Prism Diagonals for APQs

I have been told by a self-described Zeiss nut and very knowledgeable Zeiss-o-phile that the Zeiss APQs require the use of a prism diagonal for the best color correction. Can anyone provide a simple optics lesson explaning why this would be so? Is the difference from a mirror diagonal noticeable? If the APQs are used without a diagonal, are they no longer apochromatic?

Ben West

Ben,

Zeiss designed their telescope lenses including a 40mm glass path; somewhat like a tradition. This means for best color correction you will need such a piece of glass. Normaly one will use a prism diagonal. But you can also use a block of glass with the length of 40mm.
In my own observations with an APQ 100/640 I found that it really makes a difference if you use a mirror or a prism (prism diagonal bought from Thomas Baader, Germany).
Around brighter objects you will see color. But also with an AS 100/1000 (which was the main 100mm amateur refractor in former East Germany) you will see that the color correction of this semi apochromatic lens will be much worse when in connection with the prism.

Best Regards,

Michael Lalk
Greifswald, Germany

..forgot somethig:

"...much worse when *not* in connection with the prism.

Sorry,
Michael

Edward,

you are right, that the diffrence with APQ 100/1000 will be less, but using a APQ 100/1000 designed for the prism only with a diagonal will convert the perfect APQ colorcorrection down to the level of a FS 102 and it will also decrease visible down the spherical correction

Markus