Info

Fig. 3.41. Spot-diagrams for a Wright-Vaisala telescope of aperture 400 mm and f/4.0

Petzval sum (E SI v = 0). All systems fulfil the conditions ^2 Si = E Si I = Siii = 0, giving therefore a corrected flat field in front of the primary mirror. The different types differ only in the position of the corrector plate. Type A is the shortest - the length from plate to primary can be reduced to only 3 f' or less - but requires appreciably aspheric mirrors and a very aspheric plate. Baker also proposes a front baffle which loses some of the advantage of the very short plate length. (It is important to remember that, as a result of the stop-shift formulae (3.22), the stop position is - to third order accuracy - of no consequence in these systems because ^2 S i ... E S Iv are all zero). Types B and C are dimensionally almost identical: type B has a spherical secondary and lightly aspheric primary, type C a spherical primary and lightly aspheric secondary. Type D corrects the further condition of distortion (E Sv = 0) but requires significant asphericities on the mirrors, as with type A. As a consequence of the fulfilment of the condition ^2 S IV = 0 requiring |f2| = If |, all the Baker systems require an image position in front of the primary to achieve an acceptable obstruction ratio Ra. For a Cassegrain telescope with the following quantities defined as positive from Tables 2.2 and 2.3, we have from (2.57) and (2.58)

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment