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...

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Subaru
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文章 Subaru » 週一 15 6月, 2009 23:41

我都好認同, 如果要用焦點內外比較, 只適用於牛頓鏡.

焦點上睇Airy disc同diffraction ring就乜鏡都得.

Chanlunlun
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文章 Chanlunlun » 週二 16 6月, 2009 00:17

Dear Mr. Wah !,

I am also a fans of Richard Suiter and his book has been read by me several times. After studyng the articles by Roland it seems to me that he is trying to defense his product against the majestic star test.

Star test is very sensitive. All fast APO telescopes using spherical surfaces must exhibit high order spherical aberration and this aberration, very small though it is, is still reflected by by star test without mercy. It is well known that AstroPhysics making lens with all spherical surfaces and it shows such defects under star test. However it doesn't mean that the optics are bad - but if I can choose, I rather choose a telescope which passes the star test.

Quite contrary to what Christian Roland said, I had tried AB star test on some ED refractors recently at PTC. The one with non-symmtrical in and out-focus star disc shows low contrast on lunar viewing while the one with identical in and out-fcous star disc shows excellent contrast and resolution on lunar.

One thing I do agree with Roland - telescope manufacturers are afraid of star test because at last ATMers have a weapon simple enough to figure out who really makes excellent telescopes.

One last question - does Takahashi manufacture excellent telescopes ? Their lens always pass the star test. ( I haven't try their astrographs such as FSQ-106 )

Best regards
Chan Yuk Lun

Chanlunlun
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文章 Chanlunlun » 週二 16 6月, 2009 00:27

Subaru 寫:我都好認同, 如果要用焦點內外比較, 只適用於牛頓鏡.

.
Dear Subaru,

This is not true. Star test can be employed on all optical systems, no matter it is an SCT, Maksotov or refractor. If the telescope has no spherical aberration, energy should be distributed identically before and after focus. It is that simple. It is law of physics.

Most telescopes does not give identical star disc in and out focus but it does not mean that they are bad. But the ones shows identical star discs must be an excellent one.

Most of the C-8 ( and Meade 8 ) show nearly identical star disc in and out focus. And most of the beautiful planetary images are from C-8, or M-8, including Mr. Wah's Jupiter a week ago.

Best regards
Chan Yuk Lun

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willis
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文章 willis » 週二 16 6月, 2009 01:40


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Subaru
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文章 Subaru » 週二 16 6月, 2009 09:03

Chanlunlun 寫:
Subaru 寫:我都好認同, 如果要用焦點內外比較, 只適用於牛頓鏡.

.
Dear Subaru,

This is not true. Star test can be employed on all optical systems, no matter it is an SCT, Maksotov or refractor. If the telescope has no spherical aberration, energy should be distributed identically before and after focus. It is that simple. It is law of physics.

Most telescopes does not give identical star disc in and out focus but it does not mean that they are bad. But the ones shows identical star discs must be an excellent one.

Most of the C-8 ( and Meade 8 ) show nearly identical star disc in and out focus. And most of the beautiful planetary images are from C-8, or M-8, including Mr. Wah's Jupiter a week ago.

Best regards
Chan Yuk Lun
I do not agree on this, except on the classical, simple and pure reflective instruments.

For most of the multi-elements systems, it is a matter of balancing the Spherochromatism of difference zones, which the residual aberrations is beyond the 1st order. One example is the SCT, which the CA correction difference between inside focus and outside focus is noticed at high power. SCT never archieves 0 CA and SA, except at the 90% zone by design.

The question is even more complicated for astrographs because of the additional considerations for the flat focal plane.

Nevertheless, star test is a handy method to test the astronomical telescopes.

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Wah!!
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文章 Wah!! » 週二 16 6月, 2009 09:15

或者我換一個問題:
是否焦前、焦後的光斑相同, 就必然是一個(忽略場曲, 彗差等考慮, 只看球面差)極優良的光學系統?
有沒有例外的情況?

Chanlunlun
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文章 Chanlunlun » 週二 16 6月, 2009 09:48

Wah!! 寫:或者我換一個問題:
是否焦前、焦後的光斑相同, 就必然是一個(忽略場曲, 彗差等考慮, 只看球面差)極優良的光學系統?
有沒有例外的情況?
Dear Mr. Wah !,

Yes it is. It must. here is no exception. This is the law dictated by physics. There is no room for argument here if teh concern is ionly for SPHERICAL ABERRATION.

However star test can ONLY be employed to test ON AXIS spherical aberration. All other abberation ( may be to a certain extend longitudinal chromatic aberration) cannot be revealed by star test.

All TELESCOPE ( not astrograph, not photograhic lens, but telescope ) must have zero or very small amount spherical aberation within Rayleigh limit. Star test is very sensitive to this.

I have to repeat - STAR TEST CAN ONLY BE EMPLOYED TO TEST SPHERICAL ABERRATION. ALL TELESCOPES ( NOT ASTROGRAPHS ) , NO MATTER HOW COMPLICATED OR HOW SIMPLE IT IS, SHOULD COMPLY WITH THE STAR TEST WITHIN RAYLEIGH LIMIT.

I know that for astrographs they have to make balance between different abberation and some spherical aberration correction must be compromised. Then they are no good for use as telescope. And such system cannot pass the stat test but it doesn't mean that they are no good. They are good at taking pictures. They are designed for doing this job.

Star test is the majestic test. It shows how good or bad the correction for spherical aberration of a system, and only spherical aberration of that system. All kinds of telescope, if it is designed to visual purpose, should have , I repeat, spherical aberration within Rayleigh limit and they should pass the star test, no matter how complicated or how simple it is.

