Locating LX200 'GOTO' Problems in Polar Mode
by Bruce Johnston
If you're at all like me, when you first started using you scope in it's 'goto' mode, you've found that if it didn't work right off the bat, you had no real idea as to how to isolate what the problem might be. You've hopped from one star to another, only to find that in some cases, it was nearly dead on, but in the next attempt, it was way off.
The natural thing to try is to begin hopping between the brighter stars to see the results, but when that didn't work, you really had no idea of how to use any logical, methodical method of finding what the problem may be.
This discussion is an attempt to help you in that regard. Instead of just hopping around, we'll take a more organized approach and see if we can find where the problems may be. In fact, we're going to confine our testing to moving directly EAST and WEST, as well as directly NORTH and SOUTH.
The fact is, there are really only four things that will make the average scope be off in its 'goto' operations, assuming your scope doesn't have some very unusual problem. (And I can almost bet that this is exactly what you've thought you had.)
The four things that will make the 'goto' be off, assuming you have a normal scope, are:
- Polar alignment.
- Non-orthogonal forks. (Forks that aren't of the same height.)
- An OTA that isn't properly aligned to the forks.
- An R.A. or Dec main drive gear that's mounted in an 'out of round' condition.
In reality, there are five different adjustments that can cause us problems, since Polar alignment consists of two different adjustments. Therefore, from here on, we'll assume that there are five separate checks that can be made, to help in isolating which of the adjustments are the problem.
NOTE*: Let's be very clear on this point. These adjustments are NOT all that can be wrong with a scope that will cause 'goto's to be off! These are the most common adjustments that will cause 'goto' errors! There is no panacea for 'goto' problems.
In this discussion, I'll be using the terms 'left', 'up', 'down', 'above', and 'below' quite often. The purpose is, to allow those in the Southern Hemisphere to make use of the information as it's presented, in the same fashion as those in the Northern Hemisphere.
'Left' it will be 'East' in the Northern Hemisphere and 'West' in the Southern.
'Right' it will be 'West' in the Northern Hemisphere and 'East' in the Southern.
'Up' it will be 'North' in the Northern Hemisphere and 'South' in the Southern.
'Down' it will be 'South' in the Northern Hemisphere and 'North' in the Southern.
'Above' and 'Below" will mean the same in both Hemispheres.
'East/West' and 'North/South' will mean the same in both Hemispheres.
In the examples where I goof up and actually refer to a true direction, they will be referring to the direction as seen from the Northern Hemisphere. (Southern Hemisphere readers, forgive me these errors!)
Before we begin, there are several assumptions that I'll make. The first assumption is, you have a steady mount and wedge on which you have your scope. No more or less steady than any other mount of the type usually used with your kind of scope, but that it's at least 'average'.
The second assumption is, you do not have excessive 'slop', or looseness in the mechanics, anywhere in your scope. Your scope therefore, is assumed to be at least 'average' in gear mesh, as well. This also includes 'mirror flop'. If there is excess mirror flop, you will no doubt get false readings from the tests, and your chances of ever getting decent 'goto' results are pretty poor.
The third assumption is, you've already done the usual things that are needed for good 'goto' results, including Polar aligning, balancing the OTA, etc.
The fourth and final assumption is that when you look through your scope with an illuminated crosshair eyepiece, you know how to tell if the scope is above, below, left, or right of a star. This is particularly important, because we're going to be discussing in which direction the OTA is out of alignment to its star, and NOT which direction the star 'appears' in the eyepiece. On this point, I can help right now.
Aim your scope at any star and center it on the crosshairs. Now, while watching the star in the eyepiece, gently nudge the scope 'up' in the rear portion and see which way the star moves off the crosshairs. You are now looking at the star/crosshair relation for when the scope is BELOW the star! Obviously, when the star is on the opposite side of the crosshair, the scope is ABOVE of the star.
Next, gently push on the Left side of the rear of the scope and watch for left/right movement of the star. Whichever direction the star now appears in relation to the crosshair, that is the relationship when the scope is to the LEFT of the star. The opposite side represents the condition of when the scope is to the RIGHT of the star.
NOTE*: Our source for 'goto' stars will be from the list of reference stars located near the end of the LX200 manual. It lists the R.A. and Declination for each star. If you have supplemental software that will allow you to select even more stars, all the better, but the list in the book is sufficient.
We're now all set to see how the various adjustments and alignments will effect the 'goto' accuracy.
The first example will be to see if the problem is caused by poor polar alignment.
This part of the 'goto' error evaluation consists of three steps:
- Locate a star that is as close to your Meridian and as close to the Celestial Equator as is practical.
- Locate a star that is as close to the same Declination as the first, but is at least three hours different in Sidereal time to the LEFT as is practical.
