Telescope How-To

Drift Alignment Methods

Method#1Again, I have updated and published this tutorial here in response to a request.I am referring throughout to the use of a German Equatorial Mount (GEM). Also I will refer to South being 180 grid, but Northerners can use the same technique, just substitute North 0.You will need a magnetic compass, one that you can read down to 1 increments, a device for measuring angles, and a little basic handyman skill but well get to that as we work through the technique.Also you will need to determine the Latitude and Magnetic Declination of your location. These can be found herehttp://www.pangolin.co.nz/almanac/magvar.php, or also Latitude is commonly shown on most road maps, and in Atlases.

Overview:The term Polar Alignment refers to the alignment of the Right Ascension axis of a mount so that it is parallel to Earths axis. This then enables the mount to track stars by offsetting Earths rotation. For many in the Northern Hemisphere this is usually achieved by aligning to the star Polaris (with date and time offset) but in the Southern Hemisphere we cant see Polaris, and the equivalent is the constellation Octans. Unfortunately Octans is not easy to see and likely impossible from the Light Polluted skies of our major cities, but you can still accurately Polar Align without any stars at all. Heres one way.

Step 1Find South by compass and lay the tripod so that the forward leg (usually labeled N) is facing South. At this stage just standing back with the compass and getting the tripod roughly in line is sufficient. Now level the tripod head. Here I am using an old carpenters level that I cut down so that it fits in my accessory case. Check level over three points.

Step 2Heres the part where you need to make up a simple tool (or get someone to make it for you). Here I have an aluminium bar screwed to the centre of the tripod head, and centred over the Azimuth adjustment lug. Note it has a scribed centre line and is about 600mm or 2ft long.You dont need to be as fancy as this. A nice piece of timber or even a two or three foot wooden ruler held in place with a weight will do the trick. Just make sure that you dont use any ferrous metal, and the bar is centralised to both the tripod head centre bolt and the Azimuth adjusting lug.Now you need to put a bit of good old boy scout skill to practice. Earths magnetic field is variable and the compass does not indicate the true line of North/South. Now you need to apply the Magnetic Declination for your location that you looked up earlier. Mag Declination will be described as a number of degrees either East or West. If it is East you need to subtract that number from 180, if its West, add it to 180.For example the magnetic Dec at my current location is 11 East and therefore I set my compass to 169 which will therefore point me to true South. Simply move the tripod a little left or right until the compass is so aligned.

Step 3Fit the mount head ensuring that the azimuth adjusting screws as indicated by the red arrow are centralised, and youre halfway there. The mount is now reasonably parallel to Earths axis in one plane.

Step 4All that remains is to either elevate or depress the mount head to your Latitude. Note that there is likely a Latitude scale on the mount head for this purpose but I am advising you to forget it. Very rarely have I seen a mount that actually has an accurate latitude scale, in fact my EQ3 is out by 3 and my EQ6 by 4Following are three different was that you can easily set Latitude accurately. The first is with a protractor and plumb line. I have fitted a string that is too thick to be practical simply so that it shows up in the photo. A 180 protractor is probably a better idea than the 360 version shown.The second is using and engineers protractor, which has a spirit level in it.And the third is using a digital inclinometer. Note that it doesnt make any difference whether you align from the mount head saddle or the counterweight shaft.

SummaryNow your mount is polar aligned accurately enough for all night visual observing sessions, although you may need to make a minor adjustment in Dec occasionally. If you are using go to then now is the time to go ahead and complete a three star alignment.With a little practice youll find that you can align this way in only a few minutes. If you always observe from the same place then all you need to do to set up subsequently is mark the positions of the tripod legs on the ground.However if your goal is long exposure photography your initial Polar align will not be accurate enough and you will need to proceed from here to Drift Alignment.Here are two excellent Drift Alignment tutorials.

http://wcs.ruthner.at/index-en.phphttp://www.andysshotglass.com/DriftAlignment.htmlhttp://nightskyinfocus.com/diyprojects/diy-autoguider-part-4-autoguiding-and-polar-alignment/http://wcs.ruthner.at/index-en.php

