Re: Orthogonality question

Roland Christen

Hi Jimmy,

Your analysis is correct.

Best way to address orthogonality is with proper shimming of the telescope so that the optical axis points exactly at the pole when the mount is properly polar aligned and the Dec axis is sent to 90 degrees.  This eliminates the polar donut hole that results if the scope is not mechanically orthogonal and allows the scope to point to all positions in the sky.

Next best way is to compensate for orthogonality with software. We do have a method to do this but have not yet implemented it in our keypad. It is fully tested and works very well for 95% of the sky, but of course cannot compensate for the donut hole of the pole. We are working on a simple keypad modeling routine and should have it done for our next update.

The routine is quite simple and straight forward. The result can be used to either adjust the orthogonality, altitude and azimuth mechanically, or it can be simply added to the CP4/5 controller as a model to compensate for orthogonal, altitude and azimuth errors. The first will get you great pointing + better tracking, the second will get you great pointing.

Beyond that we will also implement a simple tracking model in the keypad that measures the drift rate along a constant Dec line for any object which you wish to image. It will allow unguided imaging for that object over the arc that you measured. In my tests with the 17"F8 astrograph on a 1600 encoder mount I measured only 3 points along a 3 hour image arc and was able to track the object with nice round stars in a series of 30 minute exposures.

Neither of these routines is a full sky model such as you would do with APCC-APPM on a fixed observatory mount, rather they are designed for quick setup in the field with minimal effort.

-----Original Message-----
From: jimmyjujames <jimmy_an@...>
To: main <>
Sent: Wed, Nov 20, 2019 7:45 pm
Subject: Re: [ap-gto] Orthogonality question

[Edited Message Follows]
If OE is short for orthogonality error.
An Orthogonality error (OE) is only east-west when pointing at zenith or along meridian.
OE is always in line with the counter-weight shaft.
When pointing at NCP with counter-weight shaft pointing down, OE is north-south.
 What is an orthogonality error (OE)?
 With the counter-weights down and scope pointing at NCP.
 An orthogonality error will cause the scope's viewing point to be above or below NCP.
 To the left or right of NCP is not an error.
 You can move Dec to remove any left or right offset.
 Rotating RA from east side to west side will move the scope's
 viewing point around NCP tracing out half of a circle around NCP.
 Scope will hit the pier before the other half of circle completes.
 Radius of this circle is your (1x) orthogonality error.
 Diameter of the circle is 2 times (2x) your orthogonality error.
 If you move the counter-weight shaft to west side and horizontal/level with ground, the scope's 
 viewing point will be either east or west of NCP by (1x) your orthogonality error.
 If you move the counter-weight shaft to east side and horizontal/level with ground,
 you will miss NCP by 1x on the other side of NCP.
 1x on one side of NCP, meridian flip and 1x off on other side = 2x orthogonality error when
 flipping from east to west side.
 If you move scope from NCP to zenith to Dec=0 to SCP, your scope's viewing point will be east or
 west of meridian by your 1x orthogonality error everywhere along that path.
 You will have to shim the front or rear ring to bring your viewing point back to NCP.
 After minimizing your orthogonality error,
 future meridian flips should result in star in FOV on both sides.
 If not then you may have flexure and/or worm needs re-meshing and/or
 something is loose and needs tightening.
As always, I may be wrong again.

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