Eyepiece cheat codes: Averted vision

Getting the most from your visual observations involves using a few "tricks of the trade," or cheat codes, as gamers might view them, at the eyepiece. I've picked up several over the 30 or so years I've been visually observing, but the most useful and well-known is the use of averted vision.

Averted Vision

One of the first things you hear about when you start observing is "averted vision," looking slightly away from a dim object to see it better. This uses the rods in your eyes to best effect. I would posit that there are several variations of averted vision (AV), what I call close, medium, and extreme. 

In all three variations, which are really just part of a continuum, you are usually not keeping your eye fixed on one spot, but moving it around slightly to find the sweet spot where you can get a fleeting glimpse every now and then of the object or detail. When logging my observations, I'll sometimes refer to the percentage of time I can actually get glimpses of an object to indicate how dim it was. For example, I may see a brighter or larger galaxy 50% of the time, but a tiny dim one only 10%, sometimes even less. 

Consciously trying to determine this number is a fun way of determining how difficult the observation is for you and how it compares to others. You might be surprised how infrequently you are getting glimpses of an object, despite being absolutely certain you're seeing it. This gives more meaning to terms such as "bright," "difficult," or "extremely dim."

Medium AV

Medium-sized objects such as some galaxies, globular clusters, more compact open star clusters, and some double stars respond well to what I would call medium averted vision. This is the most common variation I use as an observing "generalist." In medium AV, with the object centered, you are usually looking about 1/5 to 1/3 of the way from the object to the edge of the field of view (FOV) to see it best. Depending on your own eyesight and equipment, you may find looking in one particular location consistently yields the best results, but more often it just pops out randomly as you move your eye around the periphery of the object. Be aware that your eye has a blind spot, but by moving your eye around, you will likely not notice it.

Above: Image of M22 by StudentAstronomyGroupUoC, CC BY 4.0, via Wikimedia Commons. Edited to simulate eyepiece view with annotations.

A good example of using medium AV is observing the line-of-sight "double" star 55 Cygni. This one has a 4.9 magnitude primary component and an 11.1 magnitude secondary, with a separation of 22.7 arc seconds at position angle 174. Close doubles with a big difference in magnitudes are a fun challenge. At first glance it's not easy to spot the secondary in my 6-inch reflector from my light-polluted Redneck Observatory. But it's not particularly hard if I use medium averted vision at around 150x. It pops into view if I look about 1/5 of the way to the edge of the FOV with the pair centered, but blinks out when I look directly at the primary. Give it a try or a similar one that is a good fit for your telescope.

Close AV

Close averted vision is useful for observations of tiny objects like Saturn's dimmer moons or detail in the core of a galaxy. When viewing Saturn, I notice that the brighter moons require a bit more distance (medium AV) and the dimmer moons often require I look right next to them (close AV), sometimes seeing them if I look directly at Saturn itself, which also helps keep my orientation fixed. I recommend a free app called Moons of Jupiter and Saturn. There is also a paid app for iOS called SaturnsMoons, although I have no experience with that. Apps like Sky Safari also will show the positions of planet moons. Just zoom in on the chart.

Above: Image of Saturn and its moons by Kevin M. Gill, CC BY 2.0, via Wikimedia Commons. Edited to simulate eyepiece view with annotations.

Close AV is also useful for seeing more detail in larger or brighter galaxies. For example, the other night I was observing NGC 5907, a striking 10th magnitude edge-on galaxy in Draco about 11 x 2 arcminutes in size. I can pretty easily see the ghostly slash-shaped galaxy in medium powers in my 10-inch in a Bortle 4.5 sky, with a hint that the center is slightly brighter. This core area pops out a little better if I use close AV, looking right next to it. 

Above: Image of NGC 5907 by By Kết Nối, Việt Nam, Public Domain, via Flickr. Edited to simulate eyepiece view with annotations.

M82 in Ursa Major is another good galaxy to practice AV on. It will likely yield more detail in close AV. You might even be able to pull out a brighter core or some mottling in dimmer and smaller galaxies with this technique.

Left: Image of M82 by David Warrington from England, CC BY 2.0, via Wikimedia Commons. Edited to simulate eyepiece view with annotations.

Extreme AV

I call it extreme, because in this case you're really trying not to look directly at anything in the eyepiece. Instead you're trying to take in the whole FOV at once rather than focusing on a particular spot. Just think of relaxing your vision and letting the entire view wash into your brain. Your eye is moving around trying to soak up every photon to make some sense out of the scene. It's almost like you're trying to pull your eye back into your head a bit to get the widest field possible to register.

This is useful for extended objects or dimmer objects such as a dim but rich star cluster. A good example is NGC 6645 in Sagittarius. It's a beautiful and interesting cluster for a darker sky, but often ignored because of all the other flashy stuff nearby (M8, M20, M17, etc.). 

Left: Image of NGC 6645 by Mike Durkin from Forest Hills, NY, derivative work: Winiar, CC BY-SA 2.0, via Wikimedia Commons. Edited to simulate eyepiece view with annotations.

Extreme AV is pretty much essential for most nebulae other than planetaries. Coupled with a nebula filter (I use an NPB narrowband filter), this can often yield great results where you can see the shape of the brighter portions of nebulae. 

Left: Image of NGC 2174, the "Monkey Head Nebula" by Nigel Hoult, CC BY 2.0, via Flickr. Edited to simulate eyepiece view with annotations.

Sometimes even a small object that's extremely faint can benefit from extreme AV when you're trying to get any hint at all that something is slightly brighter than the surrounding sky. You just have to experiment, as everyone's situation can be different.