Method#2Drift aligning an equatorial mountUsing RecticleFeature

Polar aligning an equatorial mount for astrophotography purposes is not as easy as just pointing the mount to Polaris and commencing an imaging session. For astrophotography purposes good polar alignment is absolutely necessary! The effects of bad polar alignment are fieldrotation and images that seem to drift right out of the field of view of your cameras. Both are very frustrating and very easy to experience if one hasn't spent the time performing an adequate job of aligning the mount.There are different ways of going about the alignment process, but my favorite is the drift alignment method. I have found that after one gets the hang of it, this method is extremely accurate and one can actually see how well the mount is aligned in real time. Any and all errors are clearly visible right on your computer screen. I perform drift alignment with my CCD camera in prime focus, and use K3CCDTools3 and its reticule feature for monitoring the drift of the mount.I learned how to drift align my mount from a post by Charlie Hein in thecloudynights forum. I could try to regurgitate what I read there, but I rather quote his post in it's entirety as his explanation of drift alignment is so spot on. Here it is:Lawrie, drift alignment really isn't anywhere near as hard as it sounds (although it does sound pretty daunting) - I avoided learning how to do it for way too long myself for that very reason.

Understanding what you're trying to do is key. Hopefully, I can explain it in a simple to understand way. In fact, I may over simplify for you just because I don't have any idea what your experience level may be. Because of this, please don't think I'm talking down to you if my explanations seem too simplistic. I just want to make sure that I present this in an easy to understand way. On the other hand, please feel free to point out any portion of the following that you don't pick up on, and I will do my very best to clarify it for you.That said, let's get started. I'm not sure what kind of mount that you have, but I'm going to step out and make a guess that you do not have a fork mount on an equatorial wedge, but rather you have a GEM (like a CG5, SkyViewPro, LXD-55 or 75, or a similar mount). If this isn't the case then let me know, although it really does not change things too much.

It's important to note that while it's not exactly essential to do a rough polar alignment on your mount, it will save you a *lot* of time in the process, because the closer you are to being right on, the less you have to move your mount around to get it right on. Just guessing about where North is and how high Polaris is may put you farther out of alignment than the mechanics of your mount can compensate for, which would force you to physically move the mount in order to get it "in the zone".

This would be very painful to discover 15 minutes or even longer into the process, so I would *strongly* recommend that you at least level your mount and sight Polaris through the polar scope (or the hole where one would go) before getting started. It will save you a *lot* of time from here on out!Alignment Controls

If you can't see Polaris, then point the mount due North, and set your latitude as closely as you can. This is far from being an accurate way to do it, but it's better than just guessing. Also, try to make sure at this point that your East - West (right-left) adjustment bolts (on either side of the mount as opposed to the front and back of the mount, which adjust up and down) are set so that there's plenty of travel in both directions - it would be very painful to find out that you couldn't move the mount more in a direction because you started out too far to one side or the other! I say this because the farther you are out of perfect alignment, the more travel you may need to get there, so it really pays to keep this in mind.

While we're talking about how you're setting up, let's also touch on where you're setting up - you need to have a clear view straight over your head and to the south (behind the mount), and you also need to have a view either to the East or West that is as low to the horizon as you can get it for the drift alignment to work.

Now, set up your camera in the telescope and get it focused. Start up whatever program you have (you mentioned K3CCDtools so I will proceed from the assumption that you have it), and bring up your preview window.

Find a star. Any star will do to get the camera focused, but at this point you may as well choose the first star you will need for your drift alignment - this is a time saver. Here's an easy way to figure out where in the sky this star is.

First, move your mount so that the counterweights are parallel to the ground, and your scope is facing straight up. This will put your scope on the meridian (an imaginary line running from north to south straight over your head).Now, looking at the body of your mount, move the scope in DEC toward the south (the rear of the mount) so that your scope and the mount look something like a "T" laying on its side (if you look at the scope and mount from the side). This points your scope roughly at the celestial equator.

Take a look at this area of the sky and pick a star that is somewhere in this general vicinity - it does not have to be exactly there, just in the neighborhood. The important parts to consider is that you don't stray too far from here, and that the star is easily visible on your preview screen. Use this star to focus your camera - it doesn't necessarily have to be a critical focus, but the sharper it is the better.

