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User's Guide v1

Patent applied for

Telescope Drive Master

A Rigorous Supervisor for Telescope Gears

User's Guide


(Obsolete product)

Telescope Drive Master (TDM in short form) has been developed for compensation of periodic and non-periodic tracking errors of mass produced equatorial telescope mounts which errors occurred on account of mechanical manufacturing and assembling inaccuracies.

This device consists of two main parts:

  1. A high precision rotary encoder mounted subsequently onto the RA shaft directly without any detachable clutch or gear, and
  2. An electronic control box.

Principles of operation:

The electronic controller unit compares the signals incoming from the high precision rotary encoder mounted onto the RA shaft with time signal of the internal quartz oscillator. If the angular velocity of the RA shaft deviates from the prescribed (sidereal or King Rate) velocity value, it accelerates or delays telescope's driving clock. In this way, a high precision, feedback regulated, real-time rotational speed control has been created. Corrective action happens via autoguider input port of telescope's driver unit if its own.

Advantages of using TDM

•1. Eliminates telescope mounts' tracking error down to negligibly small values. The followed object (without large scale proper motion) deviates from its theoretical position on the CCD surface in smaller degree than 1" during 5-10 minutes long observation more than in 95% of duration of total exposition time. (The data above depends on optical refraction of the object position observed in the sky and accuracy of polar alignment.)

•2. It is not necessary to install autoguider telescope or to use guider chip/camera.

•3. It does not need to find bright guide star for tracking (the observation process can be automated on much easier way).

•4. TDM can speed up the high volume observation work on remarkable way since exposition can be started immediately after the scope turned onto its target (and potential RA-gear backlash worked off).

•5. High density filters (e.g. H-alpha) do not have any influence to the performance of tracking.

•6. TDM is very easy to use due to its self-test functions.

•7. TDM compensate each element of tracking errors coming from the whole RA drive chain (between motor shaft and RA shaft) since the rotational sensor has been mounted onto the RA shaft directly. It means that TDM inspects very slow rotation of RA shaft continuously in real-time and will correct it immediately if it is necessary.

•8. TDM is a standalone application so it does not need any PC support or other external device for using it.

•9. Thanks for the built in, temperature compensated, high precision quartz-oscillator clock-signal the accuracy of the telescope's drive system is not a relevant parameter.

•10. The electronic module of TDM can be used for other mounts as well so you need to change the mechanical adaptor only if you have several mounts or want to change the type of permanent your mount.

TDM has been developed as "easy-to-use" device, so you can subsequently attach its mechanical adopter onto the bottom of RA shaft of most popular mass-produced telescope mechanics easily, without disassembly of the mount. This is a "Plug-and-play" device so you do not need any special mechanical or electronic study or background to install and use it.

Limitations of Using TDM

Although Telescope Drive Master is really an extremely accurate system, it also has some limits of course.

•1. Using TDM you need very accurate polar alignment.

•2. There is no optical feedback from the certain part of the sky observed so your telescope needs to have a sturdy and rigid mount which is not cheap. (This system is insensitive regarding flexure of mechanical parts and weak quality RA bearings.)

•3. TDM cannot follow objects in the sky having remarkable proper motions (e.g. comets and asteroids close to their perihelia).

•4. This version of TDM does not compensate air refraction; this is why the applicable length of expositions can be varied depending on position in the sky observed.

At the same time, these limitations above do not decrease the practical usability of TDM considerably because of the reasons below:

•Ad 1. TDM helps you in polar alignment activity because, killing periodic error, you can also use RA drift during your Scheiner-method runs.

•Ad 2. It is not rational (and no cost-effective at all) to purchase TDM for low priced and weak quality beginner mounts. At the same time, if you have a sturdy and rigid hi-quality or semi-pro mount, the price of TDM is just on a gadget-price level. (The weakest mount which can be rational to use with TDM is the Synta/SkyWatcher EQ6 or Orion Atlas EQ-G. This is why there is no TDM adopter offered for cheap mounts.)

•Ad 3. The number of object in the sky having remarkable proper motion is fewness compared to the rich other observing possibilities.

•Ad 4. Refraction is annoying aberration mainly just too close to the horizon; so on 30-40 degrees above horizon or on higher altitudes this problem does not cause bigger problem than explained above. This version of TDM has been developed and optimized for 5 to 10 minutes long exposures maximum, and for "photometric altitudes" above horizon and for using focus lengths up to 2-3m maximum.

