Lots of views and no responses, so I'll have a crack.
The TDM system adds a very high resolution encoder to the centre of the mount itself (replaces the polar scope I think).
This allows the system to monitor where the mount is *actually* pointing, rather than where it thinks it is pointing.
So corrections, very accurate ones, can be applied very quickly, which would remove things like PE and even wind gusts and bumps I would imagine.
So assuming your polar alignment was pretty much perfect, you could get away without guiding for quite reasonable length exposures.
No guidescope or guidescope flex to worry about, one less camera, no issues with having to find a guidestar, accidentally guiding on a hot pixel etc.
Downside is that if the seeing is all over the place, it won't help you, or if your polar alignment is off.
Apparently the new model allows a guidecam as well.
I would think it would work great with an adaptive optics setup, the TDM would make sure the scope was tracking perfectly, with almost no PE etc. and the AO would take care of refraction/seeing/slight abberations and so on.
It is pricey though, about $2000, but if you already have an old EQ6 or CGEM, then you are payinh $2000 to get a mount that has a PE of potentially < 2" which would mean all you have to worry about is your polar alignment.
It is transferrable between different mounts by utilising different adapters, so there is some potential for changing around if you get a different mount.
I would think it should be possible to 'push' the mount around and not have it lose it's marbles, as the TDM would know how far you moved it.
Downside:
PA has to be very good, no accounting for atmospherics (though if the seeing is that bad, do you really want to image anyway?)
Atmospheric seeing conditions are around @2~3 arc sec's at best (guesstimate avg)...so you cannot RA track/guide better than this. By track...lest say if you are RA tracking at perfect sidereal rate...the stars will still oscillate around regardless.
TDMs are expensive...auto-guiding is cheapish and can attain good results with guiding. I'm not sure what CGEM users get but lest say around +/- 2 arc seconds with PHD. Seeing will be on top of this as it would with a TDM.
TDM works in RA only...what if the polar alignment is "off"?
IMHO...the best and far cheapest option is off axis auto-guiding.
I guess if you are autoguiding as well, then the autoguider has less corrections to do, the mount should already be tracking solidly with the TDM and the corrections made very quickly so you end up with very smooth performance, no sudden movements should mean tighter images.
Or if using adaptive optics, then you are cleaning up the seeing, as any adjustments required would then easily be within the AO capability.
Just like the Ovision worm for G11 mounts, you want the base performance of the mount to be as smooth and error free as possible, the TDM might deliver this performance from cheaper mounts, saving you the hassle of selling and buying a more expensive mount. You can keep what you have already and improve on it.
I'd jump on it if it was a $500-$1000 add-on, at $2000+ it gets expensive.
Greg the TDM is a very high quality encoder which is bolted to the RA axis and attached to a control system. This gives very accurate readings of the absolute RA position and movement faster/slower then sidereal rate is removed via control of motor speed. It does not matter too much whether the PE is uneven or spiked as the encoder updates the control system many times per second which then corrects for errors. Basically a mechanical/software solution for average to poor worms. In theory you could use such a system to flatten the PE curve to AP like precision, in the real world hmmm. I like my EM200, basic as a farm tractor but very precise worms
There is always a mechanical lag on corrections, the motors have to move considerable mass. So rough worms may be tough to correct for completely because of that. I have seen a site that coated worms of EQ6 type mounts with some sort of super lubricant that claimed big improvements in PE.
Perhaps using both would get good results - the mechanical smoothness needed plus the accurate encoders.
The PE becomes very smooth and almost non-existant if you look at the graphs for the TDM on an EQ6.
I would say if you were going to add a TDM then yeah, it would make sense to give the mount a go-over and clean up and lube the gears well. The TDM can also be added to CGEM, G11 etc.
A very interesting link Peter. That seems to be the future of mounts. More accurate encoders are becoming more common as an option for mounts.
I agree that more accurate encoders are the way to go, but you also need to consider where in the design chain they are used.
If i understand it correctly, the TDM is an aftermarket unit that effectively sends precise guide commands to the scope. Thus the system is still at the mercy of how the mount handles guide commands, unless it totally replaces the hosts motor control system.
If a hi prec encoder is designed into a mount from day one, great, but how you meld an "add on" into a variety of "proprietary" scopes may have interesting consequences.
Considering most mounts already have a PEC ability, i always wondered why someone didnt put a "mid precision" encoder onto the worm.
Assuming say a 180 tooth worm wheel, and that you could make a PEC model up, you could get away with a 180x less precise encoder for the same tracking accuracy, and have no need to fix anything to the RA or DEC axles, thus making a much simpler installation.
As the encoder is on the worm, any gearbox PE is eliminated,
and a single turn PE model can be used for the remainder.
Much much cheaper for a "retrofit" type installation.
Thoughts????
