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



>From: "Jack Schmidling" 
>To: atm{at}shore.net
>Subject: Re: ATM Focuser Tilt
>Date: Tue, 24 Jun 2003 07:32:49 -0600
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>From: "Dwight K. Elvey" 
>
>>  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.....
>
>We seem to have a cause and effect problem here.  I never suggested that
>coma was caused by decentering obstructions.
>I am saying that improper colimation produces coma and this can also be seen
>as a decentered diagonal shadow.
>
>I do not doubt that somewhere out there is the on axis star and there will
>be no coma and the diagonal will be centered when defocused.
>
>The point of collimation is to get that star or position in the center of
>the field...no?
>
>All I ever claimed is that if the diagonal shadow is not in the center of of
>an out of focus star in the center of the field, the scope is not colimated.

Hi Jack
 This is not true. A Newton will always have an off centered shadow
of the secondary. The amount of noticeable off center increases the farther
you are away from focus. If it doesn't look off centered when sufficiently
out of focus, the telescope has problems. It could be a bad focuser, as an
example.
 A properly collimated telescope should show about the same amount
of secondary offset on both sides of focus for the same size of observed
secondary shadow.
 Just because you can see secondary shadow offset does not mean that
the telescope is not collimated. If it is not collimated, you may see
excessive offset of the secondary shadow. Still the secondary shadow is not
the defining factor, since it is known to have some offset until one is
close enough to focus that the diffraction effects dominate.
Dwight


>When brought to focus, this star will exhibit a "flair" if
you do not want
>to call it coma.
>
>> I still think you are misunderstanding something about the alignment.....
>
>No doubt but that is the point of this discussion.... to under stand it.
>
>>  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.
>
>You lost me on this one.  I see the shadow offset in different directions on
>either side off focus.
>
>js
>
>
>
>

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