Astronomy Photos ByAcian

2017年10月1日

光學望遠鏡

59 神託使者帕高

簡介

看着這篇文章的你們,相信或多或少也對天文有一定的興趣。也許大家也曾經嘗試過仰望星空,但你們又知道這星空之中隱藏着多少秘密嗎?其實夜空中除了一顆顆的星星,還埋藏着使用光學望遠鏡才能觀測到的深空天體和行星的表面。然而你們對光學望遠鏡又有多少認識呢?接下來文章會簡單介紹一般用以觀測天體用的光學望遠鏡的歷史及其分類。


歷史

很多人都以為是伽利略發明了望遠鏡,可是這是錯誤的。望遠鏡普遍相信是在1608年由一名荷蘭的商人漢斯・利普西 (Hans Lippershey) 所發明的。他當時利用兩塊透鏡,發現調節兩塊透鏡之間的距離後能看到放大且清晰的影像。其後意大利的科學家伽利略 (Galileo Galilei) 1609年運用其原理製作現存記載中第一台用以觀測天象的折射式天文望遠鏡。1611年,德國的科學家開普勒 (Johannes Kepler) 對望遠鏡作出改良,改善了成像的質素並且將望遠鏡的視野擴大等問題,令開普勒式望遠鏡成為現今折射式望遠鏡的主流。1668年,英國的天文學家牛頓 (Issac Newton) 製作了第一台反射式望遠鏡。反射式望遠鏡除了能夠解決折射式望遠鏡色差(見下文)問題,而且易於製造以及具有更短的鏡筒,方便攜帶。1672年,法國天文學家洛冉・卡塞格林 (Laurent Cassegrain) 將反射式望遠鏡改良,設計出現時被廣泛應用的卡塞格林式望遠鏡。1938年,德國光學設計家伯恩哈德・沃爾德馬爾・施密特 (Bernhard Woldemar Schmidt) 把折射及反射的原理二合為一,發明了施密特式折反射式望遠鏡。後期在1944,光學工程師德米特里·德米特里耶維奇·馬克蘇托夫 (Dmitry Dmitrievich Maksutov) 發明了另一種設計的折反射式望遠鏡,名為馬克蘇托夫望遠鏡(見下文)。

用途及分類

就着字面上的解釋,望遠鏡是用以觀測遠方的物體或景象,然而以天文觀測的角度來說,能看多遠反而沒有太大意義。試想像在一個漆黑的環境裏,遠處的一座燈塔跟一塊與燈塔同樣大小、同樣距離的石頭相比,哪個會更容易用望遠鏡觀測?當然是燈塔,因為石頭是不會發光的。光學望遠鏡的主要用途是將細小的影像放大、收集更多的光以及將影像變得清晰,為的是讓使用者更容易觀測一些肉眼難以看到的天體及其特徵。而光學望遠鏡一般分為三類,分別是上文提及過的折射式望遠鏡、反射式望遠鏡及折反射式望遠鏡。

1.
折射式望遠鏡

折射式望遠鏡以凸透物鏡及目鏡組成,光線經過物鏡後被折射形成影像。折射式望遠鏡一般分為伽利略式及開普勒式兩種。伽利略式使用凹透鏡作為目鏡,因此成像是正立的,然而這種望遠鏡的視野會較窄,也就是說目鏡中看見星空的範圍會較細。相反,開普勒式使用凸透鏡作為目鏡,雖然其成像是上下左右倒轉的,但是這種望遠鏡擁有更廣的視野。
折射式望遠鏡所使用的透鏡不會形成像差,使成像顯得比較清晰及銳利,同時密封的鏡身使影像不容易受到氣流影響,可以增加影像的穩定性。但缺點是會因不同波長的光線(即不同顏色光)有不同的折射率,令不同顏色的光線不能聚焦到同一點上而形成色差。此外,愈大口徑的物鏡就具有愈長的焦距,同時也會增加磨製鏡片的難度,因此製作成本會較高及難以製造出大口徑的折射鏡。


