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More accuracy in polar alignment.

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#1 youngamateur42

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Posted 15 October 2013 - 07:47 PM

So my eq mounts have motor drives that are a real great feature, but I have been having some difficulty in getting them pointing perfectly north. The 12.5" and the 6" do not have polar alignment scopes, so I am unsure how to get it absolutely perfect. And also, when and if I do get them perfect, is there a way that I can set it up the same every time?

#2 Stargazer78

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Posted 16 October 2013 - 12:04 AM

Get a compass and try to get as close to north as you can. If you drift align, do your azimuth first. Then do your elevation.you should only need to do your elevation once unless you travel pretty far. However once tear down and set back up you will have to redo your azimuth.

#3 Roy McCoy

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Posted 16 October 2013 - 12:57 AM

Hi Justin,

You might try using Kochab’s clock. http://www.arksky.org/Kochab.htm

If you have a telrad then the following is hopefully straight forward. If you don’t then you can estimate using a finderscope. Dr. Clay Sherrod describes this in his article above.

Anyhow, this is how I interpret or use the clock when I polar align. It is approximate but very close, good enough for me, and quick.

In my mind you will be close enough simply placing the Telrad on the day of month in the following chart. This would be exactly true if you aligned on each night at 9:00pm. Most likely you will be within 2 hours of 9:00 (usually 7:00pm) so simply shift it or don’t even worry about it for visual. Just place it telrad center on the approximate date.

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#4 Roy McCoy

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Posted 16 October 2013 - 12:58 AM

Lock in your scope to your mount in it’s correct equatorial position.

Now adjust the mount until your telrad displays the following.

Always put the 2 degree circle of the Telrad through Polaris. Actually you want Polaris just inside the 2 degree circle.

Now adjust the mount a little more until the center of the telrad is near date shown on the chart while leaving the 2 degree circle on Polaris.

Your scope is now pointed north and it is at the correct elevation.


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#5 Roy McCoy

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Posted 16 October 2013 - 12:58 AM

October 15 at 9:00pm

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#6 Roy McCoy

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Posted 16 October 2013 - 12:58 AM

At 11:00 pm October 15, the North center pole moves down and over by 2 hours. Enough to worry about? - not for me, but you may want to.

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#7 Roy McCoy

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Posted 16 October 2013 - 12:58 AM

Notice that Nov 15 at 9:00pm (below) and Oct 15 at 11 pm (above) are the same due to the 2 hour time shift.

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#8 Roy McCoy

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Posted 16 October 2013 - 12:58 AM

Now look back at all the above settings and you will see that they all fall in the same quadrant if you are setting up between 7:00 pm and 11:00 pm during the fall months.

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#9 Roy McCoy

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Posted 16 October 2013 - 12:58 AM

Winter settings will be in the lower right quadrant all season.

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#10 Roy McCoy

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Posted 16 October 2013 - 12:58 AM

The spring settings will be in the upper right quad.

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#11 Roy McCoy

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Posted 16 October 2013 - 12:58 AM

Summer quad.

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#12 Roy McCoy

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Posted 16 October 2013 - 12:59 AM

Good enough for visual – for me. Simply print the below chart and put the Telrad center on the approximate date and 2 degree circle on Polaris and enjoy.

Best Regards,

Roy

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#13 Datapanic

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Posted 17 October 2013 - 10:30 AM

Here's my method:

Proper Celestial Alignment of A German Equatorial Mount

It is assumed that the mount is level and nearly aligned to the North Celestial Pole (NCP). You will need a catalog of Brighter Stars, such as published by the United States Naval Observatory or other reference that publishes the Right Ascension and Declination of brighter stars to use for reference. You are also North of the Equator, if you are South, some steps will need to be reversed.

Alignment of the Polar Axis to the NCP: Select a reference star that is near the zenith and center it in the telescope using crosshairs. Note the Instrumental Declination on the Declination Circle Vernier. Perform a Meridian Flip - rotate the telescope on its axis 12 hours in RA and 180 degrees in Declination so the telescope is on the other side of the mount. Center the same reference star using crosshairs and note the Instrumental Declination on the Declination Circle Vernier. The average of the two Declination readings is the Instrumental Declination of the mount. If the Instrumental Declination of the mount exceeds the known Declination of the reference star, then the Polar Axis of the mount is above the NCP by the difference of the Instrumental Declination and the known Declination of the reference star. If the average is less than the known Declination, the Polar Axis is below the NCP by the difference. Adjust the Polar Axis so that the Instrumental Declination is the same as the known Declination of the reference star.

Adjusting the Declination Circle Vernier (may be done during step one): Take half the difference of the two Instrumental Declination readings from the first step and adjust the Declination Vernier or Circle by that amount. Go back to the first step and repeat until no further adjustment of the Declination Circle Vernier or Circle is needed and the Instrumental Declination and the known Declination of the reference star are the same.

Placing the Polar Axis on the Meridian: Select a reference star that is about 90 degrees from the Meridian and at least 20 degrees above the horizon. Note the reading of the Declination Circle Vernier. If the reference star is East of the Meridian and its Instrumental Declination is greater than the known Declination of the reference star, the Polar Axis is east of the NCP by the difference of the known Declination and the Instrumental Declination of the reference star. If the instrumental declination of the reference star is less than the known Declination, the Polar Axis is West of the NCP by the difference of the known Declination and the Instrumental Declination of the reference star. If the reference star used is West of the Meridian, these offsets are reversed. Rotate the entire mount horizontally according to how the Polar Axis is East or West of the Meridian. You may find it easier to just set the Declination Circle Vernier to the known Declination of the reference star and rotate the entire mount horizontally East or West until the reference star is centered in the telescope.

Placing the Optical Axis Perpendicular to the Declination Axis: (Note that this step may not be possible depending on the design of the mount) Select a reference star that is near the Zenith and set the RA Circle to read 0 (zero) hours. Perform a Meridian Flip and center the reference star. Note the reading of the RA Circle Vernier. If it took less than 12 hours in RA to rotate to the other side of the mount, the Optical Axis is higher on the South end of the Polar Axis. If it took more than 12 hours, then the Optical Axis is higher on the North end of the Polar Axis. Adjust as necessary so that it takes exactly 12 hours in RA to rotate the telescope to center the reference star.






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