The Beaufort/Bortle Scale for Seeing & Transparency - how do you measure?

[SuburbanMak]

I am reasonably comfortable now with the difference between seeing and transparency but am finding that I am capturing them in a pretty vague way in my observing notes.  I use phrases like 'steady" or "a bit milky" without them being part of a graduated comparative scale.   Are there easily observable (i.e. without using instrument readings) scales for these things akin to the Bortle scale for LP  or Beaufort scale for wind?   And if so what are the observable phenomena or checks that you use to make a judgement?  I have remembered my Ladybird Book of Weather phrases like "0 - Calm, smoke rises vertically from chimneys"  or  "2 - Light Breeze, small twigs and branches moving"   & found them useful my whole life, it would be great to have a similar set of "rules of thumb"  for seeing and transparency. 

My goal I guess is to get to a simple graded scale I can use in notes for visual observing probably no more complex than  0 - No observation possible, 1-Poor, 2-moderate, 3-good, 4 very good, 5-exceptional.     When either condition is either very, very good or very, very bad they are easy to describe but I'd be interested in what heuristics people use in the field to gauge seeing and transparency in between the extremes and map those to the scale above (like the smoke/chimneys thing.) 

Do you start, for example, with a quick look at a Double, if so which and what is the impact under different conditions?  Do you look out for particular diffraction patterns creeping in? 

Anything you use to shape your sessions, choose or discount particular targets or equipment selections based on field judgements about seeing and transparency would be really valuable experiences to read for me and much appreciated. 

 

Edited March 10, 2021 by SuburbanMak

[Jiggy 67]

SkySafari has a built in scale with a drop down menu for seeing when you are recording your observations which is quite useful 

 

[SuburbanMak]

3 minutes ago, Jiggy 67 said:

SkySafari has a built in scale with a drop down menu for seeing when you are recording your observations which is quite useful 

 

Perfect ! Thank you - this  is exactly the thing...

[Pixies]

Here's the Pickering seeing scale (with animated examples)

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

 

Transparency is harder to define. Do you include light pollution? You could have perfectly transparent skies above the centre of London and not see much. You can use NELM (naked eye limiting magnitude) but this purely objective and will vary from person to person, site to site. Useful for your own records or adding context to you observation reports

 

[vlaiv]

Seeing yes - look at above examples, but transparency - no.

Or rather - you'll have to work harder to asses it.

Transparency affects light throughput. One way to measure it would be to look at series of threshold star magnitudes. Say you know that in steady clear skies you can reach 13.1 mag star with your scope.

You would need to examine stars around 13mag to see which one you can see - say you see 12.8 on particular night. Transparency knocked those 0.3 mags off.

Problem with above approach is that seeing also affects what can be seen. If turbulence is strong - star image is spread over larger area and peak intensity falls down (that is why we see stars sparkling at night). You'll need experience to be able to "subtract" effects of seeing on max magnitude and relation to transparency.

Be sure that you account for atmosphere number - look at reference stars near zenith where atmosphere number is close to 1.

If you have imaging gear - than it is easier - you need to do photometry on reference star. Seeing is easier with camera too - take 2s exposure and measure star image FWHM (take multiple short exposures and note average FWHM and standard deviation to be more precise of how steady seeing is).

[SuburbanMak]

Thank you - those Gifs are great, makes me realise that some of what I have occasionally interpereted as an inabilty to focus and attributed to scope-cooling, general equipment problem or user error is in fact a Seeing issue!

On Transparency I do think there is a link with LP particularly in an area like mine where the river causes frequent mist at low levels once the dew-point is reached, the worse the transparency at ground-level the greater the impact of localised LP (not necessarily as you say much impact on LP at the zenith).  On Sunday for example I noticed slight mist-cones beneath the street lights as I walked out to my observing spot & sure enough struggled as the evening wore on.

Also for both measures I am finding the picture can vary greatly between say 30 degrees and the zenith, almost worth noting readings for each. 