圖:折射式望遠鏡光路圖

2.
反射式望遠鏡


反射式望遠鏡以凹面主鏡、平面副鏡及目鏡組成,透過反射光線而形成影像。反射式望遠鏡一般分為牛頓式及卡塞格林式兩種。牛頓式望遠鏡中,副鏡被放置在鏡筒前端,將光線反射到側面的目鏡(見圖:牛頓式光路圖),其設計較為便宜及簡單,容易自製。卡塞格林式望遠鏡中,副鏡雖然同樣地被放置在鏡筒前端,卻把光線反射到後方,穿過主鏡中心的洞口進入目鏡(見圖:卡塞格林式光路圖)。光線反射後在鏡筒走過兩次,因而縮短了鏡筒本來所需的長度。
反射式望遠鏡沒有使用折射的原理,因此解決了色差的問題,而且製造凹面主鏡所需要的材料價格亦相對便宜。反射鏡為便於磨製,一般會採用球面鏡的設計,以致平行進入望遠鏡的光線不能完全聚焦於同一點上,形成球面像差(見圖:球面像差/球面像差成像),影像變因此變得模糊。此外副鏡架也會阻擋着部份光線進入,加上光線會進行繞射(光線遇到障礙物時會偏離原來直線傳播的物理現象),因此會減低了集光力。
     


   圖:牛頓式光路圖  


          圖:卡塞格林式光路圖


圖:球面像差 
   

                                    圖:球面像差成像(中心圖像為没有球面像差)

3.
折反射式望遠鏡

顧名思義,折反射式望遠鏡結合了上述折射及反射鏡的原理。以卡塞格林式望遠鏡作為基礎,在副鏡的同一平面上加上特殊形狀的折射透鏡,以彌補反射式望遠鏡的不足。折反射式望遠鏡一般分為施密特-卡塞格林式及馬克蘇托夫-卡塞格林式兩種。前者的構造使用施密特修正板(特殊形狀的透鏡)以改善球面像差造成的問題。後者的構造則使用球面透鏡以修正主鏡造成的像差,雖然修正鏡較易磨製,但卻比較重。
如上述所說,與反射式望遠鏡比較,折反射式望遠鏡的優點在於球面像差得到修正,加上跟折射式望遠鏡同樣密封的設計能增加影像的穩定性及防止塵土進入鏡身。此外,由於光線進入透鏡後在鏡筒內反射兩次,因而大大縮減了鏡筒本來所需的長度,使望遠鏡方便攜帶及較容易製作大口徑的主鏡片。然而跟反射式望遠鏡一樣,其副鏡會阻擋部份光線進入,減低集光力。
     
                

              圖:施密特-卡塞格林式光路圖        ​        圖:馬克蘇托夫-卡塞格林式光路圖
總結


閱讀過這篇文章後相信大家都對光學望遠鏡有更深入的認識,以及了解到不同種類的光學望遠鏡各有其優缺。天文愛好者可以就着自己觀測天體的興趣及經濟能力等因素決定使用或購買什麼類型的望遠鏡,以更深入探索星空中隱藏着的秘密。


2017年8月23日

Finding Aliens


59 紙皮


Introduction


Ancient people were fascinated by the dark sky, most of them wanted to understand the meaning or the mystery of the universe. Apart from that, they also wanted to find other lives which were similar to human. “What is it like out there?”, asked the ancient people when they looked at the starry sky. It was perhaps a simple question like such that ignited the very first desire for astronomical research in the human history. Among various subtopics like the form of existence of the universe, the possibly surviving outer space creatures have also been grabbing much attention. Specialised studies on such a topic using scientific approaches are known among astronomers as Search of Extra-Terrestrial Intelliengece (SETI).


