"Seeing" & "Transperancy" - How to determine, please.

[mental4astro (Alexander)]

Hi all,

Though I've been at this game for some 30 odd years, as the vast majority of my observing is from my light polluted home sky, I've never really bothered with describing the sky quality in terms of "Seeing" or "Transperancy". Now, as my interests develop further, I'm coming across a lot more with these terms. To make matters more confusing for me, these terms are being noted on differently named scales and with varying scale parameters.

Can someone explain these a little more clearly for me, please?

Which are the more accepted rating methods for these, and how do the various methods work, such as the "Pickering" for seeing, and "NELM" & "TLM" for transperancy?

I'm sure that many other astro-followers could also benifit from this too.

Cheers,

Mental.

[Paddy (Patrick)]

Excellent question Alex and one quite on my mind, assuming we ever get transparency here again.

I have tried using the scales in Brendan Gazzard's article on IIS "How to determine seeing and transparency" but increasingly find this system doesn't work for me. In regards to transparency, my skies are usually at least 6 if there is no obvious cloud cover, but recently high level moisture has meant that observing fainter objects is quite disappointing as galaxies float in murk. So this scale seems insufficiently sensitive for a dark sky. The scale for seeing works a little better, but it would seem to me that one based on the highest magnification that can used with stable images might be better.

I look forward to some useful answers!

[Robh (Rob)]

Hi Alex,

Transparency is rather a confusing term as most often it is a measure of skyglow. You can measure skyglow fairly accurately with a sky quality meter. However, if the air is smokey from burn-off or foggy due to vapour, this also reduces transparency. So, in the end the best way is to simply estimate the naked eye limiting magnitude (NELM). However, this has its limitations as one person's NELM will be different to another's because of variations in eye sensitivity due in part to night pupil size. Confusingly, transparent thin cloud cover can reduce overall transparency but passing heavy cloud can completely block views yet allow good transparency in non-cloud areas.

Naked eye, the old star flicker assessment can give a rough "poor, fair, good or excellent" seeing. In telescopes, estimates of seeing can vary according to magnification. At low magnification, the seeing might appear good but at higher magnification you might judge it as worse.
The Pickering test is more accurate but still ultimately subjective.
See ...
http://www.damianpeach.com/pickering.htm
It is to be noted that the size of the airy disk and diffraction rings will depend on the aperture size. Higher magnification is required to detect the diffraction pattern in larger apertures. At higher magnifications, atmospheric disturbances are more noticeable.

I'll be interested to get other people's viewpoints on it.

Regards, Rob

[ZeroID (Brent)]

I concur, would be very useful. Add to this 'Bortle Rating' and other terms and confusion reigns supreme.

Some sort of defined standard would be excellent. Astroplanner for instance, rates better Seeing on an ascending scale and Transparency on an descending scale (or the other way round ).

I understand it is a very subjective value based on a lot of criteria but it must be possible to at least get common scales, values and terms.

[astrospotter (Mark)]

Agree completely and look forward to more thoughts on topic of just what makes a 4/5 or a 5/5 night for both seeing and transparency. I also wonder if 1/5 means you should have stayed home and so I never actually log 1/5 as that is a skunk night and not worth observing anyway. 0/5 means it is raining

Seeing is how much turbulence the atmosphere provides between you and the stars and transparency is how much muck is between your scope and the stars. (just to clarify and I know we all know this definition)

For ORs try to specify NELM and SQM as a minimum. NELM accounts for both seeing and transparency but varies perhaps a lot between observers like up to 0.5 or even more. So calibrate yourself to others around you to see where your 'scale' lies. I have found I am about midway for a 50 year old.

If you are patient you can do NELM using the finnish triangles but I tend to find groups of stars that range from 5 through 7 that I know are not doubles and see which one is the lowest mag I can see averted over 50% of the time. I realize this is a 'quick dirty' way but it works ok for my own notes. Finnish triangle info: http://obs.nineplanets.org/lm/rjm.html

Transparency is the really tricky one. I have a few favorite galaxy groups that have mags from 14.5 to 16 that are easy to find and I see how dim I can make out the faint ones. 15.5 is a fairly transparent night, 16.5 is a total limit of 5/5 transparency and depends on the core and being at zenith in my 18" dob and being at a dark site of SQM 21.6 or so.

