Losmandy vs Vixen - Difference in imaging

Poita (Peter) · #1 08-12-2011, 09:36 AM

One just has to look at the difference in the Losmandy vs Vixen bars to see that the losmandy is a much beefier mounting solution.
However I'm wondering if anyone has ever done an actual comparison of images taken with one and the other with the same scopes?

I sort of take it for granted that the losmandy system is better, but then I think about a balanced scope moving slowly and wonder how much flex difference you would really get between the two on reasonable sized scopes.
I've had lots of people state that the Losmandy is 'much better, just look at it!' but has anyone quantified the difference in actual use? I'm assuming there will be a noticeable one.

Poita (Peter) · #2 08-12-2011, 09:37 AM

This is what happens when the rains set in, one's mind starts wandering.

gregbradley · #3 08-12-2011, 02:58 PM

This sort of difference would show up in go-to pointing accuracy, tracking errors that come down to flex and difficulty in achieving a perfect polar alignment.

Reading the Paramount MX manual about this point makes it clear you want all scopes to be totally rigid and no flex in order to achieve the highest go-to and tracking accuracies.

My CDK 17 has a dovetail plate twice the size of a regular Losmandy plate and it needs it. Its correct engineering. You want things way overstrong.

Greg.

frolinmod · #4 08-12-2011, 03:43 PM

My EdgeHD 14 came standard with one of those thin orange Celestron "D" style dovetail bars. A normal Losmandy "D" dovetail bar for a C-14 is 3/4-inch thick. The Celestron version is very thin by comparison. I removed that bad joke of a dovetail bar and replaced it with an Andy Homeyer scope cradle bolted to the Paramount ME versaplate.

Merlin66 (Ken) · #5 08-12-2011, 04:39 PM

Fair question.....
I have the C9.25 and a C11, both initially were mounted using the standard "Vixen" type dovetail. I've recently gone to the Losmandy version....
Why?
Hmmmm, it just feels better!
I do spectroscopy on a NEQ6pro mount and in all honesty can't say I found any significant difference.....

g__day (Matthew) · #6 08-12-2011, 11:00 PM

My C9.25 came with the small Celestron dovetail, I replaced it asap with Losmandy rails. Total chalk versus cheese comparision there!

Merlin66 (Ken) · #7 09-12-2011, 12:20 AM

I agree....one is Orange and the other is Black..... ;-)
I think many of us have changed over, but the question as asked was:
is the losmandy and better (more rigid?) BY MEASUREMENT.

g__day (Matthew) · #8 09-12-2011, 01:29 PM

The simple answer is yes, Losmandy is better.

It's a pretty simple engineering formulea (high school engineering studies) to plug turning moment and material flexure for Aluminium. http://metalab.uniten.edu.my/~Halina/EXP3~1.pdf

But even forgetting design consideration - the Losmandy D saddle cross bar is the same length but considerably thicker than the Vixen cross bar.

Assume its the same material, same length, same load and flexure is linear. So a crude estimate is under the same load if the Losmandy bar weights 4 times more than the Vixen one - it's because it's four times thicker, so its flexure will be four times less!

Shape will slightly alter this - but as a reasonable guide - weight you bars and the difference in weights I would expect to be the difference in flexure.

Finally if the Losmandy bar is 1/2" thick by 4 inches wide and carries say 10kgs of gear, placed horizontally (so maximum stress when the scope is parallel to the ground) and bar secured top and bottom of the saddle - plug that in with a flexure under load given a coefficient of elasticity and you will get exactly how much elastic flexure will occur!

Poita (Peter) · #9 09-12-2011, 02:01 PM

I'll take a look at the calculations, it has been a while since I did Materials Science.

I keep getting the same answers though, things like 'I replaced that piece of Vixen Spaghetti with Losmandy Beef' but I'm yet to see any data showing errors before that disappeared after.

I'm sure that there *would* be flexure differences, but would like to see just how much, and if it translates to actual differences in imaging. Especially on say a 10" scope or smaller.
It is a common upgrade that people spend money on, I'm just wondering if it is one that gets any actual perceivable benefit to those with medium to small scopes. I'm always interested in data on common upgrades to work out actual bang for buck, rather than anecdotal.

