Non-perpendicular error

[jambouk]

This question does not relate to a problem I am having, I am just interested to know more about it.

1. Is there a way to work out how much NPE there is in a mount?

2. How does a mount differentiate this from cone error during a three star alignment? Or does the mount only work out the cone error during a 3 star alignment and just uses the value of NPE left in the handset?

James

 

[vlaiv]

NPE and Cone should have different "signature". Don't know how its related to 3 star alignment, but here is diagram that can explain difference between the two:

Cone is named after "cone" that optical axis is covering in the sky:

NPE will have different path on the sky when tracking (both of perfect trail and to that of cone):

So if you observe circles painted on celestial sphere - cone one will be "inside" (or "outside") of perfect - and "parallel" to it - they will not cross. (higher dec or lower dec than should be with constant offset).

NPE will cross at two points, and half of it it will be on one side and other half on the other side (higher dec for half revolution around RA then lower dec for half revolution of ra with two points of exact dec on opposite side of ra - actual DEC difference will be a sine curve thru ra revolution).

[jambouk]

Thanks for that. I'd previously thought about the cone with cone error, but not with NPE, so that is useful. Having both errors would be weird, and I guess that links back to my original thoughts of how does the software differentiate the two. Maybe the machining and assembly of modern mounts mean there really is minimal NPE, and that even if a small amount  is present it is just so insignificant it probably wouldn't be noticed by the user. Maybe.

James

 

[vlaiv]

I just realized that my description is not correct.

There still are two circles as explained - but when scope is moving in DEC - and it gets more complicated than this - both cone and orthogonality error need not be aligned - in RA/DEC plain (true RA/DEC plain). Both can be at a certain angle to it, so overall behavior can be quite different - impact both RA and DEC apparent / true position in mutually dependent way.

I think that 3 star align is really not concerned with cause of error, and is trying to correct it based on error it creates rather than cause of it - by measuring true / supposed position and their difference, so it is very likely that it can compensate regardless of actual cause, but I also think that it can't compensate fully unless error is due to very trivial case of either.