Enhanced Tracking and Guiding

With Dob-Driver II

 

Understanding The Fundamentals

 

 

This report should help you improve your understanding of critical functions in the Dob-Driver II hand controller, mechanical subtleties in mount behaviors, and how these relate to effective use of drive systems when you home in toward “arcsecond territory”.  Simply by knowing some basics you can produce large improvements in both visual high-mag tracking and in astro-imaging without resorting to major mount rebuilding or contemplating the purchase of a “better” mount.  Remember that this report is about extremist details, not about “normal” enjoyment of using a telescope with a Dob-Driver.  It comes directly from the engineers instead of grapevine-political positioning or marketing rhetoric so common with competitors on the internet.  So if you consider yourself to be the obsessive type – dive in -- read on!

 

 

Background

 

Telescope motion is a system: Electro-Mechanical

 

Accurate tracking in any telescope system can be separated into two primary categories… electrical and mechanical.  This paper will discuss them as such in separate sections.

An altaz mount is far simpler to use than an equatorial mount.  It has no setup requirement like equatorial mounts and does not even need to be level.  Equatorial mounts, on the other hand, almost always use worm-gears for control of the main axes, tilt the tube at compound angles while tracking, experience polar error similar to altaz mounts though to lesser extent, and end up with a host of subtle dynamic deviations.  So in the end, though a Dob-Driver II will operate either mount method with very high motor precision, each mount choice has its give-and-take, both mechanically, and from the convenience of use perspective.

 

Driving with a serious intention:  What’s your vector Victor?

 

Any uniform velocity in any given direction can be defined with only two points.  This is a vector, and is the way equatorial mounts work – with only one heading (Westward in Ra) required to counteract the planet rotation.  There are no other vectors, just a slight occasional Dec axis adjustment, and the speed is relatively constant such as sidereal, solar, lunar, planets – unless Mr. Atlas decides someday to put a different record on and pirouette in a new fashion – the world will turn the same for all!  But Dob-Driver II is NOT just a vector machine although it does that too -- it is a trajectory machine!  A trajectory needs more than 2 points to define the curved path that is actually a continuous +/ – change in velocity slope profiles.

 

 

 

So… let’s get started… show me the key elements!

 

 

 

Electrical Controls

 

The basic Dob-Driver givens:  1) Since these use a stable-cut quartz crystal, and the hand control microprocessor executes exactly 1-million instructions per second with not much else to do but be totally focused on motor behavior, we know the time progression to a very high precision.  2) Since the motor rotor itself is fully digital, we also know the position to a very high precision at any given time on a continuous basis.  3) Both of those things allow us to set a velocity on both axes in any direction and maintain it or bend it in a curvilinear fashion to our will with absolutely insane regularity at anytime we want, even changing in thousandths of a second as requirements dictate.  4) The only remaining uncontrolled parameters are the operator precision in centering an object (or an autoguider that is doing the same), and the mechanical aberrations described in the next section following this one.

 

Entering Track Mode:  The act of entering track mode identifies the first point in the trajectory curve.  We call it the “anchor point”.  Every time you enter track mode it will start a new anchor point, even though it continues at this time to use a previous tracking data set if you tracked something (or even the same thing) before, until more data is accumulated.  Prior track data is cleared when you have recently powered-up and are tracking the first object, or when we detect the telescope has been slewed to a new position that is largely far away from the last position tracked, or you entered track mode again and new data is being collected from your pushbutton adjustments while observing.  Note that you have likely just used pan mode to place the view where you want it to be before entering track mode.

 

Re-Entering Track Mode:  What if I don’t like the view position – I want to shift the view a bit and do precision tracking again?  You re-position the object or tracking star where you want then exit track mode (the T lamp blinks) then you enter track mode again.  A new anchor point is established anytime you do this.  Fine-position shifting of an image is a lot easier in track mode than pan mode so you see this is an important thing to know – try it on the Moon sometime like I did when moving at 500x from one crater over to a mountain ridge – you reposition the view first, then exit and re-enter track mode.

 

Re-Centering in Track Mode:  Once track mode has been entered, all subsequent corrections with pushbuttons or by an autoguider camera device must return to the exact same position that was there when track mode was entered originally, else the time/displacement calculation that is supposed to adjust for tracking errors will not be correct.  Note- an autoguider camera input is normally used in guide mode, but can be also used in track mode, though track mode is not recommended for imaging purposes it can be nice to track objects for very long periods unattended.  For the vast majority of Dob-Driver uses, visual errors of re-centering an object or star reference point to the exact location when track mode was entered is not very important, since there are some very-fancy proprietary algorithms combined with “fuzzy logic” principles that are there to deal with that.  In the case of very-high-mag observation and duration imaging however, accurate re-centering becomes more important.  The Dob-Driver II will calculate, store, and immediately use new trajectory data when it knows you are done re-centering – this is 7 seconds with no button pushing.

