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Old 02-09-2014, 10:12 AM
Roger273 (Roger)
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Direct Drive Mounts

Hi All
I'm looking at replacing my EM10 mount, and would be interested in any feedback on the ASA DDM60 direct drive mount. ASA claim you can run unguided with their high resolution digital encoders, but is anyone on iceinspace doing this with the DDM60? It's a similar price to the Paramount MX so any advice on which way to go is much appreciated.
Thank you
Roger
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Old 02-09-2014, 01:10 PM
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Digital shaft encoders really do not alleviate the need for guiding, as stars often show movement in Dec due standing atmospheric waves.

Drift from the refracted pole will vary from night to night with atmospheric density height, and I haven't seen a in imaging system yet that is immune to small focal plane shifts due gravitational bending moments changing with the camera's orientation.

I've also been intrigued to know what happens to a direct drive mount when the power fails?? With no gear to lock it in place I'd imagine things would get very interesting.

I'm sure they are a fine mount, but direct drive is not a feature I'd rate as essential.
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Old 02-09-2014, 02:13 PM
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multiweb (Marc)
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Originally Posted by Peter Ward View Post
I've also been intrigued to know what happens to a direct drive mount when the power fails?? With no gear to lock it in place I'd imagine things would get very interesting.
Spring loaded clutch held by a solenoid? Big clap in the dark.
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Old 02-09-2014, 07:02 PM
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Big clap in the dark.
Ceftriaxone will help clear that up
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Old 02-09-2014, 07:49 PM
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Hi Roger,

Again, I don't have a DDM (but I do have an ASA scope), but once the mount is tracking to within an arc second or two, the atmosphere takes over. Ask any imager, there are some nights where you just can't seem to make things work, then you actually look at the stars and see the wild scintillation characteristic of really bad seeing. No mount can fix that (at least in the amateur sphere).

So your question is, do I stump for the DDM and hope I don't have to guide, or do I get a PMX or AP mount and live with the guiding?

I have a PMX, and apart from some problems which did go on for quite a while, it has been great, once I replaced a dud motor. You can PEC train these things to very small residual errors and set up sky models to remove most of the pointing errors, but still most imagers guide anyway. Once you have made that decision, i.e. to guide, then PEC isn't really necessary, a decent polar alignment is all that's required. Even better at long focal lengths is to use AO and only bump the mount when required, if you PA is good, that won't be very often. One of the drawbacks of using a Newtonian scope like the ASA N12 is that you have very limited back focus from the corrector, so no AO for me

SO direct drive or not, probably not the question you need to ask. What you get with your mount is also worth consideration. I'm not sure what you get with the ASA mount, but with a PMX you get a lot of software as well. Then there's support, the SB guys run this forum thing, which is frustrating when you have a problem, not sure about ASA, though they have a rep in Melbourne, but I suppose Bintel or Peter are the SB reps in Aus.

HTH
Stuart
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Old 02-09-2014, 09:15 PM
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Logieberra (Logan)
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Quote:
Originally Posted by Roger273 View Post
Hi All
I'm looking at replacing my EM10 mount, and would be interested in any feedback on the ASA DDM60 direct drive mount. ASA claim you can run unguided with their high resolution digital encoders, but is anyone on iceinspace doing this with the DDM60? It's a similar price to the Paramount MX so any advice on which way to go is much appreciated.
Thank you
Roger
The new MX+ looks exciting. Can't go wrong with any of the high end guys - AP, SB and ASA (in no particular order! )
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Old 03-09-2014, 04:40 AM
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It seems to me that the direct drive is not going to give you anything better than what you would get with a decent mount (AP, Paramount).
Geoff
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Old 03-09-2014, 09:37 AM
Roger273 (Roger)
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Hi
The feeling I'm getting from Peter, Stuart and Geoff is that the PMX is the way to go, and to forget about direct drive. I'll be in Melbourne next week so I will go and talk to the ASA guy anyway. If I change my mind and go with the DDM60 I will put my experience up on the forum.

Thanks for all the feedback
Roger
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Old 03-09-2014, 09:40 AM
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I would read independent reviews on the DDM before the retailer here.

There is a Tak NJP in the trader now at a great price, and these track as well as an AP, PMX/PME/ASA, at half the cost
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Old 03-09-2014, 10:35 AM
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I think that one advantage of the direct drive mounts with encoders is that the main feedback loop runs at fairly high speed, so they can react quickly to control the effects of wind gusts - regardless of whether or not they are being guided. A conventional mount does not have an equivalent feedback loop, so you must rely on mount stiffness (or an AO) to deal with wind.
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Old 03-09-2014, 12:02 PM
clive milne
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I think that one advantage of the direct drive mounts with encoders is that the main feedback loop runs at fairly high speed, so they can react quickly to control the effects of wind gusts - regardless of whether or not they are being guided. A conventional mount does not have an equivalent feedback loop, so you must rely on mount stiffness (or an AO) to deal with wind.
Yes, that is basically my understanding as well.
There are a few qualifiers however. Irrespective of the power and accuracy of the drive/ encoder assembly, the maximum useful guiding frequency will be a function of the resonant frequency of the telescope structure and the tuning parameters (proportional, integral & derivative) applied in the control algorithm. It is likely to be the case that the factory default settings will be different from optimum settings derived by experiment in the field.

