Big scope tiny mount

[CraigT82]

I see there are a few of these little harmonic drive mounts like this one around now, with pretty huge rated payloads.

https://www.teleskop-express.de/en/artesky-166/mounts-goto-harmonic-drive-334/wd-20-18033
 

Can a mount of such little mass really hold a big heavy scope? I don’t mean drive it around the sky and point it in the right direction/track accurately without breaking, but actually hold it still against breeze and vibration (e.g. by touching the focuser, walking around the scope etc). This one seems to be compatible with Eq5 style tripods( m10) so couldn’t even put it on a big beefy tripod? 
 

I am tempted at some point to swap out my AZeq6 for something like this one day to reduce the e amount of weight that I need to move around, but I’m just not convinced a tiny mount head like this can actually keep a 25kg, 1.5m long scope held steady. 

[Alan White]

Interesting question Craig, I have been thinking exactly the same.
Many of these are available with a light carbon fibre tripod too, 
surely not steady enough for visual users who need to keep touching the focuser and telescope.

[Astronomist]

This topic reminded me of a picture I saw in the Askar 203mm thread yesterday:

I know its an extreme example, but technically this mount should be able to handle that (24kg) load with a counterweight (it is slightly over the 'without counterweight' weight capacity).

I simply cannot see that this would be in any way stable, in fact it looks a bit like an elephant balancing on a cocktail stick. I suspect It would suffer terrrible low frequency vibrations due to the lack of a substantial dampening mass.

It would be nice to hear from someone who has used one of these strain wave mounts near to its rated capacity, but i think most of them are in use with lightweight portable setups, which they lend themselves very well to.

[dweller25]

My interest peaked when I heard how much the harmonic drives could hold and how little they weighed, but I have seen a Youtube video that says without a counterweight there could be a risk of toppling over with heavier scopes.

So the counterweight and counterweight bar adds to the weight of the setup, loosing some of the advantage !

Edited August 10 by dweller25

[CraigT82]

Yes toppling over would be a concern with any of these mounts when used without CW and a heavy scope on board. Probably not so much an issue if pier mounted but the pier would need to be much bulkier to cope with the extra deflection forces an big one side load like that would exert?

 

[dweller25]

However, here is some real world experience….

https://www.cloudynights.com/topic/932222-askar-185-and-zwo-am5/?hl=%2Baskar#entry13613140

 

[CraigT82]

8 minutes ago, dweller25 said:

However, here is some real world experience….

https://www.cloudynights.com/topic/932222-askar-185-and-zwo-am5/?hl=%2Baskar#entry13613140

 

Yeah I did see that thread, doesn’t answer my question though.  I’ve no doubt that the motors and gearing can point and track the stated payloads accurately for imaging.

I want to know how it can hold the scope steady in the presence of external forces. I guess what I really want to know is, if you were to look though the big scope visually, whilst focussing, would the image dance around or would it remain steady? 
 

In my head (and I’m no engineer) you need a mount head and tripod with heavy mass to hold a big scope steady. Or is it the case that these little mounts are sufficiently stiff and without free play that they can hold a much heavier scope steady without all that mass?

Edited August 10 by CraigT82

[Elp]

The tripod is the critical component here not the mount head. If you've ever handled a HD mount you'd find there's little to no mechanical flex, you certainly can't physically move any axis with a HD drive on it whilst it's still, with power off they'll fall under load but it's not a very quick motion (there's a video on YT), most if not all RA axis have a brake for such a scenario so it doesn't move when power cuts.

Physics tells you if you mount something off centre balance (scope) higher off the ground you're likely to get more flex too, so goes back to the tripod.

From my limited visual experience when viewing you're not constantly touching the scope, it's strictly hands off, again dampening is the factor here, again the tripod.

Edited August 10 by Elp

[Elp]

You are supposed to test the east/west heavy balance prior to using it CW-less, common sense will tell you that but the manuals also state so. If it's likely to be off balance in such a situation it's why they recommend adding CWs as a option, or use a heavier tripod or fixed pier, again why people then get heavier mounts to compensate for using such large loads. The benefit of HD drives is the torque they generate, so can be physically smaller in size whilst moving heavy loads, if you've ever seen an ABB or Kuka (others also) industrial robot arm in action you wouldn't believe they could handle the weights that they do, with so much easy acceleration and deceleration, note they're usually bolted to the ground with solid foundations, in astro cases again back to tripod.