Now we need to line up the camera and figure out which direction the image is oriented. Bring up the recticle display in K3. Now slew the telescope so that the star moves in a left-right (or right-left) direction, and note how well the star tracks the horizontal line of the recticle. You need to rotate the camera in your focuser so that the star follows the horizontal line of the recticle as closely as you can possibly get it. Be prepared to spend some time learning how to do this step, but once you figure it out, it'll take less and less time to accomplish.

Once the star tracks exactly with the horizontal line of the recticle then your camera's orientation is set. Now you need to figure out which way North, South, East and West are in your view. This is easy to do. To find North-South, just lightly push on the South side of your scope (on the end that the light is coming in) towards the North while watching the display. Push just enough to be able to see the star move in your display. Your star will appear to move to the South. Now you know the North-South axis. You can use a similar routine for East-West - lightly pushing on the West side of your scope towards the East makes the star appear to move West.

Whew! All that to get to here! Now we're ready to actually look at the East-West drift of the mount. As counter-intuitive as this sounds, we look for this East-West misalignment by watching for a North-South drift of the star in our display. Slew the scope so that the star you've just used to focus and orient your camera with is placed exactly on the line of the recticle that runs East-West. Try to place it as exactly on the line as you can. We want to bisect the star with this line (cut the star in half).

Now we watch for the star to drift off the East-West line in one direction or the other, and we move the mount using the East-West (side to side) adjustments to correct this drift. The rules for this are pretty simple at this point:

Step 1 - Correcting East-West misalignment

If the star drifts South, the polar axis is pointing too far East.If the star drifts North, the polar axis is pointing too far West.

Depending on how far out of perfect alignment you are, the star may start to drift immediately. You can make adjustments as soon as you can positively detect the direction it's drifting in, using the above rules. At first, you will probably want to make a fairly large correction. Watch that you do not lose your driftng star off the edge of the screen while making your adjustments - if it looks like that is going to happen then center the star and then continue to move the mount if you need to. As you get closer to nailing the alignment, make smaller and smaller moves. I find that sometimes it's advisable to adjust past where you think the perfect point is so that you get a sense of what you are accomplishing by moving the scope. The bottom line here is that you are aiming for having the star stay perfectly bisected by the East-West line of your recticle for a longer period of time than you would want to expose for without guiding - a good time frame is five minutes with no drift - the longer it can stay right on the line the better your alignment is. I've had the star stay perfectly bisected for over a half hour (I lose track of time chatting with folks while it drifts), which is a very good alignment.

Now that you have drifted out the East West misalignment, you need to do the same for the North-South axis. Leaving your DEC axis exactly where it is, unlock your RA axis and move it either to the East or West, whichever direction gets you closest to the horizon. Find a star in the general vicinity, just like you did earlier. Your camera should still be focused and correctly oriented, so all you need to do at this point is figure out where North-South and East-West are again, using the same trick you used earlier. Once that's settled, you're ready to go.

Once again, bisect the star on the East-West line of the recticle, and watch for drift in the North-South direction. However, this time we're checking to see if the mount is too high or too low, and we use the adjustments at the front and rear of the mount to move the mount up or down. An added wrinkle here is that the rules are different depending on if you are looking at the Eastern horizon or the Western horizon:

Step 2a - Correcting North-South misalignment (using Eastern horizon)

If the star drifts South, the polar axis is pointing too low.If the star drifts North, the polar axis is pointing too high.

Step 2b - Correcting North-South misalignment (using Western horizon)

If the star drifts South, the polar axis is pointing too high.If the star drifts North, the polar axis is pointing too low.

As before, we're looking to keep the star bisected on the East-West line for as long as we can stand to watch it - at least five minutes is a good rule of thumb, longer is always better. Once you have this down, you might want to go back to check the East-West just in case you accidentally messed something up along the line - that's your call.And that's that!

No question about it - this procedure takes time - time to learn (repetition and familiarity make it faster), and time to perform (repetition and familiarity make it faster). It taxes your patience, but it is definitely worth the trouble!

