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  #21  
Old 05-09-2014, 01:00 PM
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marc4darkskies (Marcus)
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Here is a very interesting post in an ASA forum about a member starting up a DDM in a remote observatory after power off:

"... Mount is switched off and parked, As it is equilibrated with the telescope and CCD ...mount keeps perfectly parked.. When I want to begin a session..I open the roof... switch on remotely the mount and by means o Teamviewer software.. looking through a webcam very sensitive sited inside the observatory.. I use Autoslew in order to move the mount aproximately to the zenith. Then I click Homefind and in few seconds the mount finds the exact values. Then I open a planetary programm (The SKy6) and synchronize with a near star. From this moment..I can begin any astronomical session automatically ( Used ACP and CCDAutopilot)"

The bold text is mine. Summary points seem to be:
  • Needs to be balanced to maintain a parked position!
  • When starting up from power off you need to point to the zenith first (using a remote camera)?!
  • Then and only then can you tell the mount to find the home position!
  • Then you have to sync on a star before starting your session!
The poster then goes on to say this always works and he doesn't understand where the problem is!

I'll tell him where the problem is! Paramount: 1) switch on 2) tell the mount to home (from any parked position) and start a session. And when the power is off the Paramount won't move - guaranteed!
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  #22  
Old 05-09-2014, 01:38 PM
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Terry B
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Quote:
Originally Posted by marc4darkskies View Post

snip


I'll tell him where the problem is! Paramount: 1) switch on 2) tell the mount to home (from any parked position) and start a session. And when the power is off the Paramount won't move - guaranteed!
And for my Tak NJP it is similar. Turn the power on. Unpark the scope. slew to any star- usually within 20 arcmins. Take an image and platesolve to fine tune the pointing then slew anywhere.
Fine balancing is impossible due to the odd shaped spectrograph hanging off the end of the scope. Doesn't seem to matter though. The motors are powerful enough to cope.
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  #23  
Old 05-09-2014, 05:17 PM
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Bassnut (Fred)
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Quote:
Originally Posted by marc4darkskies View Post
Here is a very interesting post in an ASA forum about a member starting up a DDM in a remote observatory after power off:

"... Mount is switched off and parked, As it is equilibrated with the telescope and CCD ...mount keeps perfectly parked.. When I want to begin a session..I open the roof... switch on remotely the mount and by means o Teamviewer software.. looking through a webcam very sensitive sited inside the observatory.. I use Autoslew in order to move the mount aproximately to the zenith. Then I click Homefind and in few seconds the mount finds the exact values. Then I open a planetary programm (The SKy6) and synchronize with a near star. From this moment..I can begin any astronomical session automatically ( Used ACP and CCDAutopilot)"

The bold text is mine. Summary points seem to be:
  • Needs to be balanced to maintain a parked position!
  • When starting up from power off you need to point to the zenith first (using a remote camera)?!
  • Then and only then can you tell the mount to find the home position!
  • Then you have to sync on a star before starting your session!
The poster then goes on to say this always works and he doesn't understand where the problem is!

I'll tell him where the problem is! Paramount: 1) switch on 2) tell the mount to home (from any parked position) and start a session. And when the power is off the Paramount won't move - guaranteed!
Oh dear, if all this true, that's just woefull . No wonder you don't hear a lot about them. The price isn't bad though, for direct drive, but rest is just trash. Itelescope tell me the CDK direct drive is the ducks guts and very reliable. Albeit far more expensive.
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  #24  
Old 06-09-2014, 10:41 AM
clive milne
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Actually, I would suggest that the mount should be imbalanced such that on loss of power (and no breaking) the scope will automatically settle in a predetermined orientation. ie) If you made the counterweight shaft heavy on the RA and primary mirror end heavy on the DEC, the scope will automatically park upright and pointing at the pole. You could have the imbalance torque 10% of the motor torque to achieve this without any meaningful loss of performance.

I would also suggest that the shortcomings of the ASA system are not necessarily inherent to all direct drive telescope mounts.
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  #25  
Old 06-09-2014, 04:38 PM
Roger273 (Roger)
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Is everyone just assuming that loss of power is a problem with the ASA? I don't think this thread has any comments from someone who has actually used one as yet. The following email is from a user who sounds more than happy with his ASA:

"Dear Roger,

My relationship with ASA dates back to 2006 as they adopted my proposal to produce Astrographs with fast focal ratio. Since then I have tracked the developments of all their projects. One of the highlights was, as they passed to me a DDM85 direct drive mount. Previously I gained my experience with different traditionally mounts (GPDX Vixen, Losmandy G11, Losmandy HGM Titan, AP 1200). But with the DDM 85 from the very beginning I was sure that I never want go back to a traditionally mount. I assembled off axis guider systems at my observatories but most of my images are taken without external guiding. My portable mount is an ASA direct drive (DDM 60) and does the same good job as the DDM 85. I´m really very happy with both mounts. As I know ASA has an Australian in Victoria – have a look at their website".

