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Picking a telescope

Advantages and Disadvantages of Telescope Types
System Type Advantages Disadvantages
Refractor Lens Telescope High contrast Expensive and bulky at large apertures
Very sharp planetary detail at long focal lengths Possible chromatic aberration (achromats)
At the same Aperture and focal length,
Refractor will have a sharper image than a Reflector 		Cost much more per mm than a Reflector
Low maintenance
Reflector Mirror TelescopeGreat aperture for the money Requires cool-down
Free of chromatic aberration Requires occasional collimation
Very easy to use as a Dobsonian Contrast slightly reduced by obstruction
Maksutov-Cassegrain Catadioptric TelescopeCompact and portable Heavy for its size
Long focal length Long cool-down time
Excellent contrast Narrow field of view
Practically no chromatic aberration


Aperture - What diameter do you want on a telescope?

Simple. No matter the type or the purpose of your telescope, buy the biggest Aperture you can afford. But not so big it creates issues.
The bigger the aperture, the more light gathered and this will create better views. The moon is usually the exception for light gathering needs.

What Is Telescope Aperture?
The aperture of a telescope is the diameter of its primary optical element. In a refractor telescope, it’s the diameter of the lens;
in a reflector telescope, it’s the size of the mirror.

This measurement, usually given in millimeters or inches, sets the foundation for what the telescope is capable of.
In general, the more aperture the better, but larger telescopes come with added challenges and cost.

A beginner refractor usually have a 70mm to 90mm aperture. A mid-sized Dobsonian (reflector) telescope may feature a 200mm (8-inch) mirror.
Unlike magnification, which can be changed with different lens eyepieces, aperture is fixed and directly tied to performance.

Large apertures are highly beneficial for viewing detail and photographing planets, nebulae, or galaxies.

Aperture affects two essential qualities:

  • Light-Gathering Power
    A larger aperture collects more light, making faint galaxies, nebulae, and star clusters visible. For deep-sky observing and astrophotography, this is critical.
  • Resolution
    The wider the aperture, the finer the details you can resolve. This means sharper views of lunar craters, the cloud bands of Jupiter, or the spiral arms of galaxies.

    Below breaks down what different Apertures can show you, or what you can expect to see at common aperture sizes.
    Here are five main aperture classes of hobby telescopes:

  • 40mm - 65mm
    Good for Moon gazing and viewing the phase of Venus. Can see/detect rings on Saturn.

  • 70mm – 90mm
    Perfect for a detailed Moon, distinct views of planets, and bright double stars.
    Limited for deep-sky objects. Adramada galaxy and The Orion Nebula (M42) are visable.

  • 100mm - 120mm
    Good for hobbiest trying to get alot more out of planets while trying to see more of nebulae.

  • 130mm – 150mm
    A versatile choice. Many more galaxies and nebulae are within reach under dark skies.
    Clearly see Jupiter's two main equatorial cloud bands and it's distinct polar regions.

  • 200mm and above
    Serious visual astronomy. Spiral galaxy structure, planetary detail, and faint nebulae begin to shine.

    • Knowing what I know now:
    - 114mm Aperture is needed for anything more than great moon viewing.
    - 130mm Aperture if you want to see any detail in a Nebula.
    - 150mm Aperture if you want to see any real detail in anything.
    - 200mm Aperture, just know it will cost you alot.

    f-Number - what f-Number do you want with a telescope?

    This depends on how you wish to use your telescope. There is not a telescope that is great at all things.
    Some may be great at faint deep space objects. Some may be great at viewing planets. Some might try to be able to do both, but not excel to the same degree at either.

