Quote:
Originally Posted by Rod
I’m very much looking forward to seeing this come together. A cassegrain will be one of my retirement projects so I’ll be interested to learn here. A couple of questions to start with:
I’m assuming you prefer the DK because the classical cassegrain has the complexity of the hyperboloid secondary and simple optics are easier to build to a high standard?
I’ve read some say the spherical secondary on the DK does not need collimating, although it must be well centred. Is that true?
Rod.
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You are spot on about optical complexity. That is why I compared the D-K to the Newtonian. One of the great advantages a Newtonian has over any other reflecting telescope, and few people realize, is the fact that the secondary mirror can be mass produced to very high precision without any hand figuring. Thus the typical Newtonian has only a moderately aspheric mirror that needs hand figuring. The next in line of simplicity is the D-K with its spherical secondary that can be figured against a not too difficult to make spherical test plate. The primary of the D-K is faster than a typical Newtonian primary, but the departure from spherical is only about 3/4 of the equivalent Newtonian's primary.
The D-K does need to be collimated but the collimation is far easier than any other Cassegrain configuration. The reason is that the secondary mirror doesn't have an optical axis, only a mechanical one. Any line that goes through its center of curvature can be regarded as an optical axis. And that means it doesn't even have to have its mechanical axis accurately centered on the primary mirror's axis.