Mount Weight Limit - Imaging vs Visual

[richyrich_one]

Why is there a difference with the recommended weight limit for a particular mount when imaging vs visual use?

[Bizibilder]

It is all down to stability and rigidity.  For imaging the mount must be capable of carrying the weight that you put on it without any shake or strains.  For visual this is not so important as your eye can compensate for any slight vibration or movement - a camera can't.

[richyrich_one]

So what is the weak link, mount head, tripod, both?

Does the use of a pier alleviate it?

[Bizibilder]

The "weak link" is a combination of everything in the mount.  The strength of the main castings and every joint has a "tolerance".  When assembled all these add up (or subtract) to give a mount that has a certain amount of "play" or backlash etc.  The rigidity of parts like the tripod do depend on how they are set up - the legs extend for instance.

A well constructed pier is far more stable and rigid than a tripod but has no effect on the ability of the mount to successfully carry a particular design maximum weight - that is down to the bearings and rigidity of the other mechanical components.

I put my C11 (plus three and a "half" counterweights) on my HEQ5 once - just to see what happened.  Obviously nothing actually broke but the mount "stiffened up" somewhat under the gross overload the it had been put under by that abuse!  I did not, needless to say, try and drive the mount with its motors - It may well have fried them.

[ollypenrice]

Weight is vastly over rated in these discussions. Yes, it does need to be respected but this is difficult because serious (ie very expensive) mount makers give a figure that is good for imaging. Others need their payload figure dividing by two for imaging.

But, but, what about accuracy? This also matters. How many arcseonds of sky land on one pixel? If lots of arcseconds of sky land on one pixel you have a nice tolerant system and errors due to bad periodic error may be insignificant on your image. But if only half an arcsecond of sky lands on one pixel then you need very fine tracking. Having either a long focal length or small pixels or both puts you into this latter category.

So a mount may be able to carry the payload of your rig without being able to guide it accruately enough for the rig to work properly.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc

[richyrich_one]

So is a pier benificial if you are already within a mount's specified imaging limit?

Would it allow you to push the overall weight nearer the visual limit and still successfuly image?

[rowan46]

not really,  what a pier does is stop vibrations from the ground, stop the weights fouling on tripod legs and because the pier is usually a permanent installation  with a permanent mount on top. polar alignment only has to be done once

[richyrich_one]

I appreciate your helpful replies.

If I wanted to put a 250PX on an NEQ6 and image with it (which I do ), do you think using a pier would make much difference?

[richyrich_one]

not really,  what a pier does is stop vibrations from the ground, stop the weights fouling on tripod legs and because the pier is usually a permanent installation  with a permanent mount on top. polar alignment only has to be done once

Sorry Rowan, just missed your post.

I think I've got it now.

[ollypenrice]

So is a pier benificial if you are already within a mount's specified imaging limit?

Would it allow you to push the overall weight nearer the visual limit and still successfuly image?

A pier is better than a tripod but, as I said in my previous contribution, you can't just base you assessment of a mount-scope-camera combination on the weight. This would be naive. Put a 15kg widefield Newt with a focal length of a metre on a mount and then put a 15kg SCT with a focal length of three metres on the same mount  and the first might work fine but the second might be a disaster.

In order to give sensible advice about the practicality of a rig we would need to know scope weight and then focal length and pixel size. I image widefield with this;

This dual rig images at 3.5 arcseconds per pixel, which requires very low tracking precision (though the mount can deliver great precision.) Adding another two scopes/cameras would double the weight but would not trouble the mount one iota. However, if I put a lighter scope on the mount, but a scope with five metres of focal length, life would get a lot more complicated. Tracking would need to be more precise. Several times more precise.

Weight is only a small part of the story.

Olly

Edit, OK so you have now specified a scope and mount. What about pixel size? Are you aiming to use a DSLR? These have small pixels. Can you specify?

[richyrich_one]

Edit, OK so you have now specified a scope and mount. What about pixel size? Are you aiming to use a DSLR? These have small pixels. Can you specify?

