Strange behavior with LX90 motor accuracy
Posted 10 May 2013 - 06:00 AM
I tested in terrestrial mode during the day, by storing the positions of a few landmarks so that I can test by slewing to them. From its home position (northwards), I stored a nearby landmark as "LM1", then slew left 90 deg. and stored another as "LM2", then slew right 180 deg. (90 deg to the right from home) and entered as "LM3".
So, location-wise, it is like this: LM2 ---- LM1(Home) ---- LM3
Now if I start at LM1, and slew to either LM2 and LM3, the motor is very accurate. From LM3 to LM1 or from LM2 to LM1 - also very accurate. But when slewing between LM2 and LM3 without going through the home position first(that is scope rotates 180 deg in the other direction), the scope will overshoot the target. Now, I thought to myself that the alignment must be off by then, but when I continue to slew it back to LM1 (home), the LX90 slew 270 degrees (cord-wrap setting on) in the direction it came from and LM1 was dead center. Then from there, going to LM2 or LM3 is very accurate again.
It seems that the scope is VERY accurate as long as it slews back through the path it came from because it has count the data clicks to get back to target. But if it goes to the target from the other direction, it will miss slightly (about 10 degrees). It don't seem to know how much to move in the other direction to get to the target. Is this normal? If not, any suggestion with what is wrong with my scope? I did a reset and retrain and it still behave the same way. By the way, I have problem with my GOTO Astro mode even after successful 2-star alignment. My Autostar firmware is already the latest version (43Eg) on my 497 HC. I suspect the way it compute the clicks (not sure the right terminology to use) to get from coordinate A to B is wrong.
Any someone come across this problem before? This scope was the 2004 classic model.
Posted 10 May 2013 - 06:53 PM
Don't feel bad, dealing with the ETX mechanics on a bad night can drive one nuts.
Good luck, Chris
Posted 11 May 2013 - 08:21 AM
Anyway, I did somehow solve the problem. I did some more search and found a 2003 thread with suggestion by "Dick" on Yahoo's LX90 user group that I should use the Alt/Az reading to see if my LX90 read 360 degrees when I rotate the scope precisely 1 turn. That is a very simple diagnosis that I am very glad to learn as a newbie.
Mine reads 353.35 degrees. That explains why it overshot the target. Dick suggested that my encoder slots may be dirty. He also suggested a workaround: to change the Az Ratio setting in the Autostar to compensate for the mechanical error. I learned how to calculate the new ratio here:
I tried that and with some trial and error my targets are now within my 7mm eyepiece. So the work around works very well.
If the problem is due to manufacturing variation of the gears and mechanical parts then the above setting should be a permanent solution. But I am worried that if its due dirty encoder slots, then the accuracy could deteriorate as the encoder gets dirtier or clears up, unless I clean up the encoder which is a rather cumbersome task to dismantle the scope. I hope my LX90 will hold its current accuracy well. Will test it with real stars when the sky clears up.
Posted 11 May 2013 - 10:22 AM
Thanks for the update- it is sure to help some frustrated 90 owner in the future!