[SuburbanMak]

2 minutes ago, vlaiv said:

Seeing yes - look at above examples, but transparency - no.

Or rather - you'll have to work harder to asses it.

Transparency affects light throughput. One way to measure it would be to look at series of threshold star magnitudes. Say you know that in steady clear skies you can reach 13.1 mag star with your scope.

You would need to examine stars around 13mag to see which one you can see - say you see 12.8 on particular night. Transparency knocked those 0.3 mags off.

Problem with above approach is that seeing also affects what can be seen. If turbulence is strong - star image is spread over larger area and peak intensity falls down (that is why we see stars sparkling at night). You'll need experience to be able to "subtract" effects of seeing on max magnitude and relation to transparency.

Be sure that you account for atmosphere number - look at reference stars near zenith where atmosphere number is close to 1.

If you have imaging gear - than it is easier - you need to do photometry on reference star. Seeing is easier with camera too - take 2s exposure and measure star image FWHM (take multiple short exposures and note average FWHM and standard deviation to be more precise of how steady seeing is).

Thank you - this makes sense, I think, what you are saying is that transparency isn't an absolute.

As a working guide though this kind of fits with your point leveraging NELM  but absolutely hear that you need to factor in seeing, especially if it is really poor... 

Transparency:  How clear is the sky?

Transparency is a measure of what you can see in the nighttime sky in spite of dust, smoke, haze, humidity, or light pollution.  An easy way to measure this is to use the magnitude of the faintest star you can see.  Ideally, this would be looking straight up at zenith.  But, to make life simpler, you can use the Little Dipper (Ursa Minor) if you can see it.  Here is the scale.

1 - If you can't see Polaris.

2 - If you can only see Polaris.

3 - If you can see the two stars on the end of the bowl of the Little Dipper (Kochab and Pherkad).

4 - If you can see any of the stars in the handle of the Little Dipper.

5 - If you can see 6 of the 7 stars in the Little Dipper.

6 - If you can see all 7 stars in the Little Dipper.

7 - If you can see stars near the Little Dipper that are not part of the stick figure.  (I envy your young eyes...)

Although atmospheric extinction will vary from season to season, and from latitude to latitude, using the Little Dipper is a simple and reasonable solution.

 

 

 

 

 

 

[Tiny Clanger]

I came across this a while ago, it might be of interest

https://project-nightflight.net/limiting_mag.pdf

[vlaiv]

51 minutes ago, SuburbanMak said:

As a working guide though this kind of fits with your point leveraging NELM  but absolutely hear that you need to factor in seeing, especially if it is really poor... 

Good idea would be to cross reference your records with AOD forecast.

https://atmosphere.copernicus.eu/charts/cams/aerosol-forecasts?facets=undefined&time=2021031000,3,2021031003&projection=classical_europe&layer_name=composition_aod550

Local transparency can be significantly different depending on your observing site. Fog near lakes / rivers and a lot of smog during winter time if people burn fossil fuels for heat can change things considerably.

Good intro article on transparency / extinction:

https://skyandtelescope.org/astronomy-resources/transparency-and-atmospheric-extinction/

[SuburbanMak]

1 hour ago, Tiny Clanger said:

I came across this a while ago, it might be of interest

https://project-nightflight.net/limiting_mag.pdf

This is useful and more reliable looking than the Ursa Minor example I found earlier - thank you! 

[jetstream]

Minor differences in transparency can make major differences on DSO. Ive had many nights of vg transparency and looking at a faint object only for it to diminish a bit. This has sent me on collimation ventures, checking for fogged up whatever, dew etc etc., thinking scope was faulty..

Thing is its hard to see these small but important differences naked eye on the Milky Way or whatever. Threshold objects will disappear under these differences.

[JamesF]

1 hour ago, SuburbanMak said:

This is useful and more reliable looking than the Ursa Minor example I found earlier - thank you! 

It is indeed interesting.  I've generally used UMi when I wanted an estimate of limiting magnitude, but I'll try to give this one a go some time.

Sadly however I think the major limiting factor these days is my eyes :(

James