This is not true. I tested those old ASTRO R76 and R60 refractors ( theuy are just achromats ) many times. They all passed the star test. They have huge secondary spectrum by nowaday standards.Subaru 寫: 由於所有用refractive element的系統有會有Spherochromatism, 所以不能單靠用star test的焦點前後測試去斷定望遠鏡的好與壞, 除非支鏡完全沒有色差. .
Actually a photovisual system can be made to pass the star test without sacrificing off axis performance. The method is to make one ( or more ) surface into an aspheric surface. Up to now no refractor manufacturer ( may be Takahashi is the exception ) , including AstroPhysics, take this step. So all three elements APO systems must bound to have high order spherical aberrations ( unfortunately they are not negligible ) and they are reflected in star test.Subaru 寫:
而現在的折射鏡, 由其係Astrophysics果d, 在設計時已經要想到拍攝時的要求, 所以有好多近代的APO鏡已經係望遠鏡同astrograph的兩用系統, 要balance好多東西, 很難單為了焦點前後的star test效果而去做.
The method of aspherize lens surface to eliminate high order spherical aberration has been widely employed in in production of premium grade photographic lens. Unless there is a very big maeket demand, production of aspheric lens element, however, is very costly.
Best regards
Chan Yuk Lun