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The 10 Critical Telescope Drive System Elements For GOTO Dobsonian Mounts
Have you ever wished that you knew more about the technical aspects of Dobsonian Mounts? This technical white paper will give you all the design details that you need to know. If you are either looking to buy a GOTO dobsonian telescope or build your own DIY dobsonian drive system, then you owe it to yourself to study this document.
For the amateur astronomer who is looking for the "best of both worlds", it is hard to beat the large aperture of a Dobsonian Telescope coupled to a precision GOTO drive system. But, what are the critical aspects of a Dobsonian mount that need to be considered. This white paper will give you the details.
It's All About The Fundamentals
The philosopher poet George Santaynan once said, "Those wo cannot learn from history are doomed to repeat it." Wise words and they actually do apply to telescope drive system design and implementation essentials for GOTODobsonian Mounts. This report should help you avoid "the mistakes of history", and improve you understanding of the critical elements of Dobsonian Mount design and how they relate to effective use of altaz drive systems.
Motors: Why do you drive when you could walk?
But now that has changed with the advent of simple motor devices controlled by intelligent software, some of which have the required astronomical precision, and in certain models the method is just like a car where you insert the ignition key and turn it on – that’s it! A telescope should be as easy as a car to operate, quickly learned and obviously controlled only by simple intent. With a driver at the wheel, then freedom comes the same way you may have experienced when you got a driver license and access to an automobile – the world became vastly larger – in this case it is no less than the universe itself.
So… let’s get started… show me the key critical elements!
The 10 Critical Elements of Dobsonian Mounts
Critical Element #1: Ease of Setup and Setup Time
A drive system should be able to complement the reason you bought such a convenient telescope. A telescope that is hard to set up, or one that is not easy to use, will more than likely gather dust. Keep this in mind as you design your drive system or look for one to purchase. The main reasons for not using a telescope as often as you’d like are typically related to excess setup time, complexity and inconvenience. Remember! The entire goal of adding a drive system to your Dob is to increase your enjoyment of the instrument and to improve its functionality.
We can’t stress this point enough… GO FOR
SIMPLICITY. MINIMIZE THE SETUP! This is the number
one critical issue!
Critical Element #2: Good Bearing Supports
These “chattering” effects are greatly reduced by substituting Virgin-Teflon. Notice we didn’t just say “Teflon”. There are different grades of Teflon material and many are not acceptable for high quality telescope bearing surfaces. The cheaper mechanical grade reprocessed Teflon, while commonly available for telescope use, might not always be the best choice. For the best performance-to-price ratio, select Virgin Teflon for your telescope. You will not be disappointed. Most Dob owners are not aware that many manufacturers use UHMW polyethylene plastic pads instead of the Teflon fluoropolymers. While some advertise that they do use Teflon bearings, they do not mention it is the remixed contaminated stuff and not virgin molecular chains. Remember… we want our bearing surfaces to be as smooth-running as possible, so it doesn’t make sense to use any material other than Virgin Teflon. This is your telescope. You’ll want it to be as good as it can be.
Roller devices may be a good alternative on large heavy telescopes, if you want to incorporate these into your design as a replacement for pads. Good drive system suppliers should have various roller devices intended to upgrade from sliding friction pads if needed. These then can be used to replace a supporting pad on a one-at-a-time basis until desired performance is achieved. On the altitude trunion cradle there are 4 pads that could be replaced with rollers, some roller types have friction in them intentionally so that balance deviations in the tube do not allow it to self-swing if all 4 are used, yet the roller function (back to physics class again) still improves that difference between static friction and dynamic friction by at least a factor of 3! The azimuth is far less sensitive to imbalance yet a good drive installation will still use rollers instead of pads, or at least an unloading method to reduce psi distribution on the pads if rollers are not installed. Because this will improve the micro-motion plus have higher power/battery efficiency plus higher slewspeeds – it’s a winning situation all around.
