1. Astronomical observation
Olivier Ravayrol

2. Summary A bit of history A bit of theory In practice
What to watch and with what?
What materials for what budgets?
Links
Questions ?
Copyright © 2013 Olivier Ravayrol - All rights reserved. 2
© 2009 Capgemini. All rights reserved 2

3. A bit of history: Key dates
1543: Copernicus's heliocentric system defined
1609: Kepler states his laws of planetary motion
1610: Galileo built a reflector and observe the satellites of Jupiter
1655: Huygens observed Saturn's ring and its moon Titan
1687: Newton built a reflector and published his law of gravitation
1774: Messier publishes its deep sky object catalog
1781: Herschel discovers Uranus
1846: Le Verrier, and Galle discovered Neptune
1905: Schwarzschild predicts the existence of black holes
1916: Einstein presented his theory of General Relativity
1929: Hubble discovers and measure the expansion of the universe
1948: Gamow develops his theory of Big Bang
1970: V. Rubin hypothesized dark matter
: The Voyager probe approached planets close
1995: Michel Mayor and Didier Queloz discovered the first extrasolar planet
Copyright © 2013 Olivier Ravayrol - All rights reserved.

4. A bit of theory: The coordinate systems
The observer on Earth is at the center mark
Imaginary lines are represented on the celestial sphere
Azimutal type: depends on the place and time !
Azimuth : origin = South [0° at 360°]
High: origin = Horizon [0° at 90°]
Equatorial type: uniform system
Declination ( projected Latitude): origin = celestial equator [-90° à 90°]
Right Ascension ( projected Longitude) : origine = Vernal pt [0° à 360°] (intersection between the ecliptic and the celestial equator)
Example : M31 => AD=00h 42’ 44’’ ; Dec=+41°16′ 07’’
The type Equatorial + is suitable for astronomy can track objects with the Earth's rotation
Copyright © 2013 Olivier Ravayrol - All rights reserved.

5. A bit of theory: Units
Apparent magnitude: used to measure the brightness of a celestial object
+ it’s high – it’s light (inverse log scale)
Polaris = 2,09 ; Vega = 0 ; Sirius = -1,46 ; Soleil = -26,78
Measure of astronomical distances
AU (Astronomical Unit) = Dist Terre / Soleil ~= 150 M km
Distance Sun / Mars ~= 1,5 AU
Distance Sun / Saturne ~= 9,5 AU
LY (Light Year) : distance traveled by 1 light year ~= Billion km ~= AU
Nearest star: 4,22 AL ; Pléiades cluster: 400 LY
Diameter of the Milky Way : AL
Distance of the Andromeda Galaxy : 2,5 M LY
Copyright © 2013 Olivier Ravayrol - All rights reserved.

6. Practice: Getting the mount aligned
Advantage of the equatorial mount
Rotational axis of the earth (almost) aligned with the polar star for the northern hemisphere => Frame alignment ease
Equatorial mount has "just" to follow the earth's rotation (manually or automatically)
Getting the mount aligned (equatorial type) :
To level the mount
Alignment on the polar star (manual or semi-automated via software)
Without drive and without GoTo: we hold the coordinates’ object through graduate axes of the mount and manually track with the AD axis wheel
With GoTo: preliminary 3 stars alignment for an automatic pointing objects via the remote control
Copyright © 2013 Olivier Ravayrol - All rights reserved.

7. Practice: Effects of light pollution
Copyright © 2013 Olivier Ravayrol - All rights reserved.

8. Practice: Sky with clear light pollution
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9. In practice: Sky with moderate pollution
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10. Practice: Triangle of summer
Copyright © 2013 Olivier Ravayrol - All rights reserved.

11. What to watch and with what ? (1/2)
Eyes
(Wait 10 minutes before observation and not look lighting then !)
You have to identify from the pole star :
From the "Big Dipper" to find the North Star
With a compass, look north to 45 °(France) to find the North Star
With software as Stellarium or applications on Smartphones
Some constellations and star clusters (the Pleiades)
The Milky Way and the Andromeda galaxy
Shooting Stars
The ISS and some satellites (Iridium)
Binoculars:
Some open clusters and globular star cluster
The moon but only with a lunar polarizing filter
The sun but only with a solar filter
Copyright © 2013 Olivier Ravayrol - All rights reserved.

12. What to watch and with what ? (2/2)
The refractor: for the wide field (extended objects)
The large open star clusters (the Pleiades)
Extended nebulae (Orion, America)
The Great Andromeda Galaxy
Moon with a filter, planets, comets
The tasks and solar prominences with filters
The reflector: for the narrow field (smaller objects)
Galaxies
Small nebulae (Horse's head nebulae)
Planetary nebulae (Ring Nebula)
Copyright © 2013 Olivier Ravayrol - All rights reserved.

13. What materials for what budgets ? (1/5)
Promote the diameter of the instrument
The brightness of the object increases with the diameter
The resolving power increases with the diameter
We can always increase the focal length (and thus the magnification) with Barlow lens
Too much magnification with a small diameter will give less sharp images!
F/D > 10 => Planetary
F/D < 5 => Objects less bright
Binoculars: allows a 3D effect
Focus on model 7 x 50 (D = 50 mm; Gr = 7x)
Beyond 7x, a tripod is required because the magnification is too large: 10 x 70 models are good but expensive !
Copyright © 2013 Olivier Ravayrol - All rights reserved.

14. What materials for what budgets ? (2/5)
Refractor or reflector ?
In the previous slide: the diameter should be preferred
The refractor may not exceed D = 100mm because prices are exponential above ! (The reflector’s mirror is less expensive to manufacture than the lens of the refractor)
Choice of instrument for observation :
Refractor: limited diameter
+: Less bulky than a large reflector’s diameter
-: For use of the planets or wide field photo
Reflector: F / D low
+ It’s doing everything (but less "stung" than the refractor)
-: It must be regularly "collimated" (for Newton type)
Copyright © 2013 Olivier Ravayrol - All rights reserved.

15. What materials for what budgets ? (3/5)
Choice of eyepieces and filters
Eyepieces
Magnification = Finstrument / foculaire
3 eyepieces => Wide field: 25mm to 35mm; 12mm medium field; Magnification 6mm to 8mm with wide angle 60 ° to 100 °
Barlow lenses
2x: increases the focal length and thus magnification by a factor 2
5x (Max): allows to see the red spot of Jupiter
Visual filters
For the Moon: variable polarizing
For the Sun: Solar Filter for sun spots (H-Alpha for the protuberances but very expensive)
For deep sky (light pollution filter removed): Type UHC or CLS
For planets: different colors can improve the contrast according to the observe planet type
Copyright © 2013 Olivier Ravayrol - All rights reserved.

16. What materials for what budgets ? (4/5)
Copyright © 2013 Olivier Ravayrol - All rights reserved.

17. What materials for what budgets ? (5/5)
Average budget for visual observation: 900 €
Average budget adapted for photo: 1630 € (without the Reflex and accessories)
Copyright © 2013 Olivier Ravayrol - All rights reserved.

18. Links Training sessions at “Ferme des étoiles” (Gers, France)
Softwares
Planetarium (Open Source) : Stellarium (
SkyChart (Open Source) :
Various softwares :
Useful links
Magasins / Fournisseurs de matériels pour l’astronomie :
Optique-Unterlinden :
La clef des étoiles (Toulouse):
Pierre-Astro :
Amateurs forum:
Association « La ferme des étoiles » :
Visibility prevision:
Nebulosity :
Copyright © 2013 Olivier Ravayrol - All rights reserved.