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Author Topic: PHD2 Guiding with mounts with high backlash  (Read 125 times)

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Offline mikejenningz

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PHD2 Guiding with mounts with high backlash
« on: 11:57:27, 14 March, 2018 »
Still battle with mount and guiding !

Owning a Celestron CG-5, I have played around with reducing backlash the DEC gears for quite some time now, with some success, but it still can hamper good, smooth auto-guiding.  I’ve never quite got it to the level where my PHD guiding works really well.  It is not only changes in direction of DEC during guiding that cause issues, it can also be when uni-directional guiding has longer delays between pulses which seem to build up into some stiction causing poor guiding and jumps in DEC, just enough to spoil an exposure.

Recently I got my system setup with a pretty good polar alignment, but with some detectable error.  I also mounted the OTA slightly tail heavy.  I discovered by chance. When I was imaging the Rosette Nebula down low to the South East, but amazingly guiding work flawlessly.  Guiding continued in DEC pushing in a southward direction steadily but manageable, caused by the mounts slight polar alignment error.  I did experiments imaging in other areas of the sky with this setup, all producing the usual guiding issues.  I concluded that the tail-heavy setup was actively keeping weight on the gear teeth, and the guiding direction was working against the gear-teeth reducing the backlash and stiction issue.
This led me to wonder how easy it would be to identify for each potential imaging position whether or not the OTA should be either nose heavy or tail heavy to achieve this favourable auto-guiding result for a given polar alignment setup.
The factors involved seem to be:
Your polar alignment error – Easterly and Northerly.
Mount position meridian – East West
Direction of Slew – RA /DEC
PHD guiding direction – North/South

From these factors it should be possible to calculate if you need the OTA to be slightly Nose/Tail heavy to combat the problems described earlier for each slewed position.
I have done some further work on this which I’d like to share, but anyone had similar thoughts?

Mick.

 

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