Don't be fooled by manufacturer. Be a wise consumer. If a TELESCOPE ( NOTICE : a TELESCOPE , not a photographic lens, not an astrograph, but a TELESCOPE, such as C8, ...etc ) cannot pass the star test, just forget it and buy another one.

Best regards
Chan Yuk Lun

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文章 syyuen168 » 週二 16 6月, 2009 09:53

Chanlunlun 寫:
Wah!! 寫:或者我換一個問題:
是否焦前、焦後的光斑相同, 就必然是一個(忽略場曲, 彗差等考慮, 只看球面差)極優良的光學系統?
有沒有例外的情況?
Dear Mr. Wah !,

Yes it is. It must. here is no exception. This is the law dictated by physics. There is no room for argument here if teh concern is ionly for SPHERICAL ABERRATION.

However star test can ONLY be employed to test ON AXIS spherical aberration. All other abberation ( may be to a certain extend longitudinal chromatic aberration) cannot be revealed by star test.

All TELESCOPE ( not astrograph, not photograhic lens, but telescope ) must have zero or very small amount spherical aberation within Rayleigh limit. Star test is very sensitive to this.

I have to repeat - STAR TEST CAN ONLY BE EMPLOYED TO TEST SPHERICAL ABERRATION. ALL TELESCOPES ( NOT ASTROGRAPHS ) , NO MATTER HOW COMPLICATED OR HOW SIMPLE IT IS, SHOULD COMPLY WITH THE STAR TEST WITHIN RAYLEIGH LIMIT.

I know that for astrographs they have to make balance between different abberation and some spherical aberration correction must be compromised. Then they are no good for use as telescope. And such system cannot pass the stat test but it doesn't mean that they are no good. They are good at taking pictures. They are designed for doing this job.

Star test is the majestic test. It shows how good or bad the correction for spherical aberration of a system, and only spherical aberration of that system. All kinds of telescope, if it is designed to visual purpose, should have , I repeat, spherical aberration within Rayleigh limit and they should pass the star test, no matter how complicated or how simple it is.

Don't be fooled by manufacturer. Be a wise consumer. If a TELESCOPE ( NOTICE : a TELESCOPE , not a photographic lens, not an astrograph, but a TELESCOPE, such as C8, ...etc ) cannot pass the star test, just forget it and buy another one.

Best regards
Chan Yuk Lun
Just like you did the other day for the two refractors. It was sad that the smaller size refractor did not pass the test.

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Wah!!
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文章 Wah!! » 週二 16 6月, 2009 09:54

Chanlunlun 寫:...
If a TELESCOPE ( NOTICE : a TELESCOPE , not a photographic lens, not an astrograph, but a TELESCOPE, such as C8, ...etc ) cannot pass the star test, just forget it and buy another one.

Best regards
Chan Yuk Lun
But...... how can we judge a telescope pass the star test or not?

syyuen168
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文章 syyuen168 » 週二 16 6月, 2009 10:00

Wah!! 寫:
Chanlunlun 寫:...
If a TELESCOPE ( NOTICE : a TELESCOPE , not a photographic lens, not an astrograph, but a TELESCOPE, such as C8, ...etc ) cannot pass the star test, just forget it and buy another one.

Best regards
Chan Yuk Lun
But...... how can we judge a telescope pass the star test or not?
Want to know more to test your 16 inches Dobs [疑惑] [疑惑]

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Wah!!
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文章 Wah!! » 週二 16 6月, 2009 10:10

Star test cannot give a quantitative result to the telescope.
Foucault test is the cheapest way to give us a PV error number.

苏鲁锭
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文章 苏鲁锭 » 週二 16 6月, 2009 13:18

Wah!! 寫:或者我換一個問題:
是否焦前、焦後的光斑相同, 就必然是一個(忽略場曲, 彗差等考慮, 只看球面差)極優良的光學系統?
有沒有例外的情況?
照相星点测试要明确光斑弥散大小。
离焦过大,就会掩盖误差。
目视星点检测要明确出瞳光的直径。和离焦大小。过大的出瞳(低倍率)和过大的离焦都不能发现小的误差。
对于目视星点检测,当出瞳在1MM一下。离焦在2MM以内,如果看不出星点弥散差别,这个反射镜的精度肯定优于1/20.
我用1/22的椭球面做过实验。

Chanlunlun
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文章 Chanlunlun » 週二 16 6月, 2009 13:24

苏鲁锭 寫:
Wah!! 寫:或者我換一個問題:
是否焦前、焦後的光斑相同, 就必然是一個(忽略場曲, 彗差等考慮, 只看球面差)極優良的光學系統?
有沒有例外的情況?
照相星点测试要明确光斑弥散大小。
离焦过大,就会掩盖误差。
目视星点检测要明确出瞳光的直径。和离焦大小。过大的出瞳(低倍率)和过大的离焦都不能发现小的误差。
对于目视星点检测,当出瞳在1MM一下。离焦在2MM以内,如果看不出星点弥散差别,这个反射镜的精度肯定优于1/20.
我用1/22的椭球面做过实验。
Dear Mr. So,

According to international practice, for reflector star test should be carried out at 40X per inch. That is, at an exit pupil of diameter about 0.6 mm.

1 mm exit pupil is too big.

Best regards
Chan Yuk Lun

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Subaru
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文章 Subaru » 週二 16 6月, 2009 22:12

其實Roland Christen所講的係star test用在refractors及catadioptrics上的limitation, 無講到話用於純反射鏡有何問題.

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.

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.

現時要用完全沒有色差的望遠鏡, 到目前為止都只有反射鏡才能做到.

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