This is a procedure for achieving the theoretical pointing accuracy of an LX200 telescope. This procedure has been applied to over 40 LX200's that I've been informed about and improved the pointing to at least the specification quoted by Meade (2 arc-mins). 60% of people acquired 1 arc-minute accuracy, and 2 people achieved 30 arc-seconds.
Most of the other pointing procedures out there have one of 4 problems:
This procedure makes a few of premises for achieving alignment:
Align Through The Eyepiece
The best way of measuring the alignment of the scope is through the eyepiece. Where it is pointing to is the best measurement of the goto accuracy. Measuring with a ruler is not accurate enough (accuracy required is too high), lasers depend on true surfaces and perfectly accurate reflecting surfaces). Spirit (Carpenters) levels are not accurate enough at the tolerances required. In the end the only thing that matters is what the scope points at. By using this as the measurement we're solving the problem at the root cause, and not causing any inaccuracy by the inaccurate measurement of our problem.
Analyse The Variables
The Lx200 is a complicated device and analysis of the errors is the first thing to achieve. There is detailed analysis of the various problems and pointing effects to do with alignment in the Lx200Alignment Document. Basically there are only 2 things that effect pointing, the alignment of the OTA and the height of the fork arms.
Dependency Of Adjustment
The 2 alignment errors (OTA and fork), mutually interact with each other. To accurately fix one error, you have to negate the other. This procedure does this by altering the OTA alignment when the OTA is level and the fork height has no effect on where the OTA points. And when the OTA is aligned it is then safe to align the fork arms as the OTA now has no effect on this adjustment.
Most LX200's are shipped from Meade with a mechanical pointing accuracy of 1 arc-minute, but have the OTA tube out of alignment with the mount, or the fork arms incorrectly positioned. The tolerances for this alignment are very marginal indeed, e.g. the fork alignment for an error of only 4 arc minutes equates to a fork height inaccuracy of 0.47mm!
Please see the associated Pointing Effects document for the reasoning and formulas for these adjustments.
If this is your first time aligning your LX200, it's strongly suggested that you follow through this procedure (without making any adjustments), to familiarise yourself with the amount of error you have in alignment and how to adjust it out.
When you make these adjustments you are taking on responsibility for them yourself, I will not be held responsible for any damage caused either directly or indirectly.
Be careful, check your scope before and afterwards is not looking twisted as far as the forks are concerned as you don't want to cause additional bearing wear.
As a thanks for writing this procedure, please notify me of your experiences, accuracy improvements and comments. This will help me to produce a better document for everyone else.
Please check back to this site frequently for updates as this procedure is work in progress and will evolve according to user experiences and ideas for improvment.
Regards
Mark Simpson
http://www.AuroraChasers.com/Astro/Lx200AlignmentProc
First of all, check your scope is structurally sound, i.e. no parts are warped, broken, and the RA clutch/Dec clutch aren't slipping.
Check your current pointing ability. We need to check that the scope can return to a place accurately. I.E. When you return to the same place that the scope is accurate to about 1 arc-minute.
Calculate out the width in arc-minutes of your reticle eyepiece (see Meade Manual) so you can estimate your accuracy mentally.
Mount scope in Alt-Az, and put the scope in Land mode. Point the scope (can be done in daylight) at a fixed object, and note the Azimuth and Altitude bearing from the handset. Now move the scope to random points of the sky with the handset, and try returning to the original location. Try it a number of times to various points and at maximum slew speed. You should get about 1 arc-minute accuracy in returning to the original position. If you want to do further testing, you can choose random points all over the sky. If doing with stars you'll need to Polar Align and watch out for drift, i.e. do the tests quickly.
If these tests pass fine, then congratulations, the good news is you should be able to achieve your 2 arc-minute pointing accuracy.
Adjust the date and time on your handset to be accurate to the second. Make sure your timezone offset is correct (watch out for daylight savings time). Also make sure your latitude and longitude are correct. Get in the habit of checking your date/time/gmt offset and site location every observing session.
Remember, never rotate your eyepiece or diagonal whilst you're aligning.