It certainly feels and looks more impressive though

bmitchell82 (Brendan) · #10 09-12-2011, 04:44 PM

Sorry Greg, that comment is completely wrong! Correct engineering is making it work efficiently and safely because any mug can put a 5" thick piece of Titanium under the telescope and it will work is it effecient? NO, is it cost effective NO. Hence this is poor engineering! If i did it that way ide find myself out of a job because nobody would ask me to design their commercial high rise buildings.

Quote:

Originally Posted by gregbradley

My CDK 17 has a dovetail plate twice the size of a regular Losmandy plate and it needs it. Its correct engineering. You want things way overstrong.

Greg.

The thickness of these plates isn't really the issue, as what you fellas are forgetting is that you have a tube that sits above that dove tail it provides strength to the over all Y axis of the bar. The real difference is in the X axis or the major axis or in another term again the strong axis of the bar which is its depth the losmandy bar is for simplicity 100mm where as the vixen is 35 i think possibly 40mm in depth so the calculation for that is;

((b*d^3)/12)*A*h^2)

b= bredth
d=depth
A= Area
h= centroid of each section from the Nutral axis of the overall shape (or section as its commonly refered to)

If you want to figure out the deflection of your dovetail in the Y axis or weak axis then you have to go into beam formule for a cantileaver beam of the distance away from where the dovetail saddle finishes for simplicity as the saddle is far stronger due to its geometry. Remembering that in most cases you have to resolve your forces into X and Y to do the proper analysis as the telescope isn't bearing vertically.

I havn't gone into half the things that should be considered during a analysis like this but it isn't just a simple 2 second Jig needless to say

So in essence Peter what it is changing is the moment capacity of the bar bending in its strong axis allowing less "Twist" to happen when the tube is at its worst orientation! Its completely a flexural issue which affects pointing in a reasonable way! as you can get Vixen dovetails on large foot print plates see William optics version.

adman (Adam) · #11 09-12-2011, 04:57 PM

I am not an engineer, but from my experience with both systems, it feels like the biggest difference in strength is in torsion - ie twisting the bar along its long axis.

Adam

Poita (Peter) · #12 09-12-2011, 05:13 PM

Brendan, that was what prompted my question, a thicker, stiffer bar may always be 'better' but where is the cutoff point.
Is the vixen already past that point for smaller scopes, or is it woefully lacking?
It is interesting that often people purchase upgrades because it is the done thing, or it is easy to see it is 'better', (like cameras with more megapixels) the question is, is it measurably better.

I have a Vixen dovetail one-piece that incorporates a very large thick plate, it is hard to imagine it flexing under the load of a 6kg scope that is moving incredibly slowly, but I'd like to actually know.

adman (Adam) · #13 09-12-2011, 05:27 PM

Peter - those William Optics-style plates would be very strong - I don't think you would have much problem with those - not with that size scope at least

bmitchell82 (Brendan) · #14 09-12-2011, 05:39 PM

Thats exactly what im talking about adman, Torsion.

What alot of people don't understand is that the strength of the materials that we are using far far far exceed the loads that are placed on them!

Even in your case Greg just because you have a CDK doesn't mean it automatically must require or impose far greater stress's on your materials. Load is indiscriminant! it will flex materials exactly the same as if its a ton of feathers, a ton of lead or even a ton of gold.

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.

Still is your telescope that accurate?

Most people go on what they feel looks about right and to be honest sometimes in my field we know that something works but the public and architects get worried because it "feels" too small!

eg. think of a normal commodore sedan about 1.8ton

would you feel safe if it was sitting on a 35 x 35 x 2 mm Square hollow section 1 meter off the ground? No? I would as its capacity is exceeds that of the car significantly and would easily hold a toyota land cruizer or to take it further.... get a aluminium can that isn't damaged in any way and stand on it... the walls of that are .4 of a mm probably far less........ yet it will hold my weight no issue and im 80kgs.