Also, it needs to be said that direct drives are (in theory) able to manage deviations due to such things as wind loading (particularly in the dec axis) at a frequency and accuracy that is simply not possible using a conventional gear train.
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Old 03-09-2014, 12:23 PM
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I have no doubt direct drive mounts can give you an near enough perfect rotational rate.

The simple problem remains: that is different to a star's real position due a myriad of atmospheric effects.

Being an "open loop" system the mount simply can't compensate in the same way as an autoguider (or autoguider+AO ) .... which close the loop.
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Old 03-09-2014, 12:46 PM
clive milne
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That is true Peter,
It is also true that a direct drive with an autoguider (or autoguider+AO) is going to have better performance than a geared drive on the same mount.

(assuming it is engineered and tuned correctly)
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Old 03-09-2014, 04:39 PM
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Originally Posted by clive milne View Post
That is true Peter,
It is also true that a direct drive with an autoguider (or autoguider+AO) is going to have better performance than a geared drive on the same mount.

(assuming it is engineered and tuned correctly)
Unfortunately I've not had an opportunity to test that thesis. What I have found true, using 5-7hz closed loop sampling, is the star sizes that my system generates are only effected by seeing.

I've certainly pondered: would better results be obtained with a drive system that tracks well below the seeing noise?

Running some back-of-the envelope numbers, I suspect my PME is already there, as being in a dome, there is no wind buffeting. Hence, over a 1 second interval, the drive would only have 2arc sec/720 seconds or a .003 arc sec error.
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Old 03-09-2014, 04:57 PM
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I always thought the big benefit of direct drive is no backlash/friction etc.
DEC would certainly benefit from No backlash,
but without the HiPrec encoder it is ( relatively ) useless for absolute precision/tracking at the arcsec level.
In RA, i would think that ( as the motor doesnt need to reverse ), tracking would merely be a function of the speed of the encoder/motor feedback loop and the accuracy of the encoder itself, incl any reading artifacts.
There has been a very good ( long ) thread on CN recently re the new encoder put out by IOptron for the CEM mount.
The technical/mechanical problems associated with the encoder itself ( when looking at the arcsec level ) shows that a lot of the real gains/losses can come from the encoder itself, vs the drive mechanism.
Very interesting as a system integration exercise.

Andrew
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Old 03-09-2014, 04:57 PM
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I think that there is a bit of a problem comparing mounts, in that maybe we don't know for certain how our mounts actually perform.

We might try to find out by tracking a star for example. We get data that shows the star wobbling about and immediately say "that high frequency stuff is seeing noise" - what is left after the wobbly bit is filtered out must be the mount error. But maybe part of the wobbly bit is also mount error - we simply do not know. It would not be surprising to find errors due to minor gear and bearing vibrations, or noise from dirt in the lubrication etc (1/100 the thickness of human hair will give you an arcsec of error). In principle, a direct drive mount could have lower levels of chatter if properly set up - but who knows.

Anyone know of any direct angle/rate measurements for a mount, as opposed to star position?

WRT your original question Roger, apart from ASA, quite a few high end systems use direct drive, so it clearly is worthwhile - eg: http://planewave.com/technology/mechanical-design/
http://www.skyvision.fr/wordpress/mo...mount/?lang=en
http://www.astelco.com/html/products/ntm/ntm.htm

Last edited by Shiraz; 04-09-2014 at 08:05 AM.
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Old 04-09-2014, 06:38 PM
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Interesting video which talks about Planewave direct drive about have way through. https://m.youtube.com/watch?v=B_Louir2r10
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  #18  
Old 05-09-2014, 08:52 AM
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marc4darkskies (Marcus)
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No doubt, direct drive would definitely be a nice-to-have. My biggest problem though is that no-one seems to have engineered a solution to the problem of unwanted movement when the power is off. This seems like a simple thing to solve and I'm puzzled why ASA doesn't do anything about it or even acknowledge or address the question . They seem to rely instead on a perfectly balanced system ... and I've never seen one of those - especially when you have a rotating camera whose load is not perfectly concentric with the optical axis! Also, what happens if power is lost while slewing - an admittedly improbable event, but a very ugly one. If I'm wrong, someone please tell me because that's probably the only reason I would never consider buying one.

Cheers, Marcus
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Old 05-09-2014, 09:35 AM
AndrewJ
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Gday Marcus

"Most" precision direct drive motors are to all intents and purposes just bloody big steppers with advanced microstepping mechanisms to control speed.
Depending on the design, the motors used in the ASA may actually have a holding torque ( like a std stepper ) that defaults to keep it stable when the coils are no longer energised??? Dunno, but if anyone has an ASA mount, maybe they could advise what happens if deenergised???
Ie if the clutches are left locked, and the mount isnt powered,
how much torque is reqd to make the axis rotate ( if at all )?

Andrew
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Old 05-09-2014, 11:01 AM
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Maybe use a UPS when likely to be active and manual lock when not. Then, provided that all plugs and cables are high quality, the only possible failure would be if the drive circuitry went bad. But we accept the very slight risk that the electronics of our worm drive machines could malfunction and slew the mount into the limits, so failure of drive electronics is probably an acceptable risk for a direct drive as well.

However, it would be worth ASA at least addressing the issue rather than pretending they don't understand the question on their FAQ page. Otherwise they are going to get killed in the marketing wars.

Last edited by Shiraz; 05-09-2014 at 11:28 AM.
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