Also, it is a worthwhile check whether with and without the diagonal, that the object still appears in the center of the reticle eyepiece.
Make sure you're mounting your scope on a solid surface that doesn't flex when you move round it. For instance if I was to align on my deck, the flexure of the wood as I walk around the scope accounts for 4 arc-minutes of error.
Check your current whole sky pointing ability, and calculate how many arc-minutes it's out by, so you can see the improvement. If you want to be thorough, use the formulas to check how much your alignment is out by. I suggest calculating the OTA error at horizontal, and then subtracting it from the fork error at vertical to come up with the real fork error.
Even if this is not your first time, it's always useful to quantify your error before fixing it.
Although a twisted fork does not contribute any error to goto pointing accuracy, it should be made fairly accurate so no additional stress is put on the bearings. I haven't heard of anybody who has had bearing wear due to a twisted fork, but if you can easily correct it, why not.
To do this step, you need a door frame. Put the LX200 on it's tripod or table, next to the door frame such that you can measure the distance between the door frame and the top of the LX200 fork.
Next power up the LX200 in land mode, and rotate in RA (via the handset), so that the forks are parallel to the wall.
Now measure the distance between the door frame and flat edge just below the curved top of the fork. Jot it down.
Now jot down the Azimuth setting from the handset.
Rotate in Azimuth exactly 180 degrees according to the handset, and then take the measurement of the other fork from the door frame.
They should be the same to within a couple of mm's. If not then you need correct the twisted fork (it should be fairly obvious from looking at the level of the bottom of the 2 fork arms against the RA bearing housing.
Lightly oil screws and insert into the 4 holes in the OTA clamps. Insert them until the screws just touch the allen key screws. (Don't force them).
Mount the scope in Alt-Az and set the handset to land mode. Find an object as close to the horizon as possible and as far distant, something that is easy to find (I use the top of a TV transmitter). It is important to be close to the horizon as the OTA error is maximum and the fork error is the least at this altitude.
Note down the Azimuth bearing from the handset. (press the Mode key until you see the RA and Dec on the handset, then press the ENTER key)
Rotate in azimuth (using the handset) by exactly 180 degrees. Now release the dec clutch and rotate the OTA back over itself so it's now pointing at the object again.
Chances are it's not and it's too far to the left or the right. Or maybe it's not in the eyepiece at all, in which case use the finder. Try to estimate the amount of arc-minutes it's out by and half the error, this is the amount of inaccuracy this is contributing to your pointing.
Now to correct the error, slew the scope in azimuth until half the error is removed, then remove the remaining error by loosening the 3 allen bolts and using the brass screws to move the OTA. Screw equally on both screws on the same plate. If you run out of adjustment, then loosen the brass screws and push the OTA back.
Repeat this step until you are down to 1 arc-minute accuracy (i.e. you're just your mechanical pointing error).
Tighten the middle bolt, then the remaining 2 on each plate.
Now run through this step one more time to check tightening the bolts didn't move anything.
Congratulations you have now made a significant improvement to your pointing ability.
If you have a Wedge, then use that method as it's a lot easier to handle because you can adjust the wedge easily, and Polaris is easy to find and Polar drift is minimal.
If you don't have a wedge, then first of all try to borrow one, otherwise if you're careful the Alt-Az adjustment will work, it's just a little more difficult.
It is possible to do this alignment during the day (in LAND mode) if you can find a suitable object high enough up (and static) that you can accurately align it in the reticle eyepiece. Use this as a substitute for the star. If you find this works well for you, then please email me to tell me about your method.
Now that we have removed the OTA alignment error, we now know the remaining error is due to the forks and will be at a maximum when the OTA is pointing straight up.
This step is easiest done at night with a clear view of Polaris. Setup the scope on the Equatorial Wedge, and adjust the scope so it's reading exactly 90 degrees on your setting circles and the OTA is pointing up.
Adjust the wedge so that the OTA is pointing at Polaris in the reticle eyepiece. Note, we're not aligning on the celestial pole, we're aligning on Polaris itself.