This is just to give you a idea of the actual strength of the matierials we have today.

Peter I hope this helps you with your issue.

gregbradley · #15 09-12-2011, 06:31 PM

You must be an engineer Brendan.

I did not fully follow your post. Do I have it right that the problem is more a twisting rather than a bending issue?

When I said strength or way overstrong I meant in all ways not just bending. So yes I see thicker isn't necessarily the only strength
vector needing to be allowed for but twisting and even shrinking and expanding.

For example my TEC feathertouch focuser has an eyepiece holder which is strong. But if the temp drops it loses its grip!
Found that out the hard way!

Greg.

bmitchell82 (Brendan) · #16 09-12-2011, 07:00 PM

Greg? Ive already gone though wall thickness and pier diameter do you want me to go though it again thickness is a major component of strength irrispective of diameter! I have analysed it along with section geometry a good 8 months ago now in a very long debate to which ACE's where present.

Im a 4th generation civil engineer (maj structural engineering) with my g grand father and grand father and uncle (structrual engineer) both infrastructure engineers I never heard my grandfather talk about the old maxim of 5 x as strong.... he designed to the materials strength with capacity factors of safety that gets upto 1.5 times the maximum loading. But im not going to go into load combinations and their realitive factors as thats nearly 2 years worth of uni and much more than this thread will accept. This doesn't mean that you have to take my opinion as Gospil but if you have facts/proof/expertise then bring them to the table and lets figure it out. I have supplied facts and the proof while my expertise is quite extensive over alot of different fields giving me logic im still learning loads in my choosen career path.

So back to topic. Irrispective of the angles that we hang these off i have just analysed for the worst that is using the smallest Second moment of inertia in the weak axis or Iyy or Geometric stiffness of the section at horizontal, anything over the side will start taking into account a deeper section to which the cube root really beefs up the Iyy tending towards Ixx which is the strong axis. Even at this a 10mm thick Aluminim plate will only flex 1 micron under 40kgs of weight if you want to tell me some specifics of your dovetail length i can tell you the simplified approximate deflection.

If you want to look at the basic analysis in the PDF its all there and if you would like to correct anything im not precious, if i have made a mistake say so.

As for the difference between the vixen and losmandy dovetails its not the actual dovetail section that makes a difference its the top plate that makes the difference eg the difference between 100mm width and 40mm width for lateral stability or torsion.

If you where to have a look at the Williams optics Vixen style dovetail you will see it has a massive top plate, Same with losmandy they have a big thick top plate and your PME has an extremely wide and if i remember quite a thick adaptor plate. Also this is the reason why they can honeycomb the plate because the depth of the section hasn't changed and the thickness still is the same more force is placed into the honeycomb ridges because they will attract more forces.

Overall this plate thickness is to take the moment couple created by torsion to the central axis of your mount to which your motors/adjustments take that moment and stop it hence your stability.. Big motors = lots of holding torque. aka your motors are the key point here and will be the limiting factor not material strength!

Thats what the point of my post is, people underestimate the strength of materials and go way over board without knowing what they are doing and then seem to sprout that they are knowledgeable or as i call these people ACE's or Arm Chair Expert s. Its extremely frustrating!

Brendan

bmitchell82 (Brendan) · #17 09-12-2011, 07:07 PM

FYI

Pier section analysis click here

gregbradley · #18 09-12-2011, 08:12 PM

You could tone it down a bit here Brendan its all friendly.
I wasn't really talking about piers but since its brought up, one paper I read says rigidity is a product of wall thickness
and the cube of diameter. That means then that diameter being cubed is the major factor. Not to say different thicknesses or
strengths/grades of steel aren't important.

But that would mean it would be better to have an 8 inch 3mm wall thick tube than a 6inch 4mm tube. This is also the conclusion of Dennis Persyk who was the first I saw to write a paper about this. Whether a steel tube is better over a concrete pier I do not know.
I am sure either would likely be good.

