Averted Vision Scale Developed by Ron Morales
Averted Vision (AV) is an essential tool for Deep-Sky observers. To allow some distinction in the visibility of Deep-Sky Objects (DSOs) Ron Morales of the Sonoran Desert Observatory developed the following scale.
Object can be seen with averted vision but once found, the object can occasionally be seen with direct vision. If an object is first noticed with averted vision but once found this object can then be seen steadily with direct vision it is considered a direct vision object as opposed to an averted vision object.
Object can be seen only with averted vision but it is held steady. Here the sweep of one's vision makes the object detectable.
Object can only occasionally be seen with averted vision as it "comes and goes" with the seeing conditions. In this case the object is seen more than 50% of the time.
Object can only occasionally be seen with averted vision as it "comes and goes" with the seeing conditions. In this case the object is seen less than 50% of the time.
Object can only be glimpsed with averted vision after a continuously viewing the field for a few minutes or more. This level of averted vision usually occurs when one carefully observes a field for a lengthy period of time. This might occur within the first 3 to 5 minutes of viewing the field.
In this level it is important that the observer has no knowledge of the exact location of a possible object. Having such knowledge prior to viewing could mislead some observers into believing that they saw something they did not actually see. One problem associated with viewing extremely faint galaxies it that sometimes an extremely faint star could be misidentified as an extremely faint galaxy.
For this level of averted vision it is suggested that the observer make a field sketch showing faint stars as well as the object in question. This field sketch can then, at a later time, be compared to an actual photograph or chart. At this level of detection are you seeing or just detecting the presence of an object.
James Whinfrey 5 April 2017
I've reproduced the scale here because I've noticed the disappearance of a few websites where I've found it in the past and you'll frequently find it quoted in our observations.
Please note that the emphasis in under AV5 is mine as I feel a sketch is the only reliable way to confirm an observation of this kind by later research. I've also created a PDF version of this averted vision scale document.