LX200 Polar Alignment Procedure

Iterative Method

by Philip Perkins

I've found accurate polar alignment of the LX200 to be a 5 part process that involves a lot more than just aligning the fork arm with the celestial pole. Each part is equally significant to the end result. This procedure has nothing pioneering or exotic about it - in fact it's remarkably standard. However it does have several additions, clarifications, and even omissions from the procedure contained in the LX200 manual. Also by virtue of all steps being contained in one text it is hopefully an easier reference than the standard manual.

The procedure assumes you have an LX200 with 3.34 ROMs and Superwedge, tripod mounted. Those lucky enough to have their LX200 permanently mounted may also be interested in some of this (ignoring the part on tripod levelling). I believe that this procedure would work equally well with the standard wedge, however the description is oriented toward the Superwedge. With 3.30 ROMs I never achieved good polar alignment using this same method, so I believe that 3.34 ROMs are very significant.

Caveat: This procedure works fine for me but may not suit everyone - in fact it may even contain errors. I'd be very interested in any feedback.
 

1. Aligning OTA with fork at 90 deg

Definition: "Spirit Level" - a standard level for carpenters, plumbers, etc, that has a bubble level in both of the horizontal and vertical axes. Widely available from almost any DIY store. The one I use is made by Stanley and is about 10" long.

1) Ensure telescope is in 'stowed' mode - corrector plate cover on and rear cell aperture covered.

2) Remove telescope from wedge and place on sturdy, level surface (ensure top bolt is removed from baseplate first). Ensure RA and Dec locks are free. Align tube with rear cell uppermost. Use cardboard or paper shims to level drive base on this surface, using bubble level in drive base as reference. This is an important step, so get best accuracy. I found the built-in bubble level perfectly accurate, but if you suspect accuracy, use a Spirit Level along the top surface of both axes of drive base to double check (swing fork arm out of the way as necessary).

3) Loosen slightly the black plastic central hub of the declination setting circle. Unless this is already loose (shouldn't be) you'd best use rubber gloves to grip this. Sight-align OTA with fork arms, swing fork through 180 deg to double check, then slightly tighten the Dec lock (slight resistance of OTA to movement). The following process sets the OTA to a best vertical position.

a) Check and set vertical alignment of OTA by using Spirit Level against opposite vertical surfaces of OTA. Achieve best compromise by getting an equal but opposite error on opposite sides. Next, check horizontal alignment by using Spirit Level on top surface of rear cell. The lower part of this surface is not perfectly flat, so position Spirit Level with care, and use this surface only as a reference check (it's reassuring if it reads level, as it did on mine).

b) Swing fork arm through 90 deg and repeat step a) With luck, you should still get equal but opposite errors on opposite sides of the OTA. If not, correct for half of the error compared with step a). If you get a wildly different reading from a) then the base is probably not level (go back to the base levelling procedure, above).

c) Swing fork through a further 90 deg and repeat step b). Continue this process until the fork arm has been through 360 deg. The first time you try it you may need to go through two complete iterations of 360 deg until you're sure you have the best compromise.

4) The idea is to get the best overall compromise between readings until you're sure the OTA is as near perfectly vertical as you can reasonably achieve. At this point tighten the Dec lock so that the OTA cannot move. Throughout the process keep an eye on the bubble level in the drive base.

5) Align the Dec setting circle (which should be loose) precisely at 90 deg. Use magnifying lens for best accuracy. Then tighten the central hub while keeping the setting circle at 90 deg. This is tricky, since the setting circle turns as you tighten the hub. Try offsetting the circle before tightening and repeat as necessary until you end up with the hub very tight and the circle at precisely 90 deg. Be careful not to nudge the OTA off its setting while doing this. The process is now complete - theoretically you only have to do it once, but it's wise to check it occasionally.
 

2. Setting geographical coordinates of observing site

1) Important to know and set the coordinates accurately. I don't know of a better way (barring a satellite terminal) than reading them from a large scale map. Use a map with a 1:25000 or better scale. Using this you should be able to pinpoint your exact location. In the UK use the Ordnance Survey Pathfinder series maps, and in France the IGN blue series also gives 1:25000. I don't know the name of the equivalent in the US.

2) Page 18 of the LX200 manual describes the process of entering the coordinates - an easy process once you know them. Note the slightly weird process for entering easterly longitudes near top of page 19 (fortunately doesn't apply to our friends in the US).
 

3. Setting time and date of observing site

1) Accurate date and time are very important for polar alignment. Many MAPUG subscribers have reported inaccurate LX200 clocks. Mine is just the same. It's important to check and reset the time before every observing session.

