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Equipment Discussions >> Cats & Casses

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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: My First SCT new [Re: aa6ww]
      #5680764 - 02/15/13 07:15 AM

Not really funny. Just optical engineering. But you are an engineer, so I should think you'd understand that.

On the other hand, I had a engineer working with me who continually asked me to correct problems on his computer and show him how to use software. Finally I became aggravated and told him, "You're the engineer. You should be telling me how to do this!" He answered, "I'm a civil engineer, not a computer engineer." I guess there are engineers, and then there are engineers.

Newts have SA corrected by having the primary figured into a parabola. SCTs have SA corrected through use of a corrector plate. The primary in an SCT has a spherical figure. It stands to reason that the corrector plate will correct the SA optimally within only a certain distance range from the primary. Position the eyepiece outside that range and SA may be introduced into the optical system. A specific observer might not notice it, but it will be there. It would probably be most noticeable at higher magnification when observing - or attempting to observe - details on the Moon or planets. Probably, though, the worst effects from increasing the focus are vignetting, reduction of clear aperture, and increase in CO ratio. These can also decrease resolution and perceived contrast for the image.

If you mostly view DSO at low power it might not be such a big deal. For example, I have a 4.5" f/4.4 Ball Scope with a spherical mirror. I've gotten decent low-power rich-field images of DSO with this little telescope. It gave me one of my best views of the Double Cluster and Stock 2. And years ago it was the first telescope which showed me M78 here in red zone suburbia. But try to pump up the power for fine detail on the Moon or planets? Forgetaboutit.


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