LX200 'GOTO' Problems by Bruce Johnston

The painless, no-tool (almost) , fix.

For a long time now, I'd experienced accuracy problems with the 'goto' option on my 10" F6.3 LX200. I'd even resorted to buying an illuminated finder scope to help me find some of the targets.

I found that even when I was using the 'high precision pointing' option, often the reference star would be completely out of my field of view, so I had to resort to the finder scope to get the star centered.  There were times when I'd find that my target was dead center in the field of view. Other times it would be too far left, right, up, or down, to even see!

My scope is in an observatory and is properly polar aligned, as well as recently having had the forks adjusted so that they are orthagonal, so those factors could be eliminated.  If either of these conditions don't exist for your scope, they will also cause poor 'goto' operation.

If you experience anything similar to mine, you may want to try what I finally did, to resolve the problem.

I can't guarantee that this method will be as successful for an alt-az scope because I've never tried it, but I see no reason that the cause and fix wouldn't be as effective for that system.

First, let me say that this technique was developed for scopes that are in a permanent location and on a wedge. However, by using the techniques described at the end of this method for making the adjustments permanent, there should be no reason that it won't work as well, on a scope that is moved from site to site.

The cause:

Image 1

After I decided to look into the matter a bit, I found that both the R.A. and Dec worm gears appear to be drilled just slightly off center. This may or may not actually be the case, but the scope functions in exactly this fashion.

(UPDATE NOTE: This indeed, turned out to be the case.  My R.A. worm gear had a slightly oversized mounting hole, and was mounted somewhat off-center.)

We'll look at the operation of the R.A. drive only, for the moment.

Drawing 'A' above, shows the hole drilled properly. Drawing 'B' shows an exaggerated view of the hole off-center, and the worm engaged with the worm gear, on the narrowest portion of the gear. In this case, the worm gear would act as if it was a smaller diameter gear, and would move the scope faster, causing the scope to overshoot the target.

Drawing 'C' shows the opposite condition. The gear would function as if it was of a larger diameter than normal, causing the drive to run slower, thus undershooting the target.

Image 2

In the two drawings above, the gear drive would be operating at the correct speed, so the target would end up in or near the center of the field of view for either case. Either of these conditions are what we'd want for the scope.

So how can we... painlessly.. make this adjustment with no tools?

Image 3

The fix:

This procedure is an iterative process, so be patient when you begin to do it. I believe you'll find it's worth the effort. I did.

First, locate a star off to your West. Use one of the stars that can be selected by name if possible. If not, then select one of the stars from the star catalog.

You don't have to select stars near the celestial equator, but it's still probably a good idea if you can. I used Arcturus as it was beginning to decline in the West as my first star.

Center it in your field of view. Use the finder scope, if necessary, to locate it and get it centered. It should now look like drawing 'F', above. Now sync on the star.

You're going to be using the finder scope quite a bit in the next few steps, so be prepared for it.

Next, select a star from the name list or from the catalog, that is perhaps 90 or so degrees East of where you're looking. It can be higher or lower in Dec. That doesn't matter at this time. I used Altair as my second star. It was the right time of year to use these two particular stars, but any two stars in the catalog will do.

Do a 'goto' to it.  Chances are, it will be like one of the stars in drawing 'G' above. That is, either on the edge of the field of view or completely out of view.

Note whether you went too far or not far enough. You can disregard how much you may be out in Dec at this time. That comes later.  Just note whether you went too far or not far enough in R.A. You can use the Dec control to simply move the target star down or up, as needed, to help bring the star into view for the present.

If you went too FAR , then unlock the R.A. drive lock, and rotate the scope by hand, about 30 degrees or so, to the EAST  and lock the R.A. lock.  Don't bother to measure the degrees; just guess.  This isn't that critical of a movement.

On the other hand, if you didn't go far enough, , then unlock the R.A. lock and rotate it toward the WEST about 30 degrees or so and lock it down.

Now, again select your first alignment star (my 'Arcturus') and do another 'goto' to it.  It will be way out now, because you just threw the adjustment off.  You'll likely need to use that finder scope to move it back into the center of the field of view. Center it and sync on it again.

