Sunday, November 27, 2005

Urban Astronomy: Confident Seeing

Extended deep sky objects which are measured in magnitudes per arc second squared, present a challenge in balancing contrast and size in the effort to detect it againsta background sky. When reading an online article on averted vision, I couldn't help but think of how an observer's skills can be associated with their confidence in their abilities.

"... seeing is an acquired ability, perfected through practice, and the more eyepiece time you accumulate, the more acute your eyes become to faint objects and subtle details..."

I appreciate these words from BINOCULAR ASTRONOMY by Craig Crossen and Wil Tirion, in their perfunctory "getting started" section, entitled Observing Techniques, on page 29. To add to this, in a Sky & Telescope column article by Gary Seronik regarding seeing M57 in binoculars he states that the observer must set their expectations of what they will see. With some of this advice, in addition to others, it is true how one's ability to see with increasing degrees of details improves over time. I look back at my logs seeing how the notes from early sessions lack the detail that I record now. Over time, my seeing ability has improved and certain level of confidence that can be found in the vocabulary that I use: sense, detect, observed. Concerning the faint deep sky objects, notably galaxies and bright nebulae, detection is the goal, and observation is icing on top.

So when I discovered the Averted Vision Scale at it got me wondering about the level of confidence I have when stating that I have detected an object. I don't think my seeing skills have evolved to the point of AV3 or greater in the copied scale below. When talking with friends about detecting an object, we have tried to limit it to a binary function: one sees it or not. Fuzzy logic is hard a fit, and one's confidence in their seeing ability will partially determine if an object was detected. The other, of course, is that the listener/receiver accepts the report as truth and with trust.

AV1 Object is seen with averted vision, however, once found, the object could occasionally be seen with direct vision.

AV2 Object is seen only with averted vision, however, it is held steady.

AV3 Object is only occasionally seen with averted vision as it “comes & goes” with the seeing conditions. In this case the object is seen more than 50% of the time.

AV4 Object is only occasionally seen with averted vision as it “comes & goes” with the seeing conditions. In this case the object is seen less than 50% of the time.

AV5 Object is only glimpsed with averted vision after continuously viewing the field for a few minutes or more.

As a shorthand aid in my logging, I created an Information Index to assign a value to deep sky objects. In a glance it tells me how much information I was able to obtain from the observation. The index assignment is temporal and dependent upon the local, immediate conditions - weather, scope, fitness, etc - when the object was observed. Over time an average can be fit that provides a detection predictability value for our skies. Below is the table for review.

"0" is an attempt with no detection.
"1" is a detection of object, location described relative to starfield.
"2" is an observation location, size, and/or shape.
"3" is an observation location, size, orientation (PA), magnitude, brightness profile and a subjective description.

When reading through it I reflected on my seeing abilities. I consider an object detected at AV1 or AV2. Applying this scale with the Information Index, I confidently report a "1”- detected based on seeing an object with a maximum AV2. I don't think I have neither the experience nor the confidence to state an object's detectabililty of AV3 or greater. I fear that I may be mistaken for "averted imagination" when the desire to detect an object is strong.

One example is NGC6946, a galaxy in Cepheus. This was prior to assigning an info index value. In my notes on a day that I suspected it, there was a remark saying "not certain." One of the principal criteria that I rely on when stating that I see an object is its location relative to the star field. Dimension, size, orientation are attributes that can be described with increasing information. On that day I recall that I sensed, suspected, the object relative to the starfield. It was a very small faint unfocused spot which I sketched in my notebook. Later I would check with software to see if this was the object. As a result, I have stated that I have seen this but upon reflecting I don't think so. If I had used the index, a certain measure of discipline could have played a role in the observation. Its surface brightness value exceeds what I have be an able to detect, and on subsequent attempts they all have been marked as "0".

So for now since we have agreed to say that we "see" or "not see", I rely on poetic license to use the terms sense or suspect to address that fuzzy ground. By noting that, it encourages me to return to an object hoping that one day a "1" will turn up.