2) Always set the actual local time whether it includes an extra hour for summer time (daylight savings) or not, and enter the offset from GMT under the "Hours from GMT" setting. This clarification is for my own benefit more than anything, since I had a slight confusion when first using the LX200. In the UK we're right on the GMT timezone, so in the summer months I thought it was correct to keep the time on GMT and to compensate for the extra hour in the "Hours from GMT" setting. Best not to do this - it's an open door to confusion. Always change the time to the true local time. In the UK during summer this means entering "-01" in the "Hours from GMT" setting - confusing, since the clocks go forward, however it is correct. In the US this means that you always enter positive values in "Hours from GMT" even though time is actually retarded from GMT. It's actually quite well described on pages 19 and 20 of the LX200 manual, providing you read it thoroughly (which I didn't:-)
 

4. Tripod levelling

1) Ensure the tripod is taught when extended by fully tightening the spreader plate tension knob (but be cautious - the spreader plate is only alloy and it's quite possible to snap an arm off). When the tripod is fully taught the extension struts should 'ping' when plucked. If all three 'ping' with the same frequency, you're in good shape :-) Try to keep tripod fully extended between observing sessions.

2) The ground underneath the tripod legs should be as solid as possible. Try to find some way of accurately marking the positions of the tripod feet. Once accurate alignment and levelling has been achieved, this helps to minimize resetting these things before each observing session. If you're on grass (preferred) it's harder to mark the feet positions (perhaps thin steel pipe sunk into the ground, which the feet slot into). My tripod is mounted on the extreme edge of a patio - not optimum for thermal stability, however easier for marking. I simply draw a circle round each foot using a soft lead pencil (my wife complains about the marks, but they rub off easily).

3) You need to know that the tripod itself is polar aligned before setting the level (if the tripod needs to be shifted to achieve polar alignment then it is likely to upset the level). Normally upon first polar alignment of the LX200, rotation and levelling of the tripod will be done as part of the overall process, so approximate polar alignment of the tripod should already be known (fine adjustments are done via the superwedge azimuth control).

4) To check level, I use the bubble level built in to the superwedge. I find that this is perfectly accurate, but if you suspect accuracy, use a Spirit Level along both axes of the top surface of the superwedge. However, be cautious, because you can't be sure that the measuring surfaces are perfectly level. For a time I used a builder's triangular level (triangle shaped level with bubble levels in both X and Y axes) on the top surface of tripod lock knob (magnetic compass removed) but found that this gave less accuracy then the built-in bubble level.

5) There's no magic to adjusting the level. It's a case of adjusting each of the inner legs in turn. Try to end up with the inner legs extended as little as possible - this gives extra stability. Don't overtighten the lock knobs - can easily distort the inner legs. I prefer not to use shims under the feet - they can easily shift or compress, especially once the weight of the telescope is added on top. Check level again after the telescope is mounted on the superwedge, and also check occasionally during the polar alignment process.
 

5. Polar alignment

I've tried a couple of alternative methods. The one that uses the viewfinder I find quite hard to use, partly because the standard LX200 finder does not have an RA prism, and because the reticle is not illuminated. Also, I found it much less accurate than this method. The drift method is very accurate but very time consuming, and not practical before every observing session (in the case of a permanently mounted LX200 it would be fine, since it would only need doing occasionally).

Assumptions:

a. Polar alignment is a complete process involving all steps 1-5, so this step assumes that all previous steps have been completed.
b. The tripod is already level and doesn't have to be shifted further to achieve polar alignment (fine adjustments can be done with the azimuth control).
c. You have a viewfinder of some sort (although this is not used for actual polar alignment).
d. You have a RA prism for the main telescope.
e. You have an illuminated reticle eyepiece. Seems a lot of people use a 9mm - OK, but this degree of magnification is not necessary (and may even make the process harder). I use a 12.5mm and this works fine.
f. You have 3.34 ROMs fitted to the LX200.

Definitions:

"Mechanical Adjustments" - make adjustments using the Azimuth and Latitude control knobs on the Superwedge, only. Do not touch the keypad controls.
"Keypad Adjustments" - make adjustments using the N,S,E,or W keys on the LX200 keypad. Do not touch the mechanical adjustments on the Superwedge.

Before starting ensure that the LX200 is firmly bolted to the Superwedge and that the tripod is level.

1) Align the finder with the OTA. First, fit the RA prism and illuminated reticle eyepiece to the telescope. Centre a reasonably bright star in the main eyepiece, then adjust the finder collimation screws to centre the same star in the finder. This is not a critical adjustment - it just helps in getting Polaris into the FOV of the telescope.

