seeing and transparency measurements

[rockystar]

Observation logging apps often ask to record the seeing and transparency conditions; how do you go about measuring these and is there pre-defined scale?

[pipnina]

I have wondered this myself. Hopefully someone comes along who knows.

[laudropb]

Most use the Atoniadi Scale which is based on how steady the view of an object is through the scope. It ranges from 1 perfect seeing object is very steady to V object is never steady. Google this for a full description.

[jetstream]

Transparency can be determined by your NELM, which is relative to each observers location. Small changes in transparency will effect the faintest star you can see.

Seeing can be a bit difficult to describe as excellent seeing can happen with cloud, but all good seeing has steady skies. Oddly enough those crystal clear night's can give some pretty poor seeing.

Picking the objects you observe by the sky conditions is very rewarding and avoids much frustration.

[Stu]

Here are a few useful links

http://m.skyatnightmagazine.com/feature/general-guide/guide-seeing-and-atmospheric-transparency

Damian Peach discusses his scale here

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

Pickering scale discussed here

http://www.skyandtelescope.com/astronomy-equipment/a-scale-of-seeing/

Gerry's point is very valid. Often crystal clear skies are associated with poor seeing so they are great for faint DSO observing. Conversely hazy or poor transparency often goes along with excellent seeing so observing planets on these occasions makes a lot of sense as they are bright enough to cut through the haze and views really benefit from the steady seeing

[rockystar]

Thanks guys, I'll have a read of those links and search for Atoniadi Scale.

What does NELM stand for? my first search brought up "National Electronic Library for Medicines" and "Navajo Lutheran Mission", neither of which I think are related to the subject :-)

[Stu]

Thanks guys, I'll have a read of those links and search for Atoniadi Scale.

What does NELM stand for? my first search brought up "National Electronic Library for Medicines" and "Navajo Lutheran Mission", neither of which I think are related to the subject :-)

NELM is Naked Eye Limiting Magnitude, a fancy way of saying the magnitude of the dimmest star you can see near the zenith. Ursa Minor is a good place to check this as the main seven stars vary in brightness down to mag 4.96 so counting those stars is an easy way to check. My skies are normally around mag 4.5 but can get to 5 on good nights.

EDIT This is where transparency and light pollution get intertwined. One approach is to check your NELM on the most transparent nights, then you can make a judgement on transparency on future nights when you observe as a reduction in NELM is likely due to poorer transparency......assuming the moon is out of the way.

[rockystar]

OK, thanks. I think I've seen your jpeg on Ursa Minor and used it to check my NELM, about the same as yours - somewhere between 4.4 and 4.7 - maybe that was the good transparency then, when I can get to 4.7.

I'll try and remember to check a constant part of the sky to work it out.

I guess seeing can also be affected by the scope not being cooled enough? And also looking too near house roof tops?

[jetstream]

Good catch rocky! There is "local" seeing conditions too...they include ground thermals, boundary layer thermals of the primary mirror, tube currents, house thermals and who knows how many else. Getting these under control can also vastly improve lunar/planetary views, allowing high mag viewing.

Collimation is also hugely important for high res viewing.

An SQM-L is very helpful to find dark skies near you...

[pipnina]

OK, thanks. I think I've seen your jpeg on Ursa Minor and used it to check my NELM, about the same as yours - somewhere between 4.4 and 4.7 - maybe that was the good transparency then, when I can get to 4.7.

I'll try and remember to check a constant part of the sky to work it out.

I guess seeing can also be affected by the scope not being cooled enough? And also looking too near house roof tops?

If I used the ursa-minor test, it wouldn't be representative of 3/4 of my sky, since the Plymouth light dome is strongest there. But I know at my house (closer to plymouth's city centre than my "dark site") the limiting mag in the best part of the sky is 4 (could see 48 ori, on the edge of visibility) And i could have sworn I saw 69 eri which would put my NELM at home around 4-4.25...

At my dark site, the dimmest I could pick out was 5-5.2... But I wasn't looking for any stars I especially knew about... I'll have to review that figure.

[rockystar]

Yeah, I was thinking about that on the way home - ursa minor at about 10 is pointing towards the horizon, so the wrong direction from the zenith. I'll have a look through stellarium and see if I can find some stars that fit through the 4 to 5 mags that imcan nicely pick out.

[Stu]

Yeah, I was thinking about that on the way home - ursa minor at about 10 is pointing towards the horizon, so the wrong direction from the zenith. I'll have a look through stellarium and see if I can find some stars that fit through the 4 to 5 mags that imcan nicely pick out.

Cassiopeia is another option with a few mag 4.8 stars near the main brighter stars

[rockystar]

Just been out to have a quick look. Only out for 10 minutes, so wasn't fully dark adapted - though not sure how much more I could have got with the moon as bright as it was - and I could just make out 35 And (just under the galaxy) which stellarium says is 4.5.

I'll have a look at Cas see if I can see any at 4.6 that I can make out.

thanks again.