- Locate a star that is as close to the same Declination as the first, but is at least three hours different in Sidereal time to the RIGHT as is practical.
We first move the scope to the star located in step #1 and sync on it. Next, we do a 'goto' to the star located in step #2. If you're quite a way off, you may have to use the finder scope to get the star into the field of view of the eyepiece. Make a mental note as to where we end up, compared to our target star. Above? Below? Don't be concerned about the Left and Right offsets we have at this point; only the 'Above' and 'Below'.
Next, do a 'goto' to the star in step #1. If your scope is mechanically sound, you should end up with it will being pretty well centered.
Repeat the process by doing a 'goto' to the star identified in step #3. Having completed the first phase of information gathering, we're now prepared to see why and how polar alignment is our culprit, and with a bit more deductive reasoning, determine in which direction the polar alignment is off.
Above are typical drawings that I, and others, often use for showing the Celestial Equator and the scope Equator for a scope that has its Pole misaligned to the left of the Celestial Pole (top drawing), parallel with it (center drawing), and misaligned right of the Celestial Pole (bottom drawing).
The problem with this view, is that it's the view as the observer would see it, more or less, while standing at their scope. As it turns out, there's a much better view for us to take that will make things much more clear and... hopefully.. more understandable. That view is the 'view' that the telescope has. We could also say that it's the view that the observer would have, if the observer was located at the North or South Pole. Then, and only then, is our view the same as for the telescope.
We'll work our way to that point of view, in stages. First, let's look at things as seen when we're just below the poles. To imagine this at our real locations, we just mentally 'tilt' ourselves so that we're almost in alignment with the Celestial Equator. Then, let's look at the entire Celestial Equator as seen from out in space.
We'll be looking at all of these views from the same direction. If we're in the Northern Hemisphere, we'll be looking South, and for our Southern Hemisphere friends, they'll be looking North.
Out first example will be a representation of the center image in Fig. 1.
In Fig. 2, we not only see the entire Celestial Equator, but we also see the Equator for a scope that is perfectly polar aligned. The Celestial Equator is represented by the red 'ring' and our scope Equator by the blue 'ring'. The two Equators are perfectly aligned and therefore, completely parallel with each other. (The Earth is but a tiny speck in the center of the two rings.)
Of course, this view would be impossible in reality, because both of the Equators extend on to infinity, but for this example, we'll limit their size. We're merely trying to get ourselves oriented so that we're eventually looking in the proper direction, and at the proper angle.
By using this point of view, we can consider the inner ring to be able to pivot around two points that are exactly in front of and behind us (Exactly on our meridian). The view behind us, of course, would be out of sight, since it would be hidden by the Earth behind us. Besides, the only part of the view we're interested in using, is the view in front of us. Again, for Northern Hemisphere viewers, that would be South, and in the Southern Hemisphere, the direction of the view is to the North.
If we can relate the image in Fig. 2 to the center image in Fig. 1, then the following three images should easily be related to all of their respective images in Fig. 1.
In Fig. 3, it is easy to see that in the left-most image, the Pole of our telescope is tilted toward the Left, or in other words, it fits the description when drift aligning, of saying that "the Northern pole of the scope is pointing East of the Northern Celestial Pole" if the alignment was taking place in the Northern Hemisphere Similarly, in the right-most image, the way that this condition is described if we were doing a drift align, would be to simply say that "the Northern pole of the scope is pointing West of the Northern Celestial Pole." Again, this would only be true as seen from the Northern Hemisphere. Naturally, in the Southern Hemisphere, the conditions would be exactly the opposite. Those are pretty obvious conditions when things are viewed in this fashion, aren't they?
Let's take the next step in making the transition from the conventional view of Polar alignment, to the more simple one that we want to use with our 'goto' discussion.
In Fig. 4, we have the three views just described, but to their right, we've mentally 'tilted' ourselves backward even more at our observatory, so that we're looking directly at the Celestial Equator. Rather than standing up, we've just decided to recline in a nice reclining chair to analyze the heavens, and we just happen to select the perfect angle for our chair, so that we see the Celestial Equator right in front of us. (Might as well relax while we learn!)
In the right hand views, which are now directly in front of us, the Celestial Equator and the scopes equator appear as straight, two dimensional lines. We can now VERY easily see the direction of tilt, if any, that our scope may have. No curves necessary here!
Finally, in Fig. 5, we've gotten to the view that we've been working toward. The drawings on the right side of figure 4 are now on the left side of figure 5, and to their right, a simple, straight line can now represent the Celestial Equator as well as the Scope's equator, and do so in a valid way. This is the view that the scope 'sees'!
NOW, we're ready to see the effects on the 'goto' operation when the scope isn't properly polar aligned in an East/West fashion.