Common use of “Ebony-Star” Formica products on Teflon is fine for manual movement of long telescopes since the hand-lever distance action is adequate. The intention there is to prevent a “vacuum lock” at the pad interface. Just like dragging a suction cup across glass, it would be easier if the glass was “lumpy”. It works when the telescope is revolved rapidly across the sky (the large amateur telescope manufacturers do not impress us in this way by showing how easy it is to move the telescope from the north to south hemisphere at 20 degrees-per-second by hand – some videos we have seen actually try to impress prospective buyers by ‘throwing’ a large telescope in a 180 degree manual swing so that we will be impressed by just how smooth the bearings are), but this will materially work against you when trying to move at speeds near stall (from one end of the dust storm on Mars to the other end of it for instance – well over 1000 times more impressive).
Ebony-Star, and similar materials like it, have bumps which will sink-into the Teflon surface as if the telescope was stationary. This makes it harder to move along at micro-speeds, similar to the extra effort a loaded truck would need when driving slow through damp dirt instead of hard pavement. Obviously a drive system would not be expected to operate efficiently then, even though it usually will function acceptably anyway. Lightweight scopes under the 10” class then no big deal. If you have a heavier scope tube over maybe 50 Lbs, then its easy to just fine-sand the bump peaks down a bit to reduce this effect, rather than replacing the surface or removing the Formica, or when you’re building one then don’t use lumpy surfaces in the first place – just cross-hatch-score the pad face to let air molecules be free!
Good drive wheels that use traction to move the axis are knurled stainless steel anyway so will just automatically do that for you and grind down the lumpies to the most-happiest point, but only where the drive wheel path is! Any Dobsonian telescope owner on the planet, and your drive system too, will thank you for paying attention to these little pieces of critically useful information.
It’s important to understand that there is latent relationship between tube balance and tracking or imaging accuracy, less so for visual use, more so for imaging. The physics behind structural dynamics is lengthy to explain and beyond the scope of this document. This is one of those times when it’s best to say, “Trust me. Your tube needs to be balanced… even with a drive system in place”. It should be enough to say that professionals understand the importance of this… and you should as well.
If your telescope does not move up or down by itself when you let go – you have a great start for sure! Remember the discussion of bearings above, removing all friction would make balance impossible. If the tube does sag or rise at some angles after a pad performance enhancement, then a simple Internet search will net you plenty of ideas for perfectly adequate, simple counterweight methods if you end up needing it. The “bail-out” position is to simply purchase one. There are quite a few options commercially available, with most being based on simple Velcro pads. The drive manufacturer likely will have variable-position counterweights available to adjust any significant imbalance condition that arises from cameras and adapters/flatteners, guidescopes etc.
Keep in mind that tube imbalance can be, and usually is, inherent in the original design itself or as a result of adding heavier (or lighter) oculars or camera equipment at the focuser. Purely visual users will commonly find balance issues to be rather insignificant since they see the target object and it is tracking fine, swap oculars and accessories – no change… so what’s the big deal? Once you have a good drive then the attraction to play with cameras becomes strong, even if you did not specifically intend to do imaging. Now when it comes to “open-shutter” time beyond just video frames you will want to be more serious about things - do not be tempted into believing that imbalance is unimportant!
Always use a drive system that allows declutching of each axis, then you can easily check the balance while the scope is in the old manual ‘freewheel’ mode instantly anytime you want at whatever angle your tube is currently at. Now how much simpler can that get!
Critical Element #4: Hand Controls and Scope Guiding
While personal visual observing may not be seriously hampered if your scope “wiggles” every time you touch the telescope, a camera will record your infraction. At this new realm of power you must have a drive with some super smooth hand control behavior. If you build your own hand control, low-force gold-plated contacts are strongly recommended instead of the rubber buttons and hobby plastics, which fail so commonly.