Drake Equation


In 1960, a young radio astronomer Frank Drake performed the first SETI experiment. He conducted the world’s first microwave radio search, aiming for receiving signals from other planetary systems. Drake used an 85-feet West Virginia antenna to point to the direction of stars Tau Ceti and Epsilon Eridani, which are in the constellations Cetus (Whale) and Eridanus (River) respectively. These two stars were chosen because they were relatively close to the Sun. Although he was not able to detect any reply, this experiment still aroused others’ interest among the astronomical community. This experiment was later named “Drake’s Project Ozma”.


20m-1.jpg
Figure 1:  The antenna used in the first SETI experiment, pointing to the direction of stars Tau Ceti and Epsilon Eridani


In 1961, Drake asked, “what do we need to know about to discover life in space?” This question helped him to set the “Drake equation”. According to the web page of SETI Institute, the annotations of the unknowns in the equation are as below.
N = The number of civilizations in the Milky Way Galaxy whose electromagnetic emissions are detectable
R∗ = The rate of formation of stars suitable for the development of intelligent life
fp = The fraction of those stars with planetary systems
ne = The number of planets, per solar system, with an environment suitable for life
fl = The fraction of suitable planets on which life actually appears
fi = The fraction of life bearing planets on which intelligent life emerges
fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = The length of time such civilizations release detectable signals into space


_211410578_orig.jpg
Figure 2: Drake Equation

Applying the equation given the estimated parameters, we could roughly get the estimation of the number of intelligent lives. Due to limitations in technology, the value N might not be as reliable as at that time, N was estimated to be 50000! However, the equation undoubtedly involves many factors which are difficult to be measured precisely. For instance, the first three variables might be affected by different research in astronomy, organic chemistry and evolutionary biology. Also, the latter variables related to the lifespan of civilization involve many uncertainties, such as the extent on economic, political and even social developments within the civilization. Although this equation consists of many uncertain factors and the estimation of number N is not rigorous, which might be very small, astronomers still believe that extra-terrestrial lives exist in outer space.


If N is as large as 50000 aforementioned, why can we not detect any signal from aliens? Some arguments describing this question were suggested If the Drake equation is true, the Earth should have already been visited by extra-terrestrial aliens. Two main hypotheses trying to explain this the question: Intelligent extra-terrestrial life is extremely rare and those civilizations have not contacted or visited the Earth.


Intelligent extra-terrestrial life is extremely rare


Biological evolution is rare or even unique to Earth. No other intelligence or life can be formed in other planets. The biological evolution requires different factors, such as whether a planetary system has those certain properties. For instance, if a right sized terrestrial planet exists in a circumstellar habitable zone,  simple life might evolve. Although it is possible that complex life is evolved from simple life, the probability of successful evolution to complexity is extremely low. Even if they are intelligent, they may not have advanced technologies to communicate with human beings.Along with non-intelligent life, those civilizations are therefore difficult for us to detect. Moreover, in extreme cases, if their technology is advanced, it is the nature of intelligence to destroy themselves or others, by using their technology to harm themselves or others. They might have declared wars, contaminated the environment or even created poor artificial intelligence to destroy the planets. Also, new life, no matter it is intelligent or not, will normally die out in the course of time. The species may die out due to the dynamic and varying planetary environments They may suffer from their planet’s catastrophes such as massive volcanic eruption or astronomical disaster like gamma-ray burst. The above arguments could possibly explain the reasons why no signal are received so far.


Intelligent civilizations have not contacted or visited the Earth


The other way to resolve the Fermi paradox is by stating that ET intelligence does exist but they have never contacted or visited us. Star may be too far away for meaningful two-way communication to be made. Right now, the furthest signal sent by the human has only travelled 50 light-years and no reply is received yet. This may imply that two intelligent civilizations are too far away that our signals have not reached them or they have not sent their reply. If the two civilizations are that far, say several thousand light years, the human might be possible to detect their existence but it might be less possible to communicate with them because of the far distance. A renowned speculation by astronomers Sagan and Newman suggests that if other civilizations exist, and keep transmitting signals to explore other civilizations, their signals and probes simply have not arrived yet. Another possible reason is that transmitting signals throughout the galaxy physically maybe very expensive. The recent scientific knowledge (special relativity) tells us that nothing can be accelerated beyond the speed of light in vacuum. The cost of materials and energy for interstellar ventures may be so high that it is unlikely to be affordable to any civilization. Even if their technology is feasible for communication, other civilizations may only receive but not understand the signal. Receiving the signal is easy but the solving the riddles it carries is difficult. Apart from that, “What should we say?”, “Who is going to speak for the Earth?” would be the most concerned. Because of these, consultations on different countries and transmission among different planets take time and a civilization may just become extinct in the process. Due to these factors, intelligent civilizations might not contact or visit the Earth.


