no engineer

My RC has a full length Vixen plate which I fit into an ADM Vixen to Losmandy adapter the Vixen dovetail is supported along twice the distance as it would be on an EQ6 saddle and it works fine AFAIC.

With the type of imaging I do (no permanent setup), I don't even consider flex an issue, as Brendan said seeing etc will probably impact more on the quality of the image I take.

Saddle plates seem to have taken a backseat in this discussion. I find the dovetail plate on my C14 perfectly adequate with the help of the 14" long Casady saddle plate and provides a great deal of support longitudinally.

[bmitchell82 (Brendan)]

the saddle plate has been the guts of the discussion its the one doing all the work the dovetail itself is mearly holding the thing flying out the back and to help transfer loadings to the main motors