Motorising a Dob

[SuperG]

I have a 10inch dob that really isnt being used. With planet season coming up id like to try and see if anyone has a super cheap way of making a motorised platform such that the dob would keep the planet in the FOV of a camera for at least a couple of minutes. Where would i get the parts in Sydney? I know you can buy dob track platforms but they are pretty expensive.

[OzEclipse (Joe Cali)]

The altitude and azimuth tracking rates change rapidly around the zenith but slowly at other parts of the sky. There are a number of ways to track a dob.

1. Equatorial Platform
2. Servo systems links to digital ALT AZ encoders
3. Track and train systems

Equatorial Platform
As you say, some EQ platforms can be expensive.
These aren't too bad:
https://opticaleds.com/custom-made-e...rms/platforms/
The synchronous motors he supplies will be 110V 60Hz unless you can arrange for 240v/50Hz motors to be installed.

Synchronous motor speed is proportional to the mains frequency not the voltage. But if you put 240v into a 110v motor you'll burn it out. If you convert AU 50Hz mains to 110v 50Hz using a transformer, the platform will run at 5/6th siderial speed. The easy workaround is to buy a car inverter from the USA to generate the right voltage and frequency or buy the inverter he advertises which has a fast/slow control which will help you centre the image.

Servo systems
The servo systems are similarly as expensive as EQ platforms. You need a Nexus or Argo Navis Digital Setting Circle connected to a Servo Cat system.

Argo Navis Digital Setting Circle
http://www.wildcard-innovations.com.au/purchase.html ~AUD400
Owner Gary Kopff is a member of this forum.

Servo Cat
http://www.stellarcat.com/Pages/servocatjr.html ~USD1400

Track and Train
I recalled a JMI track and train system that was listed unsuccessfully for several months on IIS classifieds.

I found the thread, sale was withdrawn -
http://www.iceinspace.com.au/forum/s...d.php?t=156780
It was marked as withdrawn not sold, so I'd write to Mark and see if he still has the system to sell.

The JMI TNT system is specifically designed for Meade Lightbridge or similar Lazy Susan round based dob.
http://www.jmitelescopes.com/buy_acc_altaz.htm

It can't be adapted to all dobs.


In short, the ED Jones EQ platform in kit form may be your cheapest option.

[Wavytone]

As a one-time control systems engineer there's a fourth method, though I haven't seen it attempted on amateur scopes - though I have half a mind to try as it should work well enough with say a small micro-controller - anything from a BASIC Stamp to Arduino or RaspberryPi would do...

1. Attach an auto-guider (ie a small guidescope with camera) with ST4 outputs such that the RA signals are used for azimuth and the dec signals for altitude.

2. In software, construct two "floating-integral" controllers, these are easily implemented in software, such that one controls altitude, one controls azimuth and connect the signals from the auto guider accordingly.

3. Each controller integrates the pulses from the auto guider over time, for example "up" treated as positive and "down" as negative (altitude) and "left" as positive and right as negative (azimuth). You may of course need to reverse the signs depending how you wire the motors driving the axes.

The point about integration is that a persistent error is integrated with time resulting in the controller producing a signal that cancels the error. If you this right it will probably work nicely with a time constant of a few seconds. This works very nicely with slow-moving signals as in telescopes, even with random noise added as a result of wind and atmospheric scintillation at higher frequencies which the integral controller simply averages out.

This will self-learn the right rates in a matter of seconds, much faster than the "train and track" manual method.

If you want to be really fancy you could implement a deadbeat controller algorithm, though thats tricky to tune if you haven't been taught the theory behind it. The deadbeat algorithm would be quite fast enough (two controller cycles) to correct period errors in a geartrain.