Astronomy is a fascinating hobby that allows you to explore the universe from your own backyard. To get started, however, it's essential to choose the right telescope that best suits your goals and objectives. With such a wide variety of options on the market making the decision difficult, here are three crucial factors to consider before purchasing your first telescope.

1. Understand the different types of telescopes

There are mainly three types of telescopes, each with its own advantages and disadvantages:

• Refractors telescope : Invented by Galileo, it uses lenses to collect and focus light. It is ideal for observing the Moon and the deep sky thanks to its contrast and image stability. It does not require collimation (i.e., alignment of the mirrors), which is an advantage for beginners and saves time. However, it suffers from chromatic aberrations (colored fringing effects around bright objects) which can be corrected with correctors/reducers sold separately unless it is equipped with ED or apochromatic glass lenses. Large refractors become very expensive.

  
  

  • Examples: Sky-Watcher Evostar 90/900 or 80 ED, Celestron Astromaster 102AZ

    
    

    

    
    

• Reflector telescope (Newtonian) : Operating with a concave mirror, it is perfect for observing distant celestial objects such as galaxies and nebulae of low light intensity. It offers excellent value for money at the same aperture. However, it requires regular collimation, which consists of realigning the mirrors to optimize optical performance. Another point to consider is the presence of egrets on the images (small bright peaks around the stars), caused by the spider that holds the secondary mirror. Finally, Newtonian tubes are often made of metal, but more expensive carbon models exist, thus reducing thermal expansion and therefore optical adjustments.

  
  

  • Examples: Dobsonian Sky-Watcher 150/1200, Orion SkyQuest XT8

    
    

    

    
    

• Catadioptric telescope (Schmidt-Cassegrain, Maksutov-Cassegrain) : It combines lenses and mirrors to offer good versatility and increased compactness. This type of telescope is ideal for planetary astronomy and those who want a portable instrument with very good image quality. However, they are more expensive at the same aperture than Newtonian telescopes and take longer to reach thermal equilibrium due to their closed design. Some high-end models are made with carbon tubes to minimize thermal expansion and improve optical stability.

  
  

  • Examples: Celestron NexStar 6SE, Meade LX90 ACF

    
    

    

    
    

2. Choose the right aperture and focal length

A telescope's aperture (the diameter of its objective lens or primary mirror) is the most important criterion to consider in astronomy. It determines the amount of light collected and therefore the quality of the observed image. The world's largest mirror is under construction in the Atacama Desert in Chile and will belong to the "Extremely Large Telescope" or ELT. Here's what the different apertures allow you to observe or photograph:

• An aperture of 70-100 mm is sufficient to observe the Moon and planets in detail.

• An aperture of 130-200 mm allows access to deep sky objects (galaxies, nebulae, star clusters).

• Beyond 200 mm , the telescope becomes bulky and requires a stable support.

  
  

Focal length also plays an important role. It is the path that light travels through the optical instrument to reach the observer. It is used in calculating the focal ratio. The following are the types of focal lengths:

• A short focal length (from 400 mm ) offers a wide field, ideal for deep sky.

• A long focal length (from 1000 mm ) allows for better magnification, perfect for planetary details.

  
  

The focal ratio (f/D = focal length divided by the aperture, i.e. the diameter) is also to be considered. The lower the ratio, the greater the angle of view and the quantity of light; the higher it is, the smaller the portion of sky that can be observed, but it allows for very fine details:

• f/5 or less : Ideal for deep sky (e.g. nebulae, galaxies).

• f/10 or more : Ideal for planets and the Moon.

  
  

  

  
  

3. Choose a suitable mount

The mount plays a fundamental role in the stability of the instrument and the tracking of celestial objects. Its choice is crucial for both observation and astrophotography:

• Alt-Azimuth (AZ) mount : Simple to use, this allows you to move the telescope up and down and left and right. It is ideal for beginners, but requires regular manual adjustment to track moving objects using small cranks. Although it is available at a fairly low price, its performance is limited.

  
  

  • Examples: Celestron Astromaster 70AZ, Sky-Watcher AZ-GTi

    
    

• Equatorial mount (EQ) : More advanced, it compensates for the Earth's rotation. This type of mount is recommended for long observation sessions and especially astrophotography.

  
  

  • Examples: Sky-Watcher EQ3-2, Celestron Advanced VX, ZWO AM5

    
    

• GoTo controller : Equipped with a computerized system, it allows celestial objects to be automatically pointed using a database. This requires the mount to be motorized and therefore powered by electricity.

  
  

The polar alignment

Polar alignment is essential for good star tracking. Alignment with the North Star is essential for an equatorial mount:

• Manually with a polar finder : You must orient the polar axis of the telescope towards the pole star and adjust the position using the graduations on the finder.

• With a GoTo system : The telescope uses an automated alignment process on several stars to calculate its position.

• With a motorized mount : Some models include alignment assistance with digital adjustments to make the process easier.

  
  

In conclusion

If you're planning to do astrophotography, choosing the right telescope and mount is even more crucial. Here are some key points:

• A motorized equatorial mount is essential to compensate for Earth's rotation and obtain sharp images.

• A telescope with a good f/D ratio : An f/D ratio around f/5 is ideal for deep sky, while a higher f/D is recommended for planets.

• Digital camera (DSLR) with T-ring : Allows you to attach a DSLR directly to the telescope.

• Cooled cameras : Reduce electronic noise and are ideal for deep sky photography.

• Planetary cameras : Offer high frame rates to capture details of the planets.

• Monochromatic vs. Color Cameras : Monochromatic cameras offer better sensitivity but require filters, while color cameras are easier to use.

• Guide Camera : Used with a guide scope to improve tracking of long exposures.

Buying a telescope is an exciting investment. Consider the type of telescope, its aperture and focal length, and the mount it comes with. With these tips, you'll be ready to explore the skies and discover the wonders of the universe!