Last updated: 1 March 2001

Setting Percentages For Better Tracking

From: sherrodc@ipa.net (Clay Sherrod)


Now you CAN control BACKLASH!

A very welcomed and new addition to the AutoStar database (version 2.1ek and beyond) is the capability of ETX and LX 90 users to manually key in deficiencies in altitude and azimuth BACKLASH.

Your "Train Motors" function under "SETUP/TELESCOPE" is a quick and convenient - and very accurate - method to eliminate most backlash that AutoStar detects in the drive trains of ETX and LX 90 telescopes. However, even with the "Train Motors" done accuratelly, there can be residual backlash still remaining in your telescope. This can be due primarily (in addition to poor training techniques) to your telescope being aimed at a particular angle in which the telescope is "relaxed" or not engaging fully with torqued gears. We all have experienced "spots" in the sky at which tracking can be perfect, yet your next GO TO position reveals very erratic tracking. This is due to positional-selective backlash.

The term "backlash" is frequently used (sometimes incorrectly) to described tracking and slewing errors or all sorts: "creep after beep", the "rubber band" effect, improper right ascension tracking, and perhaps most annoying - slow (if ever) response times to react when pushing particularly the azimuth (right ascension or R.A.) buttons of Autostar when set to about "6" or lower. Indeed, I have worked with many people who have reported NO response at all in attempt to center in azimuth with settings "1, 2, and 3" (the slowest).

Likewise, if the response time is non-existent or extremely slow, you will also experience a considerable "overshooting" of your object in azimuth after you "let go" of your appropriate button. This experience is particularly troublesome, jerky and gets you going back-and-forth attempting to center a high magnification. In many cases, when the motors finally DO engage the image to reverse, it is accompanied by a sudden "jerk" and an all-to-rapid reversal than puts you now uncentered in the OPPOSITE direction.

This problem is very pronounced in many ETX 90's and -125's in AZIMUTH, and there are a few complaints concerning ALTITUDE backlash. Indeed, my Altitude (I use Polar mode, so it is "Declination) had perfect response, even with the slowest speeds, but horrible in azimuth (my Right Ascension). Conversely, fellow ETX-er Dick Seymour has had more trouble with backlash in ALTITUDE and little in AZIMUTH backlas. I said "HAD"....Meade's new firmware function control has all but eliminated what was becoming a problem of epidemic proportions, not just for us, but for many of you out there in the fascinating world of ETX's.


Unannounced to those using and buying ETX telescopes, but well covered in the new LX 90 (which also uses AutoStar and its basic functions) manual, there is a new feature under "SETUP/TELESCOPE..." on the #497 Autostar that quietly showed up when v2.1ek was let out of the bag at Meade. Scrolling under "Telescope" past "type", "mount", the "ratio" settings, you will finally come to a new function entry: first "AZIMUTH PERCENT" and scrolling one further you find "ALTITUDE PERCENT."

Note that changing the percent in either axis does not physically remove backlash from a mechanical system; training the drives does that best, and can be very effective if done slowly, precisely and expertly. However, changing the Alt and Az percentages will ALLOW AUTOSTAR to compensate - or "...put a KICK into" the motor reversing process as Mr. Seymour puts it - for the long duration during which backlash must catch up with reversing gear trains....or "backlash."

This "kick" significantly reduces response time when centering at very slow speeds and allows for a much firmer and precise "stop" when the centering is completed.

setup/align [SCROLL]
"daylight savings"
"telescope" [ENTER and SCROLL]
"telescope type"
"focal length"
"azimuth ratio"
"altitude ratio"
(enter amount then MODE)
"azimuth percent" appears again [SCROLL]
(enter amount then MODE several times by to "align/easy"

DO NOT CONFUSE THESE NEW FUNCTIONS with "Altitude Ratio" and "Azimuth Ratio" which are found higher on the scroll list. The "ratio" settings are based on the actual engineering specs for the drive systems and should NEVER be changed!

If you have downloaded the v2.1ek with the new A2.4 downloader program from the Meade website you now have this new - and life-saving (well, at least "sanity-saving") - function available to you - provided you know how to use it.

It's new, and there is no information out on it other than referring to the Meade LX 90 instruction manual, available on-line or what you might glean by calling Meade Tech Support.

Here's how you can GREATLY IMPROVE the following aspects of you telescope drive system using this simple procedure:

1) your sidereal tracking of objects at high magnification will improve greatly;
2) when centering objects, even at the lowest speed settings, you will realize nearly instantaneous response when you hit the arrow key(s) to move;
3) when centering is DONE and you release the key, you will see your objects STOP, and remain where you want them most of the time;
4) slow response to "catch up" in sidereal tracking once centering is done can be eliminated, thus not having to wait a moment until the tracking is actually evident;
5) in my case, I have eliminated "creep after beep" altogether.

Keep in mind, I use POLAR configuration and my settings have been made based on my observations with backlash observed over a long period of time in that Mode; in Alt-Az and with each particular scope the effect and resultant settings will be different.



