As noted in the prior blog, I recently spent an enjoyable evening from my driveway tracking down the Owl nebula. It was a challenge to discern it amid Towson's ample light pollution with my modest 80mm refractor, but that made the accomplishment all the sweeter. It also made me revisit a wonderful book, Visual Astronomy of the Deep Sky by Roger N. Clark to refresh my understanding of the science behind the successful observation.
You don't have to be an amateur astronomer to grasp the concept of dark adaptation - we've all experienced how over several minutes you can begin to pick out dimly lit objects in the bedroom once the lights are doused. While the widening of the iris to take in more light helps, it is the gradual accumulation of "visual purple" that is the principle actor, amplifying the sensitivity thousands of times. It takes about 30 minutes to build up a full compliment of the chemical in the eye's rods & cones, and it can be undone by a brief encounter with a strong light source (even a bright star as viewed through the eyepiece). So lesson one for the suburban astronomer when stalking the Owl is to be as diligent as possible shielding your eyes once you have some night vision. A shroud of some sort to drape over your head to block out the neighbors' lights is an inexpensive but useful tactic.
Another interesting finding reported by Clark is that the eye actually does have an integration capability when it comes to gathering light. It's certainly not like film or CCD sensor, but "for the detection of the faintest objects, the light must accumulate on the retina for around six seconds." This proves yet again that patience and discipline play a key role for capturing faint fuzzies, requiring that you study a section of the field for several seconds rather than looking at a new spot if you do not immediately find something. It's actually a bit harder than it sounds - I know my tendency is for my eye to want to dart around the field in search of my target.
Averted vision is well known among observers. The eye's improved sensitivity to faint light outside the direct area of focus (the fovea) is due to a higher density of rods in that area of the retina. Clark provides diagrams that show the population of rods across the field of view, as well as pointing out the literal "blind spot" we all have. When you're behind the eyepiece it helps to know where this rod dense region is rather than simply looking askance of your suspected DSO. For the record, it is "8° to 16° toward the nose from the center of vision."
One of the more misunderstood concepts is the relationship of magnification to detecting a faint object. You often see people posting on FB or boards that you want the lowest magnification because that gives the brightest image. But that also gives you the brightest background (light pollution) as well, and anyone who has bothered to bump up the magnification of a misty open cluster can attest that the fainter stars are easier to see under that higher power. Many assume that the increased magnification darkens the background sky and makes the object easier to pick out - yet the object is also darkened, so that does not make sense. Clark explains with several graphs how the size of the object plays a significant role in our eye's ability to pick out a faint object. In fact, there is actually an optimal magnification for an object given its size, brightness, and scope aperture.The author kindly provides a handy appendix that list many DSO and their optimal magnification. For M97 and my 80mm Vixen the suggested magnification would be 140x, and indeed I had my best view at 150x.
To wrap it up, I bagged the Owl by applying most of Clark's suggestions: guard your night vision to the best of your ability, patiently study one area of the view with averted vision for 10-15 seconds, and don't hesitate to increase the magnification if your first inspection turns up no result. If you are a fan of visual astronomy I thoroughly recommend you get a copy of Visual Astronomy of the Deep Sky. He truly treats the topic as a science as well as walking you through how to study an object to get the most from it. Because it is out of print you should be prepared to pay a premium price (north of $100 for a copy available on Amazon). Of course, an even better option if you are a member of HAL would be to check out a copy from the club library as noted here. With star parties resuming this summer it's a great time to learn more about the science and practice of visual observing!