Center polaris in the reticle eyepiece, and note the direction of your Dec axis, next rotate in RA by 180 degrees, and notice how far Polaris has moved in Dec. Remove half the amount by the Dec knob (make sure scope is powered off). Recenter Polaris using the Wedge controls, and repeat. When Polaris stays centered through 180 degrees in the Dec axis, then your Dec is sitting exactly at 90 degrees.
Now center Polaris again, this time we're looking at the RA axis. Now rotate 180 degrees. Estimate how many arc-minutes the error is. Half this is the amount your fork-arms are contributing to your pointing error. You can then calculate this using the following formula and arrive at a figure for how many mm's your fork arms are misaligned by:
Misalignment(in mm's) = sin(z/60) * s Where z is error in arc-minutes. Where s is the distance between the forks in mm's.
Remove half the error with the RA knob, and push down on the top of a fork arm lightly with your hand to see which fork arm you'll need to lower (or pick the opposite arm to raise) to correct the error, put a label on the fork arm to move and note which direction you need to move it in.
Mark your fork arm positions with a knife, so you can return to the factory settings.
You can adjust your fork arms on the wedge, by making sure the arm won't twist under it's own weight. To do this, rotate in RA till you can visualise placing a line through the 2 forks and it would intersect the eclliptic pointing south. At this position, the forks can be adjusted without them twisting under their own weight. However be very careful not to strain the gears, and it is best done with the power off and RA clutch released.
Loosen the 4 allen bolts on the fork you marked earlier. Just loosen them enough that the fork arm can be moved. Don't loosen them too much, otherwise the fork arm will drop down, or may come off completely causing a lot of damage. Use wedges to move the fork up by the amount you calculated. Tighten the bolts. Note: tightening the bolts ALSO alters the height of the fork slightly. To drop the fork you might find it easier to really loosen the top 2 bolts, and then use the bottom to bolts to drop the fork load down on slowly.
Repeat this step, to test your improvement, until again, you're down to 1 arc-minute accuracy.
Altering the fork height can be a bit difficult as Meade didn't provide a Method to accurately alter the height. As you can test easily, it is a case of moving by small amounts and noticing the effects. Quite often this can be made easier by loosening the top 2 bolts, and using the bottom 2 to drop the weight gradually, or by using wedges to push the fork up by small amounts. Note that tightening the bolts will shift the arm up too.
IMPORTANT: If at all possible, use or borrow a wedge, you will get much better results and it will be a lot easier. Balancing a scope in this method may cause the scope to fall over and injure yourself, other people and/or damage the scope or tripod. Also the extra strain on the tripod leg spreader may cause it to break. What you are doing with this method is effectively creating a crude wedge with the tripod without buying the real thing that was designed for the job. It is easier to do at higher latitudes than lower latitudes using this method, but it is less than ideal. It will work if you can get your scope pointing to Polaris, but a wedge is far easier both to adjust and safer. Plus a wedge can be used for photography too. If you like to live dangerously and really don't want to get a wedge, then read on, you have been warned!
Now that we have removed the OTA alignment error, we now know the remaining error is due to the forks and will be at a maximum when the OTA is pointing straight up.
This step is easiest done at night with a clear view of a star as close to Polaris as possible. Setup the scope Alt-Az, and adjust the scope so it's reading exactly 90 degrees on your setting circles and the OTA is pointing up. Adjust or shim the tripod so that it is pointing at this star. I.E. You're trying to get your scope pointing as close to the celestial pole as possible whilst mounted in Alt-Az mode. Be very careful to make sure your scope is stable, and that the weight isn't going to tip it over. In theory the longer the tripod legs are extended the more stable the tripod will become as the base is wider.
Use shims (paper or card) for fine adjustments under the tripod legs.