Using a camera for astrophotography will call for even finer motion control. Assuming you’ve either built or purchased a system that will allow you to finely control the motions of your scope, you will need to choose a method for keeping the object dead center on the camera chip. If your preference is to perform this function manually, look for a good quality reticle. In its simplest form, a reticle is a higher powered eyepiece which has crosshairs internally mounted. Most of these reticles are illuminated, to make the centering task much easier. The use of a manual guide system is somewhat beyond the intent of this report, but here’s the basic concept. The objective of guiding your scope is to keep the photographic subject “motionless” on the camera’s imaging sensor. Locate a guide star (any star bright enough to see through the reticle) and keep this star centered on the reticle crosshairs by making the necessary pushbutton adjustments.
If you don’t thrill to the idea of making manual adjustments, you should really look for an autoguider. While this requires additional equipment, it eliminates the manual tweaking discussed above. More about autoguiders follows below.
Critical Element #5: Timing Considerations, Servo vs. Stepper
So let’s talk about timing a bit. What do we really mean by “timing” and why is this important at all? We all understand that the earth is rotating and the stars appear to move overhead. The first goal of any telescope drive system is to track the movement of the celestial objects. The “hidden” goal is for the system to perform this function smoothly as well. So, what is smoothness and why is this important? An example may help. Here’s a simple example.
If you’ve ever been to a County or State Fair, you might have tried your hand at the shooting gallery. You know what I’m talking about. You’re armed with a bb gun and a row of ducks are slowly moving from left to right in front of you. You take aim at the duck and “smoothly” move the gun barrel in perfect motion and sync with the speed of the passing duck. Pull the trigger, hit the duck and win a major prize! Now imagine that someone put their hands over your eyes while you were aiming and then quickly removed them only briefly once in a while. Oops! The duck has moved and you’re forced to aim again – and again – never very sure of the position or speed.
The same “hand of the eyes” handicap can happen with telescope drive systems. All drive systems rely on motors and feedback, of some sort. The feedback is important so that the system knows where the object is (or supposed to be). If the amount of time between “feedback points” is too great, the object in the telescope eyepiece might just appear to jump or oscillate around the target. Just as with the blinded shooter in the duck gallery, first the object is here… then it’s… there. If our Dobsonian telescope is going to be used purely for low-power visual observations, the duration of the timing step might not be too critical. Most of us, however, don’t want a system that “sort of” works. You might be tempted to counter with the argument, “You might be right, but I can always use an autoguider.” Even if this is used, you will be limited by the system’s resolution -- what is the resolving arcseconds and how fast does the system respond to “close the loop”?
Some drives used on Dobsonians (even the over $2000 club) use servomotors and optical encoders to provide the required velocity motion in both altitude and azimuth, plus and minus directions in each axis. This is not a huge problem visually, aside from some wandering around of the object in the eyepiece, but there is a problem when imaging. The encoder lines – no matter how great the resolution of the encoder, cannot resolve the motor position good enough to use with consumer servo-motors. Mainstream tests on a $2500 commonly-available servo-unit with a high quality autoguider connected could not hold 20 arcseconds from true position. The wandering is not easily detectable visually, but is unusable in imaging work. Yet the alternative quartz-clocked digital motors (microsteppers) can hold 0.5 to 2 arcseconds. Keep in mind this is comparing encoder equivalents of 8000+ with DC servos ( a very high quality expensive encoder unit) to the digital motors’ 1.2 MILLION. That’s not just a little difference – that’s a whopper! The milliseconds count and you can’t allow deviation between “check-points” like that. You will miss the duck!
Even tight-resolution autoguider cameras cannot be used to overcome true-position tracking flaws in a servodrive. A drive that dynamically adapts velocity in two axes at once within milliseconds of receiving a guide signal is the range we want to be in.