 

Guide Mode:  Move over to guide mode when the magnification is very high, or you wish to guide astrophotos.  It is somewhat similar to track mode but with some very big differences for this purpose.

 

Entering Guide Mode:  It will continue to use current “vector” data obtained.  There will be no new anchor-point or curve calculations, as all is now relative velocity to where you are currently.  If you did not track first in track mode then there will be no tracking vector data and this mode will not be useable.

 

Re-Entering Guide Mode:  No difference from the above “Entering Guide Mode”.

 

Re-Centering in Guide Mode:  Very different from the track mode though the operation and intention may seem quite similar.  Any push of a button will alter velocity at one of two speeds that is calculated by your setting in config mode (the T lamp) while the button is held down.  Release of that button will then add or subtract an appropriate very small amount of tracking speed in that axis, always less than +/-½% as calculated by the proprietary fuzzy-logic.  During the button push on-time the speed of movement will be altered as described above to “catch-up” or “fall-back” toward the intended position - there will be no backlash jerks (absolutely zero backlash deviation) and the observed speed change while holding a direction button down will be very subtle as set by your program settings in Config mode – only discernible at high magnifications with a crosshair reticle or on our Tech2000 autoguider units with a PC screen connected.  This is where the Dob-Driver internal firmware really shines.  Because the drive velocities in each axis and the direction of changes dynamically and independently adapt so that the duration or frequency of each following correction will be less.  In effect – the motors will “evolve” to meet your mount gears and structural flexing particulars in each short time frame like a child learning, then soon arrive at the perfect velocity in every axis that is needed to obtain maximum tracking precision.  This is always a “hunt-and-seek” function.  It will dynamically adapt to your mount, drive, thermal shifts, gravity orientation shift, and telescope dynamics on the fly whether you are doing it manually with a crosshair or an autoguider camera device is doing it.  This is a very important feature typically only available on major observatory mount setups.

 

Meridian crossing in Guide Mode:  In altaz mount perspective, any object will ascend to Meridian, at which time the altitude velocity is zero.  Then direction will reverse and the object will commence descent.  The opposite is true to the North when below Polaris – the object will descend to zero velocity and then begin ascent.  The Dob-Driver II will realize that zero velocity is approaching and will reach a threshold well before the zero point is reached and simply stop doing that ½% velocity adjustment since it becomes meaningless to approach zero speed by ½% at a time for each correction.  The next correction that reverses the motor will invoke a backlash move.  Most of the time it is not noticed by a telescope operator.  But in precision guiding I do not usually want to bet on the result and therefore avoid exposures that might cross the meridian – I’ll wait if it looks that close until it reaches meridian and then expose from there-on.  When using a Dob-Driver II on equatorial mountings – ignore this consideration.

 

 

 

 

 

Mechanical Structures & Drivetrain:

 

OK – Did all the easy & quicker direct stuff above with electrical controls and figured that out.  Now I’m a control freak.  Now what do I look for when that is not enough to solve the remaining tracking aberrations?

 

The basic Dob-Driver givens:  1) Since the digital motors move at a steady velocity thanks to the quartz crystal clock, regardless of load or friction changes, we can be sure the speed at any time currently assigned to a motor will not vary until the firmware tells it to do so at the appropriate times.  2) The position resolution at any main axis on the sky in any instant of time is calculated within 2 arcseconds or better.  With HRG motors this can go down to .5 arcsecond or better.  So why are the remaining deviations from my camera data showing larger than 2 or .5 arcseconds?  The answer is in the mechanical structures between the main motor rotor and the telescope main axis.  How about a good encoder on the main axis?  Find one that gives over 5 million encoder counts for one revolution with a certainty of +/- .5 arcseconds and then we can use that.  As long as you are willing to pay for 2 of them, one for each axis, and are willing to accommodate such (very large) encoders.  Gearing encoders to increase precision does not just add significant complexity or delicacy to mount handling – it adds back-in the same mechanical flaw problems you wanted to avoid in the first place!  No wonder large observatories cost many millions.  This does not even start to address gravity flexure of tube and mounting structures.  There are some bits of knowledge and devious techniques to save you a couple million bucks – read on!

