LX200 Polar Alignment Procedure

New - Drift Method

by Philip Perkins

 
In June 1996 I posted on MAPUG the Original Procedure for accurate Polar Alignment of the LX200. What follows is an update of that procedure based on the experience I have gained in the meantime. The new procedure is substantially different from the original one, and all the changes are positive ones:

This document is organised into four main parts:

This is a substantial document because it attempts to be as complete as possible. Most of this document is ancillary to the main procedure which consists of four basic steps, as seen in the Procedure for Experienced Users. As such it is intended for 'armchair reading'.  For use beside the telescope, please access the Field Document which may be printed double sided on a single sheet of paper and laminated as necessary.
 

1) Introduction

1.1) Why Polar Align?
When we do long exposure photography of celestial objects from the earth we are doing so from a rotating body. Therefore to prevent the object being photographed from trailing on the film it is necessary to solve two problems:
a)The telescope must be rotating in exact synchronism with the earth.
b)The rotational axis of the telescope must be exactly aligned with the rotational axis of the earth.A quartz clock drive solves problem a), and polar alignment solves problem b). A quartz clock drive runs happily on its own as soon as it is switched on and is thus the easiest problem to solve. As many of us have found, accurate polar alignment can be a harder problem to solve.

Key concept of polar alignment:   Polar alignment is all about alignment of the telescope mount with the celestial pole. Furthermore, by mount we mean the Right Ascension Axis  - the orientation of the Declination Axis (or the telescope within the mount)  has no effect on polar alignment. This may appear obvious, but it is easy to lose sight of this if up till now you have been immersed in the standard LX200 procedure which places a lot of emphasis on interaction with the LX200 computer. Alignment of the LX200 computer is not polar alignment - it is purely a process which Synchronises the internal computer co-ordinates with the real co-ordinates to which the telescope is aimed. It will help if you keep this in mind as you go through this process - those wedge adjustments are the very essence of polar alignment.

1.2) Pointing Accuracy
The Original Procedure was based on the Iterative Method. It worked well initially, but after some months I became aware of increasing errors in pointing accuracy when slewing between Polaris and the 'Second Star'. The Iterative Method is highly dependent on pointing accuracy. The theoretical pointing accuracy of the LX200 is 0.333 arc seconds, but there is a big difference between theory and a practical LX200 which would be considered very good if it achieved 100 times less accuracy (33 arc seconds). The reasons for the divergence between theory and practice are almost entirely to do with constraints in the mechanical accuracy of a consumer priced telescope. These constraints exist even in a new telescope - our problem is that as wear on mechanical components increases with use, so the pointing accuracy declines further. Problems with pointing accuracy may be determined as follows:

The Iterative Method can work well providing that your telescope has excellent pointing accuracy, however unless you can repeatably land on objects within an arc minute or so then the pointing accuracy is probably not good enough for accurate polar alignment. Since the new method is based on the Drift Method it overcomes this problem.
 

1.3) Introduction to the New Procedure
a)The new procedure uses the Drift Method for polar alignment. The importance of the Drift Method is that it will very accurately determine the truepolar alignment of any telescope that has a basic sidereal rate drive. It makes no other assumptions about the telescope. Although it helps to have pointing ability, no pointing ability is required for the Drift Method.

b) The most striking characteristic of the New Procedure is its great simplicity. Here are some interesting differences between the New Procedure and the Original Procedure:

The new procedure focuses directly on the fundamentals of polar alignment - the wedge adjustments. Here is a useful concept: - assume for a moment that the tripod / wedge assembly is already perfectly aligned. Then achieving a completely 'astro ready' LX200 involves nothing further than powering up and Sync-ing on a star.  (This omits some standard settings checked routinely by experienced users such as date / time / location). This may help illustrate where the focus should be in polar alignment.
 