Freeze Frame!!
You've made the transition from the 'old' way of looking at our Celestial Equator to the 'new' way, so it's the ideal time to jump in and have you make the first real test for an alignment error! Using Fig. 6 as a guide, select a star (Star #1) that's near the Celestial Equator and also near your Meridian. Sync on it. Now, do a 'goto' to Star #3, several sidereal hours to your right. Disregarding EVERYTHING ELSE, notice how far off to the left or right you ended up. Next, do a 'goto' to star #1 and again, see how far off to the left or right you are.
If you're off in the same direction, by about the same amount, then your R.A. drive gear is pretty well centered on its pivot point and you're okay. If you UNDERSHOOT in one direction and OVERSHOOT in the other, your R.A. gear is traveling in an out-of-round manner, and you need to correct this before going on.
If you have this condition, go to the following link and read more about the cause and cure of the problem, then return here for the rest of the discussion.
Improving `Go To' accuracy on the LX200
Continuing on...
Now that I've interrupted the flow of things, I'm repeating the steps described earlier. We begin the test by selecting a star near our meridian and also near the Celestial Equator, just as we would for doing a drift align. In Fig. 6 that is represented by "star" #1. Having centered it in the crosshairs of the scope and done a 'sync' on it, we then locate a star that's Left of us by several hours in Sidereal time, and do a 'goto' it. (Star #2). We go back to Star #1, which will (should!) come back to our center of view, and do a 'goto' to a star that's to the Right of us by several hours in Sidereal time.
If we had no polar alignment problems, we'd end up with our crosshairs at the same height as the target star in both 'goto' operations. However, we're assuming that we are off in Polar alignment, so let's see what we can expect if the pole of the scope is Left of the Celestial pole:
Just as in the top example in Fig. 4, the 'equator' of the scope is also tilted to the Left, along with the Axis of the scope. This means that when we did a 'goto' to the Right, the scope ends up pointing ABOVE the target star, and when we do our Left 'goto' we end up with the scope pointing BELOW our target star.
If instead, the axis of the scope was tilted to the Right, which would mean that our scopes pole is to the RIGHT of the Celestial pole, we get the opposite condition, as seen in the above drawing, where the scope ends up ABOVE the star when going to the Left, and BELOW it when going to the Right!
If you find that this condition exists, then you know that you must polar align more accurately. Furthermore, by looking at the direction of tilt, you can determine in which direction you must adjust the scope to correct for the mis-alignment.
This is a big aid in determining what's wrong with your 'goto' operations, and it's QUICK!
You would also find that in all of these cases, your 'goto' distance would be somewhat SHORT in the East/West distance traveled, but it would be much less pronounced than the North/South error. We'll discuss that part shortly.
That explains what we'd see if our polar alignment is off to the East or West, but what about when it's ABOVE or BELOW the Celestial Pole??
As shown, we can visualize the scopes equator as simply having been 'tilted' toward or away from our point of view when the scopes axis is either 'above' or 'below' the Celestial Pole. On the left, the scope is tilted so its pole is BELOW the Celestial pole. In the middle, all is properly aligned, and on the right, it's tilted ABOVE the Celestial pole. This tilt pivots around two points that are directly East and West of our location, or perpendicular to the pivots used in the previous example.
Shifting these views to where we're looking at these tilts from directly in line with the Celestial Equator should be easy now, since we've already done that in our previous example. We've even chopped off the portion of the image that's behind us, since all we really want to see is the portion that the scope will follow in our actual line of sight.
Let's immediately transform our drawing into simple lines, where we can more easily visualize the track of the scope as it is moved Left to Right. With this type of view, it still might be a bit difficult to visualize what the movement of the scope would be if it had been centered on a star on the Celestial Equator, then was moved Left or Right. But keep in mind, this is only a representation of the EQUATOR of our scope. It isn't necessarily where the scope is pointing.
Fig. 12 above, shows us the path of the scope equator, as well as that of what the scope would follow for the two conditions, if it had been aimed at a star on the Celestial Equator and then moved Left and Right.
Above, we see three stars along the Celestial Equator. One is at our Sidereal time, or close to it, and the other two are separated by several Sidereal hours Left and Right. After having done a 'sync' on the center one, then doing a 'goto' to the other two, we see that when the pole of the scope is pointing BELOW the Celestial pole, the scope will end up pointing BELOW both the Left and Right target stars. When, however, the scope is pointing ABOVE the Celestial pole, the scope will end up pointing ABOVE both of the target stars.
If we find that either of these conditions exist, we not only know what's causing the problem, but in which direction to move the Altitude adjustment to correct for it. Nice and simple.
