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Celestron Auto Focuser - Issues

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#1 Skoviak99

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Posted 03 December 2020 - 10:14 AM

I just received a new Celestron Auto Focuser for my Evolution 6 telescope. The unit installed and calibrated without issue. The problem is when making moves. So using the Hand Controller, if you make a move using the move up/down it works correctly, so Jog one direction and it moves, Jog in the other direction and it moves. However if you make a goto type move, i.e. 10200 to 10400, it will move directly to 10400. From 10400 to 10600 it will move directly to 10600. Then however if one moves in the opposite direction, from 10400 to 10200, it moves first 500 units from the starting location, so in this example it moves to 9900, then reverses to the target 10200. So it always gets to the target in this particular direction, but makes a 500 unit detour before going there.

 

If I use the laptop software CPWI it has the same issue, you can jog or goto a new target in one direction directly, but if you try in the opposite direction it first makes that 500 unit jump then reverses to the new target.

 

This is also true with the SkyPortal App from a mobile device. In this case the graphical  "Rotary Knob" will work directly in one direction, then have the 500 unit glitch in the other direction. Also true with the small incremental moves of +1 or +10 and -1 or -10 in the App. One direction will always increment  +1 or +10 directly, the other direction will always first move 500 units then reverse and go to the -1 or -10 increment that was selected.

 

The Backlash setting was zero, but I tried setting it to 0, 10, 50 or 99 with no effect on the 500 unit jump. This feels like either a bad servo or some type of firmware issue. Has anyone seen anything like this?


Edited by Skoviak99, 03 December 2020 - 10:16 AM.


#2 jrcrilly

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Posted 03 December 2020 - 10:41 AM

From the device manual:

 

NOTE:When using the “Move To” command to automatically
move the focuser to an index position, the focus motor will
always approach focus in the positive direction (i.e. with
the focus knob rotating counter-clockwise and the primary
mirror moving outward). This is done to ensure the best
accuracy of the “Move To” command, as it eliminates any
potentially “lost” index position steps due to backlash in the
telescope’s focuser system. If the desired index position is
inward (i.e. a lower number) from the initial position, the
focus motor will move 500 steps inward past the selected
index position before reversing direction and returning to
the selected position.


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#3 ryanha

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Posted 03 December 2020 - 01:55 PM

The way that an SCT focuses is that the knob is pushing the primary mirror "in" or "out" in the tube.

 

Because of gear/mirror slop, you always want the last move to be "against gravity" that way you minimize the chance that you will be off of the expected position.

 

So it is a sort of variation of back-lash in that it is about making sure that the last move pushes the mirror "outwards" against gravity. 

 

In apps like NINA, this is actually called an "overshoot" compensation (vs. back-lash) because the compensation is only needed in one direction.

 

This confused the S out of me at first but now it seems obvious smile.gif

 

 

--Ryan


Edited by ryanha, 03 December 2020 - 05:06 PM.

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#4 Skoviak99

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Posted 03 December 2020 - 05:17 PM

Thanks Gents, my mistake was to not consult the online version of the manual which is more comprehensive than the one that ships with, as the printed manual makes no reference to the 500 increment step, but rather just says to consult the online complete manual. So a gotcha for sure, but it sure is nice having folks that one can turn to for assistance.


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