I always take a few SQM readings and make a note. It is easy and fast (once you buy the meter). This can easily change over a night by 0.3 to 0.5. Take reading directly overhead BUT avoid the milky way (especially you guys with the galaxy core right over top as it is FAR brighter in the milky way).

Another 'overall measurement' is looking at well know but faint object like up here I use M101. If I can see the faint arms and some of the very dim stars in the haze it is a great night. But this is not very scientific at all.

For seeing all on its own a very rough estimate as mentioned already here is to look for how much twinkling the stars have. Zero means seeing is very good, constant flicker is very bad seeing.

For more science find double stars of similar magnitude with spacings suitable for your scope limits so on a great night they separate very nicely but on a poor night they merge in a fuzzy blur. My very favorite is the double double up north here but you guys have a big scope double double in Nu Scorpius (http://en.wikipedia.org/wiki/Nu_Scorpii) But you need excellent night to split both sides of Nu Sco. In 'average' seeing it appears as two stars with one being a bit elongated. I have not found nice lists of doubles for all around seeing tests so have slowly been gathering a few but basically for my 18" dob I look for stars about mag 7 that are not too hard to find with telrad and about 2 to 3 arc seconds apart so seeing judgement is easier. Both famous doubles I have listed are naked eye targets from a dark site.

[mental4astro (Alexander)]

Originally Posted by Paddy

Brendan Gazzard's article on IIS "How to determine seeing and transparency" but increasingly find this system doesn't work for me.

I can see why it doesn't work too well. Many of the target objects are too bright for their designated 'scale position'. For instance, from home, I can easily make out Omega Centauri, but not 47 Tuc, and they are set on the same position, . AND, up to one year ago, I could still make out 47Tuc, but I know the sky is terrible, .

You've also pointed out something I haven't been able to pin as a condition of Transperency - "that galaxies float away in the murk"!

I've noticed this when I've been able to see some non-galaxy objects, but have then struggled to see any galaxies, and even the brighter ones are so underwhelming, when other times they've been brilliant (using the same scope).

Originally Posted by Robh

Hi Alex,

Transparency is rather a confusing term as most often it is a measure of skyglow. You can measure skyglow fairly accurately with a sky quality meter. However, if the air is smokey from burn-off or foggy due to vapour, this also reduces transparency. So, in the end the best way is to simply estimate the naked eye limiting magnitude (NELM). However, this has its limitations as one person's NELM will be different to another's because of variations in eye sensitivity due in part to night pupil size. Confusingly, transparent thin cloud cover can reduce overall transparency but passing heavy cloud can completely block views yet allow good transparency in non-cloud areas.

Naked eye, the old star flicker assessment can give a rough "poor, fair, good or excellent" seeing. In telescopes, estimates of seeing can vary according to magnification. At low magnification, the seeing might appear good but at higher magnification you might judge it as worse.
The Pickering test is more accurate but still ultimately subjective.
See ...
http://www.damianpeach.com/pickering.htm
It is to be noted that the size of the airy disk and diffraction rings will depend on the aperture size. Higher magnification is required to detect the diffraction pattern in larger apertures. At higher magnifications, atmospheric disturbances are more noticeable.

I'll be interested to get other people's viewpoints on it.

Regards, Rob

Thanks, Rob, for defining NELM. Without knowing it, this has actually how I've been gauging Transperency, in a round about way. This is the first value I gauge when I look up at the sky- "what can I see, how faint can I go". I've even got some "usual suspects" that I use, such as the Eta Carina nebula & Omega Cent'. Then there is M7. If that's a go, then the Cloud of Sagittarius & M8. To push a little more M16 which is usually an averted vision target from home. The NELM puts a number to this

Honestly, I can't be bothered with a Sky Quality Meter. I don't see the need for me when my eyes do the same job. Where I can see it's usefulness is when we begin to compare human eyes, as you mentioned, and a subjective judgement doesn't cut it. I know I can see things naked eye some of my friend's can't, so a more objective system can help. Mind you, if you can't see thing with the naked eye that someone else can, you've still got that 'catch' at the eyepiece.