Conclusion


Although astronomers currently have not detected any signals from aliens yet, their studies will continue. In fact, we have done some experiments on searching aliens. The electromagnetic waves to search for signals is mainly used in some famous experiments including sending Arecibo Message and directly observing the planet. A careful search for non-natural radio emissions from space may allow us to detect possible alien civilizations successfully. In the future, we might detect some “useful” signals and aliens might actually be in contact with us.


Reference:
1.Drake, F. (1961). The Drake Equation. Retrieved from www.seti.org/drakeequation
2.Ward, P. D.; Brownlee, D. (2000). Rare Earth: Why Complex Life is Uncommon in the Universe (1st ed.). Springer
3.Webb, S. (2015). If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY? Springer
4.Zaitzev, A. (2006). The SETI Paradox. Retrieved from https://arxiv.org/abs/physics/0611283

2017年5月5日

火星與生命



59 Matthew


簡介

從古到今,都有一些關於火星生物到訪地球的傳聞。在科幻電影橋段中,也常有火星人侵略地球的畫面出現。一直以來,人類都對這地球的鄰居存有不少幻想。這火紅的星球離我們這麼近,環境亦與地球十分相近,到底它有沒有可能存在着或者曾經孕育過生命呢?



火星適居性

關於火星的適居性,科學家普遍認為太陽系中有一個適合生命生存的範圍,稱為適居帶。而被納入這範圍的條件包括它能否保存液態水、它的表面溫度等。一直以來,科學家不斷嘗試界定適居帶的範圍,邁克爾·H·哈特(Michael H. Hart)1978年認為適居帶的內邊緣距離太陽0.95AU(1),外邊緣是1.01AU。其後在1992年,詹姆斯·坎斯特(James Kasting)得出不同的結果:他把內邊緣定在距離太陽0.850.95 AU之間,而外邊緣則定在1.371.67 AU之間。在內邊緣以內,行星中的液態水會因高溫而蒸發;相反在外邊緣以外,低溫把行星表面的水凝結成冰層,令生物缺乏液水的供應。
火星文明


17362008_1600022513360779_7512387773311889363_n.jpg
圖一:綠色範圍是太陽系的適居帶


圖一是科學家認為的太陽系適居帶。火星距離太陽大概有1.5 AU,是位處適居帶的外邊緣,理論上只是僅僅有存在生命的可能,因此有關火星適不適合生命生存其實仍有爭議。大氣層在孕育生命上擔當了一個重要角色,它能形成溫室效應,將來自太陽的熱力保存,這些熱力會轉化成為能量供給生命和讓水能保持在液態,為生命提供液態水。然而,火星的大氣層極其稀薄,原因是火星的體積太小,引力不夠,以及缺少了磁場(有科學家指是39億年前受巨型小行星撞擊,破壞了火星產生磁場的鐵芯的熱流),所以不能阻止空氣被剝離到太空當中。但這又是否代表火星不適合生命居住呢?卻又未必。因為在擁有液態水及微薄大氣層的環境裏,微生物仍然有機會生存。況且,這適居帶的範圍並不是永遠保持不變的,它會隨著太陽的變化而移動。在往後的日子裡,太陽會變得更熱且更快地燒掉它餘下的氫供給,每11億年就會光亮10%,而且其體積會變得越來越大,適居帶因此而外移,火星亦會變得更適合生命居住。