The basic tradeoffs in a nutshell: DC servomotors can give high slew speeds when the commutator is overstressed for short periods to avoid burnout, but exhibit severe cogging behavior at low (tracking) speeds, which cannot be controlled with electronic means no matter how good the motor and encoder set is. Conversely, AC servomotors (also called DC microsteppers) have insanely super tracking position accuracy even at very extreme low speeds, but have lower maximum speeds. This is probably why we see manufacturers who use DC servos promoting high slew speeds as a key feature in their advertising, whereas the scientific sector uses microsteppers because of the extreme accuracy. The former will use high-ratio gearboxes on the DC servos to get around the cogging problem (even though it does add efficiency loss, backlash stackup, and increased loss of position accuracy from multiple gearstacking). The latter will use far lower gearstack ratios, which merely improves the situation while maintaining the desired higher slew speed ability within 20% of competitive maximums.
Servomotors can be used but they do require the extra installation time and cost of encoders applied at the main axes. Again, the encoders are an essential component of a DC servo drive system, as they provide the system positional feedback and provide indication of velocity when the axes are moving, but it is like the “duck” analogy described above, checkpoints are fewer, and plenty of extra geardown will help to keep the main rotor moving at a somewhat regular speed between checkpoints to reduce cogging behavior. A word of caution. If you go with servomotors, select the most robust encoders you can find AND the highest resolution you can possibly afford. While you might never conquer the potential “timing” problems associated with DC servo cogging, you’ll minimize the unwanted effects somewhat by using very high resolution encoders. Additional problems can be encountered by using encoders and you should be made aware of these.
If you will be attempting to minimize cost, you’ll probably purchase your servomotor and encoder separately. Installing these will take some care on your part so be careful! The last thing you want is to damage your new encoder. Go the extra mile and add a sturdy cover over your encoder system (installing one yourself or if you buy a drive system with an encoder sticking out somewhere). Many encoders, the good ones, especially the types required for this precision application, make use of an etched glass encoder wheel and they can be damaged and optically prone to dust or moisture condensates. The cover you will be adding may save you from disaster! Also, if you can afford it, purchase brushless servos for this application. This will eliminate the oxidation/contamination and wear-out of the commutator brushes normally used in consumer servo-drive systems.
Digital microstepper motors (which are actually AC servos operated by 12v DC supply via special microchip drivers) may be used for this drive application when they are specifically approved or custom designed for astronomical precision, and for some very good reasons. Stepper motors avoid the commutator brush issues, the motors are indeed brushless, there is no electrical sliding contact inside anywhere, and the motor is sealed so it will never wear out. The strategic reason that you may select a stepper motor system is its fully digital design, a much higher pole-count on the main rotor (for accurate velocities at very low tracking speeds) and their inherent encoder resolution built inside the sealed case. It’s not uncommon for well built stepper motor designs to yield well over 500,000 encoder counts in just one axis revolution, some implementations we have seen approach 2 million and higher!
Critical Element #6: Electronic Hand Controls Really Need These Minimum Details
Slew ability: Allow yourself the ability to electrically swing-about in the sky without needing to push the scope by hand or declutch axes. This is very important when homing-in on an object or simply ‘scanning’ the area around it just because you want to look around. Quite useful too if you are using a ladder at the eyepiece. Let alone the ability to cross large distances with “hands-off” just to be a cool-guy while you pretend to lay-back while the telescope does it’s thing toward a new target, but that’s only a nice-to-have. Sometimes this function is called called ‘Pan’.
Micro-Slew: You absolutely must have an ability to make fine eyepiece corrections to center any object in the view with smooth and sure movement that gets the object exactly where you want it to be, typically in the center of view though many telescopes have a ‘sweet-spot’ that may not be in the center – and then that is where you put it. Without this function you have no control and only hope for potluck. This is usually a function included in the primary track-mode of a high-quality hand control. Some of the better ones also have a ‘guide’ function for imaging which is far tighter in the precision placement and maintaining of location during imaging.
