THE FLASH MOVIES EMBEDDED IN THIS PAGE MAY TAKE A WHILE TO LOAD GUYS - BEAR WITH IT :)
If you want to take good astro images, your polar alignment needs to be pretty well spot on. This is particularly important if you're not guiding. Without a good polar alignment you'll certainly have sausage shaped stars which you may be happy to accept initially, but eventually you'll want to do something about it.
Using a polar scope is fine up to a point, but it's not particularly accurate, unless you're using a top-end mount. Drift alignment, however, works regardless of the quality of the mount, but is considered by some to be a bit of a "black art".
I am a firm advocate of drift alignment, mainly because I use it with some success. I'm also the first to admit that, initially, it can be difficult to get the hang of it. But bear with it, and it will repay the effort you put in. And it really isn't that difficult :)
I read everything I could on polar alignment when I started this infernal activity, and dismissed drift alignment as "too laborious", until I read a webpage by Peter Kennett. Peter's website is here,(edit: sadly, not any more) and the first thing you'll notice is that it uses frames - not good (sorry Peter!). You may also notice that most of the links in the left hand margin don't work. This is because the website is no longer maintained (Peter now works abroad apparently, and doesn't get the opportunity). Now the stuff on Peter's website on drift alignment is nothing short of brilliant in my humble opinion, and gave me the confidence to give it a go. And, as they say, I've never looked back! ;)
I was concerned that the information may be lost forever as the website is no longer maintained, so I got Peter's permission to reproduce the entire content, including a rather nifty simulator, on this blog. He has kindly agreed to this, and the result is below. Please note that this is all © Peter Kennett, including the Flash content - I do not claim credit for any of what follows :) I have added the occasional comment within the text, perhaps where I think things could be explained slightly better - all such comments are in red.
Because this content may be something that viewers may want to return to, I've made it a page instead of a simple blog entry. Makes it easier to find. :) Oh, and by the way, you'll need an Illuminated Reticle Eyepiece (who said this hobby was cheap?!)
Drift Alignment by Peter Kennett
When you set up your equatorial platform to track the heavens, you already know that you need to align the polar axis of the mount to the celestial pole, and for visual use you can simply use the star Polaris if you live in the Northern Hemisphere. Any basic, rough alignment to the pole will allow the mount to track the sky with enough precision for visual use, GOTO computer systems, and very short photographic exposures.
However, for long exposure astrophotography, you need your mount to be very accurately aligned to the celestial pole. A simple rough alignment or just using Polaris will not do. In these simple alignments you are certain to be either too far North or too far South (altitude error) or too far East or too far West (azimuth error). Some mounts come with excellent polar alignment scopes built into the RA axis of the mount and enable quite accurate alignment ... once you learn how to calibrate and use them. But for very long exposures even this may not be accurate enough.
One way to be absolutely sure you have precise polar alignment is with the DRIFT TECHNIQUE to accurately check the polar alignment of the mount and correct any errors. This technique will give you a very high degree of precision, enough for very long exposures with no tracking error due to misalignment.
The technique is basically simple. You monitor a star in an illuminated eyepiece (I use a 2 x barlow with my eyepiece - working on the basis that the higher the magnification the more accurate the alignment) and watch for any tracking error in DECLINATION (DEC). If the mount is not accurately aligned you will see the star drift. You then correct the position of the mount and recheck the star for continued drift. You will use two stars to check the alignment. The two stars are located in a specific area of the sky that will show the greatest amount of error in either altitude (mount too high or too low) or azimuth (mount too far East or West (think of this as Left and Right, don't get too hung up on cardinal directions)
STEP ONE: Level the mount: To begin, you should make sure your mount is level. You can skip this step, but every time you correct either the azimuth or altitude you will mess up the other! This will mean you will have to re-do the alignment many times before you have it very accurate. Level the mount.. it will save you a LOT of time!