Other comments from anyone who has used an ASA mount would be greatly appreciated.
Roger
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  #26  
Old 06-09-2014, 05:25 PM
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Bassnut (Fred)
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ASA product support generally is another matter that needs considering. At least one high profile user on IIS and another serious user in OZ im aware of that wasnt public with his experience had the same absolutely jaw droppingly bad experiences with ASA engineering and support. I mean expensive product engineered so badly it was totally unusable, and next to zero support. Things may have improved lately, but I wouldnt go near them myself, very risky. I
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  #27  
Old 12-09-2014, 01:02 AM
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Shiraz (Ray)
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Interesting question, so asked ASA what protection there is against mount failure with power loss. Their response in summary was:
- a reasonably balanced mount will just stop when power goes off - due to bearing friction.
- the mount software detects if the scope contacts anything when under power (position is not as expected) and powers down - unlike a geared mount which will keep trying to drive.
- the mount software can do a "homefind" on power up.
- there are more than 300 DDM out there and they know of no examples of scope damage due to power outage.
- they haven't fitted a brake because there is no need for one.

FWIW, have confirmed that my EQ6 and EQ8 both stop dead and hold position if the clutches are released, even if slewing (ie as would happen with power out on a DDM). Provided that the loads are fairly well balanced, some extra torque is required to overcome bearing stiction, as ASA state. Of course, the main servo loop in a DDM will need to be tuned to the load, so good balance is essential anyway. The idea that a DDM will somehow fall over or continue slewing if power fails seems to be just another urban myth - plausible, but not real.

Looks to me like the DDMs could be far more robust (in principle) than has been suggested - they just require a slightly different set of procedures and assumptions.

Edit: just looked up the homefind process - the mount does not need to be manually positioned pointing to the zenith to initialise, it's just that this is where it will most quickly find the reference marks that tell it exactly where it is.

Last edited by Shiraz; 14-09-2014 at 10:02 AM.
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  #28  
Old 12-09-2014, 06:12 AM
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Quote:
Originally Posted by Shiraz View Post
- the mount software detects if the scope contacts anything when under power (position is not as expected) and powers down - unlike a geared mount which will keep trying to drive.
That may be true for stepper motor drive systems, but I'm not aware of any servomotor drive systems that keep trying to drive. So far as I am aware, when they hit something they all detect the excess motor drive current and immediately stop dead in their tracks. Paramounts also sound the claxon after stopping, just to embarrass and humiliate you further. :-)
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  #29  
Old 12-09-2014, 07:43 AM
AndrewJ
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Gday Frolinmod

Not sure re the Meade Classics, but none of the later Meades "measure" current for this purpose. ( Hence why motor cards can burn out )
They monitor the encoder and if the motorcard has been told to move the motor and the card needs to use a duty cycle that exceeds a set value to try and move the axis and still NO movement is detected over about 16 seconds, it declares a motor fault.

Andrew
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  #30  
Old 16-02-2016, 12:34 AM
GeorgeC
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This thread is a bit old, but I only just stumbled across it.
I own a DDM60Pro mount. Regarding the balance question... Yes, the mount must be very accurately balanced to get good performance. I use a system of sliding weights to get near perfect balance.
If there is a power cut (not happened yet!) then there would only be a problem if it happened during a fast slew. If it happens during tracking the residual friction in the bearings will keep it in a fixed position.
The issue of unguided imaging is vexatious. If your scope has ZERO flexure, in all planes, in every position, then it is certainly possible. Some people get 45 minute unguided images.
Even with a sturdy carbon fibre tube I can not achieve it and so guide.

Older DDM mounts did have to be calibrated at the start, but ASA now put absolute encoders in, and the mount knows where it is when switched on.
Some time later this year there will be Version 2 of the mount which promises to be even better (but probably a bit pricey!)
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  #31  
Old 17-02-2016, 09:55 PM
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Peter Ward
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After talking at length about the subject of direct drive mounts with a well known Californian mount manufacturer, seems the elephant in the room is the rigidity of the mount in the first place.

Big mounts have often have big gears for a reason.

The simple fact is a large diameter gear will hold things in place with significantly more rigidity than a magnetic field.....and without deflection compensation, direct drives would be pretty "rubbery" indeed.

Sure gears also have errors....but these reduce through simple geometry with gear size (assuming the gear is equally well made) and absolutely encoded geared mounts are easily as capable in being sampled many times per second to effect a perfect tracking rate with far less susceptibility to be deflected by buffeting in the first place.

Wind buffeting on the OTA may or may not be sensed as a rotational rate change (mechanical stiffness of the OTA may not be great) hence its a bit of a red herring to say it can be compensated for all the time.
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  #32  
Old 18-02-2016, 03:39 PM
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Shiraz (Ray)
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Quote:
Originally Posted by Peter Ward View Post
After talking at length about the subject of direct drive mounts with a well known Californian mount manufacturer, seems the elephant in the room is the rigidity of the mount in the first place.

Big mounts have often have big gears for a reason.

The simple fact is a large diameter gear will hold things in place with significantly more rigidity than a magnetic field.....and without deflection compensation, direct drives would be pretty "rubbery" indeed.