CharlieEffects of Bad Alignment

I had no clue on how to properly align my equatorial mount until I read the above explanation by Charlie. I now keep a print out of the directions on me every time I go out to image.The picture to the right is a perfect example of how an image will look like if your equatorial mount is poorly aligned. The night I took this image I was in a hurry to align the mount so I could get going imaging M51. Can you blame me? You will probably have to click on the picture to enlarge it, but in it you will see streaks that go from top right to bottom left as well as stars that look elongated. These streaks are actually hot pixels which decided to make there presence felt between my regular dark frame acquisition sessions which I do between light imaging session. Actually, this image was one of the reasons why I decided toPeltier coolmy DSI Pro!At any rate, the downward direction of the streaks shows that my mount was poorly aligned.I hope that the information here will help some out there as much as it has helped me. Drift alignment of your equatorial mount is an important concept to master as an astrophotographer. Good luck and clear skies.

Method#3There seems to be a lot of confusion lately, as to what 'drift alignment' is all about. What is it? How does it work? How does a person make use of it? With a little luck, I may be able to explain how it works, fairly painlessly. Once that's understood, using it from then on should be a breeze. In the following discussion, I'll be emphasizing Drift Alignment as applies to a wedge mounted LX-200 or other SCT, but the principle for drift alignment is actually the same for all scopes. I'll just be using the LX-200 as a reference for where the user is looking, compared to the scope orientation, and what will be seen from that perspective.Let me first say that Drift Alignment, likeALLpolar alignment techniques, is used to get the polar axis of your telescope parallel with the polar axis of the Earth. Nothing more; nothing less. It's just that it's done by watching selected stars 'drift' in Declination while viewing them. From the type of Declination drift, we determine in which way our scope is mis-aligned, and we can correct it accordingly. Thus, the name of 'drift align'.There are really only two directions that your scope can be off. You can be either 'left' or 'right' of the Celestial pole in question, or you can be 'above' or 'below' it. Of course, you can be any combination of the two at one time, and it's very likely that, in fact, you are when you start. We'll see how the 'drift' method is used to close in on both of these directions, one at a time.I can also define polar alignment a slightly different way and it will still mean the same thing. Polar alignment is the act of setting up your telescope in such a way that the North end of its polar axis points toward the North Celestial Pole and its South polar axis points toward the South Celestial Pole. That seems as if I'm just saying the same thing but a little differently.... which I am... but if you keep that particular definition in mind as we progress, I think you'll find that it will make following discussion even easier.Unfortunately, before we can look at the idea of 'drift alignment', we had best get a couple of termsWELLunderstood! This part can get especially confusing if a person is looking directly through the eyepiece one time, then perhaps through a diagonal the next time. This isn't truly complicated, but if you're not careful, you could easily get confused, so we're going toelaborate on it.

This drawing is a view of our scope as seen from its West side, when it's aimed approximately at the Celestial Equator and fairly near the meridian, as seen from a mid-latitude in the Northern Hemisphere.Notice that the polar axis of the telescope runs right down through the center of the scope mount, more or less parallel with the forks. TheSOUTHERNpole of the scopes axis aims down through the ground and at the Southern Celestial Pole, while theNORTHERNpole aims toward the Northern Celestial Pole.What's even more important, is to notice that the direction of 'North' is toward the physical TOP of our scope tube. The 'top' side being that side where our finder scope is also mounted, more or less.The direction of 'South' is obviously in the opposite direction, and is toward the wedge, or down toward the forks.I guess that it can be accepted that when I talk about aiming your scope 'North' or 'up', it means that theFRONTof the scope is going to be rotated in the direction of what we just defined as the 'North' direction, or toward theTOPside of the scope tube! Likewise, aiming the scope 'South' or down, means it will be moved in the direction of theWEDGE,or theBOTTOMof the scope tube!. Fair enough?

The tricky part is seen in this view. It also is a view of the scope as seen from its West side, but the scope is aimedEASTthis time. It's still aimed near the Celestial Equator, but it's been rotated rather toward theEASThorizon! Things look a lot more different now, but the fact is, our rules are still in place. If the front of the scope is moved 'up' or 'North', the front of the scope will still be moved toward the TOP of the scope (where the guide scope is still located, more or less). Aiming the scope 'South' or 'down', will still mean pointing the front of the scope more toward the 'bottom' of the scope, or toward the forks or wedge. This isn't so bad, is it?