早期研究

早於17世紀,荷蘭天文學家克里斯蒂安·惠更斯(Christiaan Huygens)撰寫了一本名為《Cosmotheoros》的著作,他認為宇宙中充滿著外星生命,亦指出了一些其他星球存在外星生物的要素,例如必須要有液態水,而且水在不同星球會有不同的性質,以保持液態。但這似乎都只是他的猜想,因為他並無提出任何不同星球的水的具體性質。而他在火星的表面上觀測到陰影和亮點,都是他認為火星有水並且存有生物的證據。他又嘗試從宗教角度推論,認為既然上帝創造其他行星的原因就是為了孕育生命,而且外星生命沒被聖經否認,所以猜測其他星球有生命的存在。他的理論在當時是屬於異數。
18世紀,英國天文學家威廉·赫歇爾(Frederick William Herschel)對火星做了一些研究,再與地球作比較。他發現火星無論是一日的長度、旋轉軸傾斜的角度,以及擁有季節變化這些特徵都與地球十分相似。種種有趣的比較,令赫歇爾產生了火星有生命的想法。
到了19世紀,義大利天文學家斯基帕雷利(Schiaparelli ,Giovanni Virginio)仔細觀察火星,他在火星表面上隱約看到了一些筆直的線條,連接著陰暗的地區,就像地球上的河流連接著大海,且線條如此直令他聯想到是人工的產物。由於1869年蘇伊士運河完工(當時的工程奇蹟),他的發現被認為是火星上大規模的人工結構。有趣的是,他把這些線條稱為“canali”,即義大利文的水道,但這字傳播開來的時候,被誤譯為“canal”,即英語的運河,故他的發現被後世稱為火星運河。這發現令火星有生命的猜測傳得更激烈。這些猜測於1913年終於被推翻,人們發現這些運河其實只是因為當時望遠鏡質素欠佳而造成的觀測錯誤。

17425995_1600022503360780_8712691413829718630_n.jpg
圖二:斯基帕雷利製作的早期火星地圖

現今發現

2011年,佐治亞理工學院研究生露捷德拉·歐嘉(Lujendra Ojha)和研究人員在火星照片中發現了一些沿著隕石坑,峽谷和山脈的斜坡下降的黑條紋。條紋會在温暖的季節裏變深,在冷凍的季節裏變淺,最後消失,然後在翌年再次出現,他們把這些條紋命名為季節性斜坡紋線(Recurring slope lineae,簡稱RSL)。其後NASA透過火星勘測軌道飛行器(Mars Reconnaissance Orbiter,簡稱 MRO)上的成像光譜儀來研究這些RSL的光譜,他們發現當條紋褪色時,水合鹽的跡象亦隨之消失。因此他們認為這是火星上有流動液態水的證據。

為了更深入了解火星的氣候、地質等,以研究其「可居住性」,NASA派出的「好奇號」於201286日降落火星,展開探索。蓋爾撞擊坑(Gale Crater)是好奇號的主要探索範圍,該撞擊坑是方圓數千公里內的最低點,科學家認為水曾經匯集到坑內,也滲入地下。他們認為,即使地表水乾了,地下水也可能持續存在,令生命得以長期維持。好奇號也在火星上首次檢測到元素硼(Boron)。在地球,硼(或它的其他形態)是核糖核酸(RNA)(2)形成的一個組成部分,通常發現在乾旱的地方與蒸發的水。若火星存在硼,而當時的地下水的溫度及酸鹼值合適,火星便有可能存在生命。
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圖三:好奇號在火星岩石中檢測到高濃度氧化錳

