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



>From: "Jack Schmidling" 
>
>From: "Mel Bartels" 
>
>> No.  What you are saying is that any offset shadow or obstruction placed
>> in front of the tube would magically produce coma when there was none
>> before.......
>
>Not quite but I will certainly do the experiment that  you suggested.
>
>I suspect it has something to do with the fact that this obstruction is a
>special case in that it is supposed to be at the optical center.
>
>> Coma does not result from obstructions whether centered or not.  Coma for
>> paraboloids is an off-axis issue, not an on-axis issue......
>
>I understand but explain this then....
>
>A defocused star that shows the diagonal shadow in the center of the star
>image, shows no coma when brought to focus.  All stars that display
>off-center diagonal shadows display coma when focused that exactly
>corresponds with the off-center position of the diagonal when defocused.
>
>js
>

Hi Jack
 Something is wrong here. When focused, the centering of obstructions
does not effect things like coma. Coma is caused by unequal light paths
from off axis use of the parabola.
 It may be that your focuser isn't tracking correctly. This may
cause the kinds of problems your talking about. I still think you are
misunderstanding something about the alignment. The off centered shadow and
coma may be tracking together because of something you are doing,
systematically wrong. It may just be a problem with the way your focuser
tracks.
 It may also be that you have a slight triangular astigmatism
that you are mistaking for coma. These can be tricky to distinguish when small.
 Still, the fact that you see the secondary shadow not being
offset when sufficiently out of focus that the shadow is not dominated by
diffraction, tells me that something is wrong. Dwight

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