Thinking About Making A Mount.

[Stub Mandrel]

Having seen an EQ6 mount and suddenly discovered how big they are (I didn't know the tripod has 2" legs... so I'd guessed it was about 20% smaller, a big difference in bulk!) I've been thinking of making a big mount.

Lots of choices so I'm interested in other people's thoughts.

Fork, mono-arm or GEQ.

My thinking is a mono-arm or GEQ will be easiest to mount a scope on. GEQ will be more compact and drive will be easier but Mono arm has advantages in use.

Bearings

Much maligned plain bushes can take big loads and tend to be smooth, but are more likely to suffer stiction and demand higher tolerance machining. I think the practical choice is angular contact ball bearings vs. rollers. Rollers take bigger loads and are less likely to 'brinel' the races if left in the same position for ages. Balls might be smoother in use. Obvious option is to use a size allowing bearing swaps. Can anyone advise bearing sizes used on HEQ5 or EQ6 mounts?

Body material.

Cast iron is deal in terms of weight, vibration deadening, ease of machining. But all that weight is potential downside as well. Fairly expensive.

Steel no advantages over cast iron if solid, but can be fabricated by welding. Cheap.

Aluminum alloy, not as stiff unless made bigger section. An aluminium body as stiff as a steel/CI one will be bigger but lighter. Easy to machine, very expensive for big chunks!

One option is to use the headstock of a small lathe as the main body. A mini-lathe headstock is a cast iron box with space inside for DEC drive. Plenty of material for fitting to an Alt-Az base for polar alignment.  It would include a spindle with a bore that could be sleeved for a polarscope and a nice big (80mm) flange suitable for putting a demountable DEC assembly on it. Could end up bulkier and heavier than a bespoke arrangement.

Drive

I was wondering about alternative drive modes to worm gears for the final drive (not intermediate). Apparently rubber wheels and belts are used by some top-end mounts, but I'm worried about the lack of stiffness even with a high-tension kevlar belt. That said these belts are used for high-load CNC systems and Avalon mounts.

DFM Engineering use a friction drive with a hard steel roller on a steel disc - great but their discs are about 30" diameter, a bit bigger than I had in mind...

My thought is to build a 3D model to check feasibility and strip away excess material, and ensure it can be machined with my kit..

 

Alternative is just to dive into the bits box, which has some big bearings and stuff in it already.

 

This may never fly, but random comments and ideas welcomed.

[Davey-T]

Are you planning on it being portable Neil ?

Dave

[Stub Mandrel]

Semi-portable - i.e. not permanently in place, but it can be heavy.

[Davey-T]

I think the " easiest " way to get accuracy is direct drive with permanent encoders, probably the most expensive too 

Dave

[iapa]

when designing, is it worth considering motorizing the alt and dec for PA?

 

[JamesF]

Were I going to built my own mount I think the first questions I'd need to answer would be what OTA I wanted to put on it and whether I might care about field rotation.  Possibly whether I wanted it to be electronically-controlled, too.  Those three things might have a significant impact on the final design.

As regards bearings, I can't help thinking that I own two motorbikes, one of which is quite capable of breaking the motorway speed limit in first gear and the other will do somewhere upwards of one hundred and sixty-mumble mph.  There's potentially a huge amount of stress on the steering head and swing arm bearings, yet they're really nothing astonishing.  Oh, and the second has an aluminium box section frame.  I've stood the Ducati on the front wheel under braking (on a racetrack) which must put a massive load through the headstock.  So perhaps in the telescope world accuracy is more important than "massiveness"?

James

[Stub Mandrel]

8 hours ago, JamesF said:

Were I going to built my own mount I think the first questions I'd need to answer would be what OTA I wanted to put on it and whether I might care about field rotation.  Possibly whether I wanted it to be electronically-controlled, too.  Those three things might have a significant impact on the final design.

As regards bearings, I can't help thinking that I own two motorbikes, one of which is quite capable of breaking the motorway speed limit in first gear and the other will do somewhere upwards of one hundred and sixty-mumble mph.  There's potentially a huge amount of stress on the steering head and swing arm bearings, yet they're really nothing astonishing.  Oh, and the second has an aluminium box section frame.  I've stood the Ducati on the front wheel under braking (on a racetrack) which must put a massive load through the headstock.  So perhaps in the telescope world accuracy is more important than "massiveness"?

James

I want it to be take something up to the weight of a 200mm  newt, no field rotation and electronic control.

Your bike (unless it's a harley) has suspension whose movement will completely mask any flexure in the swing arms (yet even mountain bike owners claim benefits from adding stiffeners to fork brake bridges etc.) but I doubt you want steering accurate to less than an arc-second even at mumble-speed :-)

I've uses an online calculator to compute the deflection of a 1" square, 6" long steel bar with a 20lb load (150PL + counterweights?) on the end (OK I wanted a simple calculator that didn't ask me for a moment of inertia, just a cross section!) The result was 0.000576". Assume the load is a telescope pointing up at the zenith, so what's the angular error introduced by that deflection if the scope is 1.2m long?

0.0055 degrees = 20 arc-seconds.

Obviously this is the worst case if the scope is rotated from horizontal to vertical, and as it would happen very gradually it could be guided out, but it does show that what we think of as 'rigid' in everyday terms can be quite flexible when considered in terms of pointing telescopes. It also makes you realise how much the scope itself flexes... it's a wonder we can image anything!

I think what is really needed is enough rigidity to resist vibration or flexure under things like wind load and smooth enough action to allow guiding to eliminate other errors.

10 hours ago, Davey-T said:

I think the " easiest " way to get accuracy is direct drive with permanent encoders, probably the most expensive too 

Dave

Best suited to a permanent installation though

 

10 hours ago, iapa said:

when designing, is it worth considering motorizing the alt and dec for PA?

 

Yes it will be motorised and guided mount.

 

[newbie alert]

I'd of thought  making your own would be far more expensive and prob not as accurate as a massed produced one..its not just gears/bearings and a bit of lube..all the electronics etc that work in conjunction which each other..encoders and flimsy wires on circuit boards frighten the life out of me..  it if you're capable and have an understanding of these things then good luck to you...hope it wows you..

[Stub Mandrel]

My other hobbies are model engineering and electronics, so I've got a well equipped workshop and reasonable skills.

I've already fitted gearboxes and stepper motors to my EQ3 and made a goto/tracking controller for it.

I'd hope to at least equal the accuracy of a commercial mount but as my musings on Nige's question concluded, it's more the smoothness of action and rigidity than absolute accuracy that matters. In engineering you can try and build in precision or design in adjustment. With a mount, guiding can provide the adjustment to cover up minor errors, especially where their effects are gradual (e.g. even top quality mounts use PEC to overcome errors in the wrom gear).

[newbie alert]

Good luck..hope it works out for you..

Mandrel mount coming soon