So long as viewers, when looking through the eyepiece, rotate their heads and align themselves as in the first image, things might seem okay. "Up' is still toward the top of their heads, even if it's not in the direction away from gravity. At least, it's 'up' to these people.But what about the viewers in the next picture? These people have simply stepped over to the eyepiece without any contortions. They are perhaps relatively short, or just don't want to rotate their heads at an angle. If the scope is to be moved 'up', will these viewers see it move 'up'? That is, will 'up' still be toward the top of the viewers heads?NO!According to what we have just stated, 'up' is defined as being toward that same part of the telescope as before, and 'down' is toward the wedge! If the viewers were to see star movement in the eyepiece that was supposed to be 'North' or 'up', the viewers would see the star moving toward the 'left' and 'up', on a diagonal! That's because the viewers don't really understand what 'up' and "North' is, as is used in astronomy.It can get even worse. If a person is using a diagonal, then the appearance of the directions change, and movement of the scope and/or any object being viewed, can get evenMOREconfusing!With all of this in mind, the point here is, be SURE of what you're seeing when you are looking for directional movement. Don't assume that just because an object is moving toward the sky over your head, that it represents 'Up", "Down", "South", or "North".PROVEit ahead of time! You can do this by very gently pushing up on the front of the optical tube when you're aimed very nearly at your meridian. The scope will move 'up', or 'North' ('South', if you're in the Southern Hemisphere) and the star being viewed WILL move theOPPOSITEdirection, no matter what you see! This observation, andONLYthis observation, is the one that can be trusted for determining what direction the observed star is 'moving'.Okay. The painful part is over. Let's get started.Alignment for the Northern Hemisphere.

The above drawing is my feeble attempt at showing what the Celestial Equator would look like to an observer at Northern mid-latitudes, when facing SOUTH. It is a non-panoramic, two dimensional representation of a panoramic, three dimensional object. The result is the sinusoidal shape you see here.

You see that the South Celestial Pole is significantly below the horizon. Naturally, the North Celestial Pole is in the opposite direction, behind the viewer and somewhere roughly half way between the horizon and overhead.

Here, we have a star that's following along the Celestial Equator as the evening passes. It rises in the East, passes our meridian, then sets in the West.I believe it's safe to say at this point, that the star always stays the same 'distance' from the South Celestial Pole throughout the night. This simply means that it stays at the sameDeclination! I realize that these terms are basic to amateur astronomers, but bear with me. The reason for terms as I re-define them, should soon become apparent.Now it's time to set up the scope. You've got it as close to Polar aligned as you think it needs to be, so just aim it South and get ready to look through the eyepiece at our 'star'. This, of course, means that you, also, are facing South.(Never fear; for the folks in the Southern Hemisphere, we'll soon see things from your point of view. But follow along anyway. There are fewer images from your point of view.)

Ooops! From the looks of things, we aren't properly polar aligned, and we didn't even know it! It seems that the South axis of our telescope is aimed somewhatRIGHT(West) of the South Celestial Pole! Although we haven't tested it yet, we can see in the drawing that the 'arc' for our telescope as it rotates on its axis, is going to follow an arc that's off to theWESTof the Celestial Equator!Naturally, if ourSOUTHaxis isWESTof theSOUTHCelestial Pole, then theNORTHaxis isEASTof theNORTHCelestial Pole! Think about that for a moment and make sure you fully agree with it. It's crucial.

What this rather messy picture is showing, is that, like the star that always stays the same distance from it's Celestial axis, the telescope is going to also remain the same distance from it's own axis, improperly aligned or not. In this case, the scope was set to point at zero degrees in Declination. However, no matter what the declination had been set to, the scope would still rotate around it's axis in the same manner.NOTE!:This also is a good image to use to re-emphasize what was said earlier about what is meant by 'Up/Down', 'North/South' as relates to the view in the telescope.Since the scope tube rotates around it's Polar axis as it moves from the East to the West, the 'Up', or 'North' part of the telescope is that part which is directlyAWAY FROMthe Polar axis of the scope (toward the 'top' of the scope) , and the 'Down', or 'South' refer to that part of the tube which points directlyTOWARDthe Polar axis of the scope (toward the wedge)! Therefore, a 'drift' of a star that seems to 'drift' down', will apparently drift TOWARD the Polar Axis. Don't confuse these directions as referring to the 'up/down' of this page!The beginnings of seeing some 'drift' in the star is becoming apparent, but in reality we'd be a bit more refined that this. When we wish to check and adjust our telescope for being out of alignment in an East/West direction, we would actually select a star that is within 10 degrees or so in Declination from the Celestial Equator and perhaps within 20 or so minutes in Right Ascension of our meridian. That's where this particular error will be most apparent, especially when we use a high powered eyepiece and the error is only slight.