A Loose End
I said earlier that if we had our polar alignment off in such a way that we would end up either higher or lower than the star, we'd also find that we ended up seeking somewhat short in our 'goto'. Let's see why.
When we do the 'goto', the scope doesn't 'know' that it's mis-aligned, so it will move the 'proper' distance in R.A. to reach the target star. Now, that 'proper' distance would also be the distance of the radius of some circle. In the left drawing, we see that the distance to the target star lands exactly on the edge of this circle. If, however, we actually move 'up' or 'down' in getting there, we still will move that same R.A. distance. Again, in the left image, it shows that if we moved higher than ideal, we'd still fall on the edge of that same circle.
In the right drawing, it shows that where we actually ended up, was not only higher, but when we draw a line from the final spot where we stopped, down to the height where the star actually was, it will fall short in its movement. In this example, we did a 'goto' to the Left, so we ended up slightly Right of the star, as well as above it. Had the 'goto' been to the Right, we'd have ended up slightly Left of the star. This would be true whether we ended up above or below the star. Expect this to happen in these cases. It's normal.
Continuing....
Is it possible for us to have both 'tilts' off at once, as above? Of course. In fact, it's more than likely that they will both be off if one is. The significant thing to remember is how to tell which is off and in which direction, even when it's a combination of the two. For the above example, we'd find that the combination of the two might look something like this:
The East/West tilt is trying to make the scope end up ABOVE the star on the left, and BELOW the star on the right. At the same time, the North/South tilt of the scope is making the scope be pointing below the celestial pole, which is in turn, trying to make the scope be BELOW the star on the left and BELOW the star on the right.
Hmmm. Since the scope is being oriented both ABOVE and BELOW the target star on the left, it turns out that in this case, the scope pretty much ends up ON the star on the left.
Since in both cases, the tilt of the scope is trying to make the scope end up BELOW the star on the right, it ends up VERY below the star on the right!
This example shows us the perfect reason that you need to check your 'goto' in both directions. If you had the above condition and only tested by going to the left, you'd REALLY be puzzled later, when your accuracy was so far off!
The 'tilts' in our examples have been pretty extreme. Hopefully, however, by going to this extreme, the principles we've been covering has been made more clear than if we had used more subtle examples. It's not too likely that your scope setup would be this far out of alignment without you realizing it. (Or WOULD you???? )
Moving from Left/Right 'goto's to Up/Down 'goto's.
For the following tests, we'll find that our 'goto's will only change 90 degrees, so that we're doing 'Up/Down' 'goto's. But an interesting thing will soon become apparent as we move into this area. We're going to find that the error patterns we seen so far with our Polar alignment problems, will have the same 'shape' as the two following mis-alignments. That is, you'll see one that creates a straight, but angled result, and another that creates a curved, but equally offset result. But we're getting ahead of ourselves. Let's see what all of this means.
Mis-Aligned Forks
We've been pretty lucky so far, when it came to adjustments that were off. It didn't take a lot of effort to correct things yet. Unfortunately, the next check you can make, isn't so easily corrected if it's off significantly. We can easily check for mis-aligned forks with a slight modification of our testing procedure, but correcting the problem is another matter!
The first thing you'd want to know about forks that are misaligned. is that this condition will have NO effect whatsoever on doing a 'goto' to the East or West. (Bet you didn't know that, did you?) This nasty condition will only show up when you make changes in Declination. However, the way it shows up is by making the scope actually be off in R.A. as the Dec is changed. Let's see why.
We've saved ourselves some time here, by jumping ahead to where we're seeing that each of the fork arms is perfectly perpendicular to the Celestial Equator, but the axis of the scope is NOT parallel! In short, the forks are not aligned. Since the forks aren't properly aligned, we see that the OTA in this case, will begin to point more and more to the right as we aim it higher, and more to our left as we aim it lower. The OTA, remember, is pivoting around the 'rod' that runs between the two forks.
If we had aligned the OTA with a star directly in front of us .... on the Celestial Equator, then did a 'goto' to the target star directly above it, we'd end up with the OTA pointing off to the right of the target star. Likewise, when we do a 'goto' to a target star below the Celestial Equator, we'd be aiming to the left of the lower target star. The further we went in either direction, the worse we'd be off to the left or right. (R. A.). The fact is, the situation really IS worse than it looks.
I've taken the OTA off of the unit so we can see the star located on the Celestial Equator that we've done a 'sync' on. But the significant point here is, as one goes away from the Celestial Equator in either direction, the PHYSICAL distance between the lines of R.A. tend to converge, as we know. This means that the further we go away from the Celestial Equator, there is an even greater R.A. error than if the lines of R.A. were parallel.
