[sac-forum] Re: --[AZ-Observing]-- Re: Good Seeing Predicted for Tonight

-- "Spencer wrote: 
> Forgive my ignorance. I can, conceptually, grasp the benefits of longer
> focal lengths for detailed imaging.  But, I cannot visualize the
> mechanics of how you're varying FL to this extent on your Newtonian.  


Darrell,

To increase focal length, I am simply placing a Barlow lens in the system. For 
the Mars image, I used a 5x Powermate, which would indeed be only 5x if I 
inserted an eyepiece into it.  In fact, there is a color filter wheel 
downstream of the Barlow before the camera is inserted.  This increased 
projection distance results in the 5x increasing to about 7x in practice.  So 7 
multiplied by the native focal length of 1400mm gives you about 10 meters of 
focal length.  The bad news is that the focal ratio then becomes f/38.

Tom

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