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    Default A gregorian with both primary and secondary paraboloids?



    Probably a stupid question but..

    In the gregorian telescope, does the secondary have to be ellipsoid or could it also be paraboloid like the primary?

    If that's possible and considering a 8'' primary, what focal lenght each of them would have? what diameter would the secondary have?

    Thanks in advance !

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    Default Re: A gregorian with both primary and secondary paraboloids?

    No, This optical configuration needs the two foci that an ellipsoidal secondary provides. It's not difficult to make one, it just needs different math to figure than a paraboloid requires. The down side is that to use it requires a star diagonal with will invert the image, but not reverse left to right which will cause some confusion navigating the instrument. Also, glare reduction is a pain, the extra baffling can increase central obstruction in shorter instruments, even beyond what a cassegrain has. My conclusion, just build an f/6 newtonian. They are not much longer, require a shorter mount, have a smaller central obstruction, great at rich field and planetary and have only one surface to grind, if you don't buy the commonly available primary.

 

 

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