Who TDM can be recommendable to; who can use it advantageously?

•1. TDM is firstly recommended for the observers who intend to take a lot number of shots per night about different parts of the night sky quickly and easily. (E.g. supernova patrols, comet and/or asteroid hunters, observers of cataclysmic variable stars, etc.)

•2. Those amateurs who make compound images with extra long total exposure time superposing dozens of short expo shots about the same object. (E.g. "artist" deep-sky photographers.)

•3. For amateurs or professionals who have backyard or institutional observatory for permanent telescope installations (with permanent and good polar alignment).

•4. TDM can be an extremely advantageous application for robotic telescope owners who needed to find bright enough guide stars within the field of view manually so far.

•5. For "tourist amateurs" who do not want to drag an extra tube and CCD just for autoguiding purposes up to the peak of the mount escaping from light-polluted regions.

•6. If you want to use a narrow band filter (e.g. H-alpha) but you have a dual chip CCD or just an AO-7/AO-8 adaptive optics as guiding equipment, your guiding chip will be in almost total darkness... But TDM will help.

•7. If you have just a tiny guiding chip at the bottom of a small aperture tube, you probably will not always be able to find an appropriate guide star... TDM will help you again.

•8. TDM together with AO-7/AO-8 adaptive optics is the best equipment that you can have!!! TDM will eliminate the periodic error of your mount (independently of the magnitude of its amplitude) and AO-X will eliminate the rest of the deviations like scintillation and/or refraction. This is the ideal, ultimate set of serious astrophotographers!

•9. Who is not satisfied with his/her mid-ranged telescope mount's tracking ability but does not want to spend another couple of thousands of euros/dollars for a top rated one (which has much more PE then TDM...), those will appreciate this cheaper solution.

Situations where TDM cannot substitute the autoguider CCD or Webcam

•1. If you need very long (10-60 minutes or even longer) exposure time as one shot. It can be rational in a desert or on high topographic altitudes (above two or three thousand meters) or at other locations with extremely dark and clear sky.

•2. In case of celestial objects with relatively fast apparent proper motion (e.g. comets and asteroids around their perihelia).

•3. If you cannot achieve good polar alignment or your telescope's imaging elements can move during exposure or your mount is not sturdy enough or, maybe, overloaded.

Why is TDM better than PEC software?

•1. Although, almost all of the better quality mounts have PEC (Periodic Error Correction) function can be trained, this "self-training" procedure is complicated, long and uncomfortable process. Moreover, the shape of PE curve of every mount may vary considerably by its loading, actual position, ambient temperature, etc.

•2. PEC, as its name indicates, can be used for decreasing of periodical tracking deviations only, which are repetitive in time. At the same time, the movement of the scope during tracking process has a lot of non-periodic element (e.g. dirt on the parts).

•3. Due to the fact that PEC is not a feedback regulated system but just a normal time-controlled correction, it cannot be able to manage periodic errors longer than the main period (worm rotation period) itself. At the same time, the drive chain contains several different rotational frequencies next to each other which generate new frequencies much longer than the base period (this is so called "harmonic beating"). It means that there are no two identical PE curves so the trained correction will not be effective enough after a few periods (except direct drive systems).

Which types of mounts can be used with TDM?

At the moment (May, 2008), there are mechanical adaptors for the below-mentioned mounts:

•1. Fornax 50, 51, 100, 150

•2. Synta EQ6, SkyWatcher EQ6, Orion Atlas G

•3. Celestron CGE

•4. Astro-Physics 1200

•5. Losmandy G11

The number of supported mounts is growing continuously so please find new mounts supported by TDM on this website: http://www.mda-telescoop.com/

Is it possible to install TDM onto home-made mounts or other mass-produced mounts as well?

Yes but, obviously, it is necessary to make an adaptor attaching the encoder onto the RA shaft especially for that certain mount. At the same time, it is not a good idea to make this part "in the kitchen" as a home-made product because incorrect design and/or manufacturing can have negative impact on the accuracy of the system. (Please, send a mail to: This e-mail address is being protected from spambots. You need JavaScript enabled to view it )

What kind of drivers can be used with TDM?

Supported GoTo drivers (and tested so far):

•1. FS2

•2. SkyWatcher SynScan / SkyScan

•3. Astro-Physics non-goto and goto

•4. Pulsar

•5. Koordinator 2000

•6. Celstron Nexstar

•7. Synta EQ6 non-goto hand-controller (after modification)

Can I use TDM with other type of controller as well?