2) On the Keypad, select Telescope - Align - Polar. Set the Declination of the OTA to 90 deg and RA on the drive base to 00, as described under b) Polar on page 31 of the LX200 manual. However the description of the RA setting in the manual is somewhat obscure.

Declination setting: rotate the OTA until pointing roughly toward Polaris. Then accurately align the OTA until the Declination Circle reads exactly 90 deg. Use a magnifying lens to accurately align the 90 deg mark with the pointer (I use a magnifier with integral red light). Don't be tempted to set this to 89.2 deg - you're aligning on the celestial pole, not Polaris - the LX200 knows the exact position of Polaris and will move the OTA to this position at the next step. Lock the Dec drive.

RA setting: quite simple really - you are aligning three things:
1. The pointer on the upper (moving) part of the fork arm base.
2. The '00' mark on the RA setting circle.
3. The pointer on the lower (fixed) part of the drive base.
First, align the '00' mark on the RA setting circle with the fixed pointer on the drive base. Then rotate the fork arm until the upper (moving) pointer is aligned with the '00' mark (and the fixed pointer). I recommend using a magnifier to check this alignment too. Lock the RA drive.

3) Press Enter on the Keypad and the LX200 will slew round to where it thinks Polaris is. Using the finder and Mechanical Adjustments *only*, bring Polaris near to the centre of the finder FOV. Next, through the *scope* eyepiece, continue using Mechanical Adjustments to move Polaris to the exact centre of the illuminated reticle.

4) Press Enter again, and the LX200 will slew to another star, usually near the southern horizon, for the second part of the alignment process. Use Keypad Adjustments *only* to centre this star in the illuminated reticle, then press Enter.

[Sometimes the LX200 selects a star that is obscured from view. If this happens, press Enter so that the LX200 escapes out of Polar Alignment mode**, then select a bright star this *is* visible, and 'sync' on this - see steps 5 and 7 below, for this procedure]

** With 3.30 ROMs I seem to recall that pressing 'Star' escaped out of of Polar Alignment mode, but with 3.34 ROMs no key apart from 'Enter' escapes out of this mode (regression issue?)

5) The standard polar alignment procedure is now complete. Next comes an additional section which yields very precise polar alignment.

As a precursor to this section, select a second star for the alignment process. This can be the same star that the LX200 selects, but doesn't have to be (especially if the star is obscured or close to the horizon). Select a star that is as distant from Polaris as practical, which means that it will be fairly close to the southern horizon (but not so close as to get a significant atmospheric refraction error). Preferably this star will be in the LX200 alignment star library (but only for ease of selection). Just so we're not being too pedantic, there's no real difference to alignment accuracy if the star is a good bit east or west of the southern horizon. For example, in spring or early summer (at +51deg), Spica is a good star to select. We'll call this star the "Second Star".

6) Press Star - Name - Polaris - Goto. The LX200 will slew to Polaris. Note the position of Polaris on the illuminated reticle eyepiece. Draw an imaginary line between Polaris and the centre of the reticle. Carefully note the halfway point on this line. Using Mechanical Adjustments *only*, move Polaris to this halfway point.

7) Press Star - Name - "Second Star" - Goto. The LX200 will slew to the Second Star. Using Keypad Adjustments *only*, move the Second Star to the exact centre of the eyepiece reticle. Then "sync" on it (press and hold Enter button until keypad beeps).

8) Repeat steps 6 and 7 until the alignment error of Polaris is zero, or virtually zero. It should take no more than 3 or 4 iterations. With each iteration the alignment error of Polaris should be significantly reduced. At this point the polar alignment process is complete.

The significant differences between this procedure and the LX200 manual are:

a) Don't go back into polar alignment mode each time to centre Polaris

b) Don't centre Polaris each time - halve the alignment error.

c) Don't wait 15 minutes between each iteration

I found a) and c) redundant, and merely add a lot of time. Also by not going back into polar alignment mode you can choose the second star. This method is a lot quicker than any other 'refined' method I've tried - it's sufficiently quick that it can be used before every observing session where a high degree of alignment accuracy is needed.

With this procedure I regularly achieve very precise polar alignment that cannot be further improved by the drift method. For example, with the illuminated reticle eyepiece (x200) centred on a star such as Arcturus I record no measurable drift due to polar alignment error over a period of 90+ minutes. This is perfect for astrophotography.

Two further things that need to be done for optimum guiding: Periodic Error Correction (under "Smart" menu) and Declination backlash correction ("Backlash" menu item). The procedures in the LX200 manual work fine for me, except don't make any N/S corrections during PEC training. Rotate the reticle so that any drift occurs down the vertical centre line (but if done after the above procedure there should be no N/S drift anyway:-)