And yes, I too go for the 'flicker test'. If things are really bad, I'd be noticing this when chasing down my usual suspects. Otherwise, it's the next cab off the rank. But I do like the Pickering scale you mention as it's a finer scale.

Originally Posted by astrospotter

Another 'overall measurement' is looking at well know but faint object like up here I use M101. If I can see the faint arms and some of the very dim stars in the haze it is a great night. But this is not very scientific at all.

For seeing all on its own a very rough estimate as mentioned already here is to look for how much twinkling the stars have. Zero means seeing is very good, constant flicker is very bad seeing.

Mark, thanks for the 'Finnish triangles'. It's a little more objective, though still naked eye reliant. But these are the eyes I'll be using at the eyepiece too.

I somewhat too use M83 as you use M101. If I can see the 'bar', good. If I struggle to even make out the core, it's very poor. Even with the planets or the Moon, if the image is degraded at 100X, it is bad.

I'm inclinding towards a 1 to 5 scale for Seeing, with 5/5 as good as it gets, & if a sky quality meter isn't being used, then the NELM. Such a scale is fine enough for impirical use. If you use the Finnish Triangles, or any other system, this can be tweeked even finer. I'm pleased to have asked this question, if nothing more I've learnt what some of the different systems are and how they work, so when I next come across them, I'll know. I can also see why there isn't one standard, as susually these systems are reliant on our human eyes, so at best we can have a 'feel' for what an author is accounting.

At least I'm more one the ball with both aspects now. Interesting for me, I was making these judgements without actually knowing the technical names for this, . Cool.

Ta.

[astrospotter (Mark)]

Oh, if you are asking the numbers I use then that is 1-5 for both seeing and transparency where 5 is about as good as it gets.

But the whole absolute how I would rate a 4 VS someone else is the part that is not really calibrated.

Yes NELM is per individual but it is rather close to an absolute for one individual and as you say, it is THAT individual that will be looking at the eyepiece AND later someday looking back at an OR and seeing NELM of 6.9 with SQM of 21.8 (then of course saying to one's self: ... AHHH, THAT was a great night!).

Using tight doubles is the better way to judge seeing alone as long as the ones you use from season to season are about as far apart, within the limits of your primary optics and matching in magnitude (both with each pair and with the different pairs you use). Matching means within about a one mag range and for my scope around mag 8 would be nice. you need at least 7-9 pair with the one you use at any given time over 60deg altitude ideally I should think. I am sure there are papers on this topic ... somewhere.

[Rob_K]

If I can put two bob's worth in, I'd say that while measures that assess and compare sky conditions undoubtedly have use, in the end they are irrelevant to simple enjoyment of the hobby. Even a newcomer quickly learns to assess his/her sky conditions at home by looking up, and you can easily tell whether it's going to be a bad, good or cracker night's observing.

Human's are funny in that we seem to need to overcomplicate things - not just that, we need to invent complexity and jargon to control the field! This is why we end up with a multitude of scales, all invented by earnest people with a wish to stamp their imprimatur on observing through a telescope. And astro equipment. And astro-imaging.

Don't even worry about it, unless you want to join the rat-race. Just plonk the telescope on the back lawn, whack in an eyepiece, and marvel! Err, provided the transparency is better than 4.0 NELM or 20.5 SQM and the seeing is whatever on the Pickering scale, or is that Bortle, or is that 1-5?

Gimme a break!

