From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


Hi Ken
 If you follow the alignment procedure, the rest of the way,
you can't get this condition. Remember, you are adjusting the primary to
place the optical axis in the correct location. If the focuser itself is
bad then you can get this. In this case, the draw tube doesn't move along
its axis.
 You seem to be making the assumption that a flat secondary
can distort or tilt a wave front. It can't do this. If you have a good
focuser and you:

1. frame the secondary
2. center the primary spot with the secondary 3. adjust the primary so that
the spot will be
    on the peep hole as seen from the peep hole

 The focuser will be along the optical axis of the primary
as reflected from the secondary. Remember, the secondary is flat. It can't
tilt the wave front without shifting the axis along with it. This means
that if you can see the reflected peep hole on the center spot, you are on
the axis and there is no tilt as seen by the eyepiece with respect to the
wavefront. The wave front can only be perpendicular to the axis. The
focuser can not help but move parallel, along this axis if there isn't
something wrong with the focuser itself. Dwight

>From: "Ken Hunter" 
>
>-------Original Message-------
>From: "Dwight K. Elvey" 
>
>the focuser can be at quite large angles to the OTA and still provide
excellent
viewing.
>Dwight
>
>I agree to that, what I have issue with is the implication that the focuser
can
have a large angle with the optical axis and perform well. In that case,
the image will move (as well as defocus) when the focuser is racked in and
out. It will also show different amounts of focus across the field with no
position of the focuser giving full-field critical focus.
>
>Ken Hunter
>
>
>
>>From: "Ken Hunter" 
>>
>>
>>----- Original Message -----
>>> The focuser need not be square to anything.  The
>>> important thing is that it points directly at the
>>> optical center of the secondary (that is, where the
>>> optical axis hits the secondary, not its physical
>>> center).
>>
>>THIS IS NOT TRUE!
>>
>>You can point the focuser at the exact center of the secondary
>>(even if the focuser is mis-aligned at a 45 degree or other angle).
>>If the light coming from the Primary is reflected at 90 degrees,
>>you want the focuser pointed at the exact center of the secondary
>>(neglecting the offset) AND... at a 90 degree angle.
>>
>>The proper test for focuser alignment is to use a laser or sight tube.
>>If the laser dot or imaged crosshairs DO NOT MOVE when the
>>focuser is fully racked in and out, and it is pointed to the proper
>>place on the secondary, it is aligned properly.
>>
>>Ken Hunter
>>
>>
>>
>>
>>
>
>
>
>>
>
>Happy Grinding,
>Ken Hunter
>
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