The main point here, and well worth repeating, is that when the forks are mis-aligned, as you go further away from the Celestial Equator toward the Celestial Pole, if the forks are misaligned such that the scope is tilting to the 'right' the further the scope will be off to the right. The further you go away from the Celestial Equator in the opposite direction, the further the OTA will be off to the left.
Likewise, of course, if the forks are off in such a way as to make the OTA be pointing to the left, the opposite conditions occur. This makes it very easy and quick, to check and see if your forks are indeed mis-aligned, and even by how much.
Mis-Aligned OTA
In the view above, we're looking directly down on a scope that has its forks perfectly aligned, and the axis of the forks is pointed directly South (or North, for the Southern Hemisphere). The OTA axis, however, is obviously mis-aligned and is pointing significantly to our left of the proper axis.
Now, we wouldn't notice this misalignment by just looking through the eyepiece, because if our object appeared to be off to our right, we'd simply rotate the forks and OTA until we were pointing at the desired object, oblivious to the alignment problem.
We could, in fact, make our Polar alignment check and never know that we had any problem with the OTA alignment. Like the fork alignment check above, this misalignment would only show up when doing a 'goto' in Declination. We'd actually have to be moving North/South before the problem would appear, and also like the fork alignment situation, this would only show up as a problem in our Left/Right (R.A.) position. In fact, if a person was a bit careless in their checks, they'd very possibly mistake the OTA mis-alignment for fork mis-alignment. Let's see why.
If we back up in time a little, we see in Fig. 20, a case where everything is fine. The alignment of the OTA (the red circle) to the horizontal and vertical axis of the scope (the small blue circle) is perfect. No matter which R.A. time line we follow from the Celestial Equator, up toward our Celestial pole, the two axes stay in step. Even though the 'physical' distance between these lines of R.A. changes as we increase our Declination, the OTA follows the scope axis very nicely.
There is one very important point to emphasize here, before continuing. No matter where we aim the scope, no matter whether it's East, South, North, or West, the DISTANCE between the red OTA circle and the blue mechanical center circle, will NEVER change! Let's clarify that, because it's the very crux of what all of this is about:
In our simple example, we could express the distance between the center of where the OTA is pointing and the center of the mechanical axis, by saying that it is equivalent to 1 inch, when viewed at arms length. This is true no matter where in the sky we aim the scope. The two will ALWAYS remain spaced by that amount.
Fig. 21, we see this condition, where the OTA is off to the LEFT of the scope axis. In that case, it will be off by to the left by the same amount no matter where we aim the scope. For instance, in this case, we planned on syncing on the star in the lower left circle and then doing a 'goto' to the star above it on at same R.A. We didn't realize it at the time, but in order to get the star in the center of our view, we actually had to aim the axis of the scope a bit to the RIGHT of the lower star in order to center the star in our OTA.
In the previous example, where everything was properly aligned, there was no problem. We'd just move strictly in Declination and we'd end up at our target star. But here, we're not really aimed where we seem to be. When we move up in Declination, the scope will follow the R.A. line that the scope axis is on, and not the one that the star is on! Now, since the OTA is off to the left of the scope axis by the same amount always, as we go up, the OTA will, in fact, be off to the LEFT of our target star, and the further up we go, the further off it will be.
At first blush, this would appear to be exactly what we would see if our forks were mis-aligned, with the left fork being lower than the right one. Actually, it is! So how do we tell the difference between the two conditions? We do so by not only going from the Celestial Equator and UP, but also from the Celestial Equator and DOWN!
If we look closely, we can see where the difference between the two mis-alignments will be. We saw that when the forks were mis-aligned, as we moved from the alignment star at the Celestial Equator, UP, the alignment would get worse and worse to the left, for instance, if the forks were mis-aligned accordingly. Then, as we went DOWN from the Celestial Equator, the scope would be off in the opposite direction, more and more, as we moved down.
In the case of the mis-aligned OTA, the condition is different. If the OTA is mis-aligned to the left, then the higher we go, the further the OTA is off to the left.... just as with the fork mis-alignment. But here, when we go DOWN from the Celestial Equator, the OTA again begins to go off to the LEFT again, and get worse to the LEFT, the more we go DOWN.
In Fig. 22, the example on the left shows us how the mis-alignment of the OTA would appear, compared to the proper R.A. time, if the OTA was off to the left of the scope axis. In the example on the right shows us a condition where the OTA is mis-aligned to the right. The OTA will be off to the right more and more to the right, as we go either UP or DOWN from the Celestial Equator.
THIS is how a person can tell the difference between the two mis-alignments. It's almost exactly like the way a person can tell which way the Polar alignment is off, by which way the OTA is off at the East and West target stars. (I even had the two examples follow different R.A. lines up and down, to show that it just doesn't matter; the OTA will be off in the same fashion, no matter which R.A. lines you follow.)
