Re: AP1200 GTO and backlash in its brass gear?

Timo Inkinen

Hi Konstantin,

>>have ever tried and set a backslash time other than 0?

Nope, because in my discussions with AP support they always sais that I should remove any backlash setup from GTOCP3 and do the same also for PEC (disable those). I personally think too, that it would be best to get tracking as good as it gets without any additional backlash compensation. And then (after this, if it is still required) setup backlash timing accordingly. This is a sort of perfectionist / purist way of doing setup, which I'm trying to follow here.

I would like to know how much AP1200 backlash should I tolerate? What is the normal amount of backlash for a well tuned AP1200 mount? In my mount, this amount of needed backlash compensation is quite big (=huge for an AP mount, I think). And for this reason, I should not just use backlash compensation in software or GTOCP3.

I can show you a PHD2 graph for that, where I have used PHD2's Guiding Assistant recommendations:
Normally you can get away with 50ms pulses in DEC guiding and then you need 1800 ms guide pulse for switching directions, IF you let PHD2 to do its' corrections at one go. That tells me that there is something wrong in my mount with its' backlash.
Here is the same backlash thing in close-up (zoomed-in) view:

If I do not allow this 1800 ms backlash compensation in PHD2, then I will get this kind of slow drift in DEC axis: (both ways let me image with round stars)
In that image PHD2 makes a series of smaller corrections, which gradually "pulls-in" all looseness in DEC axis (=backlash) and when gears finally engage, then direction for tracking error changes (and you need to have small enough pulse into the opposite direction, so that you don't overshoot with it). And then, after this correction drift starts again. Sort of weird balance I have found from there. Anyway, my polar alignment accuracy is somewhere in 2'...4' range and guiding in one direction (N/S) only loses star permanently on the "wrong side". That happens randomly, so I prefer keep guiding in Auto mode for DEC.

My setup is in a permanent roll-of-roof observatory seen in this image:
(Don't look at the missing cable ties, this is an early image of my current setup ;)

Br, Timo

PS. Should I delete "the tail below" from my reply into this AP group? I'm not familiar with this interface, so I need to ask :-)

---In ap-gto@..., <k.poschinger@...> wrote :

Hi Timo,

have ever tried and set a backslash time other than 0?


Konstantin v. Poschinger

Hammerichstr. 5
22605 Hamburg
0171 1983476

Am 14.10.2016 um 16:03 schrieb Timo Inkinen timo.p.inkinen@... [ap-gto] <ap-gto@...>:

Hi all,
I have bought a second hand AP 1200 GTO mount made ca. 2010 (serial 1200926).
I have found out that this unit has a DEC backlash problem, which shows up in all backlash tests taken with PulseGuise and PEMPro. 
You can see this in my backlash test picture here:
I have done lots of debugging with my mount to find out where the cause for this backlash problem resides.
Starting from the port/mounts/1200gto/Gear%20Mes h%201200.pdf document measures in gear meshing and ending into camera cable fixings plus guiding software parameter adjustments.
But despite of that all, I CANNOT get this backlash out of my AP 1200, no matter what I do on my own and with the help suggestions from AP support.
On the last weekend I took the DEC gear mesh system apart and disassembled the DEC gear mesh from the mount altogether. 
Then I drove this DEC gear meshing with standard PulseGuide test #3. Ten pulses with 500ms Pulse Duration were sent and results for that test can be seen in this GIF animation taken closely from my mount gears:  
Look up there how the foremost gear moves (or actually DOES NOT move) compared to the little gear on the left.That little gear (on the left) is attached directly on the motor shaft and the big gear (in the front) is one gear away from worm screw gear.
THis animation was taken with my DSLR camera and timer taking sequence of photos during the test #3. A total of 42 photos were taken, but only half of those were needed as there was a gear change in every other image.  
As you can see from that GIF-animation, it is quite evident that my backlash problems come from this AP1200 mount’s reduction spur gear-set play. 
There are about three motor pulses sent (each 500ms duration in this test), which are required for the foremost big gear movement to start the movement in this test. As you can see, there is no play between the motor drive shaft’s first spur gear and the second Delrin / Brass spur gear system. But there is too way too much play (!) between this third brass gear and the foremost (fourth) steel gear. Delrin and brass gears are attached to each other tightly.
I have asked what can be done for this excessive play from AP support, but they seem to be quite helpless with it as I cannot get any reply for this issue. I have sent them my latest mail on Sunday and and now on Friday afternoon here in Finland, I haven’t still received any response for that question. Before this gear mesh finding I received almost daily support suggestions in the last week for all kinds of backlash issues, which could be related to my own backlash problems. But none of those suggestions referred to this kind of backlash play.
I know there will always be some amount of play in all gear systems, but I think this much play is excessive for a quality product like AP1200. 
My own diagnosis is that there is some wear in the DEC gear set, which has gradually caused this excessive play between the brass and steel gear.

Do you know, if there are some adjustments for this reduction gear meshing, which could reduce this gear play? 
I have adjusted the play between motor gear and Delrin gear with the motor adjustment seen in this picture:
But I have not found anything else, which could fix this brass/steel gear play seen in that animation as that foremost gear does not have nay adjustment in its' attachment to this DEC unit. 

Also I would like to know, if I can I buy replacement gears for this kind of gear set somewhere (from AP or its’ re-seller in Europe)? 
You know, IF it is the wear in gears that causes this backlash seen here...
Below is more information on this problem.

Here is one image from cable ties:
Here is how this backlash shows in PHD2 software with almost regular setup:
And here PHD2 Guider Assistant’s diagnosis for this backlash (note ca. 1800ms backlash compensation need):
Here is a high resolution image of this gear set: (4000x3000 px, 1.5MB)
AP1200's spur gear is firmly attached to the shaft with this kind of screw firmly attached there:
Some PulseGuide backlash test with 0,5x guide speed:
The same test with 0,25x guide speed only:
Can someone give me some advise for this kind of backlash problem, please?
Br, Timo Inkinen

Join { to automatically receive all group messages.