To make your function changes, first determine two things:

1) Do you have backlash in BOTH azimuth and altitude....or just one? Which one?
2) If in both, which is the worse? On a scale of 1 to 5, with "5" being the worst in comparing the two, assign each axis a relative number. Dick Seymour reminds us that the AutoStar - during "Train Motors" virtually realizes which axis has the worst backlash and thus automatically will compensate MORE in that axis; therefore, it may be LESS necessary to compensate in some telescopes in the worst backlash-plagued axis since AutoStar is ALREADY doing that for you!


1. Now MODE to: 'SETUP/TELESCOPE' and press "Enter" SCROLL under 'Telescope' until reaching 'AZIMUTH PERCENT' (NOT "Azimuth Ratio") and press "Enter"; the factory preset is "01%" which is no correction at all to compensate for backlash; the same preset is found under 'ALTITUDE PERCENT', the next scroll down.

2. Meade Tech Support recommends setting both percents FIRST AT 50% and then working your way back down by halves (if 50% is too jerky, which it most definitely will be!), go to 25%...if you still see backlash, take half of that (12%) and ADD back to the 25% and thus set at 37%....if still jerky go BACK down by half....and on and on....

3. Forget all that trial and error work; here are some guidelines for you to at least BEGIN adjusting for backlash compensation using the new Azimuth and Altitude Percentage settings. Here are some suggested STARTING VALUES for axis percentages for your telescope:

a) if you have NO backlash in Altitude, but significant backlash in Azimuth (this is often the case with many ETX scopes, almost carbon-copies of one-another) then here are some setting that probably will fix your backlash and give you great results:

AZIMUTH PERCENT (you had serious backlash and poor response) - SET PERCENT AT 15% ALTITUDE PERCENT (you had no perceptible backlash here and response was good) - DO NOT CHANGE

NOTE: if the situation is reversed (i.e., the Azimuth mount has know observable backlash and the Altitude axis is very poor when centering and tracking at slow button speeds), the reverse the compensation percentages for each axis. It is interesting to note that Dick Seymour's ETX 90 has settings of 35% Azimuth and 22% in Altitude EVEN THOUGH HIS WORST BACKLASH WAS IN AZIMUTH; his explanation is a sensible one: AutoStar KNOWS that the Altitude axis has serious backlash and it, electronically through the "Train Motor" function is compensating for that!

So, experimentation with YOUR scope is the key....every telescope will be different, and react differently to these settings. Remember, you can't enter ANY percent that can't either be changed or taken out completely! So try it out on the night sky, slewing and centering at high power to look for motor response and lack of stoppng when centering.

b) if you have a LITTLE backlash in Altitude, but significant backlash in Azimuth (also very common), here are some starting settings:

AZIMUTH PERCENT - (serious backlash problems, poor response) - SET PERCENT AT 15%
ALTITUDE PERCENT (a little backlash problem and pokey response) - SET PERCENT AT 7%

c) if you have a LITTLE backlash in BOTH axes, SET BOTH PERCENTS AT 7% and start from there, increasing a bit each time until jerky motion is seen when pressing the slow speed buttons

d) if you have SERIOUS backlash in BOTH axes,


SET ALTITUDE PERCENT AT 12% to start with and work your way up until jerky "start motions" is realized, then back off slightly to lower percents in that respective axis.

These settings will likely take care of or at least get you started to remedynearly all complaints of either wild or totally non-responsive ETX / LX 90 behavior when using the arrow keys on the AutoStar touch pad; if, after making these "starting point" settings, you still experience slow response, poor tracking and lazy stopping in a particular (or both) axis, then you STILL HAVE BACKLASH! Merely ADD a few percent a step at a time in each appropriate axis until it is very tight and immediately responsive at your slowest centering speeds.

If, on the other hand, you find that response is JERKY in one (or both) axis when beginning a touch key command and the image "jumps" when starting motion, then you have OVERCOMPENSATED; simply REDUCE the percentage a few points in that axis until the reaction (particularly the "startup" when you try to center or move and object) is firm and immediate, but DOES NOT JERK the object to move it.


Many of you have undoubtedly noticed Celestron's new 11" GPS-capable telescope; it looks like a gem, indeed; it seems to not even need an observer. Aside from that "tongue-in-cheek" comment, please notice in the advertising that accompanies the telescope that the unit has a "built-in" percentage control function that automatically keys in parameters to eliminate backlash and increase tracking and centering proficiency.

Wow....Meade has that, too...so does your ETX! The only difference is that - other than training the drive motors - YOU, the user, must program in the percentage, changing a little at a time until you get it right. Before, this was not available to ETX users with Autostar, but now it is.....if you upgrade to v2.1ek (or any subsequent) firmware now available off the A2.4 (and any subsequent future) Meade Autoloader downloads.

You are really in for a surprise and quite a treat as many of your problems disapper with random and odd motor behavior - not to mention tracking variances, centering problems and frustrations and drift in certain parts of the sky. You have now grown accustomed to these problems but with a little patience and the new "percentage function" of the Meade AutoStar, these problems simply disappear quickly....literally with the touch of a button (well, perhaps "two" buttons)!

P. Clay Sherrod
Arkansas Sky Observatory

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