Beginners Astronomy

Beginners Astronomy - Star GazingIntroduction : At the very beginning I must tell you that I am an amateur
astronomer and have no pretensions to know astronomy at any great depth, though I have made a post before on astronomy  called “ Amateur Astronomy,Infinite Entertainment  “.This
article should have been titled Beginners Star Gazing but I have included a few useful astronomical information’s (for a layman)and  hence the title.
Almost everyone of us at some point of time looked at the sky and watched the stars, the sun and the moon and wondered what they are in actuality.
That wonderment in my case has lead to many questions and view the sky more closely. It started accidentally one night when I could watch the sky  with a telescope and found the sky an amazing place of infinite beauty and mysteries.


{mosgoogle right}It is not a very easy job to watch sky with a telescope (unless you use
very modern computerized go to telescopes) and find objects of interest.
The biggest objects in the sky, the sun and the moon has an angular
diameter of almost half a degree which is very small when you consider
the sky you see has an angular diameter of 180 degree at any point of
time (half of the sky is below horizon, otherwise it would have been 360
degrees.) However to catch the moon and sun is relatively easy in
telescopes. The catch comes when you want to see the other dim objects
which are the things for which you are using the telescope in the first
place.

ConstellationsLarger image of  “Constellations” To begin with you must be familiar with the sky. The best thing is just go to your terrace and lie down and look upwards. If you do this for a few days slowly the patterns of sky will be familiar to you. You need a basic skymap to guide you. I found a little book in Bengali written by Biman Bose named Nakshatra Porichoy which helped me a great deal.You can buy or download some small star map for your help and gradually develop on that.As a beginner you can use Star Maps for beginners by I.M.Levitt and Roy K Marshall.Later on Wil Tirion’s Sky Atlas 2000 may be used for medium and advanced level. In a computer you can use the free software Stellarium for easy guidance.

  If you look at the sky now ( it is October and place is India, Kolkata) you will find three bright stars almost at the middle of the sky.These are Vega, Deneb and Altair. The Indian names of Vega nd Altair are Abhijit and Shravana respectively. These three stars are popularly called summer triangle. (For your information Vega will be our pole star at around AD 13,727). Vega is in the constellation Lyra, Deneb in Cygnus and Altair is in Aquila. For your information constellations are nothing but groups of stars which form  a pattern in sky and named for easy reference and recognition. These stars may be close together or far from each other and only their position in the sky which makes them to be in the same constellation. As you go on familiarizing your self with the sky you will be able to recognize stars and constellations immediately.


Type of Telescopes : Once you know the
bright stars quite easily you can begin to navigate in sky the with your
telescope.There are generally three kinds of telescopes which are available for
visual observation.

1)     
Refractors
2)      Reflectors or Newtonians
3)      Catadioptric (Schmidt or Maksutov –  Cassegrain.)
Refractors use combination of lenses to form a image of an
object which is viewed by an eye piece.These are the oldest kind of telescopes
and was used by Galileo.

Schematic diagram of a refractor

Schematic diagram of a refractor
 

 
 

Refractor Telescope

Refractors

Relectors use parabolic mirrors to form an image of the
object and is viewed by the eye piece lense.This was inroduced by Newton and
hence it is also called Newtonians.These are by far most popular and cheap and
used by most amateurs.These can be built at home with a little patience and
hard work.

Reflector

Schematic diagram of a Newtonian telescope

Reflector telescope

Newtonian Telescope
 

Catadiotropics  are
more complicated modern version telescopes which uses both concepts and the
light is reflected several times in telescope tube which makes them very
handy,easy to use and very good resolution capabilities.Modern computerised go
to telescopes are mostly of these types. But they are costly.
 
 

Schematic Diagram of a Maksutov Cassegrain telescope

Schematic Diagram of a Maksutov Cassegrain telescope

Maksutov-cassegrain computerised telescope

Maksutov-cassegrain computerised telescope
 

For sky watching you need at least a 3 inch (75 mm)
refractor or a 4 inch Reflector or cassegrain telescope. 


Type of Mounts
: Supposing that you have acquired a telescope you have to
choose the mount of the telescope.There are two types of mounts

  1. Altitude-azimuth
  2. Equatorial

In alt-azimuth mounts telescope can be rotated in horizontal
and vertical axis .This is useless for sky watching unless you have
computerised telescope like celestron nexstar.

 

In Equatorial mount one of the axis of the telescope is
parallel  with the axis of the rotation
of earth (polar axis). As you know as the earth moves from west to east at a
fixed rate.The celestial objects moves in the sky in the opposite direction
(i.e east to west) at the same rate.Once one of the axis is aligned with axis
of earth’s rotation you can track a celestial object in the sky continuously once
you find it just by rotating your telescope in polar axis.Unless you do that
the object will move away continuously out of field of your telescope.