Once again, do a 'goto' to the Second star.

Is it better? Worse? Right on? No significant change? If it's anything except right on, you're going to again unlock the R.A. clutch and again rotate it in the same direction you just did.

Again, go back to the first star and repeat the process. Yes.. again. And then again....

The whole idea here, is to keep going until you see that there is a slight change, and you are now doing the Opposite from what you were doing when you began.  That is, if you were Overshooting when you started, you want to go until you Undershoot a bit. And vice-versa if you were doing the opposite when you began.

Just so we are in agreement about what 'overshoot' or 'undershoot' means in this particular case, if you're slewing to the EAST and the star ends up EAST of center, that's 'overshoot'. If the star ends up WEST of center, you didn't slew far enough, and that's 'undershoot'.

When you reach this stage, your image in the eyepiece will look similar to that in drawing 'H' and like drawing 'I' in the finder scope. It won't be that extreme, but the point is, you'll have your star on the OPPOSITE side of center a bit.

Now, you could actually have rotated your scope in the opposite direction had you wanted to, for your overshooting and undershooting. You'd have simply been on the opposite side of the 'egg shape' of drawings 'D' and 'E'.  I just used these directions so we'd all be doing the same thing in the same way.

To confirm that you've hit the 'sweet spot' on the gear, give it the acid test.  Sync on the star you just slewed to, then slew back to the original first star.  Look okay?  It should.  Now, slew back to the star you started this last test with (my 'Altair'). Next, slew over to a star or object in the catalog, that is significantly EAST in the sky.  Is it nearly dead-center in the field of view?  If so, you're set. If it's too far EAST, then you just may have to compromise a bit, and rotate the scope manually to the West a trifle.

What you end up with, is the fact that you'll just barely be undershooting for the Western part of the sky and barely  be overshooting for the Eastern part.

After having done this, I ended up syncing on Arcturus when it was almost at my Western sky limit, then slewing to M31, the Andromeda galaxy near the Eastern limit of my sky view, and it was dead centered in my 12.5 mm eyepiece!

The Dec Adjustment

Up until now, we've ignored any Dec error we've had. We've just moved the target star in Dec, just to center it in our field of view.

To make the Dec adjustment for the same problem, all that is needed to be done is, if you are overshooting in the Dec direction, you loosen the Dec clutch knob and move the tube North, and if you are undershooting, you move the tube South. Otherwise, it's the same thing.

You also select a star that is to the South for your first star, and one that is mostly North of it for the second one.

Also, you won't need to go to such an extreme when dealing with the Dec.  After all; you won't  be covering nearly as large of an arc of the sky, if you have a decent viewing area.  You'll be spending most of your slewing time, looking for objects that are significantly above the Southern horizon if you live in the Northern hemisphere, and vice-versa for those in the Southern hemisphere. And it will probably be very seldom that you'll be looking for objects as far north as Polaris, so you can settle for stars in your viewing area of the sky.

Making the adjustments 'permanent'

All of this is well and good, but how does a person keep the scope in sync with the settings just established?  If you're in a permanent site, then the answer is easy.  At the end of the viewing session, simply aim the scope at zero degrees Dec. as read on the hand control, and the R.A. to whatever your local Sidereal time may be, then shut it down.  Never move the scope by any other means than by the hand controls under power from then on, and you're all set.

Unfortunately, this isn't always practical.  For instance, I often want to be able to loosen the Dec knob so as to adjust the balance for various loads, such as by adding a Barlow lens, or mounting a CCD camera to the scope.

No problem.  Once you've parked the scope at zero degrees Dec. and shut the scope off, confirm that your Dec setting circle also reads zero degrees, which it certainly should.  If not, loosen the lock for the setting circle and move it by hand until it reads zero degrees then lock it down. Do NOT move the OTA; just the Dec setting circle!

Now, you can move the OTA in Dec all you want without power, for balancing or whatever. To get the scope back into sync for the next viewing session, simply manually set the setting circle to zero degrees  and lock the Dec clutch down before turning it on. 