1.4) Assumptions

a) You live in the northern hemisphere. Sincere apologies to our friends south of the equator, but hopefully some of the concepts in this procedure will be useful. The Drift Method per se can be used equally south of the equator.

b) You have a Meade LX200 - this procedure has been tested on an LX200 10" f/10 but it will work with any LX200.

c) You have a wedge - the Drift Method will work with any type of mount but the overall procedure assumes a wedge (this procedure has been tested using a Meade Superwedge but it will work with any type of wedge).

d) Have 3.34 ROMs fitted - this is only important if you wish to point or Sync the telescope according to stars in the LX200 Name database. If you point or Sync on stars according to the Meade catalogue number then the procedure will work with older ROMs.

e) You have standard accessories, such as a finderscope / Telrad (or suchlike), a right-angle prism, and an illuminated guiding eyepiece.

Basic Assumptions - Before starting check some basic settings of the telescope, such as the following. These things are normally done automatically by experienced users, but are easily forgotten:
- Ensure that the N / S switch is at the right setting for your hemisphere.
- Ensure that the LX200 is in Polar mode. (Telescope - Align - Polar.  Do not enter the Polar Alignment mode, just ensure that the Polar setting is checked)

1.5) Permanent Pier Users

This procedure talks only about tripods! This is because polar alignment is much more of an issue for the unfortunate majority who do not have permanent mounts. As a permanent pier user this procedure applies equally to you, except that you are in the fortunate position of needing to do it only once (with occasional checks). Perform the Drift Method whenever you wish to check polar alignment, otherwise simply perform Step 2 at the start of each observing session - you need do nothing else.

 

2) Procedure for Experienced Users

This section is intended for users who have some prior experience of polar aligning the LX200. It is mirrored by section 3, the Procedure for New Users, which contains more detailed descriptions and may be helpful if you have problems or require additional information or clarification on any part.
 

2.1) Initial Alignment
Skip this step if the tripod is already in good alignment. Perform this step if the telescope has been moved by 1/2 degree or more geographically from a previous location.

Orient the tripod as accurately as possible in a N/S direction by magnetic compass or by 'gunsighting' it on Polaris or a landscape object that is in a known N/S direction. Then level the tripod.
For more information see Initial Alignment in the Procedure for New Users.
 

2.2) Power Up / Sync
This step must always be performed.

a) Mount the LX200 on the wedge. If appropriate, try performing the Pre-Alignment (see section 2.a in the Procedure for New Users).  Make sure the RA and Dec locks are tight and power up.  Do not enter the LX200 Polar Alignment procedure and do not adjust the RA and Dec setting circles.

b) Slew to a star in the southern part of the sky, centre it in a guiding eyepiece, select that star in the LX200 database, then Sync on it (Note: you may find it easier if you select a bright star that can be accessed by name from the Name database).
 

2.3) Intermediate Alignment
Skip this step if the wedge is already in good alignment. Perform this step if the telescope has been moved by 1/2 degree or more geographically from a previous location.

a) Select Star 19 from the Star Database (or Polaris from the Name database) and slew to it (press Goto).

b) Use wedge adjustments to centre Polaris in the finderscope (or Telrad). This should bring Polaris into the FOV of the scope guiding eyepiece.

c) Make further wedge adjustments until Polaris is roughly centred in the guiding eyepiece (this is not a critical adjustment).

If you run out of azimuth adjustment range during the above wedge adjustments, then the tripod will need shifting. First, centre the azimuth adjustment within its range, then shift the tripod by small increments until Polaris is within the finderscope. After shifting the tripod, re-level it. Then proceed with the wedge alignment as above until Polaris is centred in the guiding eyepiece.

(This step may appear to be a partial repeat of the Iterative process, however its sole purpose is to roughly align the wedge prior to the drift alignment process).
 

2.4) Refined Alignment
This step must always be performed.

This is the Drift Method of Polar Alignment. There has been no finer description of the Drift Method than that provided by Chuck Vaughn. Chuck is an amateur astronomer who has achieved worldwide acclaim for the quality of his deep space photography. Chuck has generously given  permission for his description to be reproduced below. Chuck's Web Site contains many other excellent articles and a large collection of outstanding deep space images.


All text within the following steps 1 through 7 comprising the Drift Method of Polar Alignment is Copyright by Chuck Vaughn. It is reproduced here with the author's kind permission and except for private use may not be reproduced in any form nor transmitted on the Internet without the prior written consent of the author.