Cheers -

[Lismore Bloke (Paul)]

Very interesting thread and range of opinions. My own estimations are purely personal, based on what I have seen during the session. Summarising the seeing and transparency is the last thing I do after I've examined a range of objects. I normally start by just look at a number of stars between 45 degrees and the zenith and look for the amount of twinkle, not just the first magnitude ones. If I can focus field stars to nice points in a 7mm eyepiece and they stay sharp, that's a reasonable guide to how good the seeing is. Transparency is a bit harder, normally the outer regions of galaxies are a pretty good guide. I use a range of one to ten, one is rotten, ten superb. Is it accurate, probably not terribly. Do I care whether it is accurate, not really, it's just what my eyes are seeing and just an estimate. There are so many variables, moisture, mist, thin high cloud that's hard to spot at night, dust, particulate matter from winter wood fires and burnoffs courtesy of the Department of Parks and Wildfires.

Someone else's eyes seeing through my scope and eyepieces may make an estimate somewhat different. As long as I am having a great time exploring the night sky and the wonderful things to be found there I'm happy. I make the best of the local conditions and try not to complicate things too much. In that respect I agree with Rob_K - just enjoy!!!

Cheers, Paul.

[Paddy (Patrick)]

In regards to just enjoy, well, yes. That's what it's ultimately about and it's good to not get too complicated. But as I put together observation notes, I would like to have some consistent system for also noting the observing conditions. That way if I'm not seeing much detail in something and I've noted that it's a great night then I might not expect to see more. If its poor as it mostly is at the moment, I know it's worth coming back. If I compare notes on the same object from different sessions, I know which session is more likely to have accurate recordings.

[astrospotter (Mark)]

Of course we want to enjoy the objects and that is why we like this hobby.

For me there are a few reasons to write down things like seeing, transparency, NELM, SQM or a combination of those.

One reason exactly matches Paddy's comment. I want to later be able to go back and read my notes on past observations and know about how good the night was to help understand the context. I am not capable of remembering a given observation and telescope/eyepiece used. This sort of note taking only shows it's value a year or more after the fact as a way to re-live the night. I spent 6 nights in very dark area of Australia last year and those notes are very precious to me as I a northern guy who cannot see either of the clouds or Eta Car and only get a cruddy view of Centaurus glob and galaxy. (in fact I record my observations in a little recorder so as not to make the process take much at all away from my viewing enjoyment).

The second reason is in order to go for assorted lists from the Astronomical League and such many of these clubs want to see observations along with sky conditions. These clubs only apply if you like that sort of personal goal approach to the hobby.

I totally get it that many people are not into the above approach as it can be a distraction but I doubt any of us here are not completely taken back with the views we see as that is the #1 reason I enjoy this hobby.

[mental4astro (Alexander)]

"Uncomplicated"! Ahhh, music to my ears...

I'm not after complicating things. I don't have the time to complicate things. I'm a chart 'n' scope fellow.

It's more a case like Paddy & Mark state, there are moments that you just would like to record the conditions. So, for me, if there is a simple, no-tech option, that's all I'm after. It's for those occasions that some type of tangible gauge of conditions being called, has a way of doing so, that everyone can follow. I won't be going the Sky Quality Meter way. I can't justify the machine to the way I chose to follow my hobby. But I'd also like to understand what the is ment when those quantities are also expressed.

[Rob_K]

Sorry, I didn't wish to denigrate those who wish to record their observing conditions as accurately as possible - it's a free world and anyone can add whatever level of complexity to their observing they like!

But two things concern me. The first is that people who are new to observing might read this thread and think that observing is a really complicated thing. It's not. Accurately recording sky conditions for every session will not make you a better observer. But it may assist you in gauging what you might see for given conditions as a planning aid, or satisfy a personal, organisational need.

Secondly, the idea of REALLY accurate recording of sky conditions as a comparative measure to let others know what might be visible under given conditions is a waste of time IMO. There is the difficulty of doing it across a range of sites (other than with a SQM, which itself is fraught with pitfalls). I'd guarantee Paddy's assessment would be different to Alex's, would be different to pgc's, etc. There are other variables which would render fine accuracy fairly pointless. With given equipment, the main ones of these are observer experience and observer eyesight. For instance, ngcles could try to show a new observer something at the limit of vision and they wouldn't see anything. And a younger, experienced observer might also be able to show an old codger a thing or two, under the same sky conditions.

