Quote:
Originally Posted by Paddy
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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,
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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).
Quote:
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.
Quote:
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.
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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.