 

Drift Method of Polar Alignment

1. Set up your scope as normal. Use one of the many methods to roughly align using Polaris. This can be as rough as just getting Polaris in the center of your sighter scope. The closer you get here, the shorter time you will spend drifting. (Hint: Level your tripod - it makes life easier)

2. Put a diagonal and illuminated guiding eyepiece in your scope. About 200 power is the minimum needed for adequate sensitivity during drifting. Rotate the eyepiece so that a star moves parallel to the crosshairs in Dec and RA when using the slow motion controls. Align it so that Dec is up and down (North & South) and RA is right and left (East & West).

3. Find a star very near the meridian and about +20 degrees declination and align it to the center of the guiding eyepiece. Let your scope track (you may guide in RA only if you wish) and watch for Dec drift (up or down). Unless your alignment is very close, you will see drift in 5 to 30 seconds.

4. If the star drifts up, turn the azimuth knob that makes the star move right in the field. If the star drifts down, turn the azimuth knob that makes the star move left in the field (These adjustments are reversed for a Newtonian). After adjustment, use the slow motion controls to re-center the star. Repeat this until there is no drift for at least 5 minutes. Note: If you see drift in less than 5 seconds at 200X, you are probably 10 or more eyepiece fields off in azimuth. Give the knob a good crank. This may have to be repeated 3 or 4 times to notice the drift slowing. If you don't see any drift for 30 seconds or so, you may only be 1 or 2 eyepiece fields off. Make your azimuth adjustment accordingly. If after adjustment the star drifts in the opposite direction, you went too far.

5. Find a star on the equator and within 15 degrees of either the Eastern or Western horizon. Repeat (2) and use the guidelines from (3) and (4). For Eastern horizon: if the star drifts up, adjust the elevation to move the star down. If the star drifts down, adjust the elevation to move the star up. For Western horizon: if the star drifts up, adjust the elevation to move the star up. If the star drifts down, adjust the elevation to move the star down. Repeat until there is no drift for at least 5 minutes.

6. If you made a large correction in elevation (several degrees or more), go back and check the azimuth, otherwise you are done. With a little practice, it ought to be possible to complete the procedure before the end of twilight. Try it in your back yard until you are confident. I have found this method to be accurate enough for astrophotographs of up to 3 hours for small fields (less than 1 degree) and up to 2 hours for larger fields (up to 5 degrees) for declinations between +70 and -70 degrees. For exposures longer than these and closer to the poles, a photographic method of polar alignment that is applicable only to permanent installations may be required.

7. It must be emphasized that the star cannot drift at all for 5 minutes to achieve the exposure times stated above. If you judge the drift by bisecting a star with a line in an illuminated eyepiece the star must stay bisected for the full 5 minutes. Experience has shown that if the star has drifted as little as half it's diameter then field rotation will begin to creep into long exposures.


All text comprising the Drift Method of Polar Alignment is Copyright by  Chuck Vaughn


Notes from my own experience with the Drift Method

How accurate does polar alignment have to be?
Lack of accurate polar alignment causes field rotation. The degree to which field rotation becomes visible depends on the declination of the object, the length of the exposure, and the size of the field being recorded. For example, when recording a long exposure of a wide field object near the pole, polar alignment needs to be highly accurate, whereas when recording a short exposure of a small field object near the equator, polar alignment does not need to be very accurate. As a rule of thumb, Michael Covington states: "Align to within 1/4 degree of the pole, and nothing much will go wrong." 15 arc minutes is quite a large error considering that with the Drift Method it is easily possible to be within 1 arc minute of the pole. Bear in mind that in the above procedure Chuck naturally assumes his own quality standards, which are nothing short of excellent. We should all aspire to excellence, however if you feel that you are still on your way there, you may be happy to accept slightly lower standards in the interim. In practice I have found that a slight drift during the 5 minute period can yield virtually undetectable field rotation over a 2 hour exposure provided that the object is not at high declination. By 'slight drift' I mean that the star may have moved either side of the guiding line during 5 minutes, but is still so close that it 'brushes' against the line.

