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Polar Alignment Procedure | ||
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The drift method of polar alignment is very accurate and after you have done it a couple of times, is very quick and easy. I have only used this method with a fork mounted SCT but I believe it will work with any scope / mount configuration. Polar alignment is not as critical as people seem to make out for CCD imaging, and in fact I usually set my polar alignment very slightly off perfect to get rid of the dreaded guiding backlash in DEC. 1) Put the wedge on your tripod. Turn the tripod so that the front of the wedge is facing north (this is the lowest part of the wedge). Set the latitude adjuster on the wedge to your latitude. 2) Put the telescope on the wedge and set the forks to 0º RA, the forks will now be level and pointing towards Polaris. 3) Now using line of sight along the forks, adjust the wedge in azimuth (left / right) and altitude if necessary, to Polaris. If you cannot see Polaris, then just aim your wedge north and set the latitude adjuster to your latitude. Ok, you are now roughly polar aligned. Switch the telescope on and put it in Polar mode. Put an illuminated reticule eyepiece in the telescope. Slew to a star on the eastern horizon which is close to DEC 0º. After a minute or so, check if the star is drifting up or down in the eyepiece (disregard any left / right drift). If the star drifts up in the eyepiece – turn the altitude wedge adjuster so the star moves down in the eyepiece. If the star drifts down – turn the altitude wedge adjuster so the star moves up in the eyepiece. Re-centre the star and repeat the procedure until the star doesn’t drift up or down for 3 or 4 minutes. Now slew to a bright star on the southern horizon (or as close as you can get), with your RA being as close to 0º (close is good enough). After a minute or so check if the star is drifting up or down in the eyepiece (disregard any left / right movement). If the star drifts up – turn the azimuth wedge adjustment so the star moves right in the eyepiece. If the star drifts down – turn the azimuth wedge adjustment so the star moves left in the eyepiece. Re-centre the star and repeat the procedure until the star doesn’t drift up or down for 3 or 4 minutes. If you made a lot of adjustments then you will need to repeat this whole procedure. Now you are accurately polar aligned, tighten the wedge down and mark the floor where the tripod feet are. This way next time you come out, all you need to do is put the tripod in the same place and you will be close to aligned. I leave my tripod and wedge outside permanently and bring my scope in every morning. I only polar align every month or so as the tripod and wedge do not get moved so I am permanently polar aligned. If you cannot see your eastern horizon, then it is possible to use the western horizon, just reverse the moves – for a star drifting up, move the azimuth wedge adjuster so the star goes left in the eyepiece, and for a star drifting down, move it right. Perfect Polar Alignment Using A CCD Camera Perfect polar alignment is only ever desirable if you have a permanently mounted telescope. The reason for this is it is fairly time consuming, but the benefits are great. If your polar alignment is spot on perfect, then the telescope / camera will not need to guide in DEC as it will not drift up or down for the whole night. Having said that, I use this technique to check and adjust my polar alignment, whenever I feel it necessary. You will need a fairly well polar aligned scope and a focused CCD camera. Because of the focal reducer I use, the up / down is reversed on the laptop screen compared to the eyepiece, you will need to know if this is true of your system too. Find a brightish star on the Eastern horizon DEC 0º (or thereabouts), guide on the star on the x axis only (uncheck the y axis). You will see the guide errors in DEC (or Y) axis and you can adjust the azimuth wedge adjuster accordingly. My setup reverses the x axis on my laptop, so I have to remember that left means right and right means left!! Then do the same with a star on the southern horizon RA 0º (or thereabouts), and adjust the altitude wedge adjuster accordingly. The beauty of using the camera is that you can see movements of 0.1 of a pixel, and it is therefore more accurate. Polar Alignment Tips When moving the wedge adjusters, never move them so much that the star moves out of the eyepiece field of view. Use an illuminated reticule eyepiece, and set the cross hairs parallel with the axis of the telescope, that way if there is a lot of left / right drift you can still accurately see if the star has moved up or down. Do not spend half the night getting perfect alignment –fairly good will suffice. If you can, mark where the tripod legs stand and you will be close to polar alignment the next time out straight away. Before you start your polar alignment procedure, loosen the bolts that hold down the wedge to make the movements smoother, and tighten them afterwards to prevent image shift. Try and make sure your tripod is level before you start – it is not critical but it makes for faster alignment. | ||