Mengenal Binokular untuk Astronomi 00.08

Apabila kebanyakan dari kita ditanya alat apa yang bisa digunakan untuk melihat keindahan langit, bisa dipastikan teleskop adalah kata yang pertama kita ingat. Padahal ada alat alternatif lain yang mungkin sering kita lupakan. Yaitu Binokular, alat ini mungkin lebih dikenal untuk mengamati objek-objek terestrial. Tapi jangan salah, alat ini sangat memadai untuk mengamati objek-objek astronomi.

Binokular adalah alat yang sangat mudah dibawa kemanapun sehingga memungkinkan untuk melihat berbagai objek dengan lebih cepat tanpa harus kerepotan dengan melakukan bongkar pasang. Tapi banyak sekali jenis binokular yang beredar saat ini, sehingga kita harus pintar memilih binokular yang tepat dan sesuai untuk tujuan yang kita inginkan. Jadi, apabila anda ingin membeli binokular untuk stargazing, semoga tulisan ini bisa memberikan sedikit petunjuk.

Spesifikasi yang pertama harus diperhatikan untuk memilih binokular adalah aperture atau diameter lensa depan binokular. Semakin besar aperture berarti semakin banyak pula lensa mengumpulkan cahaya. Ukuran aperture ini bisa dilihat dari 2 angka yang biasanya tertulis di tiap binokular. Misalnya 7X35, berarti binokular ini berdiameter 35 mm dan memiliki perbesaran (magnification) mencapai 7x. Kebanyakan binokular berdiameter 35mm, akan tetapi untuk keperluan astronomi sebaiknya anda memilih paling tidak binokular yang berdiameter 40mm.

Untuk fungsi perbesaran, sebaiknya kita memilih binokular yang seperti apa? Perlu dipahami bahwa menggunakan binokular seperti menggunakan teleskop refraktor dengan dua mata sekaligus. Sehingga kita harus memperhatikan cara kerja dan kemampuan mata yang unik untuk tiap orang. Secara umum, untuk keperluan astronomi anda bisa memilih binokular dengan magnification 7x. Disarankan pula apabila anda memilih binokular dengan ukuran yang besar, misalnya 10×50, anda harus menggunakan tripod untuk mendapatkan gambar yang lebih stabil dan tajam.

Karena keunikan masing-masing mata tersebut kita harus memperhatikan spesifikasi yang lain. Yaitu exit pupil binokular, yaitu lebar berkas cahaya yang melewati eyepiece binokular. Exit pupil bisa dihitung dengan membagi besarnya aperture dengan perbesarannya. Misalnya, binokular dengan spesifikasi 7×50 memiliki exit pupil sekitar 7mm. Pada umumnya ukuran exit pupil mata manusia pada siang hari adalah 2 mm dan pupil akan membesar ketika menerima lebih sedikit cahaya. Untuk stargazing biasanya digunakan binokular dengan exit pupil sekitar 5mm. Tetapi akan lebih baik apabila exit pupil binokular disesuaikan dengan besarnya pupil mata kita. Sebagai informasi besar pupil mata manusia sangat bergantung pada umur. Secara umum besar pupil mata manusia dengan kondisi sedikit penerangan kurang lebih 7 mm, untuk orang yang berumur 30 tahun ke bawah. Dan sekitar 5 mm untuk 40 tahun ke atas. Apabila kita menggunakan binokular dengan exit pupil yang lebih besar dari ukuran besar pupil mata, cahaya yang datang tidak sepenuhnya dapat diterima oleh mata sehingga mengakibatkan gambar terlihat lebih redup.

Field of View (FOV) atau medan pandang adalah hal yang juga harus anda perhatikan. FOV ini biasanya dikenali dengan berapa derajat besarnya FOV. Secara umum semakin besar FOV berarti medan pandang semakin luas, tetapi perlu diketahui semakin besar perbesaran akan mengurangi besarnya FOV. Kebanyakan binokular memiliki FOV sekitar 6 deg sampai 7 deg.