As far as safety factor goes that varies with the application. Per Wikipedia that can be as low 1.25 in the aerospace industry due to the need for weight (in order to fly) to maybe 10X. The 5X concept I got from a consultant professional engineer on a building site regarding
strength of beams needed to do a lift. I am sure those factors are lower for buildings and as per Wikipedia it is often 2.0 as the strengths of buidlings is well known and easily measured.

In astrophotography the strength factor is merely based on experience rather than maths as I am not aware of any extensive analysis on this. But practical experience clearly demonstrates the need for exceptional rigidity for imaging (not so important for visual of course).
I am not making any comment about WO dovetails etc. Just the basic concept that any flex in your imaging system from any mounting component will lead to poor tracking and go-to performance when using a guide scope in particular but also if not.

There are numerous posts on the net asking for solutions to focuser flexures and other sources of flexure.

Greg.

Quote:

Originally Posted by bmitchell82

Greg? Ive already gone though wall thickness and pier diameter do you want me to go though it again thickness is a major component of strength irrispective of diameter! I have analysed it along with section geometry a good 8 months ago now in a very long debate to which ACE's where present.

Im a 4th generation civil engineer (maj structural engineering) with my g grand father and grand father and uncle (structrual engineer) both infrastructure engineers I never heard my grandfather talk about the old maxim of 5 x as strong.... he designed to the materials strength with capacity factors of safety that gets upto 1.5 times the maximum loading. But im not going to go into load combinations and their realitive factors as thats nearly 2 years worth of uni and much more than this thread will accept. This doesn't mean that you have to take my opinion as Gospil but if you have facts/proof/expertise then bring them to the table and lets figure it out. I have supplied facts and the proof while my expertise is quite extensive over alot of different fields giving me logic im still learning loads in my choosen career path.

So back to topic. Irrispective of the angles that we hang these off i have just analysed for the worst that is using the smallest Second moment of inertia in the weak axis or Iyy or Geometric stiffness of the section at horizontal, anything over the side will start taking into account a deeper section to which the cube root really beefs up the Iyy tending towards Ixx which is the strong axis. Even at this a 10mm thick Aluminim plate will only flex 1 micron under 40kgs of weight if you want to tell me some specifics of your dovetail length i can tell you the simplified approximate deflection.

If you want to look at the basic analysis in the PDF its all there and if you would like to correct anything im not precious, if i have made a mistake say so.

As for the difference between the vixen and losmandy dovetails its not the actual dovetail section that makes a difference its the top plate that makes the difference eg the difference between 100mm width and 40mm width for lateral stability or torsion.

If you where to have a look at the Williams optics Vixen style dovetail you will see it has a massive top plate, Same with losmandy they have a big thick top plate and your PME has an extremely wide and if i remember quite a thick adaptor plate. Also this is the reason why they can honeycomb the plate because the depth of the section hasn't changed and the thickness still is the same more force is placed into the honeycomb ridges because they will attract more forces.

Overall this plate thickness is to take the moment couple created by torsion to the central axis of your mount to which your motors/adjustments take that moment and stop it hence your stability.. Big motors = lots of holding torque. aka your motors are the key point here and will be the limiting factor not material strength!

Thats what the point of my post is, people underestimate the strength of materials and go way over board without knowing what they are doing and then seem to sprout that they are knowledgeable or as i call these people ACE's or Arm Chair Expert s. Its extremely frustrating!

Brendan

bmitchell82 (Brendan) · #19 09-12-2011, 09:13 PM

I get frustrated Greg and you changed your post after i wrote mine to which is the tone you received. I didn't call you a dummy at all but I asked you if you can see any issues with my calcs please correct me.

I have always said this and will state it again for I hate speculation, hear say and mis information, if you are guessing say so if you know state your facts it's just the way i work im sorry if you feel down trodden but don't take what I say to heart because that doesn't help get all the facts out!

gregbradley · #20 09-12-2011, 09:23 PM

No worries Brendan. I understand it is your area of expertise and there is a lot of hearsay and the Engineer necessarily works in the realm of fact and provable practical mathematics.

Greg.