Sure gears also have errors....but these reduce through simple geometry with gear size (assuming the gear is equally well made) and absolutely encoded geared mounts are easily as capable in being sampled many times per second to effect a perfect tracking rate with far less susceptibility to be deflected by buffeting in the first place.

Wind buffeting on the OTA may or may not be sensed as a rotational rate change (mechanical stiffness of the OTA may not be great) hence its a bit of a red herring to say it can be compensated for all the time.
'fraid that simply doesn't match with what is going on everywhere else in astronomy Peter. have a look around the web - every mount of larger size uses either direct motor drive or friction drive. Starting from the top-end Planewaves and all the way up to the huge VLT class scopes (the VLT has a 10m diameter direct drive az motor), it looks like nobody uses gears anymore. The primary advantages given for direct drives are rigidity, precision and resistance to wind buffeting. They are not in any way "rubbery" and having rapid feedback to respond (without gear lag) to wind deflection is a big plus.

That is not to say that cost-effective gear drives do not have a place in our small, low-cost amateur systems, but I doubt that there are any technical reasons why direct drive systems with absolute encoders would not do a better job - regardless of what a maker of good quality gear drive mounts might say.

Last edited by Shiraz; 18-02-2016 at 11:20 PM.
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  #33  
Old 18-02-2016, 10:54 PM
GeorgeC
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You can tune the motor parameters to make the mount response 'stiff' or a bit 'rubbery' if you want. There is a critical point where if it is too stiff you can get oscillations, so most times you tune it to that point and then soften it a bit. The response to wind is amazing - if you wave a piece of A4 paper near the scope you can see the mount current immediately rise to correct the deviation.
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  #34  
Old 19-02-2016, 04:19 AM
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Peter Ward
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Quote:
Originally Posted by Shiraz View Post
'fraid that simply doesn't match with what is going on everywhere else in astronomy Peter. have a look around the web - every mount of larger size uses either direct motor drive or friction drive. Starting from the top-end Planewaves and all the way up to the huge VLT class scopes (the VLT has a 10m diameter direct drive az motor), it looks like nobody uses gears anymore. The primary advantages given for direct drives are rigidity, precision and resistance to wind buffeting. They are not in any way "rubbery" and having rapid feedback to respond (without gear lag) to wind deflection is a big plus.

That is not to say that cost-effective gear drives do not have a place in our small, low-cost amateur systems, but I doubt that there are any technical reasons why direct drive systems with absolute encoders would not do a better job - regardless of what a maker of good quality gear drive mounts might say.
The decision to go with direct drives on extremely large mounts is cost driven. It is simply not practical or cost effective to make massive gears for the likes of the VLT.

Large diameter high precision gears are really expensive, and difficult to make well, which I'd suggest is the real motivation toward direct drive systems in larger amateur systems.

A fully meshed gear also has no lag. Backlash is a hallmark of cheaper systems and you should not see any in high end mounts by Astro-physics, Bisque etc.

Some mount manufacturers also accelerate the drive motor for a short time
if they reverse direction. This removes any less than perfect meshing delay...which is almost exclusively on the Dec gear, as the RA drive only needs a small rate change which can applied virtually instantaneously.

As for either system doing a better job, with absolute encoders fitted on each, the point is moot. Both are seeing limited. Both respond well to any sensed rate change from wind buffeting....and I still maintain a gear will hold a telescope more rigidly than a magnetic field.

All that said, having used a direct drive mount recently, I can say positively tracking simply isn't perfect. If you don't guide, expect eggy stars....particularly if you take 20-30 minute subs as I often do.

Their albeit perfect drive rates are open loop systems that have no way of compensating for atmospherically induced drift in RA and Dec....hence I only see "advantages" of direct drives as marketing hype rather that offering a practical difference.....unless of course you are running the VLT
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  #35  
Old 19-02-2016, 05:16 AM
GeorgeC
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If wind gusts hit a direct drive mount in the DEC axis direction the response is almost instantaneous, and has the same accuracy whether it is compensating North or South. There is bound to be a small oscillation so the correction impulses will alternate rapidly between N and S.
Even with the most precisely engineered geared system, I think it is asking a lot for the stepper motor to give a series of corrections in alternate directions without any time delays as the chain of gears do their work.

I may be wrong...
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  #36  
Old 19-02-2016, 10:55 AM
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Quote:
Originally Posted by GeorgeC View Post
If wind gusts hit a direct drive mount in the DEC axis direction the response is almost instantaneous, and has the same accuracy whether it is compensating North or South. There is bound to be a small oscillation so the correction impulses will alternate rapidly between N and S.
Even with the most precisely engineered geared system, I think it is asking a lot for the stepper motor to give a series of corrections in alternate directions without any time delays as the chain of gears do their work.

I may be wrong...
Two things. My PMEII is in a dome, so buffeting by wind gusts is as likely as that by pink elephants. A perturbed high stiffness structure will likely damp out
more quickly than one which has to be actively modulated to achieve the same effect, as like a plain old spring, it intrinsically wants to snap back to its unperturbed state.
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