#83 | ast00 – Basics of astronomy & how to use a telescope

Hi guys!

In the beginning, I wanted to be in the astronomy studies since I love sciences and I wanted to do physics. However, I really like to draw too, and I’ve thought that having more artistic and social parts could be more interesting to improve myself. So well… I’ve turned into the architectural studies. I still love astronomy though, even if I don’t talk much about it! With Bach, we used to look at the stars or the blue sky near where we live and since the revival of the space era arrived in the middle of our life, it was even more interesting to understand the astronomy and how the mechanics of the universe works.


The telescope is a tool to understand the movements of the objects in the night sky. I am sure you have already known the story behind it with Galileo Galilei or even Kepler, and you will surely find the pieces of information really easily on the net. Today, I won’t talk about the story behind the telescope, but how to use it. If you want to use a telescope, you need some basics in astronomy first to be able to set the equatorial mount up.

fig.0 – the telescope composed by the optical tube, the equatorial mount and the finderscope.

What is an equatorial mount?

The equatorial mount of a telescope is the “setting up” hardware uses for the adjustments of the observation view. It is composed by the latitude adjustment screw, the setting circle “right alignment” and the setting circle “declination”.

Lattitude adjustment and polar axis

Coordinates used to localize the objects in the sky are not the same we use in geography. But how to know where is an object if the stars are “moving” because of the Earth’s rotation? Let me explain: the Earth rotates on its axis and orbit around the sun. However, the stars don’t move! It is because we are moving that we have the feeling that the night sky is moving (and that’s why the first astronomers thought that we were in a geocentric system and not a heliocentric system! But it is another story).
The polar axis is the only localization where you can see the sky turning around the poles. Let me explain: when you are in the North pole and if you look above you, you will see that a single star isn’t moving. This star is “Polaris” and it is the reference point in the North hemisphere that we use in astronomy. So, Polaris is at a 90° angle from the ground if you are in the North pole. If you move one degree to the South, the Polaris will be at an angle of 89° from the ground! See what I wanted to explain you? In fact, depending on what latitude you are, the angle to the Polaris will be the same. So if I am in France, it would be around 50°. Easy isn’t it? This is the first parameter you need to know when you want to align your telescope. On your telescope, you can change it by raising and lowering the tube (altitude) or moving in the horizontal (azimuth). In this case, you need to align at your latitude position and to the North.
By the way, there is also another point that doesn’t move in the South hemisphere: the “Omega Octanis“.

fig.01 – The “latitude adjustment screw” part of the equatorial mount needs to be pointed to the Polaris, so it can be parallel to the polar axis.

Right ascension and declination

However, the two next parameters are a bit messy to understand. The first one is called the “right ascension” and the second one is the “declination”. What is that ?

First of all, you need to imagine an imaginary sphere outside the Earth. This sphere is called “Celestial Sphere”. In this sphere, coordinates are defined by the projection of the Earth’s equator, the Latitudes, the Declination and the Right Ascension: the equator separates the globe in two hemispheres, the North and the South. In parallel to the equator, there are lines on these two hemispheres, there are called latitudes and when you move to the North, your angle is called “declination” and is positive. When you move to the South, the declination is negative. And what happens if you are at the equator? You are at 0° of declination.

fig.02 – The circle settings of the Declination.

The equivalent of longitude, perpendicular to the equator and intersecting at the two poles, is called “right ascension”. Since the Earth rotates in 24 hours, the Celestial Globe is also composed by 24 lines, spaced by 15° from each other. The 0° origin line, the reference line, is placed at the constellation of Pisces designated by 0 hours, 0 minutes and 0 seconds.

fig.03 – The circle settings of the Right Ascension disk on the equatorial mount.

How to align the finderscope?

The finderscope is an annex scope with a low magnifying scale used to search an object in the sky with ease. However, in order to be able to see the same object through the telescope optical tube, the finderscope needs to be aligned with it. The first thing to do it is to look at a far object at daytime with your telescope. It can be the roof of a house or another unmoving object (trees can be a bit hard to use for the alignment since they can balance because of the wind). When it is okay, just tighten the angles knots of the equatorial mount. Then, you just need to loosen up or to tighten up the adjustment screws of the finderscope.
The next step is to compare the view of the Telescope and the view of the finderscope. Voilà! Your finderscope is aligned!

fig.04 – The finderscope mounts on the telescope optical tube.

What did I learn?

  • The composition of the Celestial sphere
  • How to find the position of Polaris and Omega Octanis
  • How to mount a telescope and the setting up
  • Equatorial mount, right ascension, declination, latitude adjustment
  • Align a finderscope to the telescope optical tube
  • Find a star and enjoy stargazing with magnifying lenses!

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