 

Convenience Day/Night tests:  On a new mount, particularly larger ones that take time and effort to set up, we usually will set up at a test location that allows close examination of behaviors over several days of time.  The philosophy being – why move it out and set up each night and take it down again for each test?  Also when the mount is moved each test run then how could you be too sure about changes you have made to improve it?  We will normally set up the mount just-inside a garage door bay, a window, or door perhaps, that allows us to focus “down the road” in daytime, follow stars above the horizon in the eve or mornings, and simply close the door anytime we want to stop and leave the premises without having to set it all up again.  It’s useful when making changes then evaluating them at leisure over a number of days time without disturbing the mount basic position or adding lots of setup time under an outdoor sky.  Does it pan smoothly, track smoothly, accelerate and decelerate smoothly?  No jerks?  Backlash look good when motor direction reverses?  Look below for tips.

 

Quality dollars:  Bowing to the makers of very fine mountings and telescope equipment… there is no consumer mount made on this planet that can hold arcseconds for significant durations even in highly-steady sky scintillation conditions.  Sorry to inform you.  It’s not just about perfect mechanical machining (as if that were possible), but is also about temperature and gravity effects on structural systems.  All imaging work except on bright objects should include guiding as a planned prerequisite whether you stack or not.  Eyeball with crosshairs at high mag or via a Tech2000 autoguider.  No human fabrication can be assuredly capable of the required precision on its own mechanical merit and the required emplacement details and setups themselves could be quite intimidating also with no feedback such as “guiding”.

 

Worm gears:  There is the “worm” that turns against the “wormwheel”.  The worm is helical in design and this cannot ever be cut with precision – hence the PEC problem.  Each rotation of the worm gear “worm” shows variance that is repeating each revolution, so that can be profiled with “Periodic Error Correction” training sessions.  But that is only trained on one tooth of the worm wheel, which contains between 144 to 360 teeth or 10 to 4 minutes respectively.  What about the (astronomically precise?) cut positions of the other 359 teeth on that wormwheel?  If you have been accurate to extreme with setting your PEC training efforts, it will be of only some help on the next tooth and others clear around the wormwheel since each will most-assuredly vary in true position.  Runout is another serious problem.  How “engaged” is the worm to the wormwheel?  The wormwheel most-assuredly will never be perfectly centered on its hub – that’s the part your telescope is mounted to.  And it also will certainly not be of perfect radius at every one of the tooth locations.  Better mounts use spring-loading to maintain full contact of the worm at a near-nominal pressure as the wormwheel rotates, some use the wormshaft itself as the spring-load method by making it thinner and/or longer but we do not condone this (though its better than nothing!).  Humans made these things and they will never be error-proof.  You must guide out these errors.

 

Balance:  This is the most common problem in telescope accurate tracking both on Altaz and EQ mounts.  Make absolutely sure that both axes are always in balance prior to imaging at the present angle of the telescope tube.  Many experienced imagers take special care to check that the drive motor is the thing in-charge, such that imbalance will not pull the telescope ahead of the motors’ intention on that axis, and that not so much opposite imbalance will hold the drive motor back from doing its job to drive forward accurately and uniformly.  There is another technical paper on our site that delves into balance analysis separately in far more extreme detail.

 

Belt drives:  The belt can creep-along on a slip pulley surface when tracking.  Not easily visible when looking at the belt even when slewing (and it can be no problem while slewing but is when tracking due to static friction of bearings).  Balance can be a problem with this showing one way is OK and the other way is not.  Mark a pencil mark or use masking tape to monitor the belt creep when this is suspected.  Rub some violin rosin on the belt to reduce creep or adjust instrument balance to suit.  The best solution is to reduce bearing friction or imbalance that causes the belt creep in the first place.  Another common aberration with belts or gear teeth and shafts is the interplay between belt microstretch or gear tooth flex, shaft torsion, and bearing friction.  Teflon pads (Dobs) and axle shafts (ie- giro) at slow speeds can have a tendency to grab when the static friction is too much for the load being carried – then release when belt or gear tension, or driveshaft torsion, becomes sufficient to pop it free.  You will see this ‘pop’ at high mag with a crosshair or on a camera image when a target slowly backs off from the intended guide point then suddenly lurches to catch up.  The next part about bearings gives some solution pathways for this.

