Fork/Tube Alignment


Intro:
This page describes a method for testing and correcting the alignment of the fork arms and optical tube.  For setting circles (standard or digital) to read properly, and for many methods of polar alignment to give an accurate alignment, the RA and declination axes of the scope need to be perpendicular to each other.  In the case of a fork-mounted scope, correcting this involves adjusting the physical placement of the fork arms, and of the optical tube between them.

Axes alignment is a very badly overlooked issue and is not even mentioned in the owner's manual.  Meade certainly doesn't check the accuracy very carefully at the factory.  Errors of 1/4 to 1/2 degree are common on new scopes, which translates immediately into similar sized errors in the setting circle readings, which has caused many beginners (myself included) to think that it is nearly impossible to find anything using the circles.

A basic testing method is simply to move the scope in RA and dec and compare the results with a star chart to see if the motions are perpendicular.  The most popular web-based instructions for testing is from an LX200 page.  Since those scopes are frequently used in alt-az mode (they are sold without a wedge- the computer translates alt-az coordinates to equatorial), making sense of the results with equatorially marked star-charts would be very difficult, so the method described on that page involves putting marked strips of paper horizontally and vertically on a wall and checking whether the scope's aim follows the marks on the strips as the scope is rotated on its axes.  That method requires a considerable effort to mark the strips accurately, and is only as accurate as the positioning of the marks.  Since the LX10 is equatorially mounted on its wedge, we can just use the stars directly.

Getting Started:
To start, you'll need a good star atlas or star charting software.  Also, make sure the scope is carefully collimated.

The next step is to accurately polar align the scope.  Use either a carefully applied drift method or the high-accuracy version of my polar alignment method (where you align through the main scope, not the finder).  Note when using this method, when you rotate the scope in RA with the dec at 90 degrees and look for the fixed center of rotation, that point is the polar axis of the mount (ie, the RA axis), so that is the point that should be aimed at the pole, not the center of the field of view.  Here's a deep chart of the polar region that might be useful.

Possible Errors:
Before describing the details of the testing method, we need to understand the possible errors in the mount.  There are two things to look at- the fork arms, and the tube.  Imagine setting the scope upright on a table in alt-az mode (ie, without the wedge), and with the forks on the east and west sides.  One possible error is if one or both of the forks is tilted a bit north or south.  If they lean by the same amount (either in the same direction or opposite), the angle between the axes is not affected.  But if they lean by different amounts, the top of one may be higher than the other.  This may also happen if one fork is simply mounted higher than the other.  In either case, when you move the scope in dec (when equatorially mounted, ie, on the wedge), the motion would not be precisely along a line of declination.  Imagine going from +30 to -30 degrees dec with the RA fixed.  If the fork arm tops are not at the same height relative to the base, the line of motion would be tilted a bit relative to actual lines of dec, and would not be perpendicular to the celestial equator.  This will be the first error considered in the testing process.

Technically, we could also worry about the fork arms being tilted east or west, but this wouldn't cause a significant alignment  problem unless the error was substantial.  Given the way the fork arms are bolted to the base, this is very unlikely unless:
1) One of the forks is bent
2) There's something caught between the base and a fork arm.  See below for how to loosen the arm to clear it.
3) There was a major manufacturing error.

Finally, we look at the way the tube is mounted between the forks.  Between each fork arm top and the tube is a plate bolted to the tube with a short axle perpedicular to the tube that goes through a hole in the top of the arm.  The only significant error that occurs here is if one plate is mounted slightly further along the tube than the other.  Then the tube is skewed relative to the forks.  When the scope is at 90 degrees dec, the tube will not be parallel with the mount's polar axis.  This tube alignment problem will be the second error considered.

Note that a severe misalignment in any of these cases may case the dec bearings to bind.  You can check by loosening the dec clamp and moving the scope in dec.  It should move with only light resistance (when doing this, make sure the dec clamp is properly adjusted so that it is not too tight).

Testing Fork Height:
We will now test for the case of one fork arm being higher than the other.

