Friday, 25 October 2013

Polar aligning my Meade LX200-ACF 8” on a wedge—by Mark Nickols

Meade LX200 8" telescope mounting on a wedge, in polar (equatorial) home position

(and why it took me a long time to do it tolerably well)
A personal Experience

Why Polar?
There are several reasons why you might want to do an accurate polar alignment of your scope. If you want to take photos of sky objects with exposures longer than around a minute (and less if you are really picky about your results) you'll encounter the problems of bad alignment (in both Alt/Az and Polar (equatorial) mode), tracking problems (backlash and other mechanical errors in your GOTO drive), and if you are trying to take long exposures in Alt/Az mode, rotation.

(In Alt/Az mode, the GOTO scope uses two sets of gears to track an object in the sky by moving the scope in two planes, vertical and horizontal. The cost is that the sky appears to rotate around the object being tracked. With a polar alignment, the scope is oriented along the polar axis of the earth so that one set of gears is no longer needed to track an object, once centred, and rotation is eliminated.)

I have a Meade LX200-ACF 8” (203mm aperture) Schmidt-Cassegrain telescope (SCT). It comes with an Alt/Az fork mount. There are very similar Celestron and other brand GOTO scopes. The default Alt/Az mode for these scopes is fine for viewing but not so good for long exposure astrophotography.

People will tell you that doing a polar alignment is the better way to go for astrophotography, and they're right. There are lots of reasons why, but basically it comes down to 'less moving parts', so less opportunities for error to creep in—but more of that later—and, with an accurate polar alignment, there's no rotation to worry about. Error and rotation equals movement and movement is something you do not want during long exposure photos. Again, more about that later.

To do a polar alignment you either need to have an equatorial mount rather than an Alt/Az fork mount, or if you have an Alt/Az fork mount, you need a thing called a wedge. I have a wedge.
Meade wedge mounted on field tripod

A wedge is a gadget that allows you to mount your fork-mounted SCT on the tripod it came with at an angle to the vertical, instead of mounting the scope vertically. Hence 'wedge' because what you are doing is inserting a triangular wedge between your scope-and-fork configuration and the tripod it came with.

Auto vs Manual

Doing an Alt/Az alignment on my Meade is very easy. It's all computer controlled. You (and when I refer to 'you' or 'your' in this article, I mean of course 'me', 'mine', or perhaps 'one'—but you'll get the point) switch the unit on. It finds north. It finds its “home”. It finds level. It calculates the tip of the scope. It does a GPS fix and then it finds two stars for alignment. Once that is done, depending on how accurate you were in centring the suggested stars, the scope will find almost anything in the sky simply by asking for it. Pretty cool!

Polar alignment by contrast is a very manual business.

The first thing to note is that it is more difficult to do a polar alignment in the southern hemisphere than in the northern hemisphere, regardless of what kind of mount you have. This is because there are no really bright stars (all less than magnitude 5) near to the South Celestial Pole (SCP). The star patterns you must look for are not easily visible with the unaided eye especially in light polluted sites. They can be seen with binoculars however.

The second thing to note is that a very rough polar alignment can be done just by knowing the latitude and the magnetic deviation and setting your mount accordingly, not looking at the sky at all, but that will be a VERY rough alignment, good enough for looking perhaps, but nowhere near good enough for astrophotography.

For my purposes however, I'll need to look at the sky.

A slightly better method of polar alignment is to centre the scope on the star Sigma Octantis. But I want to get closer to the pole than Sigma Octantis, which is a little over a degree from the pole, so again not really good enough for astrophotography.

Maps of the constellations are not terribly useful in this endeavour as Sigma Octantis, the southern pole star, and the rough equivalent of the north star Polaris, is not even part of the constellation lines of Octans. You can see σ (sigma) near the South pole in this diagram.

Finding the South Celestial Pole


There are guides and instructions available to help you identify the SCP, but none I've found are detailed enough. For example, I'd say the first and most important thing you need to do on a scope like mine—with a GOTO Alt/Az mount, assuming you can do an Alt/Az alignment on the scope, is to find out what the South Celestial Pole actually looks like through your scope, because although you will see pictures and diagrams of it in books, articles, and on the web, what you will need to look for depends a lot on the definition and field of view provided by YOUR finder scope and YOUR eyepiece in YOUR scope in YOUR location with YOUR level of light pollution etc..

Alt/Az check for what the SCP looks like

Area around the South Celestial Pole
Forgetting the wedge for a moment, on my GOTO scope, having done a good Alt/Az alignment, and checked by doing a GOTO to a few objects successfully, I then do a GOTO to star HIP 48752 (Hipparcos catalog) or HIP 71348, either of which is a bit fainter but a lot closer to the pole than the more usually suggested southern 'pole star' Sigma Octantis (HIP104382).

To find the pole, I look for the pattern of stars near Sigma Octantis. Here, for example, is a diagram of what I need to look for which comes from an article I found on the internet describing how to do a polar alignment in the southern hemisphere on a Meade SCT like mine.

