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Best way to do 3 star alignment

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#1 chitown


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Posted 05 December 2012 - 09:34 AM

Guys, this is what i do to point my Atlas EQ-G to align my mount, tell me what I am doing wrong:

1. Initialize the hand controller, and enter time/date
2. The hand controller tells me Polaris HA clock (for example: 07:45)
3. I look through the polar scope, and attempt to place Polaris as 07:45 on the big circle. Its hard to point exactly as 07:45 but I try my best on the circle.

Then i do first star find. and its way off.

What wrong did i do?

#2 rmollise



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Posted 05 December 2012 - 09:43 AM

Uh...not quite the way you use the polar scope.

Set up the mount, rotated in RA until the circle on the polar reticle wherer Polaris goes is on the bottom.

Adjust the RA setting circle until it reads 0 and lock it down. Don't move the mount in RA, just adjust the setting circle.

Get the LST time from the HC as you have been doing.

Rotate the mount in RA until 7:45 (or whatever) is under the pointer on the RA circle (use the circle's upper scale). Lock the mount down in RA.

Use the altitude and azimuth adjusters to put Polaris in the little circle.


#3 Phil Sherman

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Posted 07 December 2012 - 10:12 AM

You're doing everything exactly the way you should. I had the same issue until if figured out a better way to rotate the scope on the RA axis for polar alignment. After making sure that the initial position of the little Polaris alignment circle is at the top of the FOV of the polar scope, I use the hour value from the hand controller to rotate the scope using the RA setting circle to determine how far to rotate the RA axis. 7:45 is 4h 15m from the 12:00 position. I double that time (8h 30m) and rotate the mount, with clutches loose, that much using the RA setting circle. Doubling the time changes the 12 hour value from the hand controller to the 24 hour values on the setting circle. When looking at the mount from the eyepiece end of the polar scope, hand controller hour values between 6:00 and 11:59 need counterclockwise rotation of the axis while values between 12:00 and 6:00 need clockwise rotation of the RA axis.

When starting the sky alignment, the first alignment star is frequently off by a large amount. This is usually caused by errors in the home position of the mount. It's easy to have a 5-10 degree error in your home position. I fix this by loosening the clutches and doing a push-to alignment to the (first) alignment star after the GOTO has pointed the scope to where the mount thinks the target should be. Lock the clutches then don't loosen them again for the rest of the two or three star alignment procedure. When I align my mount this way, the second (and third) alignment stars are very rarely outside the FOV of my 25mm eyepiece and are always in the FOV of my finder. Cone errors at the attachment point between the scope and the mount will also cause alignment errors when doing a sky alignment. A three star alignment goes a long way towards compensating for this problem.

Levelling the tripod before attaching the mount will help your alignment. I use a small torpedo level because it's more accurate than a bubble one on the tripod. Accurate location and time settings will also help.


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