Optical test for the 24 inch mirror

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

Optical test for the 24 inch mirror

文章 Chanlunlun » 週四 22 1月, 2004 17:18

Dear ATMs,

As the starting date of work draws nearby, we have to decided which f/number will be employed for the 24 inch primary mirror. As we incline to accept the f/3.5 configuration there is a need to search for methods, whichever practical or not at the moment, to test the figure during polishing/figuring of the giant. After searching for a few days on the Web ( yes, on the web, this is a great advantage of the internet if you really want to do some academic works ….) there exist at least 3 three methods which may/may not serve our purpose :-

1. The classical Foucalt test – this is the most common method employed by ATMs to test their mirrors. The main advantage of this test lies on fact that despite its extreme simplicity it gives a quite accurate result, provided that the mirror is not very big and the focal length is not very short. In case of 24 inch f/3.5 the edge zone lies on 21.77 mm and the tolerance is only 0.04 mm for 1/8 wave P-V tolerance. While it is easy to read the position of the knife edge within this tolerance by a micrometer one will find it extremely difficult to locate the shadow on the mirror within the required precision. On the average, even a well experience ATM can only read shadow with error no less than 0.2 mm, the smaller the f/ratio the bigger this error is going to be. Hence the Foucalt test will not be my cup of tea.

Another drawback of Foucalt test is that it is not able to show the zones on mirror, especially when the mirror is big and focal length is short. (figure 1)

2. The Dall Null test – In this test the light beam is first let through a plano-convex lens before striking the parabolic mirror under test. The distance between the pin hole and the lens is so accurately adjusted that the spherical aberration introduced by the lens is just the right amount and opposite to that of the parabolic mirror under test. The end result is that the reflected beam just focus onto a single point again. The knife edge is cut onto this single focus and if the mirror under test is parabolic we will see the entire mirror darks out evenly. This is an excellent test for paraboloid and any zones on the mirror will reveal itself at once.

If everything is under control, this test enables us to work on a parabolic mirror down to 1/20 wave easily. As you can figure out, the ultimate accuracy of such test depends on the quality of the nulling lens. At the same time the parameter of the lens (curvature, refractive index…etc) has to be know accurately in advance . The distance between the pinhole and the lens has to be control to a very tight tolerance. Nevertheless all these are under the reach of an average amateur telescope maker and it is strange to notice that how little attention has been given to such a marvelous method.

However the biggest aperture of mirror that could be handled by such test is 16 inch. For the 24 inch under discussion there exists no single nulling lens which can compensate for all spherical aberration of the mirror under test. ( figure 2 )

3. The Offner null test – This is the one used for testing the primary mirror of the famous Hubble space telescope. Two plan-convex lens are used to compensate the spherical aberration of the mirror under test. It has been said that the spacing between the two lens ( d2) has to be in very strict control, they should be held by Invar bars ….etc. In testing the primary mirror of the Hubble space telesocpe, the distance between these two lens is just a little bit too big and the mirror figure is completely ruined.

However my computer ray tracing program shows that the tolerance is similar to that of a Dall null test and it should be under reach for almost any ATM. The Offner null test can tackle mirror of aperture up to 200 inch with f/ratio down to as low as f/2. So this will be the chosen test for the 24 inch mirror under discussion. (figure 3)

If any body comes up with ideas of how to test the 24 inch, please supply your comments. You are always welcomed.

Bestg regards

Chan Yuk Lun
22-1-2004
附加檔案
test3.jpg
Figure 3 The Offner null test
test3.jpg (17.61 KiB) 已瀏覽 11994 次
test2.jpg
Figure 2 the Dall null test
test2.jpg (20.25 KiB) 已瀏覽 11994 次
test1.jpg
Figure 1 the Foucalt Test
test1.jpg (21.2 KiB) 已瀏覽 11994 次

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Wah!
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文章 Wah! » 週四 22 1月, 2004 21:52

Where do you get the null lens?

Chanlunlun
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文章 Chanlunlun » 週五 23 1月, 2004 09:24

Wah! 寫:Where do you get the null lens?
Dear Mr Wah,

The only source of quality plano-convex lens that is readily available is from the Orthoscopic eyepiece. Hence I have dismantled one of my Othoscopic collection and get the required lens. ( The lens that is nearer to the pinhole, called the compensator). I'm waiting for the other one meanwhile. ( the other is called the field lens ).

Best regards

Chan Yuk Lun

23-1-2004

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Wah!
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文章 Wah! » 週五 23 1月, 2004 13:39

你會用乜方法去控制null lenses的光軸同埋之間的距離?

Chanlunlun
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文章 Chanlunlun » 週五 23 1月, 2004 21:12

Wah! 寫:你會用乜方法去控制null lenses的光軸同埋之間的距離?
Dear Mr Wah,

In order to have a precise alignment of the two lenses and accurately monitor their spacing, we need to machine a metal lens barrel on a lathe.

Actually I've made a Dall null tester before for testing the 14 inch f/5 parabolic mirror. ( The mirror that is used in the telescope which frequently occurred in Water Mouth last year ) The Dall null tester is made by PVC for advantage of more easy handling with simple tools. The Dall null tester is being easier to fabricate since it consists of only one element. It is being more logical that the Offner tester should be made of bronze ( or aluminium ), for more precise placing and alignment of the optical elements.

