Tremor in my Mount

[john.clark]

Hi:

I have an HEQ5 mount with some electronics in it called the Rajiva Update, which does the guiding - so it is not the standard Sky Mount guiding system.

It has a tremor.  I posted a question about this on the Rajiva forum & no-one had a clue what is causing it.  The tremor is somewhere between 5 & 15 cycles a second - I tested this by messing around with webcam exposure times - and is getting worse.  I really have no idea whether it is elecronic, mechanical or firmware-related.

I am posting this on the very off chance that someone else has encountered this problem.

????



Thanks in advance,

John.

[Ian]

that's an odd one. How did you discover the vibration?

5 to 15Hz is an unusual frequency too as none of the rotating parts are moving that fast when tracking...

[Mike]

Sounds like it might be an issue with the electronics putting incorrectly timed phases through the electronics.

[john.clark]

Thank you for your thoughts.

If I use a webcam in prime focus position in my 8" Newtonian, I can't miss it.  The image on the laptop screen shakes like crazy.  At 5 fps Jupiter's moons photograph as lines, not points.

I estimated the frequency by fiddling around with the "shutter" speed on the webcam.  I can only get sharp pix by shooting at 15 fps with a high "shutter speed", and then using Registax to stack the tremor away.  Naturally this is unsatisfactory, as it is only possible to do this with very bright objects; and USB 1.1 compresses images faster than 5 fps.

Unfortunately I am not an electronics expert and do not posess the kit necessary to test the circuit board.

[Ian]

I wonder, if it's a stepper motor based drive if not all of the step pulses are getting through properly. It might be worth disconnecting the cables and inspecting the ends for dirt, fluff, small animals, anything that might prevent one of the connections making properly. Have any of the cables been strained?

[john.clark]

All this is interesting, and helpful.  Electronics are the place to probe further.

The problem occurs even if I do not connect the mount to my laptop, and seems not to depend on whether the two are connected.  So I think that eliminates the cable joining them.

There could be of course an internal wiring problem.

But wait...it has just occurred to me is that I stopped using a 12 battery to power the thing, and started using a 12v mains transformer.  This was about the time the problem appeared.  There are then two possibiliites.  (1) The mains transformer could be noisy and sending bad signals; and (2) it may not be really 12v and may have fried something due to overvoltage.  I'll have to get the battery and try it again.  That may take a few days.  Hmmm...

[john.clark]

I appear to have sussed it.    I brought my battery down from Norfolk.  Between it and the mount is a black box of tricks which presumably contains a voltage smoothing circuit.  (Without access to an oscilloscope I can't think how to check this.)

Anyways, when I use the battery and said box of tricks, the tremor is nearly all gone.  I can expose the old DSI II for a couple seconds per frame and get nice point-like moons around Jupiter.  

I think without helpful suggestions on this site I would still have been looking for a mechanical cause.

So: Thank you!

Regards,

John.

[john.clark]

I paid telescope House a visit yesterday, and emerged with a power supply with a decent voltage regulator (http://www.telescopehouse.com/acatalog/Observatory_Mains_Power_Supply.html).

The mount runs smoothly with power from this transformer.  The moral is clear: don't power mounts with cheap transformers. 

[Ian]

cool, that's good to know

Glad it's all sorted now.