Choosing a Telescope
Three important characteristics of a telescopes design include:
- Aperture: The diameter of the telescopes primary lens or mirror. This determines how much light the telescope can capture and will determine the overall clarity and resolution of the object you are viewing.
- Focal Length: This is the distance between the primary lens/mirror and the eyepiece or camera sensor. It will primarily determine the potential magnifying power of the telescope.
- Focal Ratio: This is the ratio between the Aperture and the Focal length and determines the optical speed of the telescope. Its something which doesn't really effect visual astronomy but its important for Astrophotography.
Of these 3 characteristics, Aperture is the probably the most important factor to consider. Most telescope designs can in theory provide several hundred times magnification however if the telescopes aperture is not sufficient to gather enough light for a given magnification, you won't be able to get the resolution or sharpness needed provide any detail in the image.
In the images below we show a comparison as to how Jupiter might look at approx x200 magnification through both an 8" and a 4" aperture telescope under the exact same sky conditions. As you can see, at this high magnification the larger 8" aperture gathers much more light and therefore better image resolution to comfortably show a much brighter and sharper image. However pushing the small 4" aperture to the same level of magnification does not offer the same level of clarity as the smaller aperture can't collect the same amount of light needed to offer the same resolution at this magnification.
© Horizon Astronomy
However, that's not to say you can't see great views of the planets such as Jupiter using a smaller aperture telescope, you certainly can, but just be aware that the maximum clarity and detail that you can see in the image will peak at a lower magnification - See example image below.
© Horizon Astronomy
When it comes to choosing a Telescope, the good news is that most reputable telescope brands of all sizes will generally be very capable instruments and provide you with great views of our solar system and beyond, but its important to be aware of their limitations, and never buy a telescope misleadingly advertised by its 'off the scale' magnification power.
So in a nutshell, when choosing a telescope, we always recommend going for the biggest aperture you can comfortably afford but also keep in mind that the bigger the aperture, the bigger and heavier the telescope will be and its a good idea to consider any space limitations you may have at home for storing and using it, or maybe if you plan on regularly transporting it to dark sky locations.
We find as an entry level telescope, a good place to start would be a 3" to 4" Refractor or Newtonian Reflectors / Cassegrain of 5" to 6" aperture upwards. These will give you some really great views of the night sky and are a great way to enter the field of amateur astronomy.
If you would like to know more about the different specific telescope designs and some of the advantages they offer we recommend reading - Our Astronomy Telescope Designs article here
The Mount:
The Mount is the foundation of your whole setup and is often overlooked when purchasing a telescope. The reality is that a poor mount can render even the best telescopes unusable and its vital to get a mount that is not only suitable for your main area of use (eg. Visual Astronomy or Astrophotography) but also a mount that can comfortably hold the payload weight of all your equipment, including cameras, guidescopes and other accessories you may decide to use down the line.
There are many variations of telescope mounts but the two main types of mounts used in Astronomy are Alt-Azimuth Mounts and German Equatorial Mounts.
Alt-Azimuth Mounts:
These mounts offer a sturdy yet very simple and intuitive way to scan the night sky. These mounts work in 2 axis - Altitude (Up, Down) and Azimuth (Left, Right) and as straight forward as that, you simply move them up, down, left or right to where you want to point your telescope. They are perfect for visual Astronomy and are extremely quick and easy to get setup. They don't require any kind of polar alignment or additional power supply to operate and the lack of motors, counter weights and other accessories mean they are much lighter than German Equatorial Mounts.
The main drawback to these mounts however is their lack of tracking ability and so this rules out any kind of long exposure deep sky astrophotography, however point and shoot astrophotography of brighter objects such as the moon and the larger planets is still possible using a smart phone or camera.
German Equatorial Mounts:
German Equatorial Mounts (GEM) are available as manually operated or fully electric GOTO versions. These mounts also operate in 2 axis RA (Right Ascension) and DEC (Declination). The RA axis needs to be first polar aligned to the north celestial pole (or south celestial pole, if you are in the southern hemisphere) and once setup correctly, these mounts are designed to allow you to accurately track the night sky, keeping your object centered in the telescopes field of view.
A Manual GEM will require regular inputs from the user to keep the image centered in the field of view so we would recommend at least getting a mount with an RA Tracking motor fitted - Many good GEM brands do offer this as an optional upgrade. That way once your target is in the field of view, the motor will track it for you, this means you don't constantly have to manually track it and it will also allow you to begin trying out long exposure astrophotography.
Ideally however, we would recommend a fully electric GOTO GEM. These will have motors on both RA & DEC axis and will also automatically slew the telescope to your selected target by simply selecting it on the hand controller or a laptop.
One important thing to consider when choosing a mount is the payload capacity. For visual Astronomy work, you can operate the mount at its maximum capacity no problem at all, however for Astrophotography it is generally recommended that you operate between 60-75% of the mounts payload capacity as tracking errors may be introduced, because the weight on the mounts bearings and torque required by the motors are higher. These tracking errors can be noticed (particularly in the stars) when taking long exposure photographs.
However it also depends on the quality of the mount, the focal length you are imaging at and the exposure time you are using. For example, if you are at the imaging at the maximum payload capacity of a mount but at a short focal length using 30-60sec exposures, then some minor inaccuracies in tracking might not be that obvious, however as you increase the focal length and exposure times, tracking accuracy becomes more of an issue. So while the 60-75% payload rule for astrophotography is not set in stone, it is a good concept to take into account when choosing a mount as it does give you more options down the line, especially if you plan on using larger telescopes and adding other accessories such as autoguiding equipment at a later stage.
Summary: So when choosing a mount it really depends on your own personal preference. If you know you will just be casually using it for observing the moon and planets and have no particular interest in long exposure astrophotography, then an Alt Azimuth mount would be a very good option, as its light weight and quick and easy to setup.
If you are more interested in viewing deep sky objects and trying out Astrophotography then a GEM is really the only option, ideally a full GOTO version but if not, a manual version with the option to include tracking motors at a later stage would work well too. Just keep in mind the payload capacity if you plan on using a number of different telescopes and camera equipment on the same mount.
For more information or if you would like some one-to-one help and advice in choosing the right telescope feel free to contact us here