STEP TWO: Rough Align: Next you want to get a rough polar alignment by using Polaris or your polar alignment scope. This will also speed things up for you.
|STEP THREE: Aligning the Reticle: During each step below, you need to make sure your Illuminated Reticle Eyepiece lines are lined up with the East/West axis of your mount. This step may be confusing, but it really is a simple step. First, locate a bright guidestar and watch it in your illuminated reticle eyepiece (you may find it easier to switch the illuminated reticle off, find a star, then switch it back on. Even at the lowest setting, these things are a bit bright!). As you move the telescope in RA forwards and back (using the hand controller) you will see the star glide left and right. Most likely your eyepiece is not orientated properly, so the star will not line up with the reticle lines. Now move the star to one side and rotate the eyepiece until the star glides along the lines when you move the scope's RA axis (click on the curved arrow in the animation to the left)
STEP FOUR: Adjusting the Azimuth: To begin, you need to locate a star near the intersection of the meridian and the celestial equator. The MERIDIAN is the imaginary line that runs from the Northern horizon, across directly overhead, and then down to the Southern horizon. The CELESTIAL EQUATOR is zero degrees Declination. It is the imaginary line that runs East to West, 90 degrees from the celestial poles. It is the imaginary extension of our equator if you extend it out into the sky, and would be directly over your head if you lived at the equator!
You need to locate a star that lies near the intersection of both those lines.
Once you have found your star, center it in your illuminated reticle. Rotate your eyepiece so that the reticle lines run East/West (left/right) as described in step 3 above.
|Guiding and Watching: This will make sure that any movement of your star in right ascension (RA) will not be confused with any movement in declination (DEC). Remember, we are only going to be concerned with the star’s DEC drift, not RA. Many astrophotographers find it better to align the guidestar along a reticle line, instead of between the lines. Watch the star, and keep it centered with slight RA controls (East/West) as needed to correct for periodic error (PE) (I don't generally bother with this. As long as the star is more or less centred between the two lines, the fact that it may have drifted left or right doesn't really matter - it's the up/down movement you're looking for). You will notice PE as a repeated movement back and forth, East and West. This cycle is due to irregularities in your gears. DO NOT make any adjustments in DEC. You are just watching for this declination movement, not correcting it.|
|AziMuth is MAD!: Now it's time to make the adjustments to correct your Polar Misalignment in AZIMUTH. Your mount may be pointing too far east or West of the Celestial Pole. Once you note some declination drift while watching a star near the celestial equator and meridian, you are seeing errors in AZIMUTH, or East/West. The azimuth adjustment is maddening because as you note the star drifting in declination, you rotate the mount in azimuth to shift the star PERPENDICULAR to the direction it just drifted. If the star moves UP you want to rotate the mount’s azimuth to make the star move to the RIGHT in your field of view. If the star drifts DOWN you want to rotate the mount to make the star move to the LEFT. That's just MAD!!!!
Just remember… you are almost never standing upright as an astrophotographer anyway.. UP - RIGHT (you can then reverse that if the star drifts down). This is why azimuth alignment is mad! Then re-center the star and check again. Keep doing this until you no longer see any drift within 10 minutes!
NOTE: If you using a NEWTONIAN telescope, it's even more maddening! You must REVERSE the directions above. If you see the star drift UP, you must move the mount to make the star move to the LEFT. If the star drifts DOWN you must move the star RIGHT. This is just the opposite from a Refractor or Schmidt Cassegrain.(Don't be afraid to make big, bold adjustments here guys, rather than pussyfooting about with tiny little tweaks. Don't hesitate to move the star right out of your field of view, then bring it back with the buttons or slo-mo knobs once you've made the adjustment, or use another star - it doesn't matter)
|STEP FIVE: Adjusting the ALTITUDE: Your mount may pointing too far North or South of the Celestial Pole too. Once you note some declination drift while watching a star near the Eastern horizon, you are seeing errors in ALTITUDE, or North/South. To begin, you need to locate a star that is about 15 - 20 degrees above the Eastern horizon. Do not locate a star too near the horizon or atmospheric refraction will cause errors in tracking by itself, and cause some confusion when trying to gain polar alignment. Once you have found your star, center it in your illuminated reticle. Rotate your eyepiece so that the reticle lines run East/West as described in step three above. (If you can't see the Eastern horizon, the West will do, although you may have to reverse the direction of adjustment - more on that later. If you can't see either (like me), just do your best in whatever direction is more favourable :)
Guide and Watch again: As before, make sure that any movement of your star in right ascension (RA) will not be confused with any movement in declination (DEC). Remember, we are only going to be concerned with the star’s declination drift, not RA. Watch the star, and keep it centered with slight RA controls (East/West) as needed to correct for periodic error (PE). DO NOT make any adjustments in DEC. You are just watching for this declination movement, not correcting it.