This is a more realistic view of the section of sky that you'd be watching, to check for Polar mis-alignment in an East/West direction. As you can see, as the scope follows the star in Right Ascension, the star slowly 'drifts' toward the Southern Celestial Pole... or 'Down' in our eyepiece, regardless on whether we're somewhat on the East or West side of the meridian.

This gives us rule number one for drift alignment: If a star near the meridian driftsSOUTH(down) in the eyepiece of the telescope, theSOUTHERNdirection on our telescope axis is too farWESTof theSOUTHCelestial Pole, and theNORTHERNdirection on our telescope axis is too farEASTof theNORTHCelestial Pole!

Here we have the same section of the sky selected, and we're watching the same star, but in this case the Southern Pole of the scope isEASTof the Southern Celestial Pole. The star, as it progresses West, driftsNORTH(Up) in the field of view,AWAYfrom the Southern Celestial Pole.

This gives us rule number two for drift alignment: If a star near the meridian driftsNORTH(up) in the eyepiece of the telescope, theSOUTHERNdirection on our telescope axis is too farEASTof theSOUTHCelestial Pole, and theNORTHERNdirection on our telescope axis is too farWESTof theNORTHCelestial Pole!

NOTE! : Be very careful, because many manuals and instructions will be talking about the NORTHERN pole mis-alignment, and you may well be thinking SOUTHERN alignment because that's the direction you're looking!!More important than the rules to be memorized, I hope that thereasonthe rules apply, makes sense to you!NOTE # 2! : You can easily get confused about which way the star is drifting, simply because your setup may or may not cause an inverted image. To be sure, once you see a direction drift, just gently nudge the bottom of the optical tube UP a little, and see if that makes the star move 'South' according to your interpretation. It had better, because you just nudged the tube NORTH!NOTE # 3! When you set about adjusting the East/West adjustment on your wedge, be certain as to which way to turn the knob! It's all too easy to be thinking backward and turn it in the wrong direction. Think about it for a bit, before you make the adjustment!The First Two Rules for the Southern Hemisphere.

Here we see a similar drawing as the previous one, except a person in the Southern Hemisphere would be lookingNORTHto see the Celestial Equator, and it's theNORTHCelestial Pole and theNORTHpolar axis of his scope that's in front of him and below the horizon. TheSOUTHCelestial Pole, of course, is behind him and up. Also, the alignment star, which is rising from the East, will be moving from the observersRIGHT(East) to hisLEFT(West).In this example, the Polar axis isEAST(Right) of the North Celestial Pole. The star, as it is watched, will driftUPWARD(South) in the eyepiece.Southern Hemisphere rules #1 & #2

Rule numberONEfor drift alignment in the Southern Hemisphere: If a star near the meridian driftsUPWARD(South) in the eyepiece of the telescope, theNORTHERNdirection on our telescope axis is too farEAST(Right) of theNORTHCelestial Pole, and theSOUTHERNdirection on our telescope axis is too farWEST(Left)of theSOUTHERNCelestial Pole!

Rule numberTWOfor drift alignment in the Southern Hemisphere: If a star near the meridian driftsDOWNWARD(North)in the eyepiece of the telescope, theNORTHERNdirection on our telescope axis is too farWEST(Left)of theNORTHCelestial Pole, and theSOUTHERNdirection on our telescope axis is too farEAST(Right)of theSOUTHCelestial Pole!

