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#1 Jeff B

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Posted 24 July 2021 - 10:26 AM

And hopefully, the answer is a short yes or no.

 

With an offset secondary, will a collimated newtonian display an offset of the poisson dot in the diffraction patterns on one side of focus?

 

Jeff



#2 belliott4488

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Posted 24 July 2021 - 03:36 PM

I'm pretty sure the answer is "no".

 

I believe the purpose of offsetting the secondary is to ensure that the reflection of the primary is fully visible through the focuser - in other words, it has to do with full illumination, not collimation.

 

I could be wrong, though, so I'll defer to anyone with greater expertise.


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#3 Starman1

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Posted 25 July 2021 - 12:40 PM

Bruce is right--a non-centered primary reflection in the secondary is not related to collimation (unless you include even illumination production as a collimation step).

In the old days ('50s, '60s), the standard guides on collimation centered the optical axis in the secondary, which resulted in the reflection of the primary appearing toward the lower end

of the secondary, yet collimation was still attained and gave good star images.

 

In the collimated scope, the Poisson spot will be dead center in the out of focus star image.

 

The only way to notice a bidirectionally offset secondary is in seeing the secondary shadow silhouette appearing non-concentric with the focuser/secondary/primary/tool reflection concentric circles.


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#4 Jeff B

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Posted 25 July 2021 - 05:56 PM

Thanks for the nice replies!

 

Jeff


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