HowTo: Determine Seeing and Transparency

[iceman (Mike)]

Hi guys.

Brendan has kindly submitted an article with some scales for seeing and transparency. Often a lot of us just make a guess as to the seeing or transparency, and some of the new folk might not even know what the difference is.

Well this article should explain it.

It's also a good idea in practise if we all endeavour to use a common scale for these, as it will also help during the Monthly Observing Challenge, being launched soon.

The Antoniadi scale for seeing is good, I just wish it was numbered the other way around, with V. being the best instead of I. Oh well

How To Determine Seeing and Transparency

A lot of people have their own method, of course this may not be for everyone, but if you currently don't have a defined way of measuring, then this may be for you.

Thanks Brendan for contributing the article.

[GrampianStars (Rob)]

G'day Y'all
IMO seems to me to be a scale for city dwellers
I can see 47 Tuc right down to transperency 1 using this scale
whereas in a city or large town light reflection off clouds woud brighten the sky considerably

[GrampianStars (Rob)]

Seeing
The stability of the atmosphere determines how much fine detail you can spot at high power with a telescope.
If the atmosphere is very calm, and detail is very crisp (the stars do not appear to twinkle, even at the horizon), the seeing is rated 10. This is extremely rare.
If the atmosphere is extremely turbulent, and lunar and planetary detail are fuzzy even with your lowest magnifications, the seeing is considered to be Zero.
Typically, it ranges between 2 and 7. This rating is rather subjective, based on your own personal observing experience. As you get more practice, your ratings will be fine-tuned.

Transparency

Transparency is based on the magnitude of the faintest star naked-eye visible overhead.
It is rated from 1 (very hazy--with only a few 1st magnitude stars visible) to 6 (thousands of stars visible with the Milky Way quite obvious).
Light pollution plays a big role in the transparency around cities...it is rarely better than 2 or 3.
This is why dark sky observing sites are so important.

[acropolite (Phil)]

A question on seeing. The other night I waited till the wee small hours to view mars. The best way I can describe the image is that of a (tight) red sparkler, almost as though it was on fire. A couple of nights later I noticed the same thing when viewing alphacrux but of course the effect was that of a white sparkler. The image was steady and there was no "movement" apparent from air turbulence. Is this the result of poor seeing, abberation in the EP, atmospheric conditions, or some optical trait of the eye. The magnification was about 133x and the transparency very good with most of the Milky way visible to the naked eye.

[ausastronomer (John Bambury)]

Originally posted by GrampianStars
IMO seems to me to be a scale for city dwellers

I agree, I can see 47 Tuc naked eye from my mag 5 backyard skies with only fair transparancy, even when its fairly low. I can see Omega Centauri with my sunglasses on

The scale is certainly not really appropriate for someone with dark skies and good conditions.

CS-John B

[slice of heaven]

I agree with Grampianstars and Aus.
It's only good for light polluted skies. Doesn't reflect on what dark sky sites are capable of.
Needs to be revised .

Slice

[iceman (Mike)]

Point taken.. Does anyone wish to contribute a revised one which has a scale that can be used from light-polluted skies right out to dark country skies?

[ausastronomer (John Bambury)]

The Bortle Scale relates more to light pollution than transparency and also is Northern Hemisphere orientated, but its a good place to start for someone to do a Southern Hemisphere Table.

http://www.novac.com/lp/def.php

Clear Skies
John B

[Rodstar (Rod)]

Having followed this thread, it seems to me that a scale is only going to be useful if it provides for different results at different times at the same location!!!!

For example, the Bortle scale really just allows you to classify a site for its intrinsic degree of exposure to light pollution. That is to say, it would produce the same reading every night!

Obviously a city slicker is going to use a different scale to someone 1000km west of Bourke!

Brendan, your scale suits me just fine!!! Cheers mate!

Rod

[slice of heaven]

Rodstar
My backyard would vary from a 3 to a 5(maybe better/maybe worse) on the scale.
The town centre is 4kms away and the light pollution varies depending on height and density of cloud cover.
Low cloud no matter how thick creates a bright yellow dome to the west so I have to turn my back on it.
No cloud and the dome disappears.
That scale is handy for classifying sites at their best...if you use that site enough.

But your right, Brendan's scale will work fine for city dwellers.

Slice

[iceman (Mike)]

The Bortle scale is interesting.. but like Rod + John say, it's more related to light pollution than how clear the sky is.

We really need a combination, that takes into account both light pollution and how clear the sky is, and can be used on the same location to provide a different reading.

[xstream (John)]

The Pickering Seeing Scale is probably also worth a thought.

http://uk.geocities.com/dpeach_78/pickering.htm

[slice of heaven]

Good one Xstream
Lot easier with pics to guide you.
Having 10 levels will be harder to learn so I'll start with 2,4,6,8,10
until I get them right.

Slice

[[1ponders] (Paul)]

I like those Picering scale images Xstream. That makes life easier. Unfortunately it also makes me realize how cra&&y my seeing has been since I've gotten my scopes.

[xstream (John)]

The scale isn't that difficult to learn Slice after you've used it a few times.
They do put a whole new perspective on your seeing Paul.

[slice of heaven]

Xstream
The scale I've been using only has 3..horrible...ok....great. lol
With those pics to refer to it probably wont take long to learn.
More homework...sigh

Slice

[Brendan]

Thanks to all who have provided a critical review of the transperancy table.

I will freely admit that the table as it stands has not been tested in the field, It was constructed as an academic exercise taking Nth Hemisphere information and converting it to something an observer in the south could use.

On closer inspection I feel that using 47 tuc and omega cen may be a little to generous. How ever I have not been able to find adequate substitutes in my short lunch break.

Suggestions for a southern sky object that is easy to identify even for newbies that sits between mag 6 - 5

in the mean time I will hunt around myself and see what i can find.

regards
Brendan