“My dear Kepler, what would you say of the learned here, who, replete with the pertinacity of the asp, have steadfastly refused to cast a glance through the telescope? What shall we make of this? Shall we laugh, or shall we cry?”
Undoubtedly the most important instrument one might bring to observing the stars is located between one’s ears. If you have been compelled by reading these pages to venture out into the evening and locate the stars and planets mentioned here, then perhaps a modest telescope or a pair of binoculars will enhance your budding curiosity.
While any telescope or binoculars you might have in hand will serve, most are designed for terrestrial viewing. Such instruments provide adequate magnification in broad daylight.
One of the most common misconceptions among those new to astronomy is the notion that the most powerful telescope is the best telescope. Not necessarily so.
Any telescope can be made extremely powerful with the right focal length and the right combination of lenses and eyepieces inserted after the primary lens or mirror. That doesn’t necessarily mean that these fantastic magnifications often boasted about in unscrupulous advertising are actually usable magnifications in the low-light conditions encountered in the night skies.
What will become apparent to the nascent star-seeker is that the size of the primary, or objective lens or mirror of the telescope is what determines the light-gathering ability of the instrument, which determines what magnifications are actually usable. So it is the aperture of the objective that is important. This increase in size over our own eyes is what magically collects more light for us to see.
“Unlike what you may be told in other sectors of life, when observing the universe, size does matter, which often leads to polite ‘telescope envy’ at gatherings of amateur astronomers.”
Neil deGrasse Tyson, The Sky Is Not the Limit:
The next important factor in determining the magnification presented to the viewer is the focal length of the objective lens or mirror. The shape of the objective determines the distance that the collected rays of light come to focus. You may be familiar with this idea from photography: a 400mm telephoto lens has greater magnification than, say, a 50mm lens. The millimeters refer to the focal length.
Aha, you think, so a long focal length telescope is better!
Not so fast. While short focal length objectives provide less magnification, they also provide brighter images. Many deep sky objects, like Messier 33 or The Pinwheel Galaxy, are large but relatively faint and are best seen in short-focus, or rich field telescopes or binoculars.
So the size of the objective lens or mirror and the focal length of this objective is what matters. This is the f ratio: focal length divided by objective size. The lower the f number for any given objective lens or mirror, the less magnification but the brighter the image.
Since the focal length can be extended by barlow lenses and eyepieces introduced after the primary lens or mirror, I’d recommend that the beginner get a telescope with a relatively low f ratio and a nice assortment of eyepieces for different magnifications. The size and quality of these eyepieces makes a huge difference in the image quality. Many manufacturers often include an insufficient number of eyepieces or poor quality eyepieces to keep the price point low. Often a few well-selected, large diameter (2”) eyepieces will make all the difference between a good instrument and a great one.
There are a myriad of optical configurations but in the most general sense telescopes incorporate either a lens or mirror. Telescopes with a primary lens are called refractors and telescopes with mirrors are called reflectors. I tend to think that refractors are best for beginners because you point the telescope down your natural sight line.
Besides the optical quality of any potential telescope, the mounting is perhaps equally important. As a boy the first telescope I could manage to purchase was a used, uh, perhaps abused, telescope. It had an up-down, side to side mounting called an alt-azimuth. The altitude control was broken and it could only be made to stay still by over-tightening a screw on that axis. It was extremely frustrating to finally locate an object only to have it slip from view before I could get a good look.
A solid, well-made mounting and tripod or pier is essential!
I am partial to equatorial telescope mounts. Quite simply these mounts mimic the rotation of the Earth by providing a declination (latitude) axis and right ascension (longitude) axis. This feature makes it easy to follow objects as they move across the night sky along the right ascension axis. And, believe me, at high magnifications the heavenly bodies will race across your field of view as our Earth turns.
GOTO OR NOT GOTO
One of the relatively recent advances in modern telescope technology has been the introduction of “GoTo” or computer-controlled telescope mounts. These devices automatically find and track celestial objects once the mount is properly aligned. For some observers, these devices are a blessing. For others, not so much. I tend to fall in the latter camp. I may be an outlier here, but I’ve always enjoyed the hunt. Learning the constellations and guide stars necessary to locate the various objects in the sky has been a plus rather than a hindrance for me. Often I have serendipitously discovered a new object while searching for another. I doubt this would happen with a GoTo telescope.
One thing to consider regarding a GoTo telescope is the setup time. Setup on these telescopes is longer and more involved. The best telescope is the telescope you use more often and if the setup is daunting or the telescope needs batteries, you might not bother. I’ve seen a lot of these telescopes gathering dust and that’s sad.
Then there are the economic considerations of a GoTo telescope. The drives, the computer and the software all come at an additional cost. Many manufacturers skimp on the optics and the mount to put together a competitive package. I’d rather spend the money on bigger, better optics and a sturdier mount than on the GoTo feature. Your mileage may vary on this one; however, be prepared to shell out considerably more bucks to get the same quality instrument on a steady mount. If that is your cup of tea, then great! Whatever gets you out there is good.
So, if you have a telescope or pair of binoculars lying around, try pointing them at the moon or one of the objects mentioned in this column. If you are thinking about getting an instrument for stargazing look at a 70mm (or better) refractor on a sturdy equatorial mount. As far as binoculars go, 8×50 provide good light-gathering ability. 8x is the power and 50mm is the size of the lenses.
Bigger lenses in binoculars are good but higher magnification glasses are tough to keep steady when hand-held. If money is no object and you are so inclined, look at GoTo telescopes or a motor drive for your equatorial mount’s right ascension axis to keep up with Mother Earth as she spins through the night.