A Few Tips for Accurate Polar Alignment

Unlike different conventional autoguider systems, Telescope Drive Master does not need and does not have any feedback from the sky during tracking. This means you need to minimize the mechanical and adjustment problems of your telescope OTA, mount and pier such as mirror flip, tube & mount flexure, dirty and improperly installed RA bearings, swinging cables of CCD & power supplies on OTA, instable and moving pier as the most common problem sources and, finally but chiefly, inaccurate polar alignment.

If you want to achieve less than 1" (arc-second) tracking error during your 5 (maybe 10) minute exposure on "photometric altitudes" of the sky (zenith distance is less than 40-45°) without position-corrections, you need to keep the axis of the RA shaft on the (refracted) pole (see explanation below) with the best accuracy you can achieve.

As a well known and maybe the most popular (and the fastest) method to do that is using polar scope. At the same time, quality and accuracy of commercial polar scopes are quite poor in many times and, obviously, the polar scope, itself, needs very accurate calibration and adjustment before using. So this can be an appropriate way for polar alignment in case of auto-guiding but TDM needs higher accuracy. But if you have a really well adjusted, calibrated and high quality polar scope, your polar aligment can be good enough for moderate (1-2 minutes) exposures with TDM.

A much better (more accurate but a bit more time consuming) procedure for polar alignment is so called "Scheiner-method" or "Declination drift-method". As it is well known, Scheiner-method consists of two parts. Without detailed explanation of this procedure here (you can find several descriptions on the Internet), to adjust the correct horizontal position of RA shaft you need to track a star near to your local meridian and close to the celestial equator. On this way, it is possible to adjust the East-West position of RA shaft resulting very-very tight (sub-arc-minute) tolerance (which can be accurate enough for TDM) because the movement of the star image during tracking caused by air-refraction, practically, does not contain component in Declination course (if it is close enough to the local meridian). At the same time, you need to track stars on the Eastern or Western horizon if you want to align the correct altitude position of your mount using this method. The problem is, in those regions of the sky, that the objects have to suffer remarkable changes in their position during tracking from second to second due to very high grade of air-refraction, in both RA and Dec directions. So, to achieve correct altitude position, you need to compare and eliminate the difference of star-drifts in Declination measured on both Eastern and Western horizons. (In spite of your demand and efforts for maximum accuracy the altitude position of the mount will be distorted by declination drift caused by air-refraction if you use Eastern or Western horizon only. That can be fairly good for auto-guiding but not good for TDM.)

Another (maybe less known in circles of amateurs) method for polar alignment was described by Edward Skinner King in his book issued in 1931. According to his methodology, it is necessary to take a photograph using the main telescope of the instrument about the polar region of the sky. For the easiest interpretation of the result, it is useful to fix the position of the CCD on the tube in the way which is used for terrestrial imaging (zenith is up, East is on right side in the image). The length of this exposure has to be between roughly 5 and 30 minutes, depending on focal distance of your scope (the longer the exposure, the shorter the focal distance and vice versa), and consists of two parts. The clock drive has to run (with sidereal or King tracking rate) during the first half of the total exposure time, and has to be switched off during the second half of the exposure. The star-trails will show the result. If you find absolutely roundish star-images with arcs of circles, your polar alignment is perfect. If you find small (or larger...) "hoes" with straight and arched star-trails, your mount is not polar aligned. Without explanation here, if you turn this image by 90 degree clockwise, the hoes will show the direction of necessary movement(s) of the mount eliminating the position-imperfections of the RA shaft. Just avoiding misunderstandings, at the end of this method, the RA shaft will be beamed onto the refracted pole of course but not to the real pole (as the thrust-point of the rotational axis of the Earth on the Celestial sphere). So the RA shaft will not be absolutely parallel to the Earth's axis but (without explanation here) this is better position for longer tracking ability of the mount compared to the absolutely parallel situation.

