Seeing & Transparency scales

[Stu]

Simple question, are there any agreed or preferred scales for seeing & transparency.

The Bortle scale seems fairly standard for sky darkness, but there seem to be many variations for the other two. Some even seem quite confused between transparency and darkness such as this one...

http://sctscopes.net/Observing_with_an_SCT/Learning_to_See/UMi_-_Viewing/SAC_Transparency_Scale.html

Also, combining cloud cover with transparency may seem obvious, but there are situations when it doesn't work, for example I remember nights with low, patchy cloud, but above that, in the clear gaps, the transparency and seeing were both very good.

Main reason for asking is that I want to start recording my observations properly and want to settle on a format first so I can remain consistent over the years. I have intermittently made records but never kept to a consistent way of doing this. I think it will improve my observing if I keep something longer term.

Interested to hear comments and suggestions.

Stu

[Moonshane]

I'll watch this with interest. It's always bugged me using phrases like good or poor when describing seeing etc. I know now better though!

[swamp thing]

Seeing

Antoniadi scale

A scale of

seeingconditions, used by amateur astronomers, that describes the effect of atmospheric motions on an image, usually that of the Moon or a planet. Its categories are:

I

perfect, without a quiver

II

slight undulations, but with calm periods lasting several seconds

III

moderate, with some greater air movements

IV

poor, with the image in constant movement

V

very bad, making observations very difficultIt

There is also Pickerings scale of seeing which uses numbers going the other way round with 10 = excellent and 1=very poor .

Cant find any such scales for transparency.

[swamp thing]

I guess to gauge transparency you'd have to record the visible mag limit that night. Not sure there could be a scale thinking about it.

[JamesF]

I often check (naked eye) limiting magnitude against stars in Ursa Minor. I guess at any given site that would work as a relative comparison of transparency?

James

[Stu]

That makes sense James, if you know the site and know what it is like under best conditions then you can make a relative judgement. Otherwise you are always in an unknown combination of transparency and LP.

Stu

[Stu]

Thanks all.

Swamp Thing, Pickerings was the one I couldn't remember, thank you for that

Shane, what was your view? Sounded like you had an opinion

Stu

[Mike73]

I use the Antoniadi scale but find it pretty tricky to gauge when viewing DSO's, with the moon or planets its quite easy but if I'm at my dark site and its a new moon with Jupiter sitting over Plymouths skyglow the only way I've found is to trust my SQM meter in a similar way to James using Ursa Major for NELM but like you say Stu its no good if you are at a new site.

Transparency I just gauge over the evening noting high,medium or low levels clouds.

[Steve Ward]

I found these helped explain things pretty well . . .

http://www.damianpea...m/pickering.htm

http://www.damianpeach.com/seeing1.htm

Steve.

[Don Pensack]

One of the best "transparency" scales is the Bortle Scale:

http://en.wikipedia.org/wiki/Bortle_Dark-Sky_Scale

Take the naked-eye limiting magnitude figures you read with a grain of salt. They MIGHT be true for an observer with PERFECT vision, but some tests I've been part of show

the naked eye limiting magnitude can vary by as much as two magnitudes (!) among 20 observers. The only way to eliminate that scatter of results is to log and record ONLY

your own results, or to use an objective measurement of darkness with a device (like the Unihedron Sky Quality Meter).

[Stu]

Thanks all, interesting to hear other opinions.

Great links Steve, I shall read thoroughly later today.

Bortle, I've always thought, relates to sky darkness as opposed to transparency but am I incorrect in that? Does it combine both?

Cheers

Stu

[acey]

Seeing scales were devised by professional astronomers in the 19th-20th centuries and have been mentioned by other posters. One example is the Mount Wilson seeing scale (http://www.handprint.com/ASTRO/seeing2.html#mtwilson ). Seeing is generally measured in terms of the deviation from what you'd expect a perfect point source to look like through a telescope (i.e. measured in arcseconds of resolution, or in terms of the Airy disk).