Yes but, obviously, a special cable will be needed which has connectors with proper shape, pin-format and signal-level. (Please, send a mail to: This e-mail address is being protected from spambots. You need JavaScript enabled to view it )

Supported autoguider input formats at the moment (May, 2008):

•1. SBIG ST-4 (SUB-D 15pin nut)

•2. Meade LX200 (6/6 RJ11)

•3. Koordinator 2000 (SUB-D 9pin nut)

•4. Astro-Physics 1200 (old non-goto version: SUB-D 9pin nut, new: RJ11)

What do TDM's self initialization and self training functions mean?

TDM does not need to be configured previously. There is just one important rule to memorize: telescope driver has to be switched on at first and the telescope has to be moving on tracking speed when you switch TDM on. TDM, after its switching on, executes a self-test procedure during the first few seconds. The field of view is moving a little bit but not more than 10-20". Direction and speed of RA tracking (N-S hemisphere observer location), direction of autoguider port's correction (E-W position of German equatorial mount) and correction speed (aggressivity of close control) will be detected and stored during this test.

If you change any of the directions above under observing (but E-W position of German equatorial mount as usual) you will need to reboot TDM (just switch it off and on again).

TDM is able to recognize high speed movements of your hand controller or GoTo instructions and will interdict corrective actions of its own.

Patent applied for this device.

Technical features

RA shaft rotary encoder resolution: 0.125" (1/8 arc-second)

Rotary connector: SUB-D15pin APA

Internal quartz timing reference: 32MHz ± 50ppm

Sample frequency: 120 Hz

Tracking speed: Sidereal or King Rate (selectable)

Correction frequency: 1 Hz or 5 Hz (selectable)

Correction hysteresis (dead zone): ± 0.5" or ± 1" (arc-second) (selectable)

Angular error-display resolution*:  0.125" or 1" (arc-second) (selectable)

Display: Constant total brightness, 2x20-digit (red-yellow) LED-line + 1pc null (green) profile-LED

Display range in 1"/digit mode: ± 15" (± 15 digits + null)

Display range in 0,125"/digit mode: ± 2.5" (± 20 digits + null)

TDM controlling unit: 16-bit RISC Microcontroller

Power supply: EU standard 12Vdc stabilized 300mA max.

Power supply connector: EU standard 2,1mm Jack

Control switches: long life DIL Reed relays

(TDM is conductively uncoupled from telescope's controller.)

Autoguider port connector: SBIG ST-4, SUB-D 15pin female

Usable correction speed: 0,25x - 0,5x (max. 1x, but not preferred)

Serial port (USART) service connector: SUB-D 9pin female

USART protocol: 9600 baud N81, ASCii or hexadecimal

USART- PC interface cable: SUB-D9 male and female, 1:1 connected (pin-to-pin)

*: Angular error display shows the actual measured error (on 120Hz frequency). The real error is the mean value of Gauss distribution produced by incoming measured error values during unit time. So, some LEDs over the prescribed limits can be switched on when the real deviation is close to these limits (mainly in case of 1/8" high resolution display mode) but this situation does not mean that the real tracking error would be over these prescribed limitations.

TDM has been produced using leadless ROHS technology within the EU.

Installation of Telescope Drive Master

Installation of TDM means, beyond connecting of cables, just mounting the encoder onto the end of the telescope mount's RA shaft. Since the physical shape of the adapter for fixing encoder highly depends on the mount's shape so there are very different adapters for different mounts.

This is why this adaptor installation description can be found in another documentation called "TDM Installation Guide for ... Mount".

How to Use TDM

You can find an array of LEDs within the black box which indicates the momentary measured tracking error of RA shaft compared to the internal clock. The middle LED of this array is a green; whenever this green LED is on (independently from any other lights) your tracking accuracy is better than the prescribed value (±0.5" or ±1", depending on the position of Jumper4 - see below). You will find two times 20pcs LEDs (red and yellow; one LED means one digit) on both side of the green LED in this array. The resolution of this array can be switched using Jumper4: 1" per digit or 1/8" per digit (or 0.125" per digit). After switching the box on (connecting power supply), a Reset function runs during the first second of operation. (This is the LED test function as well; you will see two light-bars symmetrically running up to the ends of the LED array and backward. The total brightness of LED array is constant, so more and more LEDs will shine on less and less brightness level.)