Back to the Northern Hemisphere.Once again we're back at our scope. And once again, unknown to us, it isn't really properly polar aligned as we had hoped. Instead, we have the following condition:

What we're faced with this time is a condition in which, as we face South, the Southern pole on our scope isBELOWthe Southern Celestial Pole.... or put another way, our Northern pole of the scope isABOVEthe Northern Celestial Pole.

If we look a star as it moves across our meridian, notice that there is VERY little Declination drift of the star. That's fine, because we've already reserved that area of the sky for determining any left/right alignment error. Instead, we can select a star that is fairly near the Eastern or Western horizon, and again, near the Celestial Equator.

If we watch for 'drift' of a star that is rising in the East, we can see that there's going to be a significant driftAWAYfrom the Southern Celestial Pole, or aNORTHERN(Upward) drift. If instead, we use a star that's setting in the West, the star seems to driftTOWARDthe Southern Pole, or aSOUTHERN(Downward) drift.Either direction, near the East or West horizon, can be used for checking our up/down error. It depends strictly on your choice, which is usually determined by which one will give you the best view. Your East horizon may be blocked by houses or trees. If so, just use the Western section of the sky, but always try to be fairly near the Celestial Equator. You simply must remember which direction of star drift goes with which horizon.

This gives us rule number THREE for drift alignment: If a star near theEASThorizon driftsUPWARD(North) in the eyepiece of the telescope, theSOUTHERNdirection on our telescope axis is too farSOUTHof theSOUTHCelestial Pole, and theNORTHERNdirection on our telescope axis is too farNORTHof theNORTHCelestial Pole!

Why is it recommended that all of your drift checks be made near the Celestial Equator? Won't some other area of the sky also show the error drift? The answer is, yes it will. The reason the Celestial Equator is chosen, is because that area of the sky will show the error much more quickly. Zero degrees in Declination ... the Celestial Equator ... has the biggest 'diameter' of all of the Declination circles, so the drift error can be detected more quickly or more accurately.

It should come as no surprise to find out that when the South pole on the telescope isABOVEthe Southern Celestial pole, exactly the opposite conditions exist, so we have exactly the opposite directions of drift near the Eastern or Western horizon.The trick is, to be able to remember or figure out which is which.

This gives us rule number FOUR for drift alignment: If a star near theEASThorizon driftsDOWN(South)in the eyepiece of the telescope, theSOUTHERNdirection on our telescope axis is too farNORTHof theSOUTHCelestial Pole, and theNORTHERNdirection on our telescope axis is too farSOUTHof theNORTHCelestial Pole!

Note: In reality, it's best to select a star that's perhaps twenty degrees or so above the horizon for your testing. Your image is less likely to be distorted by heat waves from the Earth, or just plain poor 'seeing' in general.A Quick trip back to the Southern Hemisphere.

Once again, we are looking North toward a star near the Celestial Equator, and we again are looking for the star to be fairly near either the Eastern or Western Horizon. In this example, we see that the Polar axis of the scope isABOVEtheNORTHERNCelestial Pole, orBELOWtheSOUTHERNCelestial Pole, which is behind us. In this case, if the star we use is near theEASThorizon, the star will driftDOWN(North) in the eyepiece. If we are using the star that's near theWESThorizon, it will driftUP(South)in the eyepiece.From what we've already covered about the actions in the Northern Hemisphere, added to the above information, we can state the next two rules for drift alignment in the Southern Hemisphere:The Second Two Rules for Drift Alignment in the Southern Hemisphere.

Rule number THREE for drift alignment: If a star near theEASThorizon driftsDOWN (North) in the eyepiece of the telescope, theNORTHERNdirection on our telescope axis is too farABOVE theNORTHCelestial Pole, and theSOUTHERNdirection on our telescope axis is too farBELOWtheSOUTHERNCelestial Pole!

