A Guide to Terrestrial Telescopes
Terrestrial Versus Astronomical Telescopes
Telescopes don’t have to be used to point at the heavens… they can, of course, be used to view objects much closer. Telescopes that are designed for non-astronomical use are known as terrestrial telescopes.
The main difference between terrestrial telescopes and their astronomical cousins is that terrestrial telescopes need to present the image in the same orientation as it appears in reality.
Astronomical telescopes (unlike binoculars) typically present an inverted (upside down) image to the observer (some exceptions. More on that below). This arrangement would be unsuitable for telescopes intended for viewing objects on Earth, so terrestrial telescopes incorporate an arrangement to show a non-inverted image.
Certain types of Astronomical telescopes (e.g. Refractors) equipped with erect-image prism/diagonal can be used for this purpose. Others are effectively one half of a binocular - known as a monocular. They use prisms to invert the image to present it the right way up for the viewer.
Terrestrial Refractors / Conversion of Astronomical Refractors
A standard astronomical refractor can easily and cheaply be converted into a terrestrial scope with the purchase of an erecting prism/diagonal mirror (many come with these when bought such as our Explore Scientific Refractors). Erecting Prisms are simple accessories and, as their name suggest, they correct the resultant image produced by the refractor to be upright (right way up) and non-left/right reversed.
These types of setup are great as sight-seeing telescopes either at home or when out and about. Make sure you choose the right level of magnification and aperture. For terrestrial viewing, you will not typically require high magnification or large aperture. Hence, a high quality, lower magnification and smaller aperture telescopes (e.g. ~70-90mm range) will do the job well. For better clarity image, and ED (Extra-Low Dispersion) telescope is recommended.
Spotting / Bird Watching Scopes
One of the main types of terrestrial telescope are models designed primarily for bird watching, wildlife spotting and hunting. They provide a higher level of magnification than binoculars do and offer a steady view on a mount or tripod but are still relatively portable and are designed for use in the field. Larger telescopes and objective lenses offer better closeups and brighter views at the expense of portability.
Straight or angled tube?
- Can be cheaper
- Generally easier to point at the desired target.
- Easier to track moving objects
- Allows a more comfortable viewing position in most circumstances
- Better for viewing targets in high locations such as trees
The optimum choice is a normally a trade-off between image brightness and magnification. During bright daylight an objective lens of 60mm with magnification of around 30x is probably the best balance between the two. With lower light conditions a larger objective lens would be beneficial, in the range of 80mm-100mm.
Just as with astronomical telescopes, terrestrial scopes have the option of interchangeable eyepieces, wide-angle or variable zoom e.g. 15-45x. These can come with the telescope when purchased, or can be added to afterwards.
Major manufacturers include Bresser and Alpen and range in price from around $250 to $1000 depending on model.
Range Finder Scopes
Other than ‘spotter scopes’ another type of terrestrial telescope are range finder scopes. Range finder scopes are useful for any activities that requires measurement of distance, including hunting, for both shooting and archery.
Bresser is a major manufacturer and their small portable handheld or bow mounted scopes prices range from around $300.
Dangers of Daytime Use
A risk that is present during daytime that obviously isn’t around after dark is that of accidentally viewing the Sun. Viewing the Sun with the naked eye can cause damage, and never ever look through binoculars or a telescope directed even in the vicinity of the Sun, as viewing the Sun directly through optical instruments can cause permanent damage and blindness.