LX200 Pointing Effects

Introduction

Most LX200's are shipped from Meade with a mechanical pointing accuracy of 1 arc-minute, but have the OTA tube out of alignment with the mount, or the fork arms incorrectly positioned. The tolerances for this alignment are very marginal indeed, e.g. the fork alignment for an error of only 4 arc minutes equates to a fork height inaccuracy of 0.47mm!

The aim of this document is to specify and quantify the possible errors, so that the owner can determine what exactly is causing the error and has a calculation for it.

The document is being made available to the Mapug group for comments and checking of the calculations (Math isn't my strong point :-) ).

I have also written a Draft Procedure for attaining the Meade 2 arc-minute pointing accuracy.

Regards
Mark Simpson, Version 0.9b, 9th July, 2001.
http://www.AuroraChasers.com/Astro/Lx200Alignment

Notes

This is a list of the various adjustments that can have an effect on alignment. It is assumed that your LX200 has been machined correctly, all the parts are the sizes they should be and everything is structurally sound and square.

It is assumed that your LX200 is accurate (electrically) in it's pointing ability and that it is repeatable.

All measurements should be in mm's or inches, but don't mix them.

Problem 1

Problem	Forks not in line with RA bearing.
Math	N/A
Pointing Effect	None on objects that are located at infinity, just Declination bearing wear and gearbox problems if forks not in line with each other.
Inter-Relationships	None

Problem 2

Problem	One fork higher than the other.
Math	Max. Pointing Error (Arc Min) = ( sin-1 (h / s) ) * 60. Where "h" is the amount one fork is higher than the other, and "s" is the inside distance between the top of the 2 forks.
Pointing Effect	With OTA horizontal there is no effect. When the OTA is vertical the error is at it's maximum. If you then rotate the OTA 180 degrees in RA the error is equal and opposite.
Inter-Relationships	Problem 4.

Problem 3

Problem	One fork further forward than it should be.
Math	N/A
Pointing Effect	No effect on pointing as the error is removed by the firmware when the first star is synched. Just declination bearing wear and possible gearbox problems.
Inter-Relationships	Problem 4.

Problem 4

Problem	One fork twisted forward
Math	Max. Pointing Error (Arc Min) = (sin -1 ((t - ( cos y * t )) / s )) * 60. Where "t" is the height of the forks, and "s" is the inside distance between the top of the 2 forks. "y" is the amount in degrees the fork is twisted by.
Pointing Effect	This is a combination of Problem 3 and Problem 4 (see these for further information)
Inter-Relationships	Problem 2 and Problem 3.

Problem 5

Problem	OTA twisted.
Math	Max. Pointing Error (Arc Min) = (tan -1 ( x / d ) ) * 60. Where "x" is the amount one side is offset from where it should be, and "d" is the diameter of the OTA.
Pointing Effect	Declination plane is offset from being at right angles to the RA plane.
Inter-Relationships	None

Problem 6

Problem	OTA pointing higher than it should be.
Math	Adjusted out automatically by Declination scale alignment, and by firmware when LX200 is started up.
Pointing Effect	None
Inter-Relationships	None