Polar Mis-Alignment (Dec)

[JohnSadlerAstro]

Hi,

I've been trying to reduce the amounts of backlash in the axes of my EQ5, RA is working nicely now but the Dec axis still takes a second or so to start the mount moving. I understand it's a good idea to introduce a deliberate Dec trend by polar mis-aligning the mount by a small amount? How is this accomplished? Should I polar align and then shove it off by a small amount? Should the error be in Alt or Az?

John

[MikeP]

You may be mistaking this for having a slight weight imbalance so that the gears remain meshed.

Mike

[JohnSadlerAstro]

Hi,

I believe imbalance is used for RA, is it used for both axes, then?

I've seen in PHD an option for either North or South tracking, just wondering how that would work if polar alignment is spot on.

John

[Owmuchonomy]

Yes, as Mike says just have a slight imbalance to ensure there is a constant tension on the Dec gears.  I assume you are referring to guiding?  If you use PhD2 then you can also configure the software to only correct in one direction dependent upon where the Dec drift trend is.

[michael8554]

You're quite right John, Dec backlash can be counteracted with balance and PA misalignment when guiding with PHD2

So instead of aiming for a straight line along the x axis when Drift Aligning, aim for a trend away from the x axis.

Then set up your dec guiding to correct only in that direction.

Even if dec crosses the axis it should go back by itself, but set Min move to try and avoid that.

I can't give you any amounts as it will depend on your mount and backlash.

Not my idea, this is what the PHD2 developers suggest.

Have a look at the Open PHD Guiding google forum that the developers run.

Michael 

[ollypenrice]

On 24/08/2018 at 19:42, MikeP said:

You may be mistaking this for having a slight weight imbalance so that the gears remain meshed.

Mike

No, the OP's right that a slight polar misalignment and one-directional guiding can stop the guider from introducing a repetitive oscillation across the Dec backlash. We've used this very successfully here on a Tak EM200 Jnr with its notorious plastic gears.

The problem with trying to cure Dec backlash by weight imbalance is that no single fixed point of extra weight will work through the night. Suppose you run camera-end heavy: with the tube relatively horizontal that will work but, as it rises to the zenith, the imbalance reduces to zero.

I never measured the polar misalignment but simply stopped adjusting it a little before it was sensibly perfect.

Olly

[lux eterna]

Maybe this can be helpful :

 

Ragnar