How long does the Drift Method take?
Not all that long, once you are accustomed to it, and providing that you do not need the very highest standards of alignment accuracy. Bear in mind that if you will tolerate no drift at all during 5 minutes then you have to wait 5 minutes after making every adjustment. This soon builds up. The trick is not to wait 5 minutes before every adjustment. Stick with it, keep a very regular eye on the guiding eyepiece, and make an adjustment as soon as you see any drift. Any significant drift should be visible within 2 - 3 minutes in a 200x eyepiece. After a little practice you get to know how much adjustment to make for any given amount of drift. You will soon get to the stage where you need to make no more than 3 adjustments per axis, and the whole process can be complete within half an hour. Aim at doing this between sunset and the end of astronomical twilight and you will not impact your imaging time at all.

Does higher magnification help? In excellent seeing conditions, yes - in average seeing conditions, no. I use a 200x guiding eyepiece - I briefly experimented with a 2x barlow to give 400x and hopefully perceive drift more quickly. Unfortunately doubling the power also doubles the amount by which the star 'bounces around' in average seeing conditions. I found that the increased amount of rapid star movement actually impaired my ability to perceive the amount by which it was drifting. Actual drift (as opposed to star scintillation) was more clearly and cleanly visible at 200x.
 

3) Procedure for New Users

This section mirrors section 2, the Procedure for Experienced Users, except that it contains more detailed descriptions and should thus be helpful to new users, or experienced users who need additional information or clarification on any point. To new users: you will notice that this section is all to do with the alignment of the tripod / wedge assembly, because this is the essence of the polar alignment process.  The procedure is in four parts:

Initial Alignment - this provides an initial alignment of the tripod with polar north and consists of tripod positioning and levelling adjustments. It should only need doing once if your scope is located at a permanent or semi-permanent site. It will need doing every time that you use the LX200 at a new site or a site that you use infrequently.

Power Up / Sync - at this stage we mount the LX200 on the wedge, power it up, and 'Sync' on a star, which will cause the LX200 to adjust its internal sidereal time and coordinate settings so that it knows where things are in the sky. This is a necessary precursor to the Intermediate Alignment process.

Intermediate Alignment - this is the next level of refinement to step 1), and should consist only of wedge adjustments, but you may need to make a further small change to the tripod position. This step should normally need doing only once if your scope is at a permanent site but will need to be done every time that you transport your scope to a new site, or to a site that you use infrequently.

Refined Alignment - this is the final part of the process and is in fact the drift alignment procedure. It consists entirely of wedge adjustments and is capable of yielding a very high degree of polar alignment accuracy.
 

3.1) Initial Tripod Alignment
Skip this step if the wedge is already in good alignment. Perform this step if the tripod is being set up for the first time or has been moved by 1/2 degree or more geographically from a previous location.

The idea behind this process is to achieve a rough alignment of the tripod with polar north such that (hopefully) no further positional adjustments need to be made to the tripod after the LX200 is mounted (i.e. further alignment should be within the adjustment range of the wedge controls). The requirement here is to gain an understanding of the direction of polar north and to roughly align the tripod in that direction. If during daylight hours, you may be limited to the accuracy of a compass. If you happen to be setting up when the the sun is near the meridian according to your local time zone then the shadow cast by the tripod will provide a good approximation of true north. Other methods involve 'gunsighting' the tripod on a distant object which (according to a small scale map) lies in northerly or southerly direction. Soon after sunset you can 'gunsight' the tripod on Polaris directly. There are two methods you might try:
a) Construct a simple 'Polaris sight' that fits directly to the wedge. Further detail may be obtained from Rob Roy.
b) Stand a few yards due south of the tripod such that the north* facing tripod leg and extension strut are in alignment. With an arm extended and pointing towards the north* facing tripod leg, draw a line upwards perpendicular to the horizon. If the upwards motion of your arm does not coincide with Polaris, then adjust the position of the tripod until it does. This procedure sounds amazingly crude, but can work quite well in achieving an initial rough tripod orientation.
*North facing tripod leg applies to the Superwedge - for the Standard Wedge, use the south facing tripod leg.
 

