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Aligning the OTA to the Forks

MAPUG-Astronomy Topical Archive     AstroDesigns

By Bruce A. Johnston
Additional references at bottom of page

This discussion is meant for explaining the 'why' and 'how' of adjusting the OTA to the forks on an LX200. When we talk about the OTA being misaligned in this case, it assumes that the forks have been properly aligned, the scope is in Polar mode, and the scope is on a wedge. Since I haven't tested this procedure on a scope working in alt-az mode, I can't at this time, say what differences, if any, there would be to the following procedure. It's reasonable to assume that the same adjustment technique would be employed, but that a new two-star alignment would be necessary after each adjustment and iteration of the technique.

Image 1

Here, we have an assembly in which the forks are well aligned. Two of the more common methods of aligning the forks are in the MAPUG Topical Archives. One deals with using a laser and the other uses a precision rod running between the forks with the OTA removed.

Both methods have a procedure for aligning the OTA to the forks after the forks have been aligned. The OTA to fork adjustment is accomplished by loosening the adjusting screws on either side of the OTA and shifting that side forward or backward as needed.

The actual 'shifting' of the OTA is done by using two small 4-40 screws that you screw into the pivot block.  Then, you screw the small screws inward a very small amount, to shift the OTA.

The following procedure can be used as a double check, or for a final adjustment, since it uses the sky itself for determining when the OTA is truly properly aligned.

As the drawing shows, this OTA is extremely out of alignment and is pointing 'East' of the mechanical axis for those in the Northern Hemisphere. The scope is shown as being oriented  for viewing in a Southerly direction.

By using such an extreme misalignment example, it's much easier to see exactly what the effects of the misalignment will cause.

Image 2

The above is a drawing that represents several lines of longitude, or R.A.  If you prefer, think of them as the inside ribs of a domed observatory.  In both cases, the lines are well separated near the bottom, and because of the domed nature, the lines all converge to a single point.  The point, of course, is the Celestial North pole in this example, or the peak of the domed roof.

The central 'line' at this moment represents our view of a R.A. line at our meridian.  It would appear as a straight line to us, other than the fact that as it rises, it would also swing over our heads and on to the Celestial pole.  Another example of where I must use two dimensional lines to represent three dimensional space.

Assume for a moment that we were using a perfectly aligned 'scope, including the OTA adjustment. If that happens to be the case, then the OTA view, shown as the red circles, will be concentric with the mechanical center of axis of the OTA, shown as the small blue circle.  Wherever we aim the mechanical center of axis, the scope kindly points at the same spot in space.

Image 3

Here, we bring back our misaligned scope. The tube itself is still aimed at the meridian line. Since we know that the OTA is misaligned to the left, we then know that the mechanical axis of the scope must actually be pointed to the WEST, or RIGHT of where we're actually looking. (Apologies to our Southern Hemisphere neighbors.)

Okay, all well and good.  We're looking at some reference star , and the scope mechanical axis is aimed somewhat to the RIGHT of the star, in order to allow us to see it. 

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 equivelant 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.

Image 4

Here, we begin the action of moving the star in Dec.  We started when the OTA was pointing directly at our current R.A.  We must remember, however, that the movement of the scope will follow the MECHANICAL center and not the OPTICAL center. In short, it will follow the R.A. line that the blue spot rests on.

As we move North, we see that the mechanical center follows its line, which it must.  However, because the physical distance between the two circles never changes, The OTA will appear to move to the LEFT, or East.  Had there been a star directly above the reference star, it would not have shown up in the center of the field of view.

Likewise, when we move South in Dec, the OTA does not follow the Dec line that passes through the reference star, but in fact, 'moves' to the West.  The further South we go... down to the Celestial Equator at least... the more the OTA will aim wrongly to the West.

Likewise, the further North we move the scope in Dec, the further East will our view move.

Thus, if we had made this test on a scope and found the results as described, we'd know that the OTA is misaligned to the East, and we'd adjust it somewhat to the West and repeat the test.  Next time it would be more accurate, and finally, we'd find that the optical center OTA would follow the mechanical center exactly.

Of course, it isn't likely that we'll find several convenient stars, all at the same R.A. to use for the alignment test, but we can select stars that are close enough in time to give accurate results.  For instance, if the alignment test began at Altair, then slewing to Vega or Deneb would easily be close enough to show accurate results without many other factors joining in to confuse the issue. The best overall condition would be to begin with a reference star that is slightly North of the Celestial Equator, then slew to stars that are significantly North of this star.

I found, for example, that I could use the three stars listed, and after properly aligning the OTA to the forks, all three would show up exactly in the middle of my dual crosshair eyepiece reticle box, and Spica would be just slightly outside of the box, due to the off-center mounting of my R.A. worm gear.  I minimized that error by using the following link:

 <Improving `Go To' accuracy on the LX200>

When I was done, I was able to slew anywhere withn my available sky and have a MAXIMUM of one arc-minute of error!

To summarize, if the OTA is misaligned to the EAST, then as the scope is moved NORTH, the error will increase in an EASTERLY direction, and if it's misaligned to the WEST, then as the scope is moved NORTH, the error will increase in a WESTERLY direction.

A movement of the scope in R.A. only, will have no visible effect on the position of the stars.  This helps, because it means that you can do an accurate 'drift align' before adjusting the OTA.


Additional references describing Mark Simpson's theory behind his alignment procedure:
and Pointing Effects: <>

Separately, Removing the OTA from the Forks, on the Misc. topic page.