Re: ATM Alt-alt telescope mount - and image rotation

["Dwight K. Elvey" <dwightk.elvey@amd.com>]

Hi
 I haven't been following this thread so I hope this
is new information. Field rotation of Alt-Az is not
constant with time but is cyclic over a 24 hour time.
 There was some talk about this on sci.astro.amateur.
I described some thought problems to demonstrate this.
 Lets assume that we have a telescope siting right on
the equator and we track a star that goes right overhead.
To track this star, we just run the Alt at a constant
speed. We don't need any field rotation.
 Now, lets assume that the star we want to track is 1 degree
south. We know that we will now have to use both Alt and
Az drives. We know that this will cause field rotation as
well but lets look at how it is effected.
 As the star comes up from the horizon, there is little
rotation since it is almost on the same track as the
other star was. As the star just reaches over head, the
Az drive has to track quite fast, while the Alt drive
now changes direction. As you can see, the field rotation
has suddenly increased in speed to keep up with the
fast rate of the Az drive.
 This was an exagerated case but demonstrates that field
rotation isn't constant and one needs to know the rates
and positions of the Alt and Az drives, as well as the location
to determine the specific rate at that instant of time.
 I haven't worked out the formula for this but I hope this
will keep others from thinking it might be something as
simple as the sine of the angle or something.
Dwight


>From: mdholm@telerama.com
>
>
>Tom,
>
>I just (literally) did some hand waving and convinced myself that you would get 
>less field rotation with first axis east-west rather than north-south. It 
>probably depends on latitude.  Below 45 degrees this may be true.  Above 45 
>degrees the north-south orientation would yield less rotation.  At the equator, 
>a north-south, alt-alt is an equatorial.  At the poles, an alt-az is an 
>equatorial.
>
>No orientation other than equatorial gives no rotation.
>
>Sorry, I don't have the equations for that transformation.
>
>Mark Holm
>mdholm@telerama.com
>
>