Transparency refers to atmospheric extinction, for which sophisticated models exist. But for us amateur astronomers it becomes tangled (and perhaps even synonymous) with a bunch of other concepts such as naked-eye limiting magnitude, sky appearance (e.g. Bortle scale) and sky quality (as would be measured for example by a Sky Quality meter).

Imagine you're at a place with good transparency (a clear night with no apparent haze) and zero light pollution. Let's suppose it's a place like UK climate, i.e. not very far above sea-level and with a fair amount of water vapour in the atmosphere You can clearly see the Milky Way, and stars down to sixth or seventh magnitude. Now put a whole town around that place, with lights on. Suddenly the Milky Way has gone and you can only see down to about magnitude 5. The transparency hasn't changed at all - what has changed is the sky brightness, because ground light is being reflected from vapour etc in the atmosphere.

If you observe at a single location where sky brightness can be assumed effectively constant then NELM will serve as a measure of transparency: if you can see stars to mag 5.5 one night and 6 the next then most likely it's due to clearer air. But if you go to one light-polluted place that's mag 5 then another that's mag 5.5 it tells you nothing about the atmospheric extinction in those two places. Even at a single site, if it's 5.5. one night and 6 the next then this doesn't necessarily mean you've lost half a magnitude to atmospheric extinction. Haze in the air will also reflect more light to the ground, making the sky brighter.

The Bortle Scale is a measure of sky brightness, and this is what you also measure with a Sky Quality Meter. If I go to my dark site and find it completely cloudy (as often happens), and point my meter at the sky, I'll get a very dark reading. For either the Bortle scale or SQM to make sense there is an assumption of good transparency.

Really then, when we talk about transparency, what we really mean is sky brightness, with atmospheric extinction playing some role in this along with light pollution.

Historically, professional visual astronomers generally recorded seeing but not naked-eye limiting magnitude - it was recognised that the latter is far more subjective than the former. There is no general agreement on whether the limit should refer to direct or averted vision, steady viewing of the faint star in question or the briefest glimpse. Really the star should be at zenith: as we look nearer the horizon we're viewing through more and more air, so cannot see stars to the same magnitude as overhead.

So, for accuracy, any measurement of NELM should record the standard of visibility (direct/averted, steady/glimpsed), and should also record the elevation of the star concerned, so that the figure can then be adjusted (using an assumed air-mass) to give an equivalent reading for a star seen at zenith.

Few people bother to do this. Instead we make a judgement, usually to the nearest whole number, perhaps based on a star sequence such as Ursa Minor which doesn't vary too much in elevation. This is useful for our own personal records as a check on the relative sky brightness/transparency from night to night, but a far better measure is a device like the Sky Quality Meter, which is not perfect, but gives useful objective readings.

The Bortle Scale also has its uses, but for UK observers I find it to be of little value - I observe in one of the darkest parts of England, with an SQ of around 21.5 on best nights, but on the Bortle Scale it's somewhere between orange and green - I've never seen the zodiacal light, or M33 naked-eye, I have light domes on the horizon, and overhead clouds are often visible from ground light. So I have very little use for the Bortle scale, and use my meter instead.

[Stu]

Many thanks Acey, wonderful reply.

What you say makes absolute sense, and matches my experience I the past. I was struggling to put the Bortle scale to any use for similar reasons to you. My normal observing location is suburban, NELM approx 5 on very good nights, often much less and varies alot depending on the direction. Bortle does seem biased towards the best locations in the world!

I shall accept the difficulties of separating transparency from sky brightness, and settle on judging NELM from Ursa Minor or similar overhead stars, and make a subjective note on perceived transparency. At my home site I will fairly quickly build a picture of the good and poor conditions. I've observed all this for years, but have never really noted or recorded it to have a good baseline to work from, that is my intention from now on.

Many thanks for all the helpful responses.

Stu