[ollypenrice]

I've just built up an AM3 with camera lens and guidescope setup. The payload does not tax the mount but the feel of the thing is remarkably solid and it has an entirely different feel to a regular GEM. We may need to recalibrate our instincts.

There are plenty of applications in engineering in which very high stresses are carried by very small components. Take the steering head bearings on a bicycle. At the yellow dots are two small cup and cone ballbearings.  Now consider a 20 stone cyclist descending at 100kph (my cousin's husband Steve, for instance.)* Under heavy braking think of the lever (red) working against those little bearings. And they must remain stiction-free because, if they don't, the bike will become unsteerable. These are astonishing forces.

Olly

At 6'6" this amiable giant stood amongst the crowds of cyclists who'd just ridden up Mont Ventoux and said, 'As usual I'm a bloody tourist attraction!' 

Edited August 10 by ollypenrice
Forgot the pic

[wimvb]

6 hours ago, Astronomist said:

This topic reminded me of a picture I saw in the Askar 203mm thread yesterday:

I know its an extreme example, but technically this mount should be able to handle that (24kg) load with a counterweight (it is slightly over the 'without counterweight' weight capacity).

I would like to see a picture of this thing pointing at zenith

[Elp]

It's nowhere near the size and weight of that scope but seeing my SF102 pointing at zenith on the diminutive hem15 which is near half the weight of the imaging train inspires confidence. As mentioned what the mount head is mounted onto makes a world of difference, you'd need a pier extension for back end clearance.

[CraigT82]

Just seen an interesting thread on CN of people using harmonic drive mounts for visual… couple of people with large-ish scopes reporting rock steady focusing so fortunately it seems the stiffness of these mounts can overcome their lack of mass when it comes to holding a big scope steady.

Worryingly also a couple of reports of people experiencing tripod tipping issues with the large weight imbalance of a big scope and no CW (mostly in alt az mode). One person states that they have to have the tripod legs fully extended to have a large enough footprint to prevent tip over. 

[TiffsAndAstro]

40 minutes ago, wimvb said:

I would like to see a picture of this thing pointing at zenith

I'm imagining a trail of sparks on the ground

[Elp]

Probably the usual promo "marketing" spiel going on as that Sharpstar HD MK3 can carry 18Kg wo CW, and 26Kg with, so technically within spec of the scope weighing 24Kg without accessories.

I would have thought the Pegasus Nyx 101 more appropriate being able to carry 20Kg wo CW (or very similar to the Sharpstar) or Warp Astron WD20, or the ioptron hae69 which can do 35kg but isn't a light mount. Using a CW with a HD mount is kind of counter productive for purchasing one in the first place.

Edited August 10 by Elp

[Elp]

37 minutes ago, CraigT82 said:

One person states that they have to have the tripod legs fully extended to have a large enough footprint to prevent tip over. 

Testing stability is paramount prior to using one wo CW. When imaging the fireworks galaxy which is quite high up ATM, my scope mounting point was over my head height and I had to lift the scope above my shoulders to fit it to the mount. Like this though I can use it visually to view high up targets standing up fairly comfortably.

[Rusted]

I seriously looked at these harmonic mounts for each of my long and heavy, solar telescopes. 6" f/10 and 7" f/12.

My present, massive, equatorial GEM is retiring as I take down my 2-storey observatory.

Three telescopes were mounted at one point but often stalled when slewing. So I ended up carrying only the 6" H-alpha at 35kg and 1.68m in length.

Despite my mount weighing a couple of hundred kg, on a massive pier, it often struggled to control backlash in the wind inside the dome.
Largely due to incredibly poor design and construction of the [commercial] worm housings. [Beacon Hill.]
These easily flexed when stressed by the long leverage [moment] of my scopes working at 3000mm focal length.

My fear with these small SW/H mountings would be losing thousands of pounds worth of kit onto the hard ground in a catastrophic failure!
Anyone with enough money for a large APO [?] refractor owes it to themselves to understand the meaning of balance and geometry.

