24 inch telescope project - alternative design

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Chanlunlun
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24 inch telescope project - alternative design

文章 Chanlunlun » 週二 13 1月, 2004 19:19

Dear ATMs,

For the 24 inch telescope that was proposed yesterday there exists an alternative design.

The main idea is to replace the f/3.5 primary mirror with a f/7 one. The focal length will increase from the original 84 inches to a 168 inches ( 14 feet !!). The telescope will be extremely long, violating goal #2.

In order to reduce the length especially the eyepiece height, the light path must be folded which leads to a so called 「folded Newtonian」 concept. Such design becomes increasingly common among building of big amateur telescopes in foreign countries.

The only advantage of the alternative design is that instead of dealing with the difficult 24 inch f/3.5 mirror we are now replaced by the f/7 mirror which is much more comfortable and easy to work with.

However there also exist many disadvantages which include

1. An extra optical flat of diameter 8 inches is required. The making of this flat may compensate the labor saved in switching from an f/3.5 mirror to a f/7 one.

2. The 8 inch optical flat is mounted at top of secondary cage and this will raise the center of mass of the telescope. The end result is bigger unstability which require stronger structure. Stronger structure means heavier weight.

3. The whole telescope will have a total length of 112 inches, still more than 2 feet longer than the classical design.

4. There are three reflections, which implies that the view seen from the eyepiece is laterally inverted. What you see in the eyepiece not longer orientates the same manner as those printed in star chart, making location of faint galaxy difficult. Faint galaxies are the main target of this equipment.

5. Severe light loss. The 8 inches flat produces an 11% of light loss, an extra reflection also produces another 11% light loss. There is a totally 23% light loss compared to the classical design.

What a high price to pay for in order to save labor to make the primary mirror. The two mirror blanks has been ordered and work will be started within few days and I』ve still not made up my mind which design should be chosen.

Can anyone help me to choose ?

Please advice me if you can, all your opinions are welcomed.

Best regards

Chan Yuk Lun

P.S.
diagram :- the light path of the 24 「 f/7 folded Newtonian
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Wongsir
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文章 Wongsir » 週二 13 1月, 2004 21:47

Mirror God,

Are you going to grind the optical flat yourself ?
Making a 8" optical flat is really a challenge!

Wongsir
13-1-2004

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Wah!
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文章 Wah! » 週二 13 1月, 2004 23:23

點解第二次反射的要用平面,而唔用凸面呢?(好似有種卡式鏡係咁樣)
我(估~)覺得磨短焦的主鏡加細凸副鏡,比起磨長焦主鏡加大光學平面來得易.

Chanlunlun
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文章 Chanlunlun » 週三 14 1月, 2004 11:11

Wongsir 寫:Mirror God,

Are you going to grind the optical flat yourself ?
Making a 8" optical flat is really a challenge!

Wongsir
13-1-2004
Dear Mr Wong Sir,

It is not that difficult to make an optical flat, since we can conveniently use the interference test to judge the figure during polishing.

What really difficult is to grind and figure the 24" f/3.5 parabolic mirror, it takes a lot of labor and there is no simple optical test to judge the figure of such a big and fast mirror during polishing/figuring.

Anyway thanks for your concern. At the moment I incline to accept the classical Newtonian rather than the folded design. I wish to hear more comments from you, if you are please to supply.

Best regards

Chan Yuk Lun
14-1-2004

Chanlunlun
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文章 Chanlunlun » 週三 14 1月, 2004 11:45

Dear Mr Wah,

Thank you for your comment. However there are two points I would like to stress :-
Wah! 寫:點解第二次反射的要用平面,而唔用凸面呢?(好似有種卡式鏡係咁樣)

Whether it is easy or not to fabricate a particular shape of optical surface depends almost entirely on whether there exists a practical method for testing that surface. For a Cassegrain telescope the secondary surface is a convex hyperbolic and unluckily there exists no simple method to test such surface. Thus it is very difficult to figurer a hyperbolic convex surface by amateur.

On the other hand a flat can be conveniently tested by interference method. All you need is a just a table lamp with an energy saving bulb. Thus to make an optical flat is rather easy.

Rumour has been spreading widely among ATMs in Hong Kong that it is extremely difficult to make an optical flat. Rimour is just rumour, if you have tried to make one, you will soon learn how easy it is. It is much more easier than making a quality parabolic mirror.

我(估~)覺得磨短焦的主鏡加細凸副鏡,比起磨長焦主鏡加大光學平面來得易.
Big mirror is extremely difficult to figure especially when it has a short focal length. For example in figuring a 4" f/6 parabolic mirror using the Foucalt test the knife edge position can have an error as big as 50% to meet the 1/8 wave criteria. However for a 24" f/3.5 mirror the tolerance reduces to only 0.2% to meet the same 1/8 wave criteria. For a 24" f/7 mirror the tolerance raise to 0.8% but is still very tight.

Thus it is not practical to test big mirror by Foucalt test, a Ronchi test is also useless.

Another disadvantage of Cassegrain is that it has a too long a focal length, making it impoosible for low power wide angle deep sky observation.

By the way if a "small" convex secondary is used, that "small" convex secondary mirror still have a diameter of about 7 inches. Look at the proportion of the secondary of C8 and you will know how "small" the secondary convex should be.

Finally thanks for your comment. You are welcom to supply further at any time you wish.

Thanks

Best regards

Chan Yuk Lun

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