5 octobre 2006 : motorisation of equatorial mounts by pic-astro arnaud gerard

5 Octobre 2006www.astrimage.org Auteur:

MOTORISATION

of equatorial mountsby PIC-ASTRO

Arnaud GERARD

Rencontres du 28/29 Janvier 2006 2www.astrimage.org

IntroductionThis 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


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.


Tracking problemsThe 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 trackingHis 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.


Problèmes liés à l’autoguidage

• Précision des ordres de correctionLe logiciel qui calcule les corrections doit pouvoir le faire avec le moins d’erreur 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 l’ensemble mécanique d’entraînement des axes et se révèlent lors du changement de sens de rotation des moteurs. En AD le problème n’existe 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.


Problèmes liés au Goto

Il n’y a pas vraiment de problème lié au Gotoon lui demande simplement d’être 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 n’y a pas besoin d’une 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.


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 versatilePIC-ASTRO is usable on any equatorial mount equipped with stepper motors. It is easily configurable.

• Developped by amateursit moves quickly to push ever further the limits. All drawings, tools and binaries are available on www.astrimage.org.

• Its costAbout 100 € + hardware circuits and motors.

• For information and orderingA single address : www.astrimage.org.

• Need information, help or to discuss ideasForum www.astrocoolpix.net is waiting for you.


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 PADultra 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 motors4) Serial cable to pc5) it’s supplied in 12 V.


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 speedIt is determined by the number of ittimer between 2 μsteps (V1AD <= 127) the maximum speed is Vmax = 8 x V1AD.

• Serial portFor the connection to a PC, PDA or MEROP it is used for the LX200 or downloading to the PIC.


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 µstepsThis 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.


Adjustments

Rencontres du 28/29 Janvier 2006

Adjustments next

www.astrimage.org 12

Rencontres du 28/29 Janvier 2006www.astrimage.org

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 trackingThe linearization of µsteps enable an unparalleled fluidity, difficult to measure without using a lase permet d’avoir une fluidité inégalée, difficilement mesurable sans l’aide d’un laser.

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Results 2/2

• Accuracy in GotoIt 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. STEINMETZhttp://xmcvs.free.fr/picastro/index.htmNote: The measures of precision are lost in the sound of turbulence.

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MEROP smart hand controller

• Ideal complement to PIC-ASTRODenis 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.


Improvements

• Changing the power partThe 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 othersEach 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 PerformanceTest of maximum speed to show the available reserve

• Viewing µstepswith laserautoguiding records

• Use in manualwithout computer

• Use with Merop

• Steering PCSync commandPosition in real timesFocus motor

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5 octobre 2006 : motorisation of equatorial mounts by pic-astro arnaud gerard

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