Backlash: Yeah – that’s pretty important. When you are observing you don’t want to wait for motors to make the instrument respond, it could get pretty long waiting when you move back and forth. 10 seconds vs. 1 second, we want the 1 second response time. Maximum pleasure will be had when a drive control has the ability to adjust this feature directly according to your telescope specifically, and store it with power off for every use next-time, and not some ‘baseline’ telescope. Then it will work with the hand control just like you did when pushing it in any direction – it moves NOW!
Critical Element #7: Axis Clutches Save Telescope Lives Everyday
What if you forgot you needed to bring a battery after you arrived at the big star party or a remote observing site? Whoops! No problem if you have the disengage feature you already built in, thinking ahead.
What if one of your visitors pushed the tube to show off their prowess in handling telescopes, or bumped into it in the dark, not knowing (or even caring) that you had a drive engaged? Whoops! No problem should result from such routine accidents. Make sure each axis has an overlimit slip-ability so that tensions will release whenever forced in this way, slipping instead of breaking gear teeth or mounting structures. Remember you’re dealing with a large and serious telescope here, and it belongs to you. Rather important don’t you think?
Critical Element #8: The GOTO Conundrum
The most-easy GOTO systems will have all of the details calculated automatically by sighting two stars and nothing more. There should be NO other steps to complete setup requirements.
Other intermediate GOTO systems will need some more steps. Leveling the mount, leveling the tube next at horizon then/or zenith, then pointing to magnetic North for instance.
High end GOTO systems will require quite a large amount
of setup time and are typically only used by research oriented fixed
observatories or obsessive-compulsively disordered people whom do not
actually intend to participate in astronomical adventures and rarely
travel. Lots of customizing is usually involved to address a
specific research goal. There are usually no less than 16
entry-level parameters and then up to 7 stars or more that are
necessary to sight and correlate before “observation” can
begin. High machining precision is required on the mounting
and tube structures, then usually followed up with regular calibration
file runs that characterize the mount in sectors for each axis up to 5
or so axes – which is important to make setup time less than an hour in
future observing sessions.
Critical Element #9: Guiding Capability
A key drive selection criterion involves the encoder feedback loop again here. Since DC servos require the encoder units at the main axis or at the output of a gearbox, the performance of the best autoguiders, or your best efforts with an eyeball at the crosshair, will be useless beyond the encoder resolution. For example, a real good encoder by consumer standards (8096 count) at the main axis is typical of expensive commercial drives operated with DSC units (Digital Setting Circles), but it will not be able to calculate the latest position error sooner than 160 arcseconds of sky motion have passed. That’s over 10 seconds between data at the normal sidereal sky speed! A microstepper however is far more “real-time” with typically no more than 2 arcseconds of motion. That’s better than 1/10th second between accurate position data updates and most do better than that. No wonder the science community likes microsteppers. The difference is 80:1 in favor!
Critical Element #10: Inertial Effects
Consider an 8-foot long 2x4 piece of lumber with one hand holding it in the center. When you try to rotate that big stick it will be noticeably resisting your intention to accelerate or decelerate it. If you cut it in half then hold the center of a 4-foot piece it is very noticeably easier. Its not the weight, its about how far out the weight is from your hand – the “inertial moment”!
A telescope drive should have the ability to deal with this effect. Mainly when attempting to accelerate to slew speed cruising velocity and then back down to tracking speed or a full stop. The usual method will include electronically controlled “ramp-up” and “ramp-down” profiles for acceleration and deceleration respectively. Do not build or buy a drive system that does not have this feature because it will fail to achieve the slew speeds that users (and product advertisers!) crave. Some of the better ones have an adjustable setting for this, called “Maxspeed” etc., so these parameters can be stored in the hand control specifically for your telescope and your own desired behavior profile.
In the meantime, stick with that familiar KISS principle… Keep It So Simple.
Else old George would again come by to say… “Those who cannot learn from history are doomed to repeat it.”