3.1.1) Initial Tripod Levelling
After initial orientation the tripod must be levelled, and must be levelled again if the tripod is shifted during further refinement of its orientation. Ralph Pass has said that the tripod does not have to be accurately levelled to achieve good pointing accuracy, because a combination of latitude wedge adjustment and the LX200 electronics will compensate. This is useful to know, but nonetheless it can save much time to perform polar alignment on a level tripod, otherwise every time that you make an azimuth adjustment you will be modifying the latitude slightly, and vice versa. I find that the bubble level built in to the superwedge is quite accurate enough for this purpose. Adjust each of the inner legs in turn until the bubble is within the inner circle of the bubble level. Try to end up with the inner legs extended as little as possible - this gives extra stability. Don't overtighten the lock knobs since this 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 the level again after the telescope is mounted on the wedge.

 

3.2) Power Up / Sync
This part must always be performed.  There are four basic steps which are simple and straightforward.

a) Mount the LX200 on the wedge. If appropriate, try performing the Pre-Alignment (see below). Make sure the RA and Dec locks are tight and power up. Do not enter the LX200 Polar Alignment procedure and do not adjust the RA and Dec setting circles.

Pre-Alignment (optional)  If the tripod / wedge assembly is already quite well aligned (from a previous alignment at a frequently used site) you may wish to perform this step which can assist in finding stars in the daytime. This will help to get a head start on polar alignment during early twilight.
a.a) Align the telescope in RA and Declination such that the OTA is pointing due south with a Declination of 0 degrees. Then tighten the RA and Dec locks and power up.
The LX200 powers up assuming the RA to be equal to the Sidereal time, and the Declination to be 0 degrees, so performing this step will mean that the LX200 is already roughly Sync'd upon power up. From this 'rough Sync' you should be able to point to a star in daylight to perform an accurate Sync. Note the following observations which may affect accuracy:
-The Declination circle may not be accurately set and the OTA may not be pointing exactly south.
-Upon power up, the LX200 hunts for the zero point on the RA worm and after finding it, assumes that this RA setting is equal to the Sidereal time. Hence the LX200 will always be pointing further west than the real RA. At least this error is constant, so if you do not see the star you are trying to point to, try scanning east.
-The Pre-Alignment assumes that the following steps b) and c) have already been set in a previous alignment. If any changes are made in b) or c), then the Pre-Alignment will be upset. In this case, turn off the power, go back to a.a, then skip b) and c) the next time round.

b) If not already done, enter the coordinates of the observing site according to Page 18 of the LX200 manual. Note the slightly weird process for entering easterly longitudes near top of page 19 (fortunately doesn't apply to the US).

c) If not already done, set the date and time according to pages 19 and 20 of the LX200 manual. Set the time to the actual local time, then the offset from GMT under the "Hours from GMT" setting.

d) Using the Keypad controls only, point the LX200 to a star - any star - that appears readily in the southern part of the sky. From an ease of use perspective, you may wish to choose a bright star that has an entry in the LX200's Name database. For example in the summer months you might choose Altair.  (Note that if you have done a Pre-Alignment then you may try pointing directly to this star).
-Using Keypad controls, align that star in the centre of a guiding eyepiece.
-Then select this star in the LX200 database, for example if it were Altair, press Star - Name - Altair - Enter.
-Then Sync on that star, which means pressing and holding the Enter button for one second until the Keypad emits a beep.
 

3.3) Intermediate Alignment
Skip this part if the wedge is already in good alignment. Perform this step if this is your first setup or if the telescope has been moved by 1/2 degree or more geographically from a previous location.  There are two basic steps which are simple and straightforward:

Slew to Polaris: On the Keypad,  press Star - Name - Polaris - Goto, (or Star - 19 - Goto, to select Polaris from the Star Database, according to preference). The LX200 will slew to Polaris.
Note: the Keypad is used only to slew the LX200 to Polaris - do not make any positional Keypad adjustments at this point.

Align the Wedge:
a)  Make Wedge Azimuth and Latitude adjustments to acquire Polaris in the finderscope (or pointing device such as Telrad). At this point Polaris should be within the FOV of the guiding eyepiece.
b)  Make further Wedge adjustments until Polaris is centred in the guiding eyepiece. This adjustment is not critical - just getting Polaris near the centre of the guiding eyepiece FOV is sufficient.

