May 31, 2020

Tricks at the Eyepiece

Sure, you’ve got the gear, you’ve got the ultimate telescope, or maybe you just like to causally observe. But have you ever given much thought as to how to observe? The simple act of looking is so reflexive that most of us do it without a second thought. In the realm of astronomy, however, the use of a trained eye is paramount to enjoying what you’re seeing.

After years of observing, this accumulated skill may become apparent when showing off that favorite galaxy or nebula to the uninitiated; cries of “I don’t see anything,” or “you mean that smudge?” are common at star parties. What follows are some tips on how to observe and interpret like a pro, and a way to perhaps quickly guide new observers to a successful first session at the eyepiece;

The human eye, down and dirty: Our eyes have been called one of the worst optical instruments made; if you bought a camera of such poor optical quality, you’d want your money back. Plagued with difficulties in focus and subject to fatigue, it’s a miracle that these windows on the world work at all. Still, let’s see YOU take water and jelly and make an optical sensing device… I like to think of the human eyes as a pair of 8mm (about the diameter of a dark adapted pupil) oculars. Our eyes get worse with age, and our retinas are coated with two types of cells, rods, for low light conditions, and cones for bright light. Our sharp field of view is surprisingly small, a little bigger than a Full Moon which itself is only about half an arc degree wide. Think of the rods and cones as fast and slow film, for those of you old enough to remember film! The fact that cones are the only photoreceptors sensitive to color explains why the universe appears largely gray to the naked eye, and only bright objects exhibit color.

Dark Adaptation: Crucial to enjoying deep sky viewing, our eyes take time to adapt to low light conditions. Astronomers of old took great lengths to adapt their eyes, locking themselves away in darkness for up to 12 hours prior to an observing run. The good news is that you don’t have to resort to extreme lengths; 99% percent of our dark adaptation occurs in the first 30 minutes or so. This is why you see folks using red flashlights at star parties; red light doesn’t trigger those cones into action and ruin our nighttime sensitivity. Some observers even build red plastic shields for their laptops, dome lights, and cell phones!

Using low power: Many would-be observers are drawn in to buying department store scopes that advertize high magnifications of x1000 or more. The trouble is, these also magnify distortions in the atmosphere and make it next to impossible to find anything! I have a 40mm eyepiece that I bought for use with my 8” SCT telescope, and I use it 90% of the time as standard equipment. This gives you a wider field of view and may allow you to “sweep up” that target that’s right at the edge of the field.

Using high power: So, when do you use high power? Ironically, many observers swing too far in the Low power=good, high power=bad direction. Planets, double stars and that elusive planetary nebula may all require higher power if conditions allow it.

Averted vision: Our eyes are uniquely adapted for our role as a top predator; stereo vision, facing forward. To this end, we are well adapted to sensing movement across our field of view, and sharpshooters and astronomers share this common skill of utilizing averted vision. Often, a faint target or planetary detail you’re looking for may “pop out” if you shift your vision slightly, or gently rock your telescope tube back and forth. The reason the works is, again, those rod cells immediately adjacent to the cones in the center of the fovea have the greatest density and therefore the most resolution of detail.

On knowing what to look for: This comes with experience. Is my target a faint planetary nebula or galaxy or a pinpoint source like a star? Magnitude brightness for nebulae is especially deceptive, as their brightness is spread out over a larger surface area.

Arrgh… the eye patch: Most of us are either right or left eye dominant, and this is so reflexive that we may not even notice which eye we prefer at the telescope. A test for this is known as the Porta test; extend your thumb, focus both eyes on a distant target, and slowly draw your thumb back towards your face. The eye you draw your thumb back towards is your dominant or favored eye. To this end, many observers will cover the alternate eye with a patch, so as to “train the brain” and not be distracted.

Observing (with the) hood: Another variation of this is to pull a hood over your head and make your own private dark room much like photographers of yore. This is especially effective during solar viewing and imaging, and I’ve accomplished this by simply pulling an old black T-shirt over my head.

On being alert (and awake!) in the wee hours: This is the fine line we always run; caffeine is an astronomer’s friend, but to too much can constrict the pupils and limit night vision. Likewise with alcohol. Be sure to get plenty of sleep, and one non-caffeinated proven alternative that I have heard of is peppermint candy to promote alertness.

But above all, get out, observe, and do what feels natural. I generally guide folks in with a red light, as the position of the eyepiece on some telescopes isn’t always intuitive. Describe to them what they’ll see, and instruct them to look through the eyepiece and relax. I common error that beginners (especially kids) make is to grab the eyepiece and pull it towards them; tell them to work with the scope. In time, you’ll have won over another fan of the cosmos!



  1. [...] This post was mentioned on Twitter by brobof and Melanie, Ramiz Qureshi. Ramiz Qureshi said: RT @Astroguyz Tricks at the Eyepiece: The fine art and skill of "how" to look through the eyepiece of a telescope! [...]

  2. [...] HQ probably own more red lights than conventional flashlights. As we mentioned last month in our post about tips for observing, light at red wavelengths can give us star chart reading capability while [...]

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