But please don't tell me we need accurate scales for eyesight & experience too! I ain't going to the opthalmologist for a test and a scaled assessment!

Anyway, more power to people who want to tackle the issue, just no need to get unnecessarily fixated on it IMO. KISS principle I reckon!

Cheers -

Rob
(6.87326 on the Observers Eyesight Scale & 3.2657 on the Observers Experience Scale)

[mental4astro (Alexander)]

Originally Posted by Rob_K

Rob
(6.87326 on the Observers Eyesight Scale & 3.2657 on the Observers Experience Scale)

Getting a bit soft there Rob, are we?

Tell me about younger eyes, ! I showed Saturn to a group of kids some time ago, & mentioned that your could see 4 of its moons. One of them then said when it was their turn at the eyepiece "Oh, yeah! Wow! I can see 4, 5, 6 moons!" Oh, cripes .

[Paddy (Patrick)]

Originally Posted by Rob_K

But two things concern me. The first is that people who are new to observing might read this thread and think that observing is a really complicated thing. It's not. Accurately recording sky conditions for every session will not make you a better observer. But it may assist you in gauging what you might see for given conditions as a planning aid, or satisfy a personal, organisational need.

Secondly, the idea of REALLY accurate recording of sky conditions as a comparative measure to let others know what might be visible under given conditions is a waste of time IMO. There is the difficulty of doing it across a range of sites (other than with a SQM, which itself is fraught with pitfalls). I'd guarantee Paddy's assessment would be different to Alex's, would be different to pgc's, etc. There are other variables which would render fine accuracy fairly pointless. With given equipment, the main ones of these are observer experience and observer eyesight. For instance, ngcles could try to show a new observer something at the limit of vision and they wouldn't see anything. And a younger, experienced observer might also be able to show an old codger a thing or two, under the same sky conditions.

But please don't tell me we need accurate scales for eyesight & experience too! I ain't going to the opthalmologist for a test and a scaled assessment!

Anyway, more power to people who want to tackle the issue, just no need to get unnecessarily fixated on it IMO. KISS principle I reckon!

Cheers -

Rob
(6.87326 on the Observers Eyesight Scale & 3.2657 on the Observers Experience Scale)

I absolutely agree Rob. The thing I find hard is coming up with a way of recording conditions that is consistent from session to session for myself as well as accurately conveying what the sky was like.

In regard to testing eyesight, maybe someone could develop a plug-in for CCD Check and at star parties it could be run over the eyes of the participants. We'd then not only know how poorly our scopes are collimated but how poorly calibrated our eyes and brains are.

[astroron (Ron)]

In My observing reports I give my assessments of the sky conditions mainly for my own benefit,but there is also just a hint to say what the sky was like at the time if you where here
As Rob said,people like myself and Les with more experience can show people things and they cannot see them and we have much older eyes in most cases but once they are pointed out then yes they can most times see them.
There is no need to make our observing sessions more complicated than is necessary.
Cheers

[Brian W (Brian)]

Hi use, I use the Astrobyte logging software and it comes with its own definition if transparency and seeing which I have, perhaps naively, used for the last 3 years. It works for me and as it is basically a 1 to 10 type of scale I find it easy to let others know what my skies were like. Certainly it is subjective but I am a bit if a hermit so it is usually just me doing the observing.

Brian

[astrospotter (Mark)]

Forgot to mention that if you do decide to jot down a number for seeing and/or transparency in any context, try to note it like this 'seeing 4/5 ' or 'seeing 8/10 ' so that way later readers (if any) will know it was a 4 OUT OF 5 for either example here. Yes it is highly subjective and mostly for yourself for some later time. It seems 5 max is very common but we humans with 10 fixation (as well as 10 fingers on most of us) sometimes are tempted to give values of 1-10.