If you run out of azimuth adjustment range during the above Wedge adjustments, then you will have to shift the orientation of the tripod. In this case it is best to centre the azimuth adjustment within its range, then shift the tripod by small increments until Polaris is within the finderscope. After shifting the tripod, re-level it. Then proceed with the wedge alignment as above until Polaris is centred in the guiding eyepiece.

The intermediate adjustments are complete at this point and the tripod / wedge assembly should now be roughly polar aligned.
 

3.4) Refined Alignment
This step must always be performed.

During this step we achieve precise polar alignment by means of refined wedge adjustments according to the Drift Method of Polar Alignment. For this process please refer to Refined Alignment (step 4) under the Procedure for Experienced Users.
The Drift Method is described in a way that is equally suitable for new users.
 

4) Maintaining Tripod Alignment

This section describes some methods to help maintain tripod alignment between observing sessions if the tripod can be restored to a previously marked position from which polar alignment has already been completed.
 

4.1) Remote Site
A remote site would typically be a dark sky site where you may not have a fixed or marked position for the tripod. If you are a frequent visitor to such as site then you may have identified a 'semi-permanent' position for the tripod. This will avoid the initial parts of the alignment process, since you will already have a good idea of the direction of polar north. Otherwise, polar alignment will have to be 'from scratch', including the initial alignment procedure.

4.2) Permanent Site
A permanent or semi-permanent site is one from which you frequently use the LX200 and have a fixed or marked position for the tripod. In this case you can avoid much, if not all, of the re-alignment process, depending on three cases, which are listed in order of preference for least re-alignment effort.

Tripod left permanently outside. Providing that the tripod is on a solid surface and does not move at all between observing sessions, then the only factor likely to affect polar alignment accuracy is the accuracy of mechanical alignment between the LX200 base and the wedge each time it is assembled. In this case you should be able to go directly to the Drift Method and do a check / tune up between observing sessions. This assumes that high alignment accuracy is required - if less accuracy is required then typically no re-alignment should be needed. Note that it is possible for the tripod to shift, especially when mounting / dismounting the LX200, so it is highly recommended to mark the positions of the tripod feet (see below).

Tripod brought indoors between observing sessions 'in situ'. If the legs are not collapsed and nothing is touched other than to physically move the tripod then it should be possible to maintain quite accurate polar alignment between observing sessions providing that the tripod is on a solid surface and the feet positions are accurately marked. In this case you should be able to go directly to the Drift Method and do an alignment tune up between observing sessions. Expect the wedge to need adjusting with every new observing session, but the changes should not be great. The ground underneath the tripod legs should be as solid as possible. Assuming a paved or concrete surface, then once alignment has been achieved, mark the positions of the tripod feet accurately using a permanent marker (or a black marker pencil if permanent markings would be unacceptable). On grass or dirt surface it can be harder to mark the feet positions, but you might try sections of thin steel pipe sunk into the ground, into which the feet slot.

Tripod brought indoors between observing sessions and the legs have to be collapsed. The same comments apply as above, except that collapsing and extending the legs each time can have a significant effect on the accuracy of tripod alignment. In this case it would be advisable to do an Intermediate Alignment before going to the Drift Method because of the potential of a significant change in alignment accuracy. The amount of re-alignment may be minimised if you can:

a) maintain the original height setting of each tripod leg

b) maintain the original orientation of the spreader bar with respect to the tripod legs. After the tripod has been successfully aligned, mark the end of one of the spreader bar arms (using a scribe or suchlike) and also mark the inside of the tripod leg to which it is adjacent. Ensure that the spreader bar is in this original orientation each time the tripod is extended.

c) try to maintain a constant amount of tension on the tripod legs by means of the tension adjustment knob.

 
Acknowledgements
Sincere thanks to the following for their generous time in reviewing this article and for their many helpful suggestions:

 

 

 

 

All text and images Copyright © 1997-2014 by Philip Perkins. All rights reserved.