Bad backlash in just North direction #backlash


papagordygrapes@...
 
Edited

I'm working on trying to use OnStep with an EQ3-class mount (Exos Nano).

The backlash in the North direction is really bad (Dec motor).  When at a 20x guide rate, I count off something like 15 seconds before I detect movement in the viewfinder.  But in the South direction it moves near immediately.  My go-to rate is set to 3 degrees/sec.  It is pretty repeatable.  

This can't be normal?  Maybe the mount is hopeless?  

Video showing mount and motors:

https://photos.app.goo.gl/ARkLrm3KbLXsRxef8


papagordygrapes@...
 

I think maybe I was smoking crack.  Experimenting this AM in daylight is showing long delays in both North and South.  

What is usable?  


Khalid Baheyeldin
 

If the delay is symmetrical, then just adjust the backlash in OnStep.
You can do this from the 3 dot menu of the Android App.


Greg Baggs
 

Just to add a little but if you haven't already tried this you might be able to make a small mechanical adjustment to the worm gear and actually eliminate most of the backlash. I just did this for my old LXD55 as the DEC backlash was pretty bad. Now before I even explain we are talking very minute adjustments to these screws, turn them as little as possible at a time. If your mount is like mine there will be three screws for each axis. The center one is an end stop and the outer two are for tightening and equalizing the thread mesh from side to side. It's also best to put a manual knob on the worm shaft to feel the friction by hand as you make these adjustments. To reduce backlash you loosen the center screw (very slightly) and tighten the outer screws equally (also slightly). Then turn the knob turn in both directions to test and feel the backlash and that the friction is not too tight. The idea is to take out as much backlash without increasing the friction noticeably. There are probably better explanations and videos on how to do this and it's not hard at all really.


On Sat, Aug 15, 2020 at 2:37 PM Khalid Baheyeldin <kbahey@...> wrote:
If the delay is symmetrical, then just adjust the backlash in OnStep.
You can do this from the 3 dot menu of the Android App.


papagordygrapes@...
 

Thanks Greg, I did make some adjustments and it seems better now.  

There is still a fair amount of play--  like 1/4 of rotating the worm axis.  But better than it was before.  we'll see if I can make this usable with backlash adjustments.


Greg Baggs
 

glad to help. but you might keep trying to make mechanical adjustments. there is a knack to it that comes with a bit of practise. on my old lxd55 mount l have almost no backlash on both axes - i can get to maybe a degree or two at the most when turning the worm by hand. but it takes very delicate adjustments of the 3 screws. the better the mechanical worm gear mesh is adjusted i expect the easier it will be for onstep to compensate for the remainder of the backlash. just don't go too far to the point where the worm starts to get harder to turn. when you think you have it perfect, it's a good idea to turn each axis 360 degrees by hand to ensure it isn't binding anywhere. thats 144 full turns of the worm by hand on my mount. fyi i'm a new guy to onstep as well - i haven't even finished my kit builds yet. but i'm an old dog at astronomy in general. i started around the mars oppositon of 2003. anyway good luck with it.


On Sat, Aug 15, 2020 at 11:47 PM <papagordygrapes@...> wrote:
Thanks Greg, I did make some adjustments and it seems better now.  

There is still a fair amount of play--  like 1/4 of rotating the worm axis.  But better than it was before.  we'll see if I can make this usable with backlash adjustments.


papagordygrapes@...
 

I'm pretty much tightened all the way down.  Just a tiny amount on the middle screw to make it tighten down most parallel. Which is probably why there is still excessive play since I can't tighten in a way that increases friction.  Tempting to file down the tightening plate a hair to see if that gets more gear engagement


Greg Baggs
 

hmm i dont think you should need to file anything. are you loosening the middle screw a hair and tightening the outside ones about the same?


On Sun, Aug 16, 2020 at 10:57 AM <papagordygrapes@...> wrote:
I'm pretty much tightened all the way down.  Just a tiny amount on the middle screw to make it tighten down most parallel. Which is probably why there is still excessive play since I can't tighten in a way that increases friction.  Tempting to file down the tightening plate a hair to see if that gets more gear engagement


Drew 🔭📷🚴‍♂️
 

There are several good guides available on this type of mount:

http://www.thestardeckobservatory.com/LXD75_Maintenance_101.html

http://www.astronomyboy.com/cg5/
 
These may help, I am sure there are more. With the worm there are two critical adjustments. The worm shaft endplay and the worm mesh. Both are critical. Check the endplay adjustment first, it sounds like that may be your issue.

Adjusting these old mounts is critical for use with OnStep, since it has no feedback. Be sure the entire range of motion is smooth with no binding. These old mounts typically have old grease which has gone solid and can have binding issues particularly if they were left "loaded" with scope in one position for long periods of time.


papagordygrapes@...
 

This is a new mount :-)  But only a $120 mount.

Right now my GoTo accuracy with a 3-star align  (not sure if I did meridian flip or not) is about 1.5 degrees.  


Drew 🔭📷🚴‍♂️
 

The endplay adjustment is separate from the "mesh" 3-screw adjustment. The reason I think it is your problem is a) it is only in one direction b) the mesh adjustment doesn't seem to change it.

The endplay is adjusted by loosing the locknut on the end opposite  the drive gear/pulley and moving the inner "screw bushing" in until it is just barely snug.

If you want to verify it is your problem try moving the scope to a position that would favor "south" endplay. See if the backlash switches from North to South.

Do not file, etc anything. This only leads to disaster. I am expert at that part.


Howard Dutton
 

On Tue, Aug 18, 2020 at 11:40 AM, <papagordygrapes@...> wrote:
Right now my GoTo accuracy with a 3-star align  (not sure if I did meridian flip or not) is about 1.5 degrees.  
Having stars on both sides of the meridian during an align is important.


papagordygrapes@...
 

I've adjusted the best I can for now (both the 3 screws and end-play).

I attempted to set backlash during daytime at 8x speed.  Counted seconds until I detected movement.  Figured each second at 8x is 120 arc-seconds.

Right now I've got the DEC set for 1200 and RA set for 400.  I fine-tuned such that tapping didn't move.

Now I did this at 8x rather than Howard's recommended 0.5x !!

I'll see how GoTo accuracy is...