Andrew the TDM by passes the worm completely and is fixed directly to the RA shaft itself. The thing with this is it has no effect on the dec axis so if your polar alignment is out even the smallest bit the whole thing becomes a paper weight.
Greg I believe the future in mounts will be direct drive and most the problems of dealing with worms and gears will become a distant memory only to be replaced by new problems.
Andrew the TDM by passes the worm completely and is fixed directly to the RA shaft itself.
Understood fully, but all i am saying is, on a cost benefit analysis,
can a much cheaper encoder on a worm + PEC be equivalent to a really expensive encoder on the output axis of an average quality mount???
Any mount designed from the ground up with a direct drive motor and an integrated hi prec encoder "should" always have the abilty to outperform a TDM retrofitted to a crappy mount.
Horses for courses.
Understood fully, but all i am saying is, on a cost benefit analysis,
can a much cheaper encoder on a worm + PEC be equivalent to a really expensive encoder on the output axis of an average quality mount???
Any mount designed from the ground up with a direct drive motor and an integrated hi prec encoder "should" always have the abilty to outperform a TDM retrofitted to a crappy mount.
Horses for courses.
Andrew
Andrew I don't see how that could improve things as an encoder mounted directly to the axis would give an absolute position (we are here) whilst one mounted on the worm would still be giving "this is where I think we are" and would be at the mercy of the quality of the worm once again.
an encoder mounted directly to the axis would give an absolute position (we are here)
Understood, but some of the high precision encoders arent "absolute" encoders, they just have high precision.
The high precision "absolute" encoders are horrendously expensive, relative to a retrofit unit that is just used to get better tracking.
Again, all i am musing is, could a big gain in accuracy ( relative to present ) be gained by using a suitable encoder on the worm.
This wont remove PE, but it means it can be mapped on a true 1:1 basis, hence removing all the quirks of a gearbox with odd gears.
Andrew its worth a test and if it works you can sell an "Andrew worm encoder add-on" for $500 and sell them all day.
I read in the Bisque PMX manual that mostly errors in the worm (20 microns) cause the PE. 20 microns is a pretty small size. I assume that figure is larger for less well machined worms.
I read in the Bisque PMX manual that mostly errors in the worm (20 microns) cause the PE. 20 microns is a pretty small size. I assume that figure is larger for less well machined worms.
Greg.
Greg, the accuracy of the machining process will be neither here or there these days as the vast majority of worms and worm wheels would be cut on CNC machines with better than 0.01 mm accuracy as it would be the only way to meet mass production quota's. The real difference is made in what takes place after the machining process to clean up the worms and this is usually in the form of lapping with various grades of diamond grits to remove the lumps. It is very time consuming hence the extra cost. Brendan Mitchell spent quite a bit of time lapping his worm gear mesh on a simple EQ6 and gets fantastic PE figures but he has also set the gear mesh clearances' very well to. Another factor is the size and number of teeth on the worm wheel itself, basically bigger is better as small errors will be less pronounce and mount builders such as SB and AP always use large wheels on the RA for that reason. It is best to view the machining process as a raw stage before final finishing. Mass produced mounts such as those made by celestron, meade, skywatcher etc do not have the same attention to lapping as something like a PME/PMX or AP mount but they don't need to as the profit margin is not there. Other things such as worm block design and run out in the worm itself will also come into play but these will affect backlash more then anything else.
Andrew, why not give it a shot on a cheap mount and see what happens. My EM200 has the encoders on the end of the worm so you might be on to something.
Andrew, why not give it a shot on a cheap mount and see what happens. My EM200 has the encoders on the end of the worm so you might be on to something.
I only have Meades and an EQ6, so too hard.
The EQ6 doesnt have the space, and with the Meade, i know more than enough about how the firmware works to know that reverse integrating it would be more of a problem.
Getting it to track at a set rate may be OK, but how to control motor reversals ( esp in DEC ) would be more effort than its worth.
And just for info re the manufacturing tolerances, i did the numbers on a meade wormwheel a few years back
With a 5" dia wormwheel, one arcsec of error at the point of contact with the worm is approx 0.00031mm.
Puts it into perspective how small the tolerances are
I only have Meades and an EQ6, so too hard.
The EQ6 doesnt have the space, and with the Meade, i know more than enough about how the firmware works to know that reverse integrating it would be more of a problem.
Getting it to track at a set rate may be OK, but how to control motor reversals ( esp in DEC ) would be more effort than its worth.
And just for info re the manufacturing tolerances, i did the numbers on a meade wormwheel a few years back
With a 5" dia wormwheel, one arcsec of error at the point of contact with the worm is approx 0.00031mm.
Puts it into perspective how small the tolerances are
Andrew
Andrew the EQ6 is a Chinese copy of the EM200 so I imagine fitting encoders to that mount would not be too difficult. The encoders on the EM200 are outside of the case directly attached to the worms (see pic below).
Seeing will be on top of this as it would with a TDM.