I was looking at an 1100D

[sharkmelley]

I sometimes use a Celestron C11 (2.8m focal length) on the NEQ6 with guidescope and DSLR.  The C11 weighs around 11kg.  I use 4 counterweights of 5kg so I'm really pushing the mount to its limits.  Nevertheless, with the belt drive modification, I can usually guide at 0.5arcsec RMS (according to PHD) unless the seeing is bad.

I have read that the 250PX weighs 15kg - i.e. an extra 4kg for the scope and probably an extra 5kg counterweight.   I really don't think I would be comfortable with that.  And a tube of that size would be pretty unstable in any breeze.

Mark

[richyrich_one]

Am I onto a non-starter then?

[Merlin66]

I've been using a C11 (at f10) with three cameras and the spectroscope (slit gap typically 29 micron) on the NEQ6pro (4 x 5kg c'weights) for the past three years.

This is a permanent set-up in an observatory.

I now use EQmod/CdC for control and AstroArtV5 for guiding and image acquisition/ processing. No issues/ no drama.

[Thalestris24]

+1 "No issues/ no drama." I'd like to have that too!

[jskee]

Nice setup Merlin66 doesn't seem like we can get the NEQ6 pro here in the states, too bad, looks like a good mount.

[richyrich_one]

Thanks for your replies.

I am going to start a more specific thread.

[Cornelius Varley]

Nice setup Merlin66 doesn't seem like we can get the NEQ6 pro here in the states, too bad, looks like a good mount.

Yes you can. Its called the Orion Atlas EQ-G Pro http://www.telescope.com/Mounts-Tripods/GoTo-Mounts-Tripods/Orion-Atlas-EQ-G-Computerized-GoTo-Telescope-Mount/pc/-1/c/2/sc/36/p/24338.uts

[laser_jock99]

If the load is well balanced then your mount should be capable of imaging up to max pay load (and beyond). If the load is off centre then you'll break something as the drive train will be strained.

Olly's point about focal length is valid for under loaded and overloaded mounts- if the tracking accuracy is not up to the focal length you are using then you're on a loser from the start.

My permanent rig is a somewhat overloaded EQ6 (75Kg total load). But as I image at sub 1m (300mm f2.9= 870mm) drive accuracy is less of an issue than collimation accuracy! 

The other issue seldom considered is inertia. Heavy objects don't move as easily/quickly as light objects- so you could argue a heavy load is inherently more stable and less prone to vibration. Professional video camcorders are still shoulder mounted and very heavy- you don't see a news crew filming with a palm corder!  

Overloaded? Not really.

[sharkmelley]

Wow!  I never seen so many weights on an EQ6.  It certainly shows what is possible.

Mark

[ollypenrice]

If the load is well balanced then your mount should be capable of imaging up to max pay load (and beyond). If the load is off centre then you'll break something as the drive train will be strained.

Olly's point about focal length is valid for under loaded and overloaded mounts- if the tracking accuracy is not up to the focal length you are using then you're on a loser from the start.

My permanent rig is a somewhat overloaded EQ6 (75Kg total load). But as I image at sub 1m (300mm f2.9= 870mm) drive accuracy is less of an issue than collimation accuracy! 

The other issue seldom considered is inertia. Heavy objects don't move as easily/quickly as light objects- so you could argue a heavy load is inherently more stable and less prone to vibration. Professional video camcorders are still shoulder mounted and very heavy- you don't see a news crew filming with a palm corder!  

Overloaded? Not really.

Ths is a good post because it challenges the orthodoxy and this is always a good thing. The net is a place where, if one sheep goes Baaa, the Baaaas revererate around the planet...

Olly

[oldpink]

wouldn't it be more down to what the Motors can handle
after all you can have perfect balance but just too much weight for the motors to getting it moving in the first place

I'm quite sure my EQ5 would chew its gears to shreds trying to move that lol

[Sp@ce_d]

Wow I am impressed.. I count 12!! Nice shot but you should really do some flats to get rid of that dust bunny

Sent from my iPhone using Tapatalk

[g0ibi]

I use a SW MN190 on an HEQ5, I need 3 counterweights for imaging. I spend a good deal of time balancing and getting it spot on as the mount is near it's max weight allowance. But it works!!

I have upgraded to belt drive and the rig is pier mounted.

Comments please?

Ron