Alt-azimuth mount

Alt-azimuth mount
 

Equatorial mount

Equatorial mount.
  


Right Ascension and
Declination
: You can imagine the sky as big opaque vault on which
celestial bodies are embedded.As the sky moves from east to west the bodies are
also moving from east to west and their positions are fixed on the
sky.According to its position each body has a celestial coordinate just like
each place on earth has a latitude and longitude.In astronomical jargon they
are called Right Ascension and Declination. Right Ascension or R.A is like latitude
an is measured from an imaginary point of sky known as FIRST POINT OF
ARIES.Declination or DEC is like longitude and is measured 0-90 degrees from
celestial equator( celestial equator is parallel to earth’s equator in
sky).Every celestial body has a fixed R.A and DEC except sun,moon and planets
whose movements are more complicated.

 
Right Ascension and Declination

Right Ascension and Declination 


Polar Alignment : The next thing to do is to polar align your telescope.If you
are in northern hemisphere it is easy.First you have to identify the pole star
or Polaris.Then make the adjustments so that the RA axis of your mount is so
that your guider scope points at Polaris.Then make further adjustments with
your main eye piece lense so that it is in the centre of eye piece.Actually the
north celestial pole is less than ½ degree away  in RA and and less than a degree in DEC and it
is helpful for ordinary viewing.For astrophotography you need much more
rigorous and accurate polar alignment.
 
Before I return to star gazing I want to give you an
important information.The bigger the aperture of your telescope objective lense
or mirror the more light gathering power it has and you can see objects dimmer
and farther away in space.
  

Star Hopping : Now that you have a
telescope and mounted in polar axis you can start finding a celestial object of
your choice. Finding a celestial object in sky is not easy as I told you
earlier because they are very very small.Most celestial objects does not have
an angular diameter more than a few arc second or minute. It is like finding a
pin in a big warehouse.
The trick is to find it by its neighbourhood.Just like you
go to some unknown place by knowing a landmark near it.Suppose you know a star
and you know a  dim object (a nebula or
galaxy) is nearby.You go to the next closest star using your star chart until
you finally arrive to your desired object.
But to do this you have to know the  magnification and true field of view .
Magnification = Focal length of telescope (in mm) ÷ Focal
length of eye piece. (in mm)
My telescope has a focal length of 1300 mm.If I use a 25 mm
eye piece the magnification will be 1300 ÷25 = 52 that is objects are magnified
52 times.
Now the True field = Apparent Field of eye piece ÷
magnification
Apparent field of eye piece is supplied by the
manufacturer.The 25 mm eye piece has an apparent field of 52 degree. Therefore
the true field = 52÷ 52= 1˚.
 
(The following portion of Star Hopping is adapted fom an
article from NIGHT SKY INFO)

 
If you don’t know the apparent field
of the eyepiece the matter becomes a little more complicated… In order to
find out the visual field of the eyepiece in this particular case, you should
direct the instrument to a star as close as possible to the celestial equator,
set the star at the edge of the visual field and measure the time it takes for
the star to cross the eyepiece, without moving the instrument. Knowing that a
star near the celestial equator will move one degree every four minutes, you
can calculate the visual field of the eyepiece.
Now that you know the visual field
and the scale of the atlas you can make either wire or plastic rings that will
represent the field. If the eyepiece has a visual field of two degrees and the
scale of the atlas measures 8-mm per degree, the ring should have a diameter of
16-mm. The ring is extremely useful when the object to be localized is found in
an area of the sky with very few stars, normally it is not used too often.
In order to find a celestial object
with the star hopping method, the visual field of your eyepiece should be as
large as possible, so don’t use great magnification. This way you will be able
to see several stars in the field, and will have reference points. Only when
you have found the desired object should you use eyepieces with great
magnifying power.
  


The finder-scope 

The finder attached to the main
instrument is also very important. It would be great to have a finder with an
aperture as large as possible (50-mm would do best). With a six to seven degree
visual field, such a finder will point far more stars than visible with the
naked eye, thus being very easy to identify the area where the target is. You can
determine the visual field of the finder with the help of methods described so
far. 


Coping with mirror or inverted images 

According to the type of instrument
you are using, the images will be either inverted or reversed (mirror image).
Newtonian telescopes give an inverted image (north becomes south), while
instruments with a diagonal (a small mirror) will give reversed images.
The solution every time you use a
Newtonian telescope is simple: turn the maps upside down. The problem occurs
with instruments using a diagonal. A solution is to flip over your sky chart
and shine a light behind it to view a mirror image of the printing through the
paper.
 