A self-test (self-initialization) will be started after Reset automatically of which duration can typically be 3-5 seconds. (The real length of this self-test depends on mechanical features of certain mount).

  1. As the very first step of self initialization, TDM will recognize RA tracking direction. This direction will be accepted by TDM as correct tracking direction; this is different on the locations of Northern and Southern hemisphere observers. (Observer is responsible for adjusting correct tracking direction on the telescope control box/software.) During this test the extreme left/right one LED (the 20th one) will be lighting continuously and you will see shaft rotation on the LED array in 1"/digit resolution. The test has been done after the 10th LED is switched on. (If tracking is not activated or tracking speed is out of ±20% tolerance of sidereal rate then TDM will repeat this step again. If RA backlash is too large this test can be started several times.)
  2. In the second step, TDM checks the directions of autoguider input's correction; it switches the relay moving to East at first. During the test the extreme left/right two LEDs (the 19th and 20th ones) will be lighting continuously and you will see shaft rotation on the LED array in 1"/digit resolution. The test has been done after the 5th LED is switched on. If the result of this test is not satisfactory, it will make this test again but in the opposite direction. TDM measures the autoguider tracking speed applied by telescope driver which is 5"/sec (0.33x sidereal speed) in the best case. If this correction speed is out of tolerance (0,25x - 1x), TDM will step forward to the next phase but stores the measured value (e.g. zero if the autoguider cable has not been connected).
  3. In this step, TDM reveals the other correction movements of telescope controller's autoguider port. During the test the extreme left/right three LEDs (the 18th, 19th and 20th ones) will be lighting continuously and you will see shaft rotation on the LED array in 1"/digit resolution. The test has been done after the 5th LED is switched on. The correction change has to be the opposite of the previous test. If the result of this test is not satisfactory, this is not interpretable error so the whole self-test will be started again. TDM measures the autoguider tracking speed applied by telescope driver which is 5"/sec (0.33x sidereal speed) in the best case. If this correction speed is out of tolerance (0,25x - 1x), TDM will step forward to the next phase but stores the measured value (e.g. zero if the autoguider cable has not been connected). The other elements of this step are the same as those can be found in the previous point.
  4. The fourth step of the initialization process can run on two different ways, based on the previous three steps:
    1. PED-mode (Periodic Error Display mode): if neither the East nor the West autoguider relays work then the extreme 4 LEDs of the display will be switched on for a second indicating the false test result. In this case, the most probable reason behind this test result is the unconnected autoguider cable. In spite of it, the self test will be completed and TDM displays the real tracking error of the mount. (It is practical to select ±1"/digit display resolution (using Jumper4) for this function unless you have a mount with less than ±2.5" periodic error...)
    2. TDM mode (Telescope Drive Master controlling mode): if both of correction movements are active and correction speed is acceptable, the last 5pcs of LEDs in the LED array will light informing you about successful self test. (The total length of the self-test is roughly 5 seconds depending on the different mechanical features of telescope mounts.)
  5. TDM-control: after finishing self test TDM control will be activated and the system will increase or decrease the RA tracking speed if it is necessary.

There are four DIL (Dual in Line) switches (say Jumpers) for fine adjustment of TDM tracking behavior. These jumpers will change their certain features immediately so you do not need to reboot TDM.

Jumper1: Tracking speed selector (off: King Rate - 1436.47 minutes/day; on: Sidereal - 1436.07 minutes/day). In this version, King Rate means "average King Rate" but not position-sensitive, real King Rate.

Jumper2: it changes display resolution (off: 1/8" or 0.125"/digit; on: 1"/digit). The total error-range of the display in high resolution mode is ±2.5" (±20 x 1/8") which is recommended for TDM mode; or ±15" (±15 x 1") which is recommended for PED mode. Attention! Since TDM displays periodic error immediately after its switching on, the tracking error displayed (and exported on the USART port) will not be symmetrical to zero level but will depend on initial phase.

Jumper3: adjusts the frequency of correcting actions (off: 1 Hz - one correction per second; on: 5 Hz - 5 corrections per second). The recommended selection can depend on the dimensions and mass of the OTA, stability of the mount or wind-speed. (See some tips below.)

