Mount problem. Your thoughts?

[ollypenrice]

I suspect a little play in my Dec axis, probably backlash in the gears. It is not always there, it depends on where the dec axis is when you lock the clutches. Thoughts on that?

OK, so I am imaging low, tube nearly horizontal, and the guiding is lousy. Then I go to near the zenith and it gets better.

Does this seem logical to you, if backlash in the Dec axis is the cause? It does to me. The relatively vertical tube has a smaller moment than the relatively horizontal one? Yes? No?

Guiding low is always hard because of the seeing but what about the moment in Dec?

Cor, mounts, what a game!

Olly

[Bazzaar]

The moment in the DEC axis changes with vertical angle?

I dont think it will, not if you are correctly balanced. Gravity is cancelled out. Then only the mass of the load being moved is involved, and thats constant.

Could be the DEC axis gear is offset from the axis center of rotation, that would cause the amout of worm to gear mesh to change as DEC rotates.

Barry

[ollypenrice]

Thanks Barry,

I know the moment doesn't really change but if the tube is vertical any imbalance is reduced. However, I have tried an assortment of in and out of balances with no real gain.

Good point about the worm being offset.

I had another thought last night, too. It has been getting worse as the weather has been getting warmer. Maybe some differentialexpansion effect?

Olly

[mcrossley]

As you are probably aware no gear is perfectly concentric with it's central bore, so you will find that the backlash does change (usually by a small amount) as you run the gear around 360 degrees. On the Losmandy forum they recommend adjusting the backlash at the tightest point to avoid binding. Not sure if you have adjustment on the EM200?

If no adjustment, find the spot on the worm wheel where you have least backlash, then slip the clutches to move the mount into a position where you use that area of the worm wheel when imaging.

Off balancing can also help, but not too much or it induces 'bounce', and as you state, as the OTA points towards vertical the amount of off balance reduces.

Other than that, its basic stuff like making sure all your cables are routed back to the mount rather than hanging - that has most effect when the OTA is horizontal.

[andrew s]

Olly - It's difficult to comment without getting ones hands on the mount but in principle the dynamics of the mount change as the interplay of the inertia about the axes and the gravitational moments change as seen by the motors/gears.

I am sure you have balanced your mount abut all 3 axis and so are not getting caught by that issue.

One old trick is to offset the polar alignment so the Dec motor is always driving in one direction although I have never tried it.

An other option if you can do it on your mount is to loosen the dec clutch and manually move the mount then reposition it using the motors so a different section of the wheel is in use at a particular dec. Depending on the root cause this may or may not help.

Andrew

[Bazzaar]

Just so we are clear, guiding or just tracking?

"Lousy" in what way?

Could it be an optical problem of the guide system? the nearer the horizon the more atmosphere disturbance.

Barry

[ollypenrice]

...lousy in the sense that AstroArt shows a tracking error around 0.2 pixels (400mm FL guidescope, 16ic guide cam.) On the EQ sixes I get an average around 0.08.

I realize that you won't guide as well near the horizon because of turbulence but the same OTAs/guidescopes/computers do far better on an EQ6 than on a five times more expensive EM200 and I'm pretty fed up about it. The mount went back, was a bit better, and is now all over the place again.

Olly

[Ibbo!]

What sort of guide exposure times are you using?

Steve

[ollypenrice]

I've tried everything from half a second to five seconds. The mount is still erratic.

Olly

[Ibbo!]

I have an NJP(old S/H)and normally it behaves but sometimes it is a royal pain

The Losmandy G11 took me over a year to get to track something like

You have probably tried the below but it might help others

Up the guide exposure to 5 secs(I have used 15 secs)

One work around is to turn off the dec guiding in one direction or turn down the aggressiveness, amount of correction applied not sure what its called in AA as I have not used AA for a while

I find its usually down to the seeing

Steve