Center the star in the reticle eyepiece, and note the direction of your Dec axis, next rotate in RA by 180 degrees, and notice how far the star has moved in Dec. Remove half the amount by the North/South control on the handset. Recenter the star by shimming the tripod legs, and repeat. When the star stays centered through 180 degrees in the Dec axis, then your Dec is sitting exactly at 90 degrees. Note: This part of the procedure is a little more difficult to achieve when mounted Alt-Az. There will be no loss is accuracy if you get movement small enough that it sits within an eyepiece view, and you can differentiate the DEC axis from the RA axis mentally.
Now center the star again, this time we're looking at the RA axis. Now rotate 180 degrees. Estimate how many arc-minutes the error is. Half this is the amount your fork-arms are contributing to your pointing error. You can then calculate this using the following formula and arrive at a figure for how many mm's your fork arms are misaligned by:
Misalignment(in mm's) = sin(z/60) * s Where z is error in arc-minutes. Where s is the distance between the forks in mm's.
Remove half the error with the RA knob, and push down on the top of a fork arm lightly with your hand to see which fork arm you'll need to lower (or pick the opposite arm to raise) to correct the error, put a label on the fork arm to move and note which direction you need to move it in.
Mark your fork arm positions with a knife, so you can return to the factory settings.
You can adjust your fork arms in situ, by making sure the arm won't twist under it's own weight. To do this, rotate in RA till you can visualise placing a line through the 2 forks and it would intersect the eclliptic pointing south. At this position, the forks can be adjusted without them twisting under their own weight. However be very careful not to strain the gears, and it is best done with the power off and RA clutch released.
Loosen the 4 allen bolts on the fork you marked earlier. Just loosen them enough that the fork arm can be moved. Don't loosen them too much, otherwise the fork arm will drop down, or may come off completely causing a lot of damage. Use wedges to move the fork up by the amount you calculated. Tighten the bolts. Note: tightening the bolts ALSO alters the height of the fork slightly. To drop the fork you might find it easier to really loosen the top 2 bolts, and then use the bottom to bolts to drop the fork load down on slowly.
Repeat this step, to test your improvement, until again, you're down to 1 arc-minute accuracy.
As you are not pointing at the celestial pole, you will likely have to work faster due to polar drift.
Altering the fork height can be a bit difficult as Meade didn't provide a Method to accurately alter the height. As you can test easily, it is a case of moving by small amounts and noticing the effects. Quite often this can be made easier by loosening the top 2 bolts, and using the bottom 2 to drop the weight gradually, or by using wedges to push the fork up by small amounts. Note that tightening the bolts will shift the arm up too.
Congratulations, you should now have very accurate pointing down to 2 arc-minutes. If not, check your handset settings as per the section above. Then look for gear faults, encoder faults, or a warped mechanical structure. Also make sure when you're doing these adjustments you're on a solid surface that isn't flexing whilst you're walking around the scope.
To get the maximum pointing accuracy (2-arc minute) out of your LX200 everytime you need to do a few things. This is because not all alignment stars are created equally. Some combinations will produce an error due to the geometry of these stars, and NEVER use Polaris as one of the alignment stars.
A reticle eyepiece must be used when centering the alignment stars, and to find the best pairs of stars you can use the "bestpair" program, or better still use Rob Dennys ACP V1.3 Alignment Wizard. ACP has the advantage of doing a declination calibration and will produce the accuracy required everytime, and you'll also be extremely accurately polar aligned. I can't stress enough, buy it and use it EVERYTIME if you want good pointing accuracy. You can currently evaluate it for free before purchasing.
If you're having accuracy problems, mount Alt-Az and do an ACP 2-star alignment. This is because the polar alignment process is a little more difficult to execute, and there are a lot less variables in the Alt-Az alignment. If you are getting accurate Alt-Az alignments, then the method you're using for your Polar Alignment is suspect, recheck.
Also, comments, corrections, improvements to the procedure are all welcome.
I have spent a lot of time researching, building Lego models of the LX200, relearning High School trig and Math and writing these documents. Everybody who uses this document has an opportunity to contribute to helping everybody else achieve a telescope which points as it should. If you've had good or bad experiences with this procedure, please take a moment to email me.
Wishing you happy goto's.
Mark Simpson
adec@bowfort.com