Jumper4: hysteresis (dead zone) of controlling (off: ±1"; on: ±0.5"). TDM keeps the tracking error within this range compared to the internal time base (it switches the correction relays on when the tracking error exceeds these prescribed values). Its recommended position depends on practical factors like mechanical behaviour of the RA drive gear, stability of the mount or wind-speed. (See some tips below.) Important note!!! The aggressivity of this controlling system (speed of correcting action) can be adjusted by telescope controller device.

TDM operation during Telescope's GoTo movements

TDM enables ±4" tracking error, as maximum value, during controlled tracking. If the difference (error) is more than this value (due to high speed movements), TDM will reset itself and it will recheck RA rotational speed in every second. As soon as the RA speed is the tracking speed, TDM will control the telescope driver again. (During high speed movement the controller relays are in prohibited mode so TDM does not cause any problem in the step-calculation based coordinate measurement process of the telescope driver unit.) After reset the self test will not be initiated; just the controlling process starts again from zero error level. Self test runs in case of switching on the TDM again.

A few tips for using TDM on more efficient way

The mechanical features of certain telescope mounts (both their construction and manufacturing quality), the correcting speed of tracking adjusted on the telescope controller and TDM's jumpers of its own exercise remarkable influence on correct tracking.

In general, it can be declared that elimination of the mechanical noise of toothed wheel (or epicyclic) gears provide the highest challenge for TDM or for any other tracking control system (e.g. autoguider) because this type of errors produces short term elongation with quite large amplitudes in PE curve. The worm wheel error has larger amplitude but much longer period so TDM (or an autoguider) can suppress its moderated trend much easier. So, this means, it is logical to use faster (5Hz) and/or higher (0.4x-0.5x, maybe 1x) control speed (more aggressive) in these cases. At the same time, mounts driven by timing belt on worm shaft or having direct drive (motor on the worm-shaft directly) cannot tolerate such an aggressive controlling parameters in many cases so 1Hz frequency and slower modification speed (0.25-0.3x) is better. But the main rule is: just test it and use the best configuration!

In case of week mechanical quality mounts or too breeze weather it can be logical to increase the dead zone of the controlling system from ±0.5" to ±1". The accuracy of tracking will slightly decrease but it is not remarkable using shorter focus distances and expositions with average seeing. At the same time, the controlled system will became much more stabile.

If you use TDM unplugged, the actual tracking error can be seen on the display.

The extremely accurate tracking ability provided by TDM obviously demands extremely strict conditions against the instrument and observer. E.g. cables coming from the CCD, lens heating belts, electronic focuser, etc. have to be fixed. The pier should be mechanically isolated from the observer's floor; you can test the efficiency of the vibration free pier isolation on TDM's display as well.

Two most important enemies of TDM's tracking accuracy are incorrect polar alignment and air mass refraction.

We can remarkably improve the accuracy of polar alignment using a webcam and image capture and analyzing software (e.g. Drift Explorer module of K3CCDTool3). The recommended method to do it is a kind of "mixed Scheiner-King method". The horizontal adjustment (on the local meridian and celestial equator) is easy and extremely reliable using Scheiner method; but it is not true in case of altitude alignment (because of high refraction values on the Eastern or Western horizon). For altitude adjustment of polar alignment King method is far better. Additionally, it is highly recommended to check the polar alignment from time to time because the most rigid pier base can move a little bit.

Finally, we can reduce the negative impact of air refraction if we make observations on "photometric altitudes" (within 40-50 degree zenith distances if it is possible). Additionally, we should use sidereal rate around zenith and King rate around the mediocre altitudes of the local meridian (see Jumper1). At the same time, we need to avoid Eastern or Western horizons. If you keep the proposals above, the maximum duration of an exposition can be around 5-10 minutes (depending on the focus length of your scope and polar alignment).

Service USART port:

If TDM connected to PC via Serial port, we can make a log-file of controlling period. TDM sends the measured RA tracking error (in hexadecimal format) to the serial port in every second. 128dec = 80hex code represents zero deviation. USART always writes tracking error in 1/8" step resolution and it is able to write 120 digits (±15") deviations as maximum value.

You can find USART utility software (COM Port Tool Kit 3.8) on the web (http://www.compt.ru/) which saves the measured data into the file called „compt.dat"; these data can be presented as a curve in Excel.

During its self test, TDM sends different messages to the PC about test-status via USART which can be found in the next list. The very last message of the self test is 2pcs of FFhexa codes and error logging starts after this code. 2pcs of FFhexa code will also be exported after reset again.

original solarflare design by rhuk
modified by MDA-TelesCoop