[sac-forum] Re: --[AZ-Observing]-- Re: Good Seeing Predicted for Tonight
- From: Tom Polakis <tpolakis@xxxxxxx>
- To: sac-forum@xxxxxxxxxxxxx
- Date: Wed, 6 Feb 2008 15:24:19 -0500
-- "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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