    The f-number (or focal ratio) on a telescope is the ratio of its focal length to the diameter of its aperture (light-gathering lens or mirror).
    It is usually written as f/N (e.g., f/5 or f/10) and dictates the optical system's "speed".
    The f-Number can be broken into three catagories. Fast, mid-range, and slow.
    f/2 is very fast, f/6 is a mid-range speed, and f/15 is extremely slow.
    Most telescopes have a purpose and range of use.
    Some telescopes are better for faint galaxies and nebulae, while others are better for planets.
  • The scopes ability has a lot to do with your f-number.
  • A lower f-number is considered "faster" because it gathers more light per unit of time, making it highly effective for capturing these faint, expansive targets like galaxies and nebulae.
  • A higher f-number is considered "slower" and is better for brighter objects like planets - to see the planet detail.
  • f/2 to f/4 are very fast numbers.
  • f/5 to f/8 are a mid range numbers.
  • f/9 to f/15 are slow numbers.


    To find the f-number: 
              N  =  Telescope Focal length (mm)
                   Aperture diameter (mm)
    Where:
    N = Focal ratio (often written as f/N )


    Lenses - what lens diameter do you want on a telescope?

    There are four different lens diameter standards.

  • 0.965 inch / 24.50mm
  • 1.250 inch / 31.75mm
  • 2.000 inch / 50.80mm
  • 3.000 inch / 76.20mm
    • While the majority of telescope manufactures use millimeter to messure aperture, majority of telescope manufactures use inches for lens diameter,
    but millimeter for lens focal length.

    Today, the 1.25-inch is the most common (from beginner to mid-advanced) and the 3-inch is the least common.
    Unless you have money burning a hole through your pocket, you should avoid the telescopes that use 3-inch lenses.
    What visual wide-field you gain, versus the price, you are better off with the 2-inch lens telescopes.
    Unless you have no money to spend on this hobby, avoid the 0.965-inch lens telescopes. Quality of view suffers a bit.


    I have two telescopes that by default use the two diffrent lense diameters. With an adapter, they both can and do use 0.965-inch and/or 1.25-inch lenses.
    I definetly have a preference, and I found amost everyone else does too.
    My first telescope, and the last telescope I purchased, came with 0.965 (24.5mm) lenses. I also have a National Geographic that came with a 1.25-inch / 31.75mm
    lenses and you can really see the difference.


    Photo of my .0965 diameter/20mm focal lens next to my 1.25 diameter/20mm focal lens.

    For astronomical telescopes, my opinion, and the majority opinion, a 1.25-inch (31.75) lens (or eyepiece) diameter is significantly better
    than a 0.965-inch (24.5mm) diameter.

    The 1.25-inch size is the most common modern industry standard, offering very good optics, a wide field of view, and access to thousands of standard upgrades.


    Here is a breakdown of the differences and highlights of why the 1.25-inch is the superior choice over the 0.965-inch:

    1.25-Inch / 31.75mm Standard:
  • Accessibility: This is the universal standard for modern telescopes. Nearly all new telescopes are built to accept 1.25-inch accessories.

  • Field of View: The wider barrel allows for larger glass elements, giving you a much wider and more immersive view of the night sky.

  • Accessories: Just way more of them. Standard filters (for viewing lunar details or blocking light pollution) and Barlow lenses are widely
    manufactured in the 1.25-inch size.

  • Quality: Something you see and feel in person. Standard starter eyepieces in the 12.5 format are well-machined, whereas 0.965" eyepieces
    are usually made of lower-quality plastic and glass.

  • Works better for photography. If you have a special camera setup, or just wanting to use your iPhone.


    0.965-Inch / 24.5mm Standard:
  • The 0.965 smaller size is an outdated standard mostly found on cost cutting cheaper models or vintage, second-hand telescopes.

  • Tunnel Vision: The narrow barrel severely limits the amount of light and the field of view, often making it feel like you are looking
    through a straw.

  • Limitations: Upgrading or finding replacement parts like filters and diagonals is incredibly difficult because fewer modern equipment is
    made in this size.

    Have a 0.965? There is hope! What to check next:
    If your telescope is equipped with a 0.965-inch focuser, you aren't completely out of luck. You can often buy a 0.965" to 1.25" adapter
    Sometimes this adapter is called a hybrid diagonal. the adapter just slides into your old focuser, allowing you to use modern, higher-quality
    1.25-inch eyepiece lens.

    Is photography going to be your primary use? Use should consider a telescope that uses 2-inch lenses.


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