Mel Bartels On Limiting Magnitude

Mel Bartels has some thoughts on limiting magnitude (emphasis mine):

…Illumination drop-off at the edge of the eyepiece? Stated in percentages (e.g. 15% sounds terrible), should be in magnitudes (e.g. 0.06 mag, unnoticeable visually). Mirror coating reflections? Stated in percentages (e.g. 92%) should be in magnitudes (e.g. 0.04 mag loss). It is very difficult to see differences of 0.2 magnitude or less. And when the view is dimmed, both object and background are equally dimmed, leaving the contrast unchanged. Unless the view is grossly dimmed, the unchanging contrast means that the object does not lose visibility.
…

• Aperture is the biggest factor [duh!]. More aperture increases visibility regardless of magnification or power.

• Seeing the object in a larger scope then returning immediately to your smaller scope can result in a half magnitude gain.

• Observer experience is worth 2 magnitudes (I have a series of sketches of M31 from childhood onward).

• Observer variation is a half magnitude or more.

• Age matters a magnitude: young kids can see very faint stars; as we get older, our lens yellows and ability to detect fades.

• Knowing where to look and what to look for worth a magnitude.

• Averted vision is worth a magnitude.

• Dark adaption continues to produce increasing benefits for hours, ultimately worth maybe a half a magnitude.

• Field baffling is an overwhelming factor: the difference between nonexistent and fully baffled views can be worth magnitudes.

• Covering your head with a black cloth also yields improvements, perhaps on the order of a fraction of a magnitude.

• Time at the eyepiece is worth a magnitude (objects gradually become recognizable or detectable over a period of time, and then they fade after a prolonged period of continuous observing).

• Comfort at the eyepiece is worth a half magnitude.

• Rested eyes are worth half a magnitude. I often take short breaks throughout the night. Upon returning to the eyepiece I can see more until my eyes tire.

• Sky transparency is such an overwhelming factor; on rare perfect nights I’ve seen scopes perform as if they had almost unlimited aperture; let’s call superb sky transparency worth a magnitude or two.

• Filters are worth a magnitude.

• Visibility appears to correlate most with aperture, then apparent size (the greater the aperture, the greater the apparent size, limited by the full field of view).

• True binocular or two eyed viewing results in a half magnitude gain in stellar limiting magnitude and about a magnitude gain for extended objects.

Make these factors work for you and you can gain magnitudes in observing prowess. It’s like having a much larger scope on hand.