 

Bearings:  Teflon, axle shafts, rollers, load carried, and their related surface materials all play a part in microfine motion effects.  Though in industry the usual solution is plenty of oil and grease which can make your “Tin-Man” seem to move about OK, on telescope bearings this is usually not acceptable since wet lubes will pick up dust or lint or particles or at the very least for exposed surfaces like altitude bearings on Dobs there is always the risk of incidental contact and distribution with fingers or other objects used during operation or transport.  With Teflon pads you should at least compress fine sandpaper between and move the parts so that the pad surface will have the high areas removed and thereby obtain a better psi distribution for the load carried at each pad.  Then score the pad face with a utility blade scraped sideways in a cross-hatch pattern to allow release of air molecules that will typically form a vacuum lock.  On axles, like Giro altitude shafts for instance, we drill the plastic pressure-slug that is under the shaft tensioner knob and inject some grease periodically into the bolt threads there.  Then when the tensioner bolt is screwed in again it pressurizes the grease to flow from the center of the shaft outward toward the ends of the bearing sleeve.  Simple thinking in these ways can pay off large dividends to get around mechanical flexures in drivetrain components and eliminate or strongly reduce those ‘popping’ releases from static friction.  Commonly referred to as ‘sticktion’.

 

Dob Azimuth Rollers:  When using rollers instead of Virgin Teflon to support a Dob structure, they can exhibit the “pie-dough problem” that produces a ‘pop’ or small-scale lurching like described above for belts.  When rolling out a pie dough in the kitchen you always notice that the rolling pin does not want to move in a curved path – it really wants to go in a straight path.  The wider the roller the more true this is.  Oh they will certainly move and it will not be obvious in manual moves or normal tracking, but the details when it comes to arcseconds of angle can be much more subtle when guiding.  Sometimes you may hear a ‘pop’ occasionally on a heavy scope when it is quiet late at night, then it will repeat after some seconds.  It is not the drive motor but is instead the slo-mo screech of the azimuth roller on the formica, etc. surface.  This problem will disappear by rubbing some graphite (pencil lead) on the path of the rollers, we use the Tech2000 “Micro-Glide” lube, or I have also used a very thin film of grease on the roller path as it dries up soon and does not collect grit and dust particles yet remains present as a film like the dry graphite but it must be re-applied occasionally for imaging precision work.

 

Dob azimuth pivot bolt:  On more rare occasions the azimuth center pivot bolt could introduce some influence on fine guiding.  If it is wobbly and tilting around then there can be positions where it wants to bind up the free rotation, and result in a ‘pop’ sort of action again due to pressure-flexing and sudden release of wood mount structures -- more so than the actual slipping of the azimuth traction drive wheel instead that would occur from severe pivot bolt lockups.  Not that this needs to be agonized over with exotic precision shafts or any such thing.  I have readily got away with nothing more than wrapping the bolt threads with Teflon pipe thread seal tape from the hardware store to provide a more vertical snug fit and lowered friction, in the least limiting the screw threads from gouging into the wood bolt hole and making things worsen over time.  Its not hard to scheme up a better and more rigid bolt-pin-stud mounting method with a bushing on the free turning part anyway as we have done that too -- many times.  There is one interesting story where at Astrofest in Illinois I had a customer approach with his arms folded sternly and he flatly stated “It don’t turn”…  “Hey let’s go see!”  As it turned out, it was a (heavy yes to be sure) Mead 16 Dob.  A quick examination revealed that the scope could not be moved in azimuth, even by hand, even pushing hard enough to tilt the (very heavy) mount off the ground!  Loosened the pivot bolt nut and all worked fine.  He had it that tight!  The lesson is – decide which axis is the problem and look for simple mechanical corrections.

 

 

Terminus

 

This is only a rough glance at the large world of precision astronomy.  No doubt I have missed something or you need embellishment on some topic line.  Since this broad content could be expanded into book size it is probably best at this time to keep it a bit general and broad-brush.

Please direct questions or suggested expansions to

 

 info@tech2000astronomy.com

 

Perchance then on a clear and quiet night, when less burdened with the paltry circumstance that this world proffers upon our own human nature, we will speak again of these means that we hope… should ultimately expand our mind.

 

Dave Masters / Tech2000        www.tech2000astronomy.com

 

 

~ Political Election Adden-dumb ~

Paul:  “Quantum positions of two opposing points could lay indeterminately with respect to one another.”

Dave:  “Two opposing ‘party-cles’ place points that always ‘lie’ indeterminately without respect to one another.”