MOTORISATION of equatorial mounts by PIC-ASTRO

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MOTORISATIONof equatorial mountsby PIC-ASTRO
5 Octobre 2006
www.astrimage.org Auteur

• Arnaud GERARD

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Introduction

• This presentation aims to understand the needs of
  the most demanding amateur astronomers the
  astrophotographer,   to propose an economic
  solution to meet those needs and economic enough
  to put to all

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Need for astrophotography

• In order needs are
• Tracking the most perfect possible (considering
  the equipment used)
• A good autoguiding for long exposures
• An accurate goto to centrate at 100 invisible
  target
• All these needs ask specific technical problems,
  the best system is one that solves al and with
  the best result.

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Tracking problems

• The main problem is the periodic error (PE)
• It is directly linked to the mechanical quality
  of the mount and settings on it One of the
  fireworks used is the system of periodic error
  correction (PEC). But this is only a trick to be
  effective as it should be that this error is
  fully reproducible and unfortunately it is never
  the case. If the EP curve is smooth, the auto is
  the only viable solution to an acceptable result.
  
• The second problem is the accuracy of the set up
  of the mount
• There is no miracle, it must complete the set up
  of the mount using the old methods which give
  full satisfaction (method of King or Bigourdan).
  Setting station must always be verified.
• Only autoguiding allows up errors of setting
  station
• The third problem is the quality of the tracking
• His speed must be accurate and the tracking as
  flow as possible. The fluidity is directly
  dependent on the frequency with which the
  position of the mount is adjusted.

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Problèmes liés à lautoguidage

• Précision des ordres de correction
• Le logiciel qui calcule les corrections doit
  pouvoir le faire avec le moins derreur possible
  . Cette précision dépend de celle des
  acquisitions qui elle-même dépend de la fluidité
  de la vitesse de suivi et aussi de celle de la
  vitesse de correction.
• Pour ne pas perturber les calculs de ce logiciel
  il faut que les 2 vitesses aient la même
  fluidité.
• Les ordres de correction doivent être pris en
  compte le plus rapidement possible.
• Problèmes dus aux rattrapages de jeu
• Les rattrapages de jeux sont dus à lensemble
  mécanique dentraînement des axes et se révèlent
  lors du changement de sens de rotation des
  moteurs.
• En AD le problème nexiste pas réellement car il
  suffit de réduire la vitesse pour effectuer une
  correction négative (le moteur AD ne change
  jamais de sens de rotation).
• En De le problème est réel et il faut pouvoir
  calibrer ce rattrapage de façon précise bien que
  ce rattrapage ne soit pas véritablement constant.
  La solution idéale est donc de procéder à un
  décalage de la MES afin que les corrections se
  fassent toujours dans le même sens.

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Problèmes liés au Goto

• Il ny a pas vraiment de problème lié au Goto
• on lui demande simplement dêtre suffisamment
  précis pour pouvoir centrer la cible dans le
  capteur sur des déplacement de quelques degrés
  (par rapport à un objet bien visible qui sert de
  référence). Dans ce cas il ny a pas besoin
  dune vitesse élevée (30 ou 40X suffisent)
• Les problèmes se posent lors de grands
  déplacements (de plusieurs dizaines de degrés)
• dans ce cas la MES doit être précise et le
  logiciel doit bien effectuer les corrections dues
  à la rotation terrestre et au temps de
  déplacements. Par contre une vitesse confortable
  est appréciable mais attention au-delà de 200X
  il y a des risques non nuls pour la mécanique.

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PIC-ASTRO

• PIC-ASTRO is accessible
• is an affordable solution to all and fulfills
  all these needs. PIC-ASTRO was designed for
  astrophotography with the collaboration of many
  experienced and novice astrophotographer.
• PIC-ASTRO is versatile
• PIC-ASTRO is usable on any equatorial mount
  equipped with stepper motors. It is easily
  configurable.
• Developped by amateurs
• it moves quickly to push ever further the
  limits. All drawings, tools and binaries are
  available on www.astrimage.org.
• Its cost
• About 100 hardware circuits and motors.
• For information and ordering
• A single address www.astrimage.org.
• Need information, help or to discuss ideas
• Forum www.astrocoolpix.net is waiting for you.

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What is PIC-ASTRO?

