Balance issue or something else?

[swannies1983]

I have been having major issues guiding in DEC (RA is fine). See here for a few posts made by me http://www.iceinspace.com.au/forum/showthread.php?t=53901&page=3)

Basically I get really large corrections in DEC, up to 1-2 pixels from the central line. Potential causes include backlash, balance and stiction. I always tend to guide in either NORTH or SOUTH direction only. However, I have recently chosen SOUTH. The reason can be seen via this video http://www.youtube.com/watch?v=KZk7P1HMLkY I am winding the pulley/belt and you can see that movement is quite jerky when I wind NORTH (towards the ground) but smooth when I wind it up (SOUTH).

Is this simply a balance issue? I have balanced DEC when the scope is horizontal and vertical. However, I tend to slightly balance it eyepiece heavy to engage the gears. When I balance the scope evenly, the jerkiness is evident when winding both up and down.

Maybe backlash? However, there is very little movement in the DEC axis when the clutch is locked.

Just a wild guess, but doesn't PhD calibrate in he NORTH direction? While I assume the jumps won't be as exaggerated during calibration, would "micro jumps" cause a problem in the final calibration? I'm only guessing.....

[bmitchell82]

Ill ask this as a basic silly question for a start.

What is your guiding scope On axis or off axis guiding?

Secondary what is your definition of not much movement? mine is at the end of the CW bar 1-2 mm max

Have you pulled the mount completely down, made sure you have set the RA and DEC gear by shimming, Re lubricated and re adjusted.?

Ill put it on the record, i don't like PHD. But, when you do your calibration does it land exactly back on the same spot you started, if you don't just because it says its calibrated means squat, its just taken a average.

I have seen massive jumps up down left right because of poor calibration routines. If you cannot get a perfect + then start again and hence why i do not like PHD, you cannot tweak the guiding on the fly and change the huristics and it needs a massive step amount.

Maxim only needs 5 pixels and it moves left right up down done. I can get this done in about 30 seconds to a minute depending on the part of the sky that im looking at SCP or East/West.

[swannies1983]

First off all, I should make it clear that it wasn't my hand controller moving the mount. I was winding the pulley using a allen key.

I'm guiding with a 60mm f/11.6 tasco refractor.

I would also say maybe 1-2mm as well. When calibrating in PhD, the mount only requires about 3 iterations to clear backlash.

I might have to take the mount apart again and start from scratch. I didn't make all the necessary measurements to determine correct shimming sizes. I just went with the ones that were already in there.

[bmitchell82]

I have seen the "click" in my mount but that was when i had possibly gone a little too close with the meshing and it wasn't correctly shimmed. I no longer see that abberation.

[swannies1983]

After speaking to a few people, it appears that my balance, is in fact, not balanced. It relates to a stiff DEC axis. I wasn't quite sure how freely the axes should spin when the clutches aren't engaged. I have some movement but it needs a big imbalance to cause significant movement when the scope is loaded.

I did open up and clean but noticed the DEC gear was quite tight in the collar. I have been given (careful) instructions to use fine sandpaper to sand out the inside of the collar. Have others had to do this?

[bmitchell82]

the best i have ever done to my mount was use a fine stanley blade to clean up the surface between the worm adjustment housing and the main body on each side where paint had dribbled during manufacture.

I have never attacked it with sand paper. If you have to start sanding away material, send it back!

My RA and Dec does not spin easily and i generally don't sit there waiting for the weight to move it, i do a average i push the mount on the axis and observe how fast it slows down or how far it travels before stopping. once your weight is about right, youll find that the same amount of force each way will result in the same distance and speed that it slows down in.

[mswhin63]

I have seen this happen when meshing of the worms/gears is loose, and is experienced when balance is favored in the direction of travel. It also more prominent when it is spun by hand.

[swannies1983]

Yes, I also believe it is due to loose worms/gears. As I said earlier, what I thought was balanced is probably not given the fact that the axis is very stiff. I am going to touch up the collar and get some more bearings, which will hopefully help when finding the correct balance.

I have tried to adjust the worm/gear but if I do it anymore than what it's like in the video, I start to get binding. I think maybe another issue is that the spacers I have look quite brittle. In fact, I just found one which was cracked. Perhaps I need to determine the correct spacing to improve alignment.

[bmitchell82]

With those adjustments how are you making them?

[swannies1983]

Which part? The stiffness in DEC when the clutch is disengaged?

[bmitchell82]

do you still have the motors bolted in because if you have them in and bolted down you will never get it right.

[swannies1983]

Motors are out, but I have a belt/pulley system.

Anyways, I have reopened the DEC axis and given the collar a bit of a sand and put in some new bearings. The axis spins much more freely now. I guess we will see how it goes the next time I'm out guiding.

[bmitchell82]

Well i don't have too much experience with belt transfers. Though i am slightly confused if you are running a belt system how are you really getting any back lash, I was under the impression that is the reason why you go this route to virtually eliminate all back lash as you can tension up those belts to a audiable tone?

[swannies1983]

This is what a fellow IIS member said when I asked him about backlash with this setup:

"Backlash from transfer gears is eliminated (because there are no transfer gears) but you still have worm and worm gear - which is essentially geared pair. So yes, you can have backlash there."

[bmitchell82]

That is right, but im getting 1-2mm at the end of my CW bar fully extended with all the transfers in place surely yours should be in the order of magnitude 1/3rd that amount which should be well under a mm.

With my worm gear/wheel adjustment i have the motors out and with my finger turn the worm gear. I do one complete loop and you will find the tightest spot and adjust for that. essentially this will give you the tightest meshing and minimal back lash that your setup can achieve.

I aim to have it so that i can turn the worm gear just it cains your finger a bit but works a treat.

[swannies1983]

Call me stupid, but how do you actually measure it?

[bmitchell82]

you have to make a datum eg put it next to a wall chair leg something thats not going to move. if you have a magnetic dial gauge even better. then... measure it.