Aligning the OTA During the Test
If you wish, you can use the previous information for adjusting the OTA while it's on your pier or tripod. If so, here's a quick view of the block where the adjustment is done on the LX200.
Although there are better images of this item in the MAPUG archives, this drawing may serve the purpose (and maybe not!) The gray 'block' shown, is bolted to the OTA (The blue and black part) via the three 'Bolts'. shown. To be accurate, they're Hex head bolts. The bolts are in slots that are cut into the block, so when the bolts are loosened, the OTA can move forward and backward within the limits that the bolt heads in their slots will allow.
The green circle is the pivot pin on which this assembly rotates, within a bearing on one of the fork arms. There is one of these assemblies on each side of the OTA; one for each fork.
If a person wishes to move the OTA to the left, toward our point of view, all they need do is to loosen the three bolts and pull the front of the OTA to the right, as we view it. The OTA, along with the three bolts, will slide to the right of our view, which is to the REAR of the scope.
This would be a pretty inaccurate way of doing it, however, since we would have little control over how far we moved the OTA on the fork. To assist with that, there are two pre-drilled holes on the rear of the block that will accept 4-40 screws, as shown.
What a person would do, if he wished to move the OTA to the rear on the left side, is to screw two 4-40 screws into the threaded holes and run them right up against the two bolt heads that they would touch. Then, the person would back the screws out, say 1/4 turn, to leave a small gap between the bolt heads and the ends of the screws. Now, when they pull the OTA back, the OTA will only move until it's stopped by the screws, which is a small amount. Don't be surprised to find that it takes a pretty healthy pull to get the OTA to move, but when it does, you'll hear it. There'll be a slight 'clunk' sound as it moves to the screw ends.
Again, this is assuming that a person wishes to move the LEFT side of the OTA to the rear of the scope, which in our view, means moving the OTA from our left to our right. Then they tighten up the bolts and see if they've adjusted enough.
On the other hand, if a person wishes to move the left side of the OTA toward the FRONT of the scope, they could loosen the bolts slightly, bring the screws up to the bolt heads, then turn the screws in, say, 1/4 turn, which would push the OTA toward the front of the scope. Then they would back the screws out slightly and tighten the bolts.
Actually, this is a very good way of adjusting the OTA to the forks, because you can see in the real world, the effect you've had. You can first determine which way to move the OTA with the previous measurement, loosen the screws and make the adjustment while the scope is still pointed to the sky, then repeat the test. All of this actually takes less than a minute to move one side of the scope by a small amount. By repeating the measurement, then making small adjustments, then repeating the measurement, you can adjust for the true optimum setting for your scope. (Trust me; it works!)
Mixing the Adjustments
If your fork arms are orthogonal, or they're not off far enough to worry about, you don't need this little tidbit. However, if they aren't all that good, then this information may well be beneficial.
You can see by the graphs, that the patterns for non-orthogonal forks and a mis-aligned OTA are very similar to the graphs for the two charts that represent the adjustments for Polar alignment. As I said earlier, they look pretty much the same, except they're rotated 90 degrees.
Unfortunately, however, correcting for the non-orthogonal forks isn't as easy as adjusting for when your scope pole is Left or Right of the Celestial Pole. In fact, it requires that you disassemble your scope to some degree to fix this fork problem.
Fortunately, if you're willing to give up some 'goto' accuracy in one of the hemispheres, it's possible to compensate for the mechanical error, to some degree, with your OTA alignment. If you look closely at the graphs for both of the adjustments, you can see that when they're both out of adjustment ..... depending on which way they're out.. in one Hemisphere the two curves can tend to cancel the effects of each other, and in the other Hemisphere, they tend to aid each other in their error. Now, if you are in the Northern Hemisphere, for instance, and most of your observing is above the Celestial Equator, you can adjust the OTA mis-alignment to try to cancel the fork effects in the Northern Hemisphere. Likewise if you're in the Southern Hemisphere and most of your viewing is in the Southern Hemisphere.
Now, the down side to this is, your 'goto' accuracy will be much worse in the other Hemisphere! But perhaps it's worth it, so as to not have to do some serious surgery on your scope.
If this is what you want to try, then keep in mind that there is no certain position in which you can put the scope for adjusting the OTA to the desired offset. You either have to do it by trial and error, or you have to do it while you're looking at the tests above. It's still trial and error, but just as with any of the other adjustments, you can make a small change, see if it's in the correct direction or not, and slowly adjust until the effects of the non-orthogonal forks are minimized. It's worth considering, at least.
More Precision
Now you've seen all of the 'normal' 'goto' problems. Hopefully, by looking at them from a different perspective, I've made their cause more easily recognizable. However, you just might want to get better accuracy in measuring the amounts of offset. Of course, you could always use a higher power eyepiece. That certainly would help. But either way, there's another simple way of actually measuring the various error offsets that you've been seeing. Here's how you can do it.