With the scope accurately polar aligned, if the scope is rotated in RA with the dec fixed, the scope should move precisely along a line of RA.  But if we move the scope in dec with the RA fixed, that motion may not be accurately along a line of dec.  To test this, rotate the scope in RA so that the scope is on the meridian (the line overhead from north to south), and set the scope to +30 degrees dec.  Check the finderscope and find a star near this point that you can locate on your star chart.  Aim the scope at this star, and then calibrate the RA setting circle on that star.  Now swing the scope down to -30 degrees dec and find another star near there that is on your chart.  Again aim the scope at that star.  Now check the RA reading on the setting circle, and compare it to what your chart says for that star.  Note the difference between the numbers, and convert to degrees (one minute of RA = 15 arcminutes = 1/4 degree).  Since +30 - (-30) = 60 degrees difference in dec, and since 60 degrees is about 1 radian, if we treat this part of the sky as very approximately flat (ie, ignore the problems of dealing with a sphere), the error in the reading equals the angular error in fork arm height.  Since the forks are about 10 inches apart, we can convert angular error into height error as follows:  one degree =(approx) 1/60 of a radian = (10 inches)/60 = 1/6 of an inch.  Your error will probably be less, so use the conversion 1 min RA = 15 arcmins = 1/4 degree = (1/4)X(1/6) inch =approx 1mm.  Figuring out which arm is higher than the other is a good excercise in dealing with celestial coordinates and 3-D geometry.  Enjoy :)

Correcting Fork Height Error:
To fix the error, leave the scope mounted on the wedge and tripod, swing the tube down between the forks (dec= -90) and rotate the scope in RA until the fork that needs to be raised is on the south side of the scope.  Loosen (not all the way!) the two bolts (see picture) holding the bottom of this fork to the base, and raise that fork arm, and then retighten the bolts.  Try to keep this arm parallel to the other one.  If you can't raise it enough (the range of movement is rather limited), retighten the bolts, and try to lower the north fork arm.  This adjustment is unfortunately rather crude, so when you're done, retest for fork height error (recheck your polar alignment first, as it is easy to bump the mount in this proceedure), and repeat if necessary.


These two bolts hold the fork arm to the base.
You may need to remove the dec motor if you have one.

Testing Tube Alignment:
This test is easy if you're already familiar with my polar alignment method.  (If you're not, read at least this step of that method.)  Aim the scope to 90 degrees dec, and rotate the scope in RA while looking through the eyepiece.  Ideally, the stars in the field of view should rotate around the center of the field of view.  If not, fine tune the dec until the center of rotation of the starfield is as close as possible to the center of the field of view.  Any remaining error is directly due to the misalignment of the optical tube between the forks.  The angular displacement between the center of rotation of the starfield and the center of the field of view is the same as the angular displacement of one mounting plate relative to the other.  The plates are nearly as far apart as the fork arms, so the approximation used above holds:  1 degree of error requires 1/6 of an inch correction in the placement of the plates.  Your error will probably be smaller, so use 1/4 degree =(approx) 1 mm again.

Correcting Tube Alignment:
The mounting plates are each held on by three screws (see picture).  Loosen these screws and try to slide the plate lengthwise along the tube.  There is only a small range of movement available (this wasn't really meant to be adjustable, but there is at least a bit of wiggle room).  The plates can be hard to move, so you may want to remove the scope from the wedge and tripod first.  The fork arms usually press towards each other a bit, which makes the plates even harder to move, so in a worst case scenario you could loosen or remove a fork arm, but of course that requires realigning the fork arm when you put it back on.


The tube mounting plate, with three screws,
between the fork arm top and the tube

As before, this is a crude adjustment, so once you retighten the screws, you'll want to retest and possibly repeat.

LX200 owners will notice that on their scopes two of the plate mounting holes are slots rather than round holes.  Meade apparently at least tries to adjust the tube at the factory.  They even have two small #4-40 threaded holes tapped between the back (eyepiece end of the scope) edge of the plate and the slots, so you can put a screw in the holes and thread it in to press against the heads of the screws to move the plate.  But on the one LX200 I tested, the arms were at different heights, and the tube had been adjusted to try to compensate.  That works only at the pole!
 
 

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