Measuring seeing

[Somnium (Aidan)]

Hi all, is there a way to manually, reliably and accurately measuring the seeing conditions on site. I am looking into a new remote site but don't have power or internet yet and I wanted to test it multiple times before i commit to build. If I need to invest in some gear I am okay to do that

[xelasnave]

I dont know but I would think you could select say three doubles that you could quickly image and compare results with "standard" captures of good and bad.
Alex

[Somnium (Aidan)]

Originally Posted by xelasnave

I dont know but I would think you could select say three doubles that you could quickly image and compare results with "standard" captures of good and bad.
Alex

That is a good plan

[rally]

Take a few images and check the FWHM of a few appropriate stars.

Youe system would neeed to be properly polar aligned, focussed etc otherwise you wont be measuring the actual seeing but the poor quality of the images !

[Atmos (Colin)]

Plate solving 5s exposures can be a very accurate way of figuring out the current seeing conditions.

[billdan (Bill)]

Problem is the seeing varies across the sky, seeing at 30° elevation is going to be a lot more than it is at zenith. It will also vary depending on the time of night, worst before 10pm and then gets better until around 3 am.

So I don't think measuring it, is really going to help much.

Bill

[billdan (Bill)]

Thinking about it a bit more, an all sky camera would be good for checking transparency and should give you an indication of what the seeing is across the sky.

[glend (Glen)]

Originally Posted by billdan

Problem is the seeing varies across the sky, seeing at 30° elevation is going to be a lot more than it is at zenith. It will also vary depending on the time of night, worst before 10pm and then gets better until around 3 am.

So I don't think measuring it, is really going to help much.

Bill

Well in terms of absolutes, isn't it better to measure it during that period when it is at its best. And really how many imagers are running subs at 30 degrees Alt. I would suggest target doubles above 40-45 degrees up to zenith, in that period from 10pm to 3am. Of course, it is usually worse at this time of year due to earth heat retention and cool down, but if you pick similiar nights and average the results it should be useful.
I would take a small generator, or battery and inverter suitable for running my laptop and mount, camera etc and use one of the astro tools that can calculate, plot, etc FWHM. If your looking at a serious site investment, why not do that? I'd be logging SQM readings as well.

[Somnium (Aidan)]

Originally Posted by rally

Take a few images and check the FWHM of a few appropriate stars.

Youe system would neeed to be properly polar aligned, focussed etc otherwise you wont be measuring the actual seeing but the poor quality of the images !

i cant really do this, i don't have astrophotography gear that i can take up there, nothing portable. i am willing to invest in a Dob, but not a full rig

Originally Posted by Atmos

Plate solving 5s exposures can be a very accurate way of figuring out the current seeing conditions.

same as above

Originally Posted by billdan

Problem is the seeing varies across the sky, seeing at 30° elevation is going to be a lot more than it is at zenith. It will also vary depending on the time of night, worst before 10pm and then gets better until around 3 am.

So I don't think measuring it, is really going to help much.

Bill

so i will be measuring at zenith at midnight. i understand the seeing is variable but i want to get a view on the average conditions. at the moment, the best i will get at my obs is 3.5" and that only happened twice in the year. i just want to know that i am getting better than that and consistently.

Originally Posted by billdan

Thinking about it a bit more, an all sky camera would be good for checking transparency and should give you an indication of what the seeing is across the sky.

but wouldnt that be significantly under sampling ????

[billdan (Bill)]

Hi Aidan,

I just think you are going to back yourself into a corner. We get limited imaging time as it is, if you make a measurement that tells you its not worth it and therefore don't bother imaging that night, you are restricting yourself to fewer imaging runs.

Better to take the chance and fix the seeing in post processing.

[glend (Glen)]

Perhaps you could hire or borrow a small portable setup to take up there for your measurement activities. Can you hire gear from members here? Or borrow in exchange for later access, etc. Surely there is a way to get some gear on site. What area is it in? Maybe there is a local on IIS who would help out?

[Somnium (Aidan)]

Originally Posted by glend

Well in terms of absolutes, isn't it better to measure it during that period when it is at its best. And really how many imagers are running subs at 30 degrees Alt. I would suggest target doubles above 40-45 degrees up to zenith, in that period from 10pm to 3am. Of course, it is usually worse at this time of year due to earth heat retention and cool down, but if you pick similiar nights and average the results it should be useful.
I would take a small generator, or battery and inverter suitable for running my laptop and mount, camera etc and use one of the astro tools that can calculate, plot, etc FWHM. If your looking at a serious site investment, why not do that? I'd be logging SQM readings as well.

this is definitely the best thing to do, i just don't have a portable scope

[Somnium (Aidan)]

Originally Posted by billdan

Hi Aidan,

I just think you are going to back yourself into a corner. We get limited imaging time as it is, if you make a measurement that tells you its not worth it and therefore don't bother imaging that night, you are restricting yourself to fewer imaging runs.

Better to take the chance and fix the seeing in post processing.

so it is more like i just dont want to invest the money in a remote site if i am going to get 4-10 arc second seeing every night (which i currently have). it is a piece of mind thing before i dive in

[pjphilli (Peter)]

Hi Aidan
I think the good old eyeball test is as good as any. Have a look at your
present sky. What is the magnitude of the faintest stars you can see?
Note the locations of the faintest and compare this seeing with your new site. Of course also consider the level of light pollution at the new site.
In my case, which is a pretty poor location near Pennant Hills road in Sydney, a reasonably clear night is when I can see clearly the faintest fourth star in the Southern Cross (Delta Crux mag 2.8) if not I give the night up unless I am desperate. If on the other hand if I can see this and perhaps some mag 5 stars or better and a very dimmest suggestion of part of the Milky Way, this is a top night! As someone has mentioned, the night usually improves markedly into the early hours of the next morning.
So first give your new site the eyeball test, perhaps on several occasions
and compare it with your present site.
Cheers Peter

[Somnium (Aidan)]

Originally Posted by pjphilli

Hi Aidan
I think the good old eyeball test is as good as any. Have a look at your
present sky. What is the magnitude of the faintest stars you can see?
Note the locations of the faintest and compare this seeing with your new site. Of course also consider the level of light pollution at the new site.
In my case, which is a pretty poor location near Pennant Hills road in Sydney, a reasonably clear night is when I can see clearly the faintest fourth star in the Southern Cross (Delta Crux mag 2.8) if not I give the night up unless I am desperate. If on the other hand if I can see this and perhaps some mag 5 stars or better and a very dimmest suggestion of part of the Milky Way, this is a top night! As someone has mentioned, the night usually improves markedly into the early hours of the next morning.
So first give your new site the eyeball test, perhaps on several occasions
and compare it with your present site.
Cheers Peter

Thanks for the thoughts Peter, sky glow is not an issue I am worried about, this place is literally in the middle of no where. No lp whatsoever, a ways out past Bathurst on 270 acres. I am 99.99% sure the seeing will be good, it is at 1100 meters elevation and the weather comes from thousands of ks of flat ground. I just want to be 100% sure instead of 99.99

[glend (Glen)]

Maybe worth asking Allan (IIS user id) how he assessed his recent site purchase. I doubt he went to the FWHM level as he is visual only i believe.
There are some assumptions that can help