Drift Aligning

Drift Aligning

To take long exposure photographs it is important to line up your telescope as accurately as possible, parallel with the line round which the Earth rotates, and the most effective way to do this is by Drift Alignment. It’s simple but can be time consuming, dependant on how accurate you wish to be.

You’ll need a short focal length eyepiece (giving 200x magnification or more) with crosshairs. You’ll be deluding yourself if you think you can do it without one, but an illuminated one would be much easier to use.

You will be working with two stars, bright enough to see clearly at 200x magnification, one in the east and the other in the south, and both should be within about 5° of the Celestial Equator, the closer to it the better. The further away from the Celestial Equator the star is, the less perpendicular will be its movement relative to the crosshairs.

You will be allowing the telescope to track the star and be making mechanical adjustments to the altitude and Azimuth adjusters of your mount, based on what you see it do in Declination ONLY, ignoring what it does in RA. Movement of the star in RA will be because of the drive system and not the mount’s position.
Your mount’s hold down bolts should be quite firmly secure, not slack.

Should you live where you can’t see a star in the east then use one in the west but reverse the instructions given for the eastern star.

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Go to your SOUTHERN STAR, within about half an hour of due south.
Orient the lines of the crosshair up and down, parallel to the Dec and RA directions. Set the star on the crosshair, as closely as possible, and watch to see what the star does. It may take only a few seconds if the mount is badly aligned or longer if already reasonably well aimed at the pole.
Ignore what it does in RA, but note which direction it moves in DEC, that is, up or down from the RA line.

If your star drifts UP adjust the AZIMUTH knobs to move the star to the RIGHT in the eyepiece field.

If it drifts DOWN adjust the AZIMUTH to move the star to the LEFT

Then reposition the star on the crosshair again and repeat. Keep repeating the process till the star stays on the RA illuminated line for at least 10 mins.
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When happy with that step, reset the telescope to your EASTERN STAR, again as close to the Celestial Equator, as you can find a suitable star.
Set it on the crosshair, again with the Dec line up and down and the RA left and right, and watch for up or down movement.

If it drifts UP adjust the ALTITUDE knobs to move the star DOWN

If it drifts DOWN adjust the ALTITUDE knobs to move it UP.

Then reset the star on the crosshair again and repeat.
Keep repeating the process till the star keeps on the RA illuminated line for 10 mins.

When happy with that, repeat the process with the star in the south to make sure your adjustment in Altitude didn’t affect the Azimuth setting. If it did, you must do it again, but it’ll not be a long process this time.

The above is the standard way to do the job, but if your telescope has a two or three mirror light path or has some other condition that makes the star’s direction uncertain, follow this method. In fact I always follow this method so I don’t have to remember which direction I have to make adjustments.

Set a kitchen timer for 1 minute and position the star on the crosshair as well as possible and start the timer. Immediately the timer stops check carefully, how much the star has moved. Then make an adjustment to the mount axis being worked on, it doesn’t matter which direction, and reposition the star on the crosshair and restart the timer. This time you are checking to see if the star moved more or less the second time, over the same 1 minute, than it did the first time. If it moved more this time than the first, you must have moved the adjusters the wrong way, and so are moving away from good polar alignment. But at least, you now know for certain, which way you should be making the adjustments. So carry on making them the right way till you see no movement for 10 or more minutes, resetting the timer to 2 mins then 4 mins etc.

BJ.

 

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