This article described a pattern of stars, or asterism, close to the pole. The patterns in the diagram are not recognised constellations, but it's essential to have a pattern to look for. In the diagram, the authors draw a trapezium with Sigma, Tau, Upsilon and Chi Octantis at the four corners, with a 'fish hook' line drawn through a group of stars closer to the pole.  You can just as easily draw up your own patterns, but the above arrangement is reasonably prominent and recognizable. Thankfully the authors have noted the field of view via the Meade standard 8x50 finder scope—but you need to see it yourself in your own scope. For a start, as indicated in the diagram, you are possibly not going to see the full pattern of stars shown here, as Upsilon Octantis is already outside the field of view in this diagram, so you may need other hints not shown in this diagram. Notably, there are other stars also visible in this field of view (and this will vary according to light pollution) which can confuse the patterns you are looking for, but which may also give additional clues.

Also, remember this pattern rotates about the SCP, just to add to the confusion.

Assuming I have found this area in Alt/Az mode on the Meade, if I hold down the 'MODE' button for a couple of seconds in the hand controller then release, the output displays the current azimuth and declination in real time, so I can adjust the scope using the hand controller to read an azimuth of 0 and a declination of -90 deg, and I should be looking right at the pole. Assuming you can find the area of the SCP in Alt/Az mode in this example, you should have a good long look at your view of the SCP through a variety of eyepieces, if you have more than one.

This view of the sky near the pole (from a Stellarium© screen display in the diagram below) is similar to what I see in my Stellarvue 9x50 finder scope. With reference to Octans, I can only see Sigma Octantis and Tau Octantis in the FOV, so how do I know I'm in the right place?

Display from Stellarium around the South Celestial Pole
For me, the distinguishing thing about Sigma Octantis is not so much its relation to the other stars in Octans, though that is important obviously (and can be verified using binoculars)  but that it can be identified by the two stars (not part of Octans) that form a straight line of three stars, starting with Sigma Octantis (the biggest and brightest of the three), off at an angle between Sigma and Tau Octantis. If I am pointing at the right portion of the sky, this pattern is easily identified in my 9x50 finder.

So now I know roughly what to look for when I set the scope up in polar/equatorial mode.

Level the tripod and find true south.

Meade wedge azimuth control affixed to the field tripod
There is no help from the inbuilt computer to level the scope when you are in polar mode. So, the first task is to attach the wedge to the tripod and level the tripod (I'll need to adjust the level again after attaching the scope) with the wedge azimuth control pointing true south. The Meade wedge azimuth control is the only part of the wedge that is permanently attached to the tripod. It can be left in place when the scope is used in Alt/Az mode. I can find the precise current deviation (it changes slightly over time) from the Geoscience Australia website by entering my latitude, longitude, and elevation. Where I am, the deviation was 12.291 deg E as at 17 Dec 2012. The measure describes the deviation of true north from magnetic north, so south by the compass is in fact 12.291 deg west of true south. When the compass points south, true south is a little over 12 deg further to the east.

The Meade wedge comes with a compass, but I needed to buy a compass made for Australia. The dip on the Meade compass is wrong and it doesn't work when attached to the mount when the mount is level. Using my Australian compass I draw an imaginary line between the centre mounting bolt in the wedge and the centre of the wedge azimuth control which should point true south (ie: 12 deg east of south by the compass).

Set the Declination

Now I check the angle of the wedge. It is important that the tripod be level before doing this. Establishing the declination is critical but unfortunately the Meade wedge design restricts me to an approximate setting. I use a smart phone app to check the actual angle on the wedge by placing the phone against the wedge which for me is 55 deg (90 deg minus the latitude—I don't bother to use minutes and seconds in the latitude because the scale markers on the scope and the wedge do not provide a fine enough precision).

Now I attach the scope to the wedge. I check the level of the mount again, and the declination, as adding the weight of the scope can make a difference, even if the tripod is standing on a concrete base.

With the scope unpowered, I set the RA control of the scope manually so that the finder scope and diagonal are in a comfortable upright position and lock it. I set the DEC to be -90 deg and lock it. The OTA should be in line with the forks and perpendicular to the wedge. I unlock the azimuth control and rotate the scope a little. It should rotate without any wobble about an imaginary line joining the centre of the secondary mirror down through the centre of the OTA to the diagonal. This is all approximate of course.

I return the RA control so that the finder scope and diagonal are in a comfortable position for viewing and lock it. Still with the scope powered down, I find the area near the pole in my finder scope which looks similar to the FOV in the previous diagram. I use the manual controls on the wedge to centre on the pole, which is between Sigma Octantis and the two stars circled above in the curve of the 'fish hook' and somewhat closer to these two stars than to Sigma Octantis.

Note the two stars that form the fourth point in the curve from the end of the 'fish hook' pattern of stars close to the pole. The pole is between, and a little below in this orientation, the small orange circle in this Stellarium view (which is Polarissima Australis—a galaxy too faint at magnitude 13.5 to be viewed by my gear in the suburbs) and the two stars circled in the diagram (top left).

Here is the scene again, zoomed in a bit to show where the pole actually is located.

Too much 'stuff' in this diagram? Let's simplify it (but come back to this diagram later). In the 26mm plossl, which is the standard eyepiece supplied with the Meade, the view is similar to this. Again, you need to see what this area looks like in YOUR eyepiece.



You can imagine a triangle with vertices at the pole, HIP 71348 and HIP 48752. I centre on the pole again using the manual controls on the wedge. Now I switch to a 12mm illuminated reticle eyepiece to get a better lock on the pole. In the 12mm, the view looks like this...


This view in the 12mm would be meaningless if not for the knowledge that the wider views have identified the 2 key stars in this view. So the lesson here is that it is essential to get a wide field view, centre the pole via best guess, and then refine the view with higher magnification, confirm your guess, centre again, and again move to a finer magnification, and repeat. Actually after many times of doing this, I tend to stick with the 26mm eyepiece at this stage, and only use the 12mm for final adjustments (see below).

In my case, I have 35mm, 27mm (I replaced the 26mm that came with the scope), and 12mm (with reticle) eyepieces. Plus I use a 9x50 Stellarvue finder scope.

When I think I am on target, I de-clutch the azimuth control and gently rotate the scope while looking through the finder scope. The stars should rotate about a central point in my FOV. If that works, I've done well. If I am out I'll have to go back a step or two. I try looking through the 27mm eyepiece and again rotating the OTA. If stars still seem to rotate about the centre of my field of view I have done very well indeed.

So, having done a best-guess location of the SCP in my SCT + wedge with the power turned off, I need to put the scope in the polar 'home' position. I de-clutch the RA control on the scope and rotate the OTA so that the RA scale points to 0. This has the effect of placing the finder scope and the diagonal underneath the scope with the eyepiece in the diagonal pointing to the ground. I power-up the scope and check/set it to be in Polar align mode. If I have had to change the mode from Alt/Az to Polar I then need to power off the scope again so it will boot in Polar mode. When I re-start the scope I just press '0' to align on the Meade hand controller. The scope indicates this is a polar align and prompts me to put it in the home position and to set the declination to be -90 deg using the handset.  This bit seems to be important. Even if you have manually put the scope OTA at 90 deg (perpendicular to the wedge) you should use the hand controller to move it off 90 deg a little then reset it to 90 deg. When all is set press 'Enter'

The scope will get a GPS fix then rotate the OTA in the RA plane placing the finder scope and diagonal in a position where I can make use of them more easily, and skews to Sigma Octantis. Do not use the handset to do any adjustment at this point; use only the manual controls on the wedge to centre the star. Once centred, tighten all knobs on the wedge and confirm the star is centred by pressing 'Enter' on the handset. The scope then skews to a second star. On the second star, use the handset ONLY this time to centre. When done press 'Enter'

I am now polar aligned, at least roughly, but not nearly as roughly as some of the other suggested methods. To undertake longer astrophotography exposures, I will need to fine tune using the drift method, or software such as Alignmaster, or the iterative method, which I prefer, but the above procedure gets me pretty close without further correction.

Iterative Refinement

This is cheap (zero cost) and pretty easy to do if you have a GOTO mount. Having completed a two-star alignment, I instruct the GOTO mount to GOTO Sigma Octantis again (HIP 104382). When the scope has completed its skew, there will be some error, but you should be able to see the target in the finder scope. Use the wedge controls (not the handset) to correct about half the error (ie: move the star towards, but not all the way, to the correct centred position). Now GOTO the second alignment star already used. Centre it using the handset only. When centred press ENTER for about 5 seconds then release. There will be a beep and a message on the handset asking you to confirm sync. Press enter again briefly to confirm.

Repeat this exercise (going back and forth between Sigma Octantis and the second alignment star) two or three times remembering to use the manual wedge controls to correct half the error on Sigma Octantis and the handset controller to centre the other alignment star—followed by SYNCing the scope each time the second alignment star is centred. By the end of the second iteration you will have removed almost all of the error and will have a pretty accurate polar alignment, and it is at this point that I might use the 12mm illuminated reticle to further refine one more time.

Summary

The article provides a method for getting an accurate polar/equatorial alignment using a Meade LX200-ACF telescope mounted using a Meade Wedge on a Meade Standard field tripod. Although the above procedure is specifically for a Meade LX200-ACF scope on an Alt/Az GOTO mount, some principles will be able to be transferred to other scopes and mounts. My experience was that I had a terrible time over some months unsuccessfully trying to get an accurate polar alignment based on books and articles until I personalised the information that is out there – by adapting the general advice to what I could see in my eyepieces in my scope and by understanding the principles as applied to my scope. Plus another secret: observation and patience. Apart from the wedge, and a couple of eyepieces, no additional software or other expensive “add-ons” were required.

Acknowledgements

Stellarium is software available under the GNU GPL v2, Public Domain License
(The bulk of the code is GPLv2/LGPL with some sections of code with BSD-like licenses. Some images are provided for use with attribution only). It is free (as in gratis—no cost) and the license provides protection for the end user. Just download it from http://www.stellarium.org/ and use with confidence.

http://www.southern-astro.com.au/ and in particular http://www.southern-astro.com.au/php/guides/lx200gpssouthpolaralignment.php on which the current article and approach to polar alignment of the LX200-ACF in the southern hemisphere is based.