Averted vision and telescope tapping stargazing techniques

Averted gaze and telescope tapping are two techniques used to see faint objects, such as nebulae, through a telescope. You might already understand how they work based on the eye's physiology.

Explanation of the stargazing techniques: Averted Vision and Telescope Tapping

For example, when observing the Ring Nebula, staring directly at it might make it hard to see. This happens because the faint light from the nebula falls on your fovea, which contains mostly cones. Cones are designed for daytime vision, and the nebula’s light isn’t bright enough to activate them. By averting your gaze—looking slightly to the side—you allow the nebula's light to fall outside the fov, onto a part of the retina that has more rods. Rods are more sensitive to faint light, making the nebula appear brighter. Since the fovea is only about 1-2 degrees across, you don't need to shift your eyes much before the rods become engaged. The farther you avert your gaze toward the edge of the retina, where rods are more numerous, the better your chances of detecting faint objects. (Just remember to look right with your right eye and left with your left eye, to avoid the blind spot.)

The drawback to using averted vision is that the photoreceptors outside the fovea are less densely packed, reducing sharpness. To prove this, try reading text while staring at a dot in the page's margin.

Another trick is to gently tap the telescope to make the view shake. Since your rods are better at detecting motion than still objects, this slight movement makes faint objects, like the Veil Nebula, more visible. This method is particularly effective because the eye detects edges better than fill, and tapping the telescope enhances edge detection, helping to reveal faint nebulae.

One tip is to tap the eyepiece slightly when using averted vision. Peripheral vision is better at detecting edges than solid areas, and this subtle motion causes the nebula's edges to interact with rod cells, triggering the retina’s motion detection system. This system evolved to notice movement—like a tiger sneaking up—making motion more critical to detect than specific colors or details.

PulsGuide: Why it works

So what's the big deal about pulsing?

The big deal is designing the reticle illumination to complement how the eye works. Red light of course, to preserve night vision. Pulsing to keep the eye from going blank because the eye, the retina, is more than just billions of light sensors converting photons into electical signals to send on to the brain for processing. If the retina merely converted photons to electrons, it would require an optic nerve an inch across! So something must happen in the retina itself , the retina processes the image to extract the most important information and sends that to the brain. So what is the most important thing your retinas see? Well, put your self into an eat-or-be-eaten state of mind. If your sitting in the jungle, what is the most important thing in the environment that your retinas need to tell your brain about? Is it the tiger sneaking up on you, or the bush sitting next to you? How does your retinas tell the two apart? Tigers move and bushes don't. It is that motion, that change in the image on your retinas, that retinas extract and send on to the brain. Intensity and hue are secondary, and can be filled in later. As you can see in the following eye chart.

So how does the retina process information? Doesn't that take Neurons? Brains? Yes, and that's exactly what the retina is, an approachable part of the brain (ref: John E. Dowling " The Retina an Approachable Part of the Brain", 1987, ISBN 0-674-76680-6). The retina is a mesh of neurons and photoreceptors, wired and programed to extract change from the image on the retina.

Indeed this is one part of the brain that we do know pretty well how it works. We know how it works so well, how the retina extracts motion, edges, contrast, that scientists have built it in silicon (Ref: Carver Mead, "Silicon Retinas"). To illustrate this, Try out the next two eye charts.

If intensity and hue were more important than change in the image (contrast in this case) then the ring would not look brighter on one side than the other. Edges are important too. To give an example of how important edges are, think why should a pencil sketch -- line drawing-- of someone's face be recognizable as a face? After all it's just lines! But those lines are the edges of the facial features. A line sketch simplifies things down the same way the retina does. To the basic important features your brain is designed to make sense of... edges! Which leads to the next eye chart.

This chart below is sorta neat. Stare long enough and the fuzzy edged circle goes to gray. If you could keep your eyes completely still (notice how hard it is to keep them still the longer you stare), then even the sharp edged circle will grey out. Fortunately your eyes won't let you. They constantly jitter just a little bit to keep sharp edges detectable by the retina's processing. Which brings us back to why PulsGuide works.

The longer and harder you stare at a guide star hiding behind a constantly illuminated crosshair, the more you try to keep your eye from jittering, the worse you'll do because its completely contrary to how your retina works. PulsGuide, by blinking the crosshair introduces change in the image, PulsGuide works with your retinas to keep your vision at maximum sensitivity. As a result you can work with fainter guide stars, guide more comfortably, get better astrophotographs.

PulsGuide WORKS!

Learn the Sky video explaining why astronomers deliberately look slightly off to the side of the object they want to observe to master the technique.

 

Download the full PulsGuide

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