Attached please find the appearance of my used Dall null tester.

Best regards

Chan Yuk Lun
23-1-2004
附加檔案
Dall null tester.jpg
Dall Null tester for the previous 14 inch f/5 mirror. This tester is one with a PVC barrel. The nulling lens is situated behind the collar at the left, with three screws for securing purpose.
Dall null tester.jpg (49.76 KiB) 已瀏覽 11947 次

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Subaru
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文章 Subaru » 週五 23 1月, 2004 22:17

Chanlunlun 寫:
Wah! 寫:Where do you get the null lens?
Dear Mr Wah,

The only source of quality plano-convex lens that is readily available is from the Orthoscopic eyepiece. Hence I have dismantled one of my Othoscopic collection and get the required lens. ( The lens that is nearer to the pinhole, called the compensator). I'm waiting for the other one meanwhile. ( the other is called the field lens ).

Best regards

Chan Yuk Lun

23-1-2004
陳sir,

But how to measure the curvature and the quality of the required plano-convex lens?

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文章 Chanlunlun » 週六 24 1月, 2004 14:09

Subaru 寫: But how to measure the curvature and the quality of the required plano-convex lens?
Dear Mr Subaru,

There exists more than one method to precisely measure the curvature of a lens surface.

The method I used is first to measure the paraxial back focal length of length of sodium light ( around 5600 amstrong ) to a very great precision. Assuming the glass material to be BK7, ( most plano-convex lens are made of BK7 or BSC2, just as most binocular prism is made of BaK4 ) by using computer ray tracing program I'm able to simulate back the lens with axactly same back focal length. The curvature of the simulated lens is the required curvature of the actual lens.

How do I know that the lens is made of BK7? How do I know the quality of the lens on hand ?

Well, I don't. I just guess.

Best regards

Chan Yuk Lun
24-1-2004

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Subaru
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文章 Subaru » 週一 26 1月, 2004 23:10

Chanlunlun 寫:Dear Mr Subaru,

There exists more than one method to precisely measure the curvature of a lens surface.

The method I used is first to measure the paraxial back focal length of length of sodium light ( around 5600 amstrong ) to a very great precision. Assuming the glass material to be BK7, ( most plano-convex lens are made of BK7 or BSC2, just as most binocular prism is made of BaK4 ) by using computer ray tracing program I'm able to simulate back the lens with axactly same back focal length. The curvature of the simulated lens is the required curvature of the actual lens.

How do I know that the lens is made of BK7? How do I know the quality of the lens on hand ?

Well, I don't. I just guess.

Best regards

Chan Yuk Lun
24-1-2004
I see.

By the way, I believe medium to low power DSO work does not demand the highest accuracy on the optics, so the test shall be adequate.

Final test on star tells if the telescope is ok or not.

Chanlunlun
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文章 Chanlunlun » 週二 27 1月, 2004 00:26

[quote="Subaru]

I see.

By the way, I believe medium to low power DSO work does not demand the highest accuracy on the optics, so the test shall be adequate.

Final test on star tells if the telescope is ok or not.[/quote]

Dear Mr. Subaru,

I regret to inform you that your idea is absolutely wrong. If the mirror suffers from anything more that 1/8 wave rms error, even a medium power can reveal the fault obviously.

For deep sky viewing, quite contrary to what intuition tells us, small telescope will be less demanding because they don't have sufficient light grasp to show details of the objects themselves. The majority of DSO appears just more or less the same patch of dim light anyway.

This is not the same story for big telescopes. When speaking of telescope of 24 inch class, we are speaking of how many stars are resolved in the core a globular cluster, or how many details we see inside a galaxy. All these requires resolving power together with light grasp. With telescope up to 24 inch aperture we frequently use power of more than 300X to observe deep sky objects. Diffraction limited optics is certianly a must.

Calculation shows that the Offner null test can provide test on paraboloid as accurate as down to 1/50 wave by unaided eye. Hence it is the ideal test for fast and big paraboloid. May be I've made you getting a wrong idea, the measurement involved in the Offner null test has to be very very precise- during mirror testing in the final figuring stage, the Offner null test and the star test has to be done alternatively with the lens separation in the Offner null tester adjusted step by step until both test agree on each other. Only in this way can we eliminate the errors in the tester setup.

Although the stability of the atmosphere does not always allow big telescope provides as sharp as image as small ones, it doesn't mean that the precision in making the optics of big telescope can be lowered.

This is the job of an ATM to make diffraction limited optics, regradless of its aperture.

Best regrads

Chan Yuk Lun
27-1-2004

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Subaru
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文章 Subaru » 週二 27 1月, 2004 10:12

Chan sir, I see. I believe it is a bit difficult for me to express the whole idea over the Internet.

Rayleigh Criterion is still required for any visual astronomical optics. However, I believe we don't need to too emphasis on the claimed surface accuracy just like those advertised by the manufacturers in the 80-90s.

Nevertheless, I think the quality of the Offner null test lens will greatly influence the measurement of the DIY mirror. So, it's a bit difficult to estimate the outcome based on some unknown on the tester.

Making big and short focal ratio mirror is extremely difficult and I hope you will achieve a smooth and accuracy surface on your 24".

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