Image 4

So far, so good.  However, we're still faced with the possibility of wanting to move the OTA manually in the RA direction with no power. How can we 're-sync' the scope in this fashion? For this one, you must be prepared for the use of a tool to help you out....................... a piece of tape.

Since the LX200 has no real need for manual RA setting circles, you can use this to get your RA reference point.  

You've just shut the scope off, it's setting at zero degrees in DEC, and you've selected your local Sidereal time for the RA setting.  Notice the small arrow on the fork assembly, that points at the RA setting circle. Right now, it's pointing pretty much down, more or less.

Select some convenient time marking on the RA setting circle that you'll always remember... say '12 o'clock', for instance ....  and rotate the setting circle until it exactly lines up with the arrow.  Now, firmly tape the setting circle down so that it can't move when the scope is up and running under power. This alignment will be your reference point for RA from now on.  When you go to shut the scope off for the night, no longer worry about what the Sidereal time is, but simply rotate the scope under power in RA.. SLOWLY.. until your small arrow lines up with the '12 o'clock' time, and shut it down.  Naturally, you've set the scope Dec setting to zero, before aligning these two points and shutting down.

With power off, you can now move the scope in DEC manually, as well as in RA.  All you need to do to synchronize everything is to align the Dec to zero as before, and the arrow to your selected RA time reference.. '12 o'clock,  in this example ... and power up. Your gears are now back in sync.

You see, when the scope is turned on in Polar mode, it assumes that you are at zero degrees in Dec, and are pointing at your local Sidereal time in RA.  Using this method, you've satisfied that criteria.

*NOTE:  If you're willing to go one step further in the way of tools and parts, see the side point suggestion below that would make the job of finding the R.A. beginning position, even easier and more accurate.

Some side points:

1. When I was done making these adjustments, I found that not only was every object within the field of view of a 25mm or 26 mm eyepiece, but it was in the inner third of a 12.5 mm eyepiece!

2. I found that I am now able to locate objects without high precision pointing, as accurately as with it.

3. If, while making the Dec adjustments, you find that your second star also shifts East or West significantly, you are either not well polar aligned or your forks aren't orthagonal.

If you see no, or very little East/West shift, you probably are well polar aligned, and you probably don't have to worry about adjusting the forks to make them orthagonal.

4. If you have noticed that your scope has had a tendency to drift either East or West while viewing or imaging an object over time, it is very possible that it was being caused by the fact that you were operating on the high or low lobe of the drawings 'B' or 'C'. Remember that in those positions, your scope is mechanically running faster or slower than optimum.   For instance, I found that before doing this adjustment, I could only take images that had perhaps 10 seconds of exposure time before my images began to smear in R.A.  After doing it, I could expose for well over a MINUTE or more, and there was no 'drift' of the image.

5. If you want to make setting the R.A. position for power up even easier and more reliable, as well as more accurate, you might consider doing this:

  • A. Go to a local hardware or building supply store and buy an inexpensive plastic 'bubble level'. These levels have 'bubbles' built into them that can just be popped out by finger.

  • B. As you are about to close down for the night, check your sidereal time on the hand control. Then move your scope in RA until it is aimed exactly at that sidereal time, using the display on the hand control that shows your present R.A.  Pop back and forth between the two displays until they are exactly the same, and then IMMEDIATELY shut the scope off! Your scope is now aimed due South, or what your Sidereal time was when you shut it off.

  • C. Pop one of the 'bubbles' out of the bubble level and put some glue on it, then press it up against the back of the OTA (that's the same side as what they eyepiece is on). Move it until the 'bubble' is directly in between the hairs in the bubble, and hold it in place until the glue dries.

  • D. If for any reason you need to move the OTA manualy before powering up the next time, just reposition the OTA until the bubble indicates level and lock it down.  It is now in exactly the same position as it was when you powered it off, and you are right back over the same spot on the gear as you were when it was shut down. The 'sweet spot' has been preserved! 

I hope this helps someone, as it did me. Clear skies, and.......


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