1. When doing the 'goto' to a star after having done a 'sync' on the previous star, move the display on your hand control to where it displays 'RA and Dec". Once you've stopped at the target star, write down the coordinates of what the display is showing. (These coordinates are the true coordinates of the target star.)
2. While still in that display mode, using the up, down, left, and right buttons, move the scope until the target star is dead-center on your crosshairs. Write down the coordinates as they show at this time.
That's all there is to it! By comparing the difference between the two readings, you now have a very accurate measurement of how far off in any direction your scope was. If, for instance, you had to move the scope up by two arc minutes in Declination, then obviously, you ended up two arc minutes below where it actually was. No big deal.
This concludes our discussion on testing for 'goto' problems, but stick around. There's more that can be gained from your newly found knowledge!
The 2-star/3-Star Polar Alignment Technique
If you think about it, you'll realize that we've actually gathered enough information to be able to develop a new technique for Polar aligning! This technique compares favorably in accuracy to 'the drift method' , yet takes much less time. This is assuming that there aren't any other problems with your scope other than the adjustments we've been discussing. In fact, in my opinion, this method can be superior to drift alignment in most cases.
Now, let's be clear on this point: if you've established that your scope is within the published specs for doing a 'goto' then this new technique can be used in the future. Don't try to use it for making the Polar alignment adjustments the first time through. After all, until you've actually done your Polar alignment by independent methods, you won't really know if your 'goto's are within the published specs. For best results during the previous tests, use the drift method to Polar aligning the first time through. THEN, if you like, you can go back and 'tweak' your Polar alignment with this method.
The first step for your new Polar alignment technique, would be to do what you did for checking Polar mis-alignment at the beginning of this discussion. Locate a star near the Celestial Equator, sync on it, then do a 'goto' to a star to your right by several hours, then to a star to your left by a similar number of hours. While at the East and West stars, notice how far from centered... in DEC ONLY ... you are, and in which direction you're off for each star. (Remember; if the two are off from each other, your scope is tilting left or right of the Celestial Pole.) Make a small adjustment of the wedge in the proper direction, then repeat the process. After several iterations, the two stars, both East and West, should be nearly exactly the same amount off, if at all.
Now, go back to the first star you used.. the one near your Meridian... and center it. Do a 'goto' to either of the other two target stars. Notice whether or not you're below or above center. (You should be off by the same amount, if any, for both the East and West stars, so either star will do.)
Adjust your wedge for being either above or below the Celestial pole, according to what your indications show to be incorrect. Just a little bit of adjustment at first. Then repeat. Repeat until both the star you did a 'sync' on and the one that you used as a target star, are both at the same height.
Repeat the entire process, right from the beginning, since the two adjustments can interact with each other. After several iterations, you should be extremely well polar aligned.
Let me tell you what I did, the first time I decided to use this method for polar aligning, after having figured out that this could be a good method for polar alignment. Here, I actually shortened the sequence even more.
The '2-star' Part of the Alignment
First, after having made all of the checks described earlier, I skipped right past using a star near my Meridian and instead picked a target star several hours to my right (West, for me) relatively near the Celestial Equator. I did a 'goto' to it, then centered it in the display, and did a 'sync' on it.
Using the star chart in the back of the Meade LX200 manual, I found another star that was fairly close to having the same Declination as my reference star (within 15 degrees) and was approximately three hours to my left (East). I did a 'goto' to it and could see that the scope was significantly below the star, which meant the scope axis was to the left of the North Celestial pole. While watching the star, I adjusted the wedge accordingly, then slewed back to my reference star in the West and again centered it and did a 'sync' on it.
I repeated this operation three more times. By then, both stars measured as being at the same Declination. Total time for the first adjustment: Approximately 4 minutes.
The '3-star' Part of the Alignment
Next, I located a star that WAS near my Meridian and relatively close to the Celestial Equator, then did a 'goto' to it, centered it and 'synced' on it. I then did a 'goto' to the star to my left (East, for me) and looked to see if the scope was off in Declination, compared to the target star. It was below the target star, which meant that my scope axis was below the Celestial pole. I made a small adjustment to raise the pole of the scope, then went back to the original star and again did a sync on it.
Again I went back to the East star and again I slightly adjusted the altitude adjustment. Total number of repeats: 2. Total time for this loop, approximately 3minutes.
After that, I repeated the entire operation, since the two adjustments can interact with each other. This time I did an very small 'tweak' of the adjustment and went to the second step again.
I didn't have to repeat the loop a third time. Total time, approximately 10 - 12 minutes.
