Re: [APML] drift alignment

[ZenAstro <zenastro@home.com>]

At 08:40 PM 9/30/2001 -0500, you wrote:
>I don't do this very often because I'm pier mounted. But every so often I'll
>tweek my alignment. Question: on the eastern star drift we always use the
>terms "drift up" or "drift down" to describe the star movement. Is this
>along the RA or DEC axis? I'm getting movement along both axis and assume
>it's atmospheric induced or something. My drift on the meridian star is
>great; none for 15 min after refinement of my mount.


Hi Bobby,
Up and down are confusing, and should not be used.

It's north or south drift in dec that you have to pay attention to in drift 
aligning.

On the meridian correction, if the star drifts south, the polar axis is too 
far east.

On the horizon correction, if the star drifts south, the polar axis is too low.

I combine them into one easy-to-remember saying...

S-E-L.

Which means, SOUTH - EAST - LOW.

This combines both the azimuth and altitude corrections for the mount. In 
both cases, all you have to remember is what to do if the star drifts 
SOUTH. Obviously if the star drifts north, the correction is just the opposite.

If the star on the meridian correction drifts SOUTH, the polar axis is too 
far EAST. Rotate the polar axis in azimuth to the west.

If the star on the eastern horizon drifts SOUTH, the polar axis is too LOW.

All of this up and down stuff is meaningless, because it depends on the 
orientation of the eyepiece and diagonal if one is used, and it confuses 
the issue to no end.

Here is the absolutely simplest way to determine directions in the 
eyepiece, the orientation of the eyepiece and diagonal (if one is used) do 
not matter:

Turn off the drive, the stars drift to the west.

Nudge the tube towards the north in dec, and the stars move towards the 
south in the eyepiece.

For drift aligning, the only thing that matters is DEC drift, north or south.

The mount does not have to be perfectly level to successfully drift align 
if repeated iterations of a star on the meridian and horizon are performed, 
but if the mount is not level, any adjustments in azimuth or altitude will 
cause an error in the alignment of the other component.

To prove this in a thought experiment, just imagine a mount where the 
azimuth is not parallel to the horizon, take an extreme example to make it 
easy to visualize, say the azimuth is perpendicular to the horizon, then 
any azimuth adjustment will obviously affect the altitude also.  If the 
mount is not level, the azimuth is somewhere between 90 degrees 
perpendicular to the horizon and parallel to the horizon, and any azimuth 
adjustments will cause errors in altitude, just to a lesser extent than if 
it were perpendicular.







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