星點測試不可作準?

檢測望遠鏡光學部份,赤道儀機械組件,目鏡配件和拍攝器材 CCD等...
Chanlunlun
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文章: 3785
註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週三 17 6月, 2009 13:40

Dear Subaru,
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. .
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:
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.
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.

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

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Wah!!
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文章: 13264
註冊時間: 週一 15 3月, 2004 21:28
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文章 Wah!! » 週三 17 6月, 2009 14:21

Chanlunlun 寫:.............
The method of aspherize lens surface to eliminate high order spherical aberration has been widely employed in in production of premium grade photographic lens.........
I've heard "high order spherical aberration" from you for many times, but actually I don't know what does the "high order" mean? :roll: :roll: :roll:

Chanlunlun
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文章: 3785
註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週三 17 6月, 2009 15:40

Wah!! 寫:
Chanlunlun 寫:.............
The method of aspherize lens surface to eliminate high order spherical aberration has been widely employed in in production of premium grade photographic lens.........
I've heard "high order spherical aberration" from you for many times, but actually I don't know what does the "high order" mean? :roll: :roll: :roll:
Dear Mr. Wah !,

Mathematically speaking spherical aberration of any optical system could expressed as an infinite series in terms of radius of entrance beam. The coefficient of the first term in that series is called first order spherical aberration and so on.

Physically speaking spherical aberration can be cancelled by combining different lenses together. However this cancellation can never be complete. The remaining uncancelled spherical aberration is collectively called high order spherical aberration.

The only way to eliminate this residue spherical aberration is to aspherize the surface of a lens, just like to aspherize a spherical mirror into a paraboloid.

Best regards
Chan Yuk Lun

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Subaru
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文章: 6638
註冊時間: 週三 02 7月, 2003 19:27

文章 Subaru » 週三 17 6月, 2009 20:46

Chanlunlun 寫:Dear Subaru,
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. .
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:
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.
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.

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
Haha, then according to your logic, the star test cannot reveal all on axis aberrations for a telescope, since the Achromats are not perfect in terms of optical correction. BTW, the Achromats do have Spherochromatism, since difference colours have different SA corrections. Thus the star test (which I respect, expecially the in-focus one), where the before focus and after focus step, is only one of the test methods but is not the ultimate one to judge the on-axis performance of a telescope.

For aspherizing the lens elements, I do believe the premium manufacturers are taking the production considerations in terms of the cost, overall optical correction, and smoothness of the curve, to archieve a balanced product for the customers.

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