Finally, I aimed at a star on the Celestial Equator and centered it, then left the scope for 30 minutes. I returned and found there was no discernible movement of the star to the crosshairs. I repeated this for a star about 30 degrees above East horizon for 30 minutes and again, found no detectable movement of the star in Declination.
You can try it for yourself and you can decide which way you prefer.
This pretty much covers both of our major subjects unless there are any questions. You there. Near the back. You have a question? In that case, lead off the question and answer session.
Q. and A. Time
Q. Is it really necessary to select a star near your meridian as the central star for the alignment tests?
A. No, not really. But by using your Meridian as a central point, it gives you more space to select stars to the Left and Right that are several hours away. If you are too far in either direction when selecting the central star, it might limit your distance to the star choices. The further apart they are, the better. The more accurate your tests can be. This way, you could select stars that are four hours away, which makes the testing and adjusting more accurate.
Q. What about the target stars? Must they actually be on the Celestial Equator? Must they all be truly in line with each other?
A. No, but don't vary a large amount from any given Declination when checking the East/West alignments. I'd recommend staying within 25 degrees maximum variance between their Declination.
There's another alternative as well. You may find one set of stars where one is nearly at the same Declination as one to the East, but there's none to the West that's close to being in alignment with them. In that case, you might use those two for checking the East settings. Then, you may find another set that's higher or lower in Declination, and are fairly close to being in alignment from roughly the center, to the West. Use that set for the West settings.
If all else fails, then let the stars be more spread out in Declination. You'll find that after getting used to using these techniques, you can use a three star set that has significant Declination differences between all three stars. I just want a person to begin the measurements with a set that is close, so as to have the person feel comfortable with the technique.
Q. Is this also true for the Up/Down target stars?
A. Yes. If necessary, you can use one set that is to your Left, for instance for the Center/Up measurements, and another set that's off to your Right for the Center/Down measurements. Or vice-versa.
All in all, the selection of the reference star combinations isn't nearly as critical as I make them sound in the discussion. Again, I'd rather suggest being too critical than too loose, in the star selections, when one first begins to make these measurements.
Since we're using the stars listed in the Meade manual, our choice of combinations isn't all that good anyway. Still, if we try to pick from the stars that are there, we'll do quite well, even though many combinations will be significantly far from what we'd call ideal. Don't worry about it. As I said in the previous question, you'll be surprised as to just how far these stars can be out of alignment and still give you exactly the information you're looking for.
The most important thing is to remember to concentrate on one adjustment at a time!
Q. When I begin, my scope ends up so far from my target stars that I can't see them, even in a low power eyepiece. What do I do?
A. Use your finder scope if necessary, to get to your stars. As your adjustments get more and more accurate, you'll soon be able to get them into view.
Q. Why do you say that the '2-star/3-star' alignment method is potentially more accurate than the 'drift alignment' method?
A. First, we have to consider how long a person will actually spend in doing a drift alignment. Also, just how accurate is 'accurate enough' for them?
Let's assume that your scope is within the specs that the manufacturer says it is for the 'goto'. For the LX200, that's within 2 arc minutes. For an 8" F10 LX200, that means that after the 'goto', the star will be within .047 inches of being on target.
Let's also assume that a person does their 'goto' between the Left and Right stars that are three hours in R.A. on each side of their Meridian. By comparison, suppose another person was checking the amount of drift every 10 minutes between their adjustments. To be as accurate as the 2-star/3-star alignment, they would get a drift in Declination of .0013 inches! Could/would they ever see that amount of drift? I seriously doubt it.
But that person could also watch for drift after corrections, every 15 minutes, so that they'd be even more accurate! Yes, and the proponent of the 2-star/3-star method could increase the 'goto' distance to 4 hours each side of their Meridian, yet add no significant extra time to the effort. That's about as close to the horizon as one would want to go.
In that case, their accuracy would be the equivalent of having a 15 minute Declination drift of .00145 inches! If your
'goto' accuracy, after having adjusted everything, is significantly better than two arc minutes, then this technique can blow
right on past drift alignment in accuracy!
The only way for a person to come to their own conclusion is to try both methods, and whichever way best fills their needs,
use it. If you wish to try the drift alignment method, although there are many good descriptions on the 'net' of how
to do it, I personally recommend you take the link to the following one......... since I wrote it.
In closing .....
It took me quite some time to mentally gyrate things in my head until I could come up with a way that I believe makes the 'goto' operations become logical and relatively simple to understand. As an offshoot to these mental images, the '2-star/3-star' method of Polar aligning came into being.
I hope that this slightly different way of looking at the Cosmos has helped you in more easily understanding what might otherwise be some very complex subject matter.
Give the tests a decent try, and whatever else happens ....................