現時火星的大氣層是以稀薄的二氧化碳為主,但好奇號在火星岩石中發現的化學物質卻證明了過往這星球上曾有更多的氧氣。好奇號在火星上一個叫作“Windjana”的地方檢測到含有高濃度氧化錳礦物的岩石,而氧化錳是需要有充足水份及氧化條件才能形成,這證明了很多年前的火星是有充足的液態水及氧氣。新墨西哥州洛斯阿拉莫斯國家實驗室的行星科學家尼娜·蘭扎(Nina Lanza)對火星氧氣的形成作了推測,她估計42億年前火星的水在磁場消失時分解。沒有保護磁場來屏蔽表面,電離輻射開始將水分子分裂成氫和氧。由於火星的表面重力相對較低,不能留著較輕的氫原子,因此剩下較重的氧原子留在火星表面,其後不斷累積,形成當時充滿氧氣的大氣層。


在生命漫長的進化過程中,當通過了某些階段,便會建立起文明。當我們對火星擁有生命一事上找到越來越多證據,對火星文明自然有更多的幻想,而一些在火星上拍下的照片,似乎令不少人相信火星文明的存在。
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圖四:好奇號拍到的疑似金字塔
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圖五:好奇號拍到的多個球體

好奇號在20155月拍下火星的照片並傳送回地球,照片公開後被發現內有一些不像是自然產生的物件,如圖四及五中的疑似金字塔和多個球體。以金字塔為例,有人認為完美幾何形狀表明它是一種人工創建的建築,令它在其他自然岩石中脫穎而出。不過同時,亦有反面意見指出這只是人類自己的心理作用,把本來無意義的形狀想像成人工產物,認為這些所謂「發現」只是他們的一廂情願。因此火星是否曾經有文明存在依然是眾說紛紜。



總結

儘管科學家發現火星曾經有孕育生命的條件(如液態水、氧氣),但同時稀薄的大氣層及沒有氧氣的環境體積似乎不太適合生命生存。到底火星是否存在生命似乎有待一眾科學家繼續探索,而在探索過程中,我們可將火星作為人類和地球的未來參考對象,盡力避免地球變成如現在火星般死氣沉沉的景況,以及探討人類移民火星的可能性,再進一步,更深入地瞭解生命的奧秘。



註:
1. AU(Astronomical unit,天文單位)1AU=149,597,870,700米。
2. 核糖核酸(RNA,即Ribonucleic Acid),存在於生物細胞以及部分病毒、類病毒中的遺傳信息載體。

參考資料:
1. Atkinson, N. (2015). “Was Mars’ Magnetic Field Blasted Away?”. Universe Today. Retrieved from https://www.universetoday.com/30538/was-mars-magnetic-field-blasted-away/.
2. Dunkin, S., Heather, D. (1999). “Early Observations of Mars”. University College London. Retrieved from http://zuserver2.star.ucl.ac.uk/~rpif/mitc/mitcearly.html    
3. N.A.S.A. (2015). “NASA Confirms Evidence That Liquid Water Flows on Today's Mars”. Retrieved from https://mars.nasa.gov/news/nasa-confirms-evidence-that-liquid-water-flows-on-todays-mars.
4. New World Encyclopedia contributors. (2017). “Christiaan Huygens”. New World Encyclopedia. Retrieved from http://www.newworldencyclopedia.org/p/index.php?title=Christiaan_Huygens&oldid=1003315.
5. Palma, C. “The Habitable Zone”. The Pennsylvania State University. Retrieved from https://www.e-education.psu.edu/astro801/content/l12_p4.html  
6. The Event Chronicle contributors. (2015). “NASA’s Curiosity Rover Finds a Pyramid on Mars”. The Event Chronicle. Retrieved from http://www.theeventchronicle.com/media/informational/nasas-curiosity-rover-finds-a-pyramid-on-mars/
7. Wall, M. (2016). "Mars' Atmosphere Was Likely More Oxygen-Rich Long Ago" .Space.com. Retrieved fromhttp://www.space.com/33296-mars-atmosphere-oxygen-curiosity-rover.html
8. “Circumstellar Habitable Zone (CHZ) or Goldilocks Zone”(2012). Exoplanets. Retrieved from http://exoplanets.co/extrasolar-planets/circumstellar-habitable-zone-chz-or-goldilocks-zone.html