  


A little theory… 

In order to start a star hopp the
first step would be to identify on the atlas the area where your aimed object
is. After this find a star, or a group of brighter stars visible with the naked
eye and situated as close as possible to the target.
Without looking through the finder or
telescope, with both eyes opened, look along the instrument and point it to the
area of interest. The point is to center in the finder, after having consulted
the atlas, a group of stars that stand out among the rest. It is extremely
important that this point of reference should fit into the visual field of the
finder so that it may be recognized even if it is not centered perfectly.
The more difficult part is to
memorize the shape of the pattern formed by the group of stars, in some cases
you may need to return to the group after having passed by it. Confusions may
occur in case you mistake a group for another, only to finally find yourself a
few degrees away from the targeted object. The solution is to associate a
familiar shape to a group of stars (line, triangle, circle or square), thus you
will succeed in memorizing them more easily. 


And the real thing. Star hopping exercise 

Star hoping to M92
M92 is a beautiful globular cluster
in Hercules, about 26,000 light years distant from Earth. It is an easy target
even for small telescopes, and on clear nights it can be spotted with binoculars,
if you observe on a dark sky far from city lights.
For this star
hopping example let’s say that your finder-scope’s field is five degrees
and the eyepiece field is one degree. The large circle represents
the field of the finder-scope and the small circle is the field of the
eyepiece.
To get to M92 first locate the bright star Pi Herculis, shining at
3rd magnitude. Center it in the field of your finder-scope and sweep one more
field, following the line drawn from Pi to Iota Herculis. At the edge of the field
look for four stars arranged in a line, and center them in your eyepice. Move
one degree in the direction of Iota Herculis and M92 will be right in the
center of your eyepiece.

Larger image of “Star hoping to M92” 


Modern GO TO Telescopes :

Modern computerised telecopes has almost made
old star hopping techniques as a thing of past But they are costly and I would
suggest that beginners should use old inexpensive telscopes to begin with.Knowing
the sky and discovering objects are half the fun of stargazing.
GO TO telescopes computerised software to
locate a celestial object.What you have to do is just have your telescope
looking at the sky,align it with some bright stars (unknown or known to you)
and press the button to move the telescope to find your desired object.This has
made astronomy easy and almost child’s play.Some amateurs are now finding
supernovas very soon without much struggle.These telescopes even do not require
them to be on an equatorial mount. They are mounted on alt-azimuth mounts and
once it latches on  an object it goes on
correcting the alt-azimuth according to the RA and DEC of the body with time so
that it is always in the field of telescope.Some telescopes include global
postioning systems so it gets its own information about where you are.However
for astrophotography accurate and rigorous polar alignment is needed even with
these.But the most modern ones have all star polar alignment systems which
makes it also much easier.However these are quite costly at present and it is a
matter of subjective opinion whether you should buy it as your first scope.
 


Apparent magnitude of a celestial body :
 The apparent magnitude  of a celestial body is a measure of its brightness
as seen by an observer on earth, normalized to the value it would have in the
absence of the atmosphere. The brighter the object appears, the lower the value
of its magnitude. Celestial objects with apparent magnitude upto 6 is
said to be visible to a naked human eye in a dark ,clear night although genrally
we can see upto 3 rd magnitude stars. The sun has a apparent magnitude of
-26.74, full moon -12.92, Sirius the brightest star -1.47,Vega 0.03, .Polaris
1.97and Andromeda galaxy 3.44.

Useful formulas :

                      116

Resolution = ——
seconds

                       D

 D = Diameter of
telescope in mm.

 

Example: A 10″ (254 mm) telescope

       

                      
116

 Resolution = ——  = 0.5 seconds

                       254

 

The size of an image depends on the focal length of your
telescope. For example, the longer the focal length, the larger the object:

 

               fθπ

    w = 
————

               180

w = width of
image

f =  focal
length

θ= apparent diameter ( i.e Moon = 0.5˚ )

 

How faint an object a telescope can see

   m = 2.7 + 5 log D

   D = Objective lense diameter

 

Where m is the limiting magnitude. Example, our 10″
telescope:

m=2.7+5 log254

  
= 14.7

 

The faintest object a 10″ telescope can see is with a
visual magnitude of 14.7 (Pluto has a magnitude of around 13.8).

f/number = Objective Focal Length / Objective
Diameter.  Example: A 2000 mm focal length telescope that has 200 mm (8
inch) diameter yields a value of f/10.