• PIC-ASTRO is composed of
• 1) An electronic box
• it includes 2 Printed Circuits an IC logic
  built around a PIC18F252, and a power part for
  the motors (drivers L298).
• These circuits can be integrated into the mount
  (HEQ5, EQ6).
• 2) A PAD
• ultra low
• - 4 pushing buttons for movements
• - 1 potentiometer to adjust speed manually
• - 1 inverter to choose adjustment of movement
  or focus
• - 1 pushing button to lighted a red LED.
• 3) Cables to the motors
• 4) Serial cable to pc
• 5) its supplied in 12 V.

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Operating principles

• A timer is used as time base
• The timer period is chosen by the user according
  to the characteristics of his mount. The solar
  and lunar velocities are obtained by changing its
  period. This value is usually between 0.5ms and
  2ms.
• 2 timers used to generate µsteps
• A timer for each motor. They currently operate
  at up to 14KHz and generate 256µsteps by half
  step, they can generate up to 1024 µsteps by
  half-step.
• Tracking speed
• It is determined by the number of ittimer
  between 2 µsteps (V1AD lt 127) the maximum speed
  is Vmax 8 x V1AD.
• Serial port
• For the connection to a PC, PDA or MEROP it is
  used for the LX200 or downloading to the PIC.

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µsteps managements

• The smallest micro step is limited to 300µs
• (Minimum recommended IT) So 0.0045 "for a Vmax
  of 8.
• The µsteps are adjustable
• By the configuration software, we can adjust 8
  µstep by half-stepto obtain a smooth engine.
• Linearization of µsteps
• This operation is to create µsteps intermediate
  (between 2µsteps predefined) in the case of
  tracking or autoguiding. This helps improve the
  fluidity of movement, the position of the mount
  is adjusted to each IT ie every 1ms. This allows
  you to generate up 256µsteps by half-step.

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Adjustments
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Adjustments next
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Results 1/2

• Speeds
• The speed tracking is consistent with the
  expected result, the target remains at the center
  of the sensor of a Webcam for more than 6 hours
  to F 1000. PIC- ASTRO manages speeds up to 63
  per axis V1 tracking speed 1X µsteps
  linearized V2 autoguiding speed µsteps
  linearized   V3 to V16 slow speeds of 3X to
  16X in µsteps V17 to V63 fast speeds 16X max
  speed in half-step
• Accuracy tracking
• The linearization of µsteps enable an
  unparalleled fluidity, difficult to measure
  without using a lase permet davoir une fluidité
  inégalée, difficilement mesurable sans laide
  dun laser.

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Results 2/2

• Accuracy in Goto
• It depends on the accuracy of the set up of the
  mount, the measurements give - 2.5.
• Autoguiding accuracy
• measured to - 0.17 pixel but always being
  improved ( -0.11 pixel to date)
• Measurements were made by V. STEINMETZ
• http//xmcvs.free.fr/picastro/index.htm
• Note The measures of precision are lost in the
  sound of turbulence.

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MEROP smart hand controller

• Ideal complement to PIC-ASTRO
• Denis PEROTTO has developed a smart
  hand-controller MEROP.
• Merop is an autonomous box that can order any
  mount compatible with the LX200 protocol.
• Key features
• Progressive Joystick guidance for variable speed
• Using specific commands to maximize the use of
  PIC-ASTRO
• Protocol LX 200 , the possibility to change to
  over protocols
• LCD screen 4x16 backlighted
• Embedded databases Messier, NGC, IC, personnal
  database...
• Scalable and modular
• Multi-lingual, currently French and English
• Check www.astrimage.org.

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Improvements

• Changing the power part
• The serial resistors dissipate a lot of heat
  with the most powerful motors, there are waste of
  energy which can be awkward when using battery as
  power source.
• And others
• Each has its limitations. To go even further
  Astrimage the association was created to bring
  together people in various fields so that they
  can take an active part in the development of
  such projects.

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Documentation

• The documentation is being updated to version
  PIC-ASTRO A1.
• The evolution of software PIC-ASTRO is permanent.
    Successive versions are downloadable.
• Everything is available on www.astrimage.org.

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Demonstrations

• pure Performance
• Test of maximum speed to show the available
  reserve
• Viewing µsteps
• with laser
• autoguiding records
• Use in manual
• without computer
• Use with Merop
• Steering PC
• Sync command
• Position in real times
• Focus motor

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