Re: [APML] Petzval hot spot?

[Chris1011@aol.com]

In a message dated 9/18/2002 11:57:55 AM Central Daylight Time, philipp@salzgeber.at writes:


> and I always wondered why Televue offers a Petzval-Design telescope with
> about the same aperture and f/stop in the same price range like the DED and
> FSQ but without the medium-format option. In my eyes this speaks for
> Televue.


The Televue design uses an even smaller rear lens and does not produce full illumination over the 2" format. In fact, I would be willing to bet that the light dropoff starts very close to the middle of the field, but is so smooth that it is not easily seen. 

The same thing is true of most Cassegrain designs - it is virtually impossible to cover a large field without vignetting unless the secondary is enormous.

Roland Christen