Sky classification

[glennbech]

Hi,

I am new to observation and have a few questions.

1) What kind of notes do you do on sky quality (stability and darkness) on observation nights? If you keep track of it at all that is. I have seen som reports that start with a classification of seeing for example. How are they done? On my last observation session, I did an experiment where I found M45 and picked the faintest star I could possibly see. At home, I used Aladin, a star atlas program to locate it. It was this little guy here. It is in the string of stars that run south from the center. It has a visual magnitude of 9.6 - maybe not very impressive as the faintest observable star?   

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2) On my last session, the sky was a bit washed out with a blue hue, probably because of the 100% relative humidity and light scatter from nearby habitation and an airport. How much does dark adaptation really count in such a scenario. I failed to detect the Leo Triplet for example, but I felt that the task was more like telling something gray and fuzzy from something slightly darker and gray/blue :-)  

[vlaiv]

You could use something like NELM to judge sky transparency for the night. It is Naked eye limiting magnitude. You will find that it is the rough guide but usable one. It is best to keep record, or at least note such value on observing nights to create your own reference frame and build your expectations based on previous experience (and monitor progression of your observing skill). The way to do it is to look at the faintest star near zenith that you can spot with naked eye after some dark adaptation (wait a while before trying to do so). Make note what star was faintest that you managed to see (or couple, and afterwards look them up using sky atlas program and see what is their apparent magnitude). Level of dark adaptation may vary, and also some people do it using direct while others prefer peripheral vision to do this (depends how well you are trained to observe in this way).

For level of LP it is a bit harder to judge by eye, many people use SQM kind of device (Sky quality meter). It is in essence a photo detector directed at patch of sky that records level of light coming from that direction.

Seeing is "measured" or rather judged by using some sort of seeing scale. Usually Pickering scale (google it up, there are a lot of reference with examples how stars look at each level). This is done by observing star at high mag thru cooled scope (to avoid thermals).

Under bad transparency and high level of LP detection is not about can you see the amount of light that comes from target, but rather is there enough contrast to distinguish it from surrounding sky. Its like looking at the puff of smoke against gray background . Note that in bad transparency LP will become worse (same stuff in the air that reduces transparency actually reflects even more light from ground sources and creates additional LP).

Dark adaptation can be achieved even in presence of rather strong light sources around you if you pay attention to detail. Some people use eye patch for observing eye while setting up equipment. Use hood or any other opaque cloth to cover your head and shield yourself from light sources (make sure you cover yourself so there are no gaps for light to pass to you, even reflected light from ground). Dark adaptation will make a difference.

There are some other techniques that can help you with detection. Select right eyepiece to give you proper exit pupil for given conditions (many people use 2-3mm exit pupil under dark skies for best contrast and 3-4mm exit pupil under LP). If background sky is too bright - go for higher mag. Tap your scope to see if it will help. Eye sometimes responds better to moving target than stationary one. Learn to use peripheral vision properly. Train yourself at it, easiest way to do this, in my opinion, is to find a star that is on threshold of direct vision detection, and then to look at it while keeping your center of vision about a quarter of FOV left or right of it - about 10-15 degrees to either side, and you will notice that star becomes brighter. Keep doing this on faint stuff and try to locate eye placement that gives you brightest image.

You might also find this link useful : Sky Brightness Nomogram

[billyharris72]

I tend to use the Bortle scale now (https://en.wikipedia.org/wiki/Bortle_scale). While nominally a light pollution scale it also captures transparency etc. It is pretty subjective, but still useful so long you aim to be consistent in your own observations. Seeing is a different matter. The Pickering scale seems to be the main one, but I find it a bit of a faff. Again, if you are after a quick, less formal approach that still lets you describe seeing, and aiming more for internal consistency the Antoniadi scale is fine.

I  Perfect seeing without a quiver of turbulence at all.
II  Slight shimmers; moments of stillness last several seconds.
III  Average seeing; larger air tremors blur the view.
IV  Poor views with a constant and disturbing swell.
V  Bad views with severe undulations; so unstable that even quick sketches are out of the question.

The best option on LP is probably to go for a sky quality meter, but even these have limitations, since the sky is rarely of a uniform quality unless you are so far away from sources light pollution that you probably don't need to pay it too much attention. So even here, observational notes on the horizon at differnt directions and how it compares to the zenith would still be needed if you wanted a complete account.

Billy.