|AltitudE is Easy! Now it's time to make the adjustments to correct your Polar Misalignment in ALTITUDE. If you see the star moving in declination, that means your mount is not accurately aligned and you must now SHIFT THE MOUNT slightly to bring the mount closer to accurate polar alignment. The question is which way do you move it? Altitude adjustment is easy to remember ... raise or lower the mount so the star moves back towards the center of the field. That is, whichever way you see the star drifting, adjust the mount to relocate the star back towards the center of the eyepiece. That's EASY! Keep doing this until the star no longer drifts! (Just remember – you are only watching for declination drift, not RA!) If the star is drifting quickly, move it a lot further than just the center. Move it as far as you think it would go if you let it drift for 5 minutes. The more inaccurate your mount, the faster the stars will drift. This means you should make larger mount adjustments when the star moves quickly, than when the star moves only slightly. If you see DEC drift within the first few seconds, estimate how far the star would drift in 5 minutes, and move the star (mount) that far. This will speed up your alignment process.|
|STEP SIX: Repeat: Now you simply repeat the whole process as needed to refine the accuracy. Once you can go 10 minutes without seeing any declination drift, you will have very accurate polar alignment, suitable for very long exposures! This now opens up the entire cosmos to your camera! (Not strictly true! If you have bad periodic error in the motors, you'll find you can get distorted stars from quite short exposures even with a spot-on polar alignment, particularly when using long focal lengths. I have managed 5 minutes unguided with a 200mm lens, which isn't bad, but the most I could get from my 8" reflector (1000mm) was about 60 seconds. However, an accurate polar alignment certainly isn't going to do you any harm!)
This next bit of Peter's article explains why you see what you see in the eyepiece - very informative:
|UNDERSTANDING HOW IT WORKS
The Drift Method is used by thousands of amateur astrophotographers and it works perfectly. But very few really understand WHY the guidestar drifts the way it does, or why you must guide on a star in a certain part of the sky. Why do you need to look at the Celestial Equator and Meridian for Azimuth drift? Does it really make a difference? I'm a visual kind of guy, and think nothing explains things better then a good image. So here, for your viewing pleasure, I have created a simple tool to show you just what is going on in the sky, and why the drift method works!
|Let's look at the sky, from the point of view of your MOUNT. A guidestar, and indeed the whole sky, circles around it in a perfect circle centered around the Celestial Pole. If your mount is perfectly aligned with the celestial pole, your guidestar will stay within the Guiding Reticle. Look at the diagram to the right. This shows the guidestar centered within an Illuminated Reticle eyepiece. The blue line represents the path the star takes around the sky, with the Celestial Pole at the center. The reticle, riding on your mount, is rotating around the Polar Axis at the same speed as the Earth. This causes the Guidestar to stay centered, or at the most, it will move back and forth along the E/W line of the reticle with Periodic Error of your mount. The Guidestar will NOT move up or down the eyepiece. The repeated movement you see here represents several hours of exposure, far more than you would ever need, but it shows that the Guidestar will stay centered with ease.|
|Now lets consider what would happen if our telescope's Polar Axis was NOT lined up properly. If we offset it a bit, it will travel around a circle too, but not the same PATH as the sky. The MOUNT is pointing too far EAST of the pole. In this example, the guidestar's path is grey, and the telescope's path is blue. The scope is pointing at a star on the Celestial Equator and the Meridian. Both sky and telescope are moving at the same rate, and it starts with the Guidestar centered within the Illuminated Reticle. As you can see, from the view of the Reticle, the guidestar moves UP the eyepiece as the Earth rotates and the telescope moves along in Right Ascension. It is the SCOPE that is actually moving down in relation to the true path the star is taking, but from our perspective, it appears the star is moving up. This is EXACTLY what you see with the DRIFT METHOD of polar alignment. For a scope pointing at the Celestial Equator and Meridian, a star will drift UP when the MOUNT is too far EAST. This is why you must adjust the MOUNT to the WEST to correct for an upward drift!|
|So what will happen if we look at the Eastern Sky, instead of the Celestial Equator / Meridian intersection? Well, you may indeed see error if the mount is too far above or below the pole, but not any EAST / WEST misalignment! Look at the illustration to the right. This mount is still pointing too far East of the celestial pole, but it is PERFECTLY aligned North and South. If you look ONLY at a guidestar on the Eastern or Western horizon, you will NOT spot any declination error due to being too far east or West of the pole for several hours! The guidestar tracks along just fine, with almost no drift up or down. This is why it is CRITICAL that you look at the Celestial Equator / Meridian area to spot any AZIMUTH misalignment. The SAME logic holds for any misalignment above or below the celestial pole, except in this case, you must look at an Eastern or Western star to clearly show it. If you only look at the celatial equator / meridian area you will NOT spot any misalignment above or below the pole.|
|Here's a visual demo of moving the mount. If you remember from the tutorial, you can simply watch the guidestar and make it move a certain direction and that will make sure you are making the appropriate adjustment. In this animation, you can see how that works. This is the same Eastern misalignment shown above. If you remember the madness, when you see the guidestar move up - you adjust the mount to make the guidestar move to the RIGHT (with a refractor or SCT). Watch the guidestar in this example when the mount is being moved to the WEST. Click the demo to begin.|
|Quick précis guys, with a few tips based on my limited experience, if it helps:
Next, a brilliant simulator for you to practice on, courtesy of Peter Kennett:
|DRIFT ALIGNMENT SIMULATOR|
|(Click on the image above to see the simulator)|
|Hand Controller: The Hand Controller allows you slew the scope in RA and DEC, make guiding corrections, set the slew speeds, and turn the Periodic Error Correction (PEC) on and off. The display tells you what speed the slew buttons are set at.|
|Illuminated Reticle: The Illuminated Reticle Eyepiece has dual crosshairs in red light so you can see them against the black backdrop of space (not as bright as mine!)|
|Power Box: The Power Box turns on power to your mount and hand controller. There is a small RESET button here to reset the simulator in case you get all messed up. The tiny gear in the corner lets you know the RA tracking system is working.|
|Telescope Box: The Telescope Box has a Polar Viewer in the polar axis so you can see the Celestial Pole if wanted (cheat!). It also has buttons to rotate the eyepiece, select a refractor or Newtonian telescope, adjust the mount in azimuth and altitude, and slew the scope to the Celestial / Meridian intersection or the Eastern Horizon.|
|Once the simulator loads, click on the start-up screen to begin. Immediately you may see stars moving across the eyepiece field of view. Do not worry, they will loop around so you can track one when you are ready.|
This simulated equatorial mount has a random polar alignment - The mount's polar axis is pointing towards the celestial pole but is not very accurate. You will get a lot of declination drift, enough to ruin most astro photos. This is what you will now correct!
Periodic Error Correction (PEC) is NOT turned on so you may see some small RA drift. If your own equipment does not have PEC, you should leave the PEC off on this simulator too. To turn on PEC, click the tiny word "PEC" on the hand controller.
This simulator starts off with the telescope pointing at the intersection of the Celestial equator and Meridian, which is the best place to start the drift alignment procedure. Later you will be able to swing the scope over to the Eastern sky to correct for Altitude misalignment.
Press the #5 or #6 button on the hand controller to switch the slew speed to a fairly fast speed. You can play with the hand controller's arrows to get a feel for how the scope responds when slewing up/down and left/right.
Using the LEFT, RIGHT, UP and DOWN slew buttons move the scope to locate a suitable guide star. You need to quickly line up the illuminated guiding reticle lines with the EAST/WEST axis of your scope. To rotate the eyepiece correctly, use the LEFT or RIGHT arrows on the telescope control box. Quickly move the star to the left and right using the left and right slew buttons and then ROTATE the eyepiece until the guidestar moves along one of the lines of the eyepiece. The star may be drifting up or down (north/south) but this can be ignored for now.
Using a slower SLEW speed, place the guidestar along one of the reticle lines in the center box. If you select slew speed 1, this is called GUIDE SPEED and it simply doubles the normal RA tracking rate when you want to move forward, and simply STOPS it for reverse. I suggest you use a faster speed to get close to center, then switch to a slower speed when ready to begin making corrections.
You will notice that the guidestar is probably moving off your chosen line. Any back and forth movement along the RA line is due to periodic error of your RA drive. This will show up as movement in the East or West (Right Ascension-RA) direction only. You may also note a lot of drift in the North or South (Declination-DEC) due to polar misalignment.
You may correct any RA movement (Left/Right) to keep the star centered, but do NOT correct any Declination drift. You must however NOTE which way any DEC drift is.. either Up or Down.
AZIMUTH IS MAD!
Now it's time to CORRECT any Polar misalignment in AZIMUTH (East/West). This is because your MOUNT's POLAR AXIS may be pointing too far EAST or too far WEST of the true celestial pole. On your own mount you must know how to adjust the Azimuth of your mount, and on most mounts it's a large screw that slowly rotates the mount to the East or West. On the SIMULATOR, you simply press the EAST or WEST buttons on the TELESCOPE control box next to the label AZIMUTH. These move the mount in small amounts to the East or West - when looking north. That is, the north end of the scope will move slightly to the left or right, with WEST being to the LEFT and EAST being to the right. This may look backwards in your eyepiece, since the scope is pointing South.
While watching declination (DEC) drift only, if the guidestar moves UP, rotate the mount's AZIMUTH so that the guidestar moves to the RIGHT in your eyepiece. If the guidestar moves DOWN, rotate the azimuth so that the star moves LEFT. The reason you make these moves is described in the TUTORIAL. DO NOT use the slew buttons on the hand controller to do this. Use the AZIMUTH buttons on the telescope control box. On your own mount, you must physically rotate the mount as described above.
The faster the guidestar drifts up or down, the more azimuth corrections you must make. If the guidestar drifts quickly, your mount is much further away from the celestial pole, and will need to be moved a lot further.. so far in fact that the guidestar will move right out of the field of view! No worry.. you can always grab another star to finish the alignment process.
After you make the mount correction, quickly re-center the guidestar with the HAND SLEW CONTROLS. Again, you can use any star in the area if you can not find your original guidestar.
Repeat these steps by watching the guidestar's movement in DEC and adjusting the mount in AZIMUTH until you no longer see any drift for several minutes. For very long astrophotography, you should not see any drift for 5 minutes. Once the DEC drift has stopped, you can go on to the next section to correct the Altitude.
ALTITUDE IS EASY!
Now it's time to correct any North or South) misalignment. This is ALTITUDE misalignment. Your mount may be too far ABOVE or BELOW the celestial pole, too far North or too far South.
On your own scope you must now rotate the mount's RA axis and move to the EASTERN or WESTERN horizon. DO NOT move the mount azimuth or altitude position... just the scope in RA. On the SIMULATOR, press the EASTERN HORIZON button and press "YES" to slew to the correct position.
Again, be sure the eyepiece is correctly oriented so that any East or West movement in RA will cause a star to move along the reticle lines. When done, choose a bright guidestar and center it again just as you did before.
Watch for any DEC drift again. Remember, you can adjust the scope in RA (East or West) but do not adjust for any declination drift.. just simply watch for it.
If the guidestar drifts UP, adjust the mount's altitude to LOWER the star. If the guidestar drifts DOWN, adjust the mount to RAISE the guidestar. You are just reversing the direction the guidestar is moving. To RAISE or LOWER the simulator's mount, press the North or South buttons on the Telescope control box (not the hand controller). The North button will RAISE the mount. The South button will LOWER it.
Keep raising or lowering the mount until you no longer see any DECLINATION drift. Recenter the guidestar after each movement just like before.
Repeat the whole process by going back to the celestial equator and meridian and checking the AZIMUTH again. This method will allow you to get VERY HIGH precision in Polar Alignment with your mount, suitable for very long astrophotography exposures. If you have to make a large change in Azimuth, go back and re-check the Altitude again too.
After you feel you have achieved accurate polar alignment, press the small round button in the UPPER LEFT corner of the RIGHT SIDE control box. This will turn on the POLAR VIEW and show you the location of the Celestial Pole as a blue circle and cross-hair. If you have done your job well.. the pole will be centered in the red cross hairs of the mount's Polar Axis.