 

Exit pupil = Objective Diameter / Magnification = Eyepiece
Focal Length / Objective f/number

MoonSome data about celestial objects :
Moon : This is the closest of all celestial bodies fom earth and a natural satellite
of earth.Although man has already landed on moon it is the first object a
beginner would like to view from his telescope.Great details of moon are
visible even from a small telescope and it is the easiest object to
align.However moon’s movements are a bit complicated as it goes eastwards
almost 12 degrees every 24 hours.Moon filters can be used to increase contrast
and bring out details of moon surface.

 

 

Moon’s surface.
 

Planets : As
we all know our solar system has 9 planets namely Mercury,
Venus, Earth,Mars,Jupiter,Saturn,
Uranus Neptune and Pluto.From a small telescope Venus,Mars, Jupiter and Saturn
are quite a view and lots of time should be spent on that.Viewing mercury is
difficult because it is very small and close to sun but is possible.Uranus and
Neptune are small bodies like stars from small telecopes and it takes a big telescope
to get Pluto in the view.

Venus is popularly known in Bengal
as Suktara or Sandhyatara according to the time it rises in east and west
respectively.In addition to the movements like star the planets slowly move
from one zodiacal constellation to other because of their motion around the
sun. These sometimes causes viewer to see retrograde motion of planets ( they
seem to move on back gear) .


 

Venus

Venus
 

Mars

Mars 

Jupiter with its planets

Jupiter with its planets.
 

 

Saturn

Saturn.

Sun spotSun : It is the closest star from earth and even a
small telescope shows sunspots on it.Remember to take special precaution while
watching sun through telescope..use a filter from a reputed company like
celestron to completely cover the objective (the main lense or mirror) of your
telescope (do not use filters which covers the eye piece lense which can be
easily destroyed from the heat gathered by sun’s ray). Do not look at the sun
directly even with the guider scope .Without these precaution you can be
blinded for life time in a fraction of a second.
• The best time to
observe the Sun is in the early morning or late afternoon when the air is
cooler.
• To center the Sun
without looking into the eyepiece, watch the shadow of the telescope tube until
it forms a circular shadow.

 

Sun spots
 

AlbireoDouble stars : A double star is a pair of stars
that appear close to each other in the sky as seen from Earth when viewed
through an optical telescope. This can happen either because the pair forms a binary
star, i.e. a binary system of stars in mutual orbit, gravitationally bound to
each other, or because it is an optical double, a chance alignment of
two stars in the sky that lie at different distances.Albireo, which is close to
Deneb in Cygnus is a bright double star,actually a binary.There are also
multiple star systems where more than two stars are gravitationally bound to
each other.

Albireo

Orion nebulaNebula:
A nebula  is an interstellar cloud of dust, hydrogen , helium
 and other ionized gases. These are
within our own Galaxy,i.e,Milkyway.Sometimes new stars form in nebulae like
Orion nebula which is visible in the south of orion’s belt.

 

Orion nebula

Galaxy : A galaxy is a massive, gravitationally
bound system that consists of stars and stellar remnants, an interstellar
medium of gas and dust, and an important but poorly understood component
tentatively dubbed dark matter.Our solar system is part of our own galaxy
Milkyway.It is spiral galaxy with diameter about 100,000 light years and
containing 200-400 billion stars.There are billions of galaxies. The closest
spiral galaxy is Andromeda almost 2.5 million light years away and a naked eye
object in a dark clear night and visible with a small telescope.

 

Andromeda Galaxy

Andromeda Galaxy
 

PleiadesOpen clusters :
An open cluster is a group
of up to a few thousand stars that were formed from the same giant molecular
cloud and have roughly the same age. They are loosely bound to each other by mutual
gravitational attraction. Most notable open clusters are Pleiades (indian name
Krittika) and Hyades and are visible with naked eye and are beautiful viewing
objects with a telescope.


  .

Pleiades

Globular clusters : A globular cluster is a spherical
collection of stars that orbits a galactic core as a satellite. Globular
clusters are very tightly bound by gravity, which gives them their spherical
shapes and relatively high stellar densities toward their centers. Globular
clusters, which are found in the halo of a galaxy, contain considerably more
stars and are much older than the less dense galactic, or open clusters.


 

Globular Cluster

Globular Cluster.
 

Astrophography with camera mounted piggy back on telescope.A few words about astrophotography :
For astrophotography you need to have very accurate Polar
alignment of your telescope.It is done with the help of setting circles (in
modern GO TO telescopes setting circles need not be there) and using the drift
of stars.Photography can be done by DSLR cameras either mounted piggy back on a
telescope or at the back of it using a T-ring and T-adapter to latch your
camera with the telescope. In the latter case telescope itself becomes a big
telephoto lense.To take photographs of very dim objects you need to take long
exposure photographs.


 

Astrophography with camera mounted piggy back on telescope.
Camera at the back of telescope


Camera at the back of telescope

That’s all folks.

                                             
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