Originally posted by Edgar_in_Indy Very enlightening...it sounds like you are extremely knowledgeable on the subject. Based on your experience with mirror lenses, would you say that the images I posted of the panda vase at the beginning of the thread are normal for a 500mm mirror lens shooting a subject approximately 12-15 feet away and without a lens hood? Or does it look like the lens is not performing properly?
The examples you post are exceptionally poor, I think. But I must confess, that I never used such a poor lens, so I cannot say for sure, whether this is a normal behaviour or you fell victim to sample variation. The Tokina 500(8 is really not that bad at near distances and the Russian Rubinars also, don't behave that bad (there is even a dedicated "macro" version available of the Russian 1000/10 mirrors). But if I look at the washed-out Pandas, I am also not sure, that the focus is really on the Pandas. If I look at your image, I have a slight feeling, that the focal plane lies somewhat in front of the Pandas, at least the somewhat better sharpness of the paper seems to indicate that?
You can yourself read-up on mirror lenses easily, by the way. There are several quite readable books, which cover the properties of these lenses in depth:
- "Telescope Optics - Evaluation and Design", Harrie Rutten + Martin van Venrooij - the real classical book on these subjects
- "Applied Optics and Optical Design" by A.E. Conrady - the pre-computer design must-have textbook(s), today cheaply available by Dover Publishing
- "Astrooptik" by H. Laux, but I fear it is only available in German, but great info on those more esoteric designs, like the Richer-Slevogt catadioptrics.
Also exactly the issue of the inter-mirror distance was the topic I corresponded about with the original designers of one of my telescopes, when I rebuild that a couple of years ago.
Ben