Satu hal yang sangat penting pula adalah tipe prisma yang digunakan oleh binokular. Terdapat dua tipe prisma yang digunakan, yaitu porro dan roof. Berikut adalah ilustrasi jalannya cahaya dengan dua jenis prisma yang berbeda.

Untuk keperluan astronomi disarankan anda memilih binokular dengan porro prisma. Binokular dengan kualitas tinggi dibuat dari barium crown glass (BaK-4). Dan akan lebih sempurna apabila coating lensanya Fully multy-coated. Hati-hati jangan memilih binokular dengan coating lensa ruby coated, karena jenis coating lensa ini diperuntukan untuk keadaan yang terang.

Yang terakhir adalah kolimasi, kolimasi dalam pemilihan binokular ini berarti antara optik dan mekaniknya teralign dengan sempurna. Bagaimana cara mengenali binokular yang terkolimasi? Coba gunakan binokular yang akan dipilih dengan mengamati objek yang jauh, dekat, dan objek antara jarak dekat dan jauh. Apabila anda tidak dapat memfokuskan objek-objek tersebut berarti ada masalah kolimasi pada binokular tersebut. Kolimasi binokular juga bisa dilihat dengan cara menfokuskan binokular dengan menutup sebelah mata, apabila kita tidak bisa menfokuskan dengan cara ini berarti ada masalah dengan kolimasinya.

Mengenai harga binokular tentunya bervariasi bergantung spesifikasinya. Tapi tentu saja harga lebih bisa dijangkau dibandingkan dengan harga teleskop. Binokular kecil untuk astronomi harganya berkisar 25$, sedang untuk ukuran sedang bervariasi antara 50$ sampai 75$. Dan untuk ukuran besar harganya mulai dari harga 100$.

Sumber : universetoday.com, astronomy.com

SOME BLACK HOLES ARE PINK 21.17

A group of Australian astronomers have found that some black
holes are bright pink!

Black holes have captured the imagination of the public over the years
with some popular depictions in science fiction movies. They have such
intense gravity fields that they even suck in light. This is why they appear
black but Dr Paul Francis, a lecturer at the Australian National University,
together with Dr Rachel Webster and Dr Michael Drinkwater, from the
University of Melbourne's School of Physics have discovered that some
black holes are pink in colour.

The "Pink holes" were discovered using telescopes at Parkes and
Coonabarabran in the western plains of NSW between 1994 and 1998.
The work will be presented at the "Fresh Science" Conference in
Melbourne.

"These pink things were quite easy to find" said Dr Francis. "The hard
bit was proving that they are black holes. These black holes are more
than a billion light-years away, and are more than one hundred thousand
times fainter than the human eye can see. It took the combined power of four of Australia's best telescopes to identify what they were."

How could a black hole be pink? "We really don't have the foggiest idea"
said Dr Francis. "We're pretty certain that it isn't the black holes
themselves that are pink, the pink light is actually coming from gas just
outside the black hole. We think that these black holes live in the middle
of galaxies, and they are devouring anything that comes near them.
Possibly as the mangled remains of space matter, stars and gas clouds
swirl down the throat of the black holes, they emit an intense pink light."

It is well known that massive black holes devour stars and gas. Black
holes like this are called quasars, and were first discovered in the
1960s."Until now", Dr Francis said, "only blue quasars had been seen,
and it was believed that the debris swirling around black holes should
emit only blue light, not pink."

So what is different about these pink quasars? "We're don't really know"
said Dr Francis. "But we are beginning to suspect that the debris swirling
around the black holes is acting as a vast natural radio transmitter,
broadcasting intense pink light to the universe."