Not to mention the tiny footprints and flexure of foolishly tiny tripods. Obviously aimed at the small refractor imaging crowd.

A line drawn between any two feet of any tripod passes very close to the centre!!! The individual legs/feet are NOT the tipping radius of the tripod.
Large tripods are better and weight is almost everything in lowering the centre of gravity! Let me repeat this for clarity:
Even an infinitely stiff tripod will tip over at the tipping line between any two feet if you put something heavy and unbalanced on top!

My main concern with all equatorial mountings is the poor value on offer for what is really a very simple mechanism.
It is not for nothing that the trend is towards ever smaller instruments and ever increased portability.
Despite this "new" SW technology: There are still no affordable mountings which can easily handle a "real" telescope.

[Elp]

10 minutes ago, Rusted said:

There are still no affordable mountings which can easily handle a "real" telescope.

Haven't used one but I'd think a heavy duty pier tripod may work (as in the type costing close to or more than 1000).

[ollypenrice]

4 hours ago, Rusted said:

A line drawn between any two feet of any tripod passes very close to the centre!!! The individual legs/feet are NOT the tipping radius of the tripod.
Large tripods are better and weight is almost everything in lowering the centre of gravity! Let me repeat this for clarity:
Even an infinitely stiff tripod will tip over at the tipping line between any two feet if you put something heavy and unbalanced on top!

 

This point needs more air time.

Olly

[CraigT82]

Yes I think the tripod tipping issue would be the main reason I wouldn’t swap out my AZeq6 for a harmonic drive unit to carry my 300p

I’d either have to use counter weights which would remove most of the attraction of the HD mount in the first place, or either somehow make or buy a very wide stance tripod

[Elp]

6 minutes ago, CraigT82 said:

or either somehow make or buy a very wide stance tripod

I specced my Berlebach without a spreader for this very reason, I can control the amount of spread.

[saac]

My engineering pedantry anxiety is at Def Con  3 and climbing, stop conflating stiffness with toppling (tipping over) - the property of stiffness has nothing to do with the occurrence of toppling. Likewise, mass has nothing to do with load carrying capacity.  Now I know how Olly feels when folk can't tell the difference between a present participle and a gerund.  Arrrrrgh. 

Jim 

[CraigT82]

14 minutes ago, saac said:

My engineering pedantry anxiety is at Def Con  3 and climbing, stop conflating stiffness with toppling (tipping over) - the property of stiffness has nothing to do with the occurrence of toppling. Likewise, mass has nothing to do with load carrying capacity.  Now I know how Olly feels when folk can't tell the difference between a present participle and a gerund.  Arrrrrgh. 

Jim 

Not sure who this rather rude and unhelpful post is aimed at, but I’m not sure anyone here is confusing stiffness with toppling. 

My concern is that any mount without counterweights is at risk of toppling with a heavy scope load, especially in Alt Az mode (which is what I’d use). My AZeq6 would certainly topple without counterweights and my 300p mounted, and that mount weighs a lot more than the smaller HD mounts.

I’m also concerned about the small HD mounts ability to dampen vibrations seen through the scope, e.g during focusing.  Mass of the mount and tripod certainly does have an effect there, and so does stiffness in the mount head and the tripod. 

[Elp]

The ZWOs are designed to work in EQ and alt az, so within their scope weight limits they work fine. Fine slewing my am3 when I was doing visual solar induced no vibration whatsoever, and that's on a CF tripod.

Larger heavier scope, needs a larger mount and tripod, this has never changed.

Edited August 10 by Elp

[CraigT82]

28 minutes ago, Elp said:

I specced my Berlebach without a spreader for this very reason, I can control the amount of spread.

I once had quite a nasty accident with a converted wooden Leica total station tripod which was carrying a CG5 and a 8” f6 newt. There was no spreader or chain and the legs are capable of splaying completely out so the tripod is almost pancaked.

One day I had it set up on the balcony at my old flat and one spiked foot slipped outwards on the tiles and didn’t stop - sent the whole thing crashing to the ground. 

I think the Berlebachs have some kind of hard stop to the leg spread to prevent that kind of incident?