Just for info: using refracted pole for polar alignment, you will need to rotate your telescope around its RA shaft with sidereal speed while exposing stars around zenith only (within 15-20 degrees) but objects in the rest of the celestial hemisphere will be better to shot using average King rate tracking speed (which is shorter than sidereal by 4 seconds per day); even on the local meridian.

It is important to know that "King rate average" is not the position sensitive "real" King rate which highly depends on object position, from degree to degree, in the sky. But this is just a kind of mean value of the best curve-fitting algorithm for using constant tracking speed.

Each method explained above has certain advantages and disadvantages of course.

During the enormous number of tests under TDM development activity it was obviously necessary to align a lot number of mounts (different types) to the pole almost day by day. Based on these experiences, the most efficient combination of these methods (both in time-efficiency and achievable accuracy) has seemed to be the strategy described below.

For historical record: after more than three or four dozen alignment procedures I have discovered Mr. Garzarolli's software on the Internet called "Alignmaster". (Find: http://www.alignmaster.de ) If you have just a few euros to purchase this time efficient software, you do not need to read this long description and do not need to do the job below...

So, for "manual polar alignment" (without software support) it is recommended to make polar alignment in five steps. (But the points 1-3 are important for software supported processes as well.)

•1. The very first step is providing horizontal plane of the pier platform. (Use an accurate bubble glass to do this.) It is more important than you could expect it because, only in this way, you can separate the result of horizontal and altitudinal adjustments. (If the pier platform is not perfectly horizontal, any adjustment will affect the other direction as well; which is not a huge problem of course but you will need to run much more iterations completely eliminating deviations.)

•2. The second step is the rough alignment by naked eye or compass just based on the direction of the RA shaft.

•3. Use polar scope for rough alignment if your mount has it. It is important to adjust the optical axis of the polar scope matching to the geometrical axis of RA hollow-shaft before polar aligning. Please read instructions provided by your telescope producer for using polar scope on proper way. (At the end of this stage, alignment accuracy can be good enough for auto-guiders but TDM needs more precise adjustment if you want to use longer focal distance.)

•4. Use Scheiner method for Azimut (or horizontal) adjustment only. (Scheiner method is not detailed here; instead, you can find several descriptions on the Internet.) According to the explanation above, while horizontal alignment can be very accurate, the altitude alignment is very difficult and time consuming due to the high refraction rate of air mass at low altitudes.

For precise horizontal alignment it is highly recommended to use a sensitive webcam (e.g. Philips SPC900NC) and an auto-guider and image evaluating software which can help you to measure declination drift of the star-centroid (e.g. K3CCDTools3 or Astro-Snap Pro, etc.). The software can indicate 0.1"/minute drift in both declination and right ascension so it is absolutely not impossible to adjust horizontal position of RA shaft in a couple of arc-second accuracy eliminating declination drift completely. This printed screen below shows the absolutely perfect alignment (after King based altitude-alignment, see in the point #5 below) where both Dec and RA drift is 0.0"/min). Be careful: roughly 50% of the total amplitudes comes from star-scintillation!!!

As mentioned above, do not worry about altitude alignment using Scheiner-method with highly refracted stars.

•5. Finally, for perfect altitude alignment, as soon as the horizontal position of the RA shaft is correctly adjusted by Scheiner-method, you can use King-method just for altitude alignment described above.

So, please remove the webcam from the tube and attach a long exposure digital camera onto your OTA that can continuously perform as long expositions as 30 minutes at least. (In general, the orientation of the camera on the focus tube is very important - see above. But in this special case, since you have already corrected the horizontal position of the mount using the "first half" of Scheiner method in 100%, camera's position is not essential because the rest of star-trail errors in King-type polar photograph should come from altitude misalignment problems exclusively - if the mount platform was originally adjusted in horizontal plane.) Take a shot with and without clock drive (onto the same single exposure - see below) and you need to adjust the height of RA shaft only according to the King-shot image and test it again.

Before alignment

Under adjustment

Polar position aligned

Just for info: King-type polar photograph is very good and useful tool not just for running polar adjustment but for checking your mount's position whether it is well aligned or not. If you have star trails using TDM on your images taken under 5 minutes long exposure time (except using long focal distances on the Eastern or Western horizon which is the worst case scenario...), you need to test your mount's alignment using King-type polar images before doing any other corrective actions.