Rule number FOUR for drift alignment: If a star near theEASThorizon driftsUP(South) in the eyepiece of the telescope, theNORTHERNdirection on our telescope axis is too farBELOWof theNORTHERNCelestial Pole, and theSOUTHERNdirection on our telescope axis is too farABOVEtheSOUTHERNCelestial Pole!Tying it all togetherYou've seen how to detect and correct the various 'drifts' of a star for drift alignment. There are a few real world facts that should also be pointed out.First, the best way for you to detect drift is to be using an illuminated crosshair eyepiece. Have the crosshairs aligned such that one set is parallel to the East/West movement of the telescope. Do this by centering the alignment star, then running the scope back and forth in Right Ascension to be sure the star stays close to parallel to the crosshair. That way, a relatively small movement in Declination can easily be detected.Another method that's often used if you don't have the illuminated eyepiece, is to position the scope so that the star you're observing is positioned at either the Northern or Southern edge of the field of view in a conventional eyepiece. That way, you can detect the North/South drift, seeing the small drift effects on the star.Second, do not... NOT... worry about 'drift' in Right Ascension. That has nothing whatsoever to do with Polar alignment, but simply has to do with the drive characteristics of your R.A. motor. In fact, what you may well want to do is to 'guide' on the alignment star in R.A., and hold it near the center of the crosshairs... or the edge of the field of view.... depending on the method used.Third, once you've been through the two alignments, left/right, up/down, go right back to the beginning and repeat the procedure. Unless you have your wedge PERFECTLY level.... not likely ... there will be interaction between the two adjustments. That is, when you go back to the left/right adjustment, even though you thought you had it perfect the first time, it in all likelihood will need a small amount of re-adjustment. Likewise after you've made this small adjustment. It probably will have effected the up/down adjustment that you've just done. Don't be surprised about it, and don't let it irritate you. It's the nature of the beast.Now, because of the fact that you'll most likely be going back and forth between the two adjustments, getting closer each time, it's easier on your nerves if you don't spend a lot of time on one particular directional adjustment before going to the other one. You'll be ping-ponging between these two adjustments, if you're trying to get the drift down to as close to zero as you can. If you keep going back to, say, the adjustment for left/right alignment of the pole that you're using as a reference, and finally get it to ideal, you may want to pull your hair out when you find that you have to go back and 'tweak' this adjustment again! It's probably better, as you near the 'ideal' settings, to make a very small correction, watch it once to make sure you improved it some, then go to the other check and do the same. That way, both adjustments will slowly get closer and closer to ideal, at the same time. (This is strictly a personal opinion, based on my own experiences, and you may not agree with it at all.)Finally, there is a matter of 'how good must the adjustments be?' That's really a subjective matter. Naturally, in the ideal world, the answer would be that anything less than perfect isn't good enough. But, at least where I doMYimaging, it's far from the ideal world!If a person is doing strictly viewing, then Polar alignment isn't critical at all, so long as your 'go to' feature, if you have one, functions adequately. For astrophotography or CCD imaging, it's quite another matter. In my personal opinion, if a person has a permanent mount, then it's worthwhile to spend an evening every few months, to get the drift down to the minimum they can get, no matter what the scope application might be. Under these conditions, a star viewed with high power would show no visible drift after 15 minutes or so.On the other hand, if you are moving the telescope between viewing sessions, the limiting factor is all too often, the time it takes to get the drift minimized. One doesn't want to spend most of the evening making Polar adjustments, only to have a short part of the evening left for imaging or photographing.Even then, the amount of tolerable drift is going to vary with the type of imaging you do. If you take a lot of relatively short, unguided images, then the accuracy of the adjustments would probably be determined by the field rotation one gets, while taking a full series of images of the same object. If, on the other hand, you self guide or auto guide, the factors to consider are going to be not only potential field rotation, but the extra work of guiding in both R.A. and Dec. For a camera with self guide, your images will certainly turn out better if you don't require Dec corrections in your longest shot. Otherwise, you're potentially faced with camera over-correction, which can lead to trailed star images.As you can see, on this subject, I leave it mostly up to the user. That's the person that has to consider all factors, and by experience with time, will determine for themselves what the accuracy needed will be. As for me, I have a permanent mount for my telescope, and I try to make sure that when doing CCD imaging, my self-guiding camera NEVER has to make Dec corrections. Instead of using an eyepiece for determining the drift of a star, I simply select the star, begin to 'self guide'; with the Dec corrections turned off, and assure myself that after 15 minutes or so, there has been no more than approximately 1.5 arc seconds of drift in Dec. That's usually good enough......... for me.I hope this discussion helps someone to get a better handle on how to make best use of the 'drift alignment' technique for their scope.

Page 1 of 18