For further information contact Niall Byrne, Media Liaison, at
ScienceNOW! In Melbourne on 0417 131 977, email niall@byc.com.au,
or Dr Paul Francis, (02) 6249-2824 (w), (02) 6257-9263 (h) Photos and
background information will be available on the website from the day
of presentation on www.asnevents.net.au/sciencenow

Lunar Data Supports Idea That Collision Split Earth, Moon 21.14

Analysis of data from NASA's Lunar Prospector spacecraft has
confirmed that the Moon has a small core, supporting the theory
that the bulk of the Moon was ripped away from the early Earth when
an object the size of Mars collided with the Earth.

Scientists presented this result and other findings today in a series
of papers at the 30th Lunar and Planetary Science Conference in
Houston, TX. Their data show that the lunar core contains less than
four percent of the Moon's total mass, with the probable value being
two percent or slightly less. This is very small when compared with
the Earth, whose iron core contains approximately 30 percent of the
planet's mass.

"This is a critical finding in helping scientists determine how the Earth
and Moon formed," said Dr. Alan Binder of the Lunar Research Institute,
Tucson, AZ, principal investigator for Lunar Prospector.

Similarities in the mineral composition of the Earth and the Moon
indicate that they share a common origin. However, if they had simply
formed form the same cloud of rocks and dust, the Moon would have a
core similar in proportion to the Earth's. A third theory suggests that the
moon was captured fully intact by the Earth's gravity.

Based on information first gathered during the Apollo era, scientists
suggested that the Moon was formed when a Mars-sized body hit the
Earth during its earliest history. "This impact occurred after the Earth's
iron core had formed, ejecting rocky, iron-poor material from the outer
shell into orbit," Binder explained. "It was this material that collected to
form the Moon.

"Further analysis of Lunar Prospector data to refine the exact size
of the lunar core and the amounts of elements like gold, platinum and
iridium in lunar rocks -- all of which are concentrated with metallic iron
-- is required," Binder added. "This will do much to pin down for good if
the 'giant impact' model of the formation of the Moon is correct, or if the
Moon formed in a different manner."

The current data come from gravity measurements conducted by Dr. Alex
Konopliv of NASA's Jet Propulsion Laboratory, Pasadena, CA. His results indicate that the Moon's core radius is between 140 and 280 miles (220 and 450 kilometers). This result is consistent with independent magnetic data, evaluated by Dr. Lon Hood of the University of Arizona, Tucson, which suggest that the core radius is between 180 and 260 miles (300 and 425 km).

In other results from Lunar Prospector, Dr. Robert Lin of the University of California at Berkeley, Dr. Mario Acuna of NASA's Goddard Space Flight Center, Greenbelt, MD, and Hood also found that a broad section of the southern far-side of the Moon has large localized magnetic fields in its crust. These fields occur opposite the large Crisium, Serenitatis and Imbrium basins -- three of the "seas" that cover much of the Moon's near side. This result supports earlier evidence linking strong magnetized concentrations on one side of the Moon with young, large impact basins
on the other side.

Results of efforts to map the composition of the lunar crust have surpassed the expectations of the spectrometer team, led by Dr. William Feldman of the Department of Energy's Los Alamos National Laboratory in New Mexico. Data obtained are so good that the distribution of thorium has been mapped with a resolution of 36 miles (60 kilometers). At this amount of detail, scientists can detect individual deposits rich in thorium and related elements. Their current observations suggest that thorium was excavated by impacts of asteroids and comets, and then distributed around craters, rather than being deposited by volcanic activity.

Lunar Prospector conducted its primary mapping mission at an altitude
of 63 miles (100 kilometers) for almost one year after its arrival in lunar
orbit on Jan. 11, 1998. In December and January, the spacecraft's altitude was lowered to approximately 15 miles by 23 miles (24 kilometers by 37 kilometers). Analyses of data from the lower-altitude observations are expected to further improve scientific understanding of the origin, evolution and physical resources of the Moon.

The $63 million mission is managed by NASA's Ames Research Center, Moffett Field, CA, and was developed under NASA's Discovery Program
of lower-cost, highly focused small scientific spacecraft.

Further information about Lunar Prospector, its science data return,
and relevant charts and graphics can be found on the project website
at: http://lunar.arc.nasa.gov