For very detailed explanation, please find the relevant pages of Kings' book at the bottom of this text.

.

A few other tips which can be useful information for running short and successful adjustment and to take perfect pictures using TDM

• - As soon as you are very close to the final position of the RA shaft, it is necessary to move the mount in extremely fine steps of course. If you want to aim at the refracted pole with your RA shaft achieving sub-arc-minute accuracy, you obviously need to move the mount on the same resolution scale. It means your mount, ideally, has to have adjustment screws with fine threads and large diameter knobs, the front surface of the adjustment screw has to be spherical and centralized, the central pin (rotation center) of the mount (both horizontal and vertical) has to be mounted on backlash-free way, the surface of the rotational platforms have to be clean and flat with smooth friction, etc. If your mount does not have smoothed enough (let's say "precise enough") mechanical adjustment possibilities, you will have difficult and time consuming job...
• - The best way to control the movement of the mount during fine alignment is using real time webcam's image. Calculating arc-second per pixel ratio and using scaled cross-hairs on the screen you can fully control any movements, even in sub-arc-minute scale.
• - It is highly recommended to use webcam for horizontal adjustment of drift method. Preferred setting is 15 frames per second and minimum duration of a record depends on accuracy. If you are quite far from the requested direction, 1-2 minutes (or even less) observation time can provide obvious result. At the same time, if you are very close to the final position, scintillation will easily cover and hide the real trend of misalignment; so you need 5-10 minutes recording time minimum to discover the residual position error. Be careful!!! The very short (sub-second), high frequency movements of star image during recording mainly come from scintillation but not from mechanical sources.
• - Thanks for TDM, both Declination AND Rectascension drifts can be used for fast polar alignment approach during Scheiner-method. (This can be named as a kind of "Combined One-position RA&Dec-Drift-Method".) Due to the fact that TDM eliminates both periodic and non-periodic errors of RA gears of any mount, it is possible to achieve quite good alignment in one session: you just need to provide both curves on the screen to be onto each other with 0'/min drift, using a star close to the local meridian and celestial equator. At the same time, you need to know that, although zero Dec. drift can provide very accurate horizontal position but, altitude adjustment based on RA drift in this celestial position never can be as accurate as horizontal because the trend of RA changing caused by altitude misalignment is on minimum level in this position (top of a sinus curve). But this method can substitute a missing polar scope.
• - It is very important not to leave cables hanging on the tube!!! You need to fix but without tightening them!!! TDM is an extremely sensitive controlling system but just a thin cable swinging by the wind can destroy roundish star images using amateur mounts not robust enough.
• - If the RA bearings (or their installation) of your mount are not precise enough, they can lift (or push) RA shaft out from its requested position. Although TDM controls its rotational speed correctly but cannot provide its good position in space of course. In this case you will see slight movements of the star-image in Declination but this can be visible in pictures in extreme situations only.
• - Do not move around your scope during exposure! Please test it: using TDM as soon as you jump a bit next to the tripod (or next to a mechanically not well isolated pier), TDM display will immediately show this vibration.
• - Almost every telescope is moving a little bit for a couple of moments after its positioning. This is why it is good practice to wait just a few seconds between the target-locked status and starting exposure.
• - You need to test the limits of exposure time with the system of your own. An Excel sheet will be communicated on this website soon to calculate the shift of a star image in arc-seconds per minute on easy way caused by refraction, depending on star-position in the sky. But this is just a theoretical result; mechanical performances of certain mounts, polar alignment, focus length, pixel size, different behaviour of certain telescope drivers, your individual demand (or tolerance level) regarding "roundish" star images, etc., so a lot of different effects will decide the maximum possible duration of exposures taken with your instrument using TDM.
• - Almost every pier (even standing in concrete) can have small movements in long scale of time. So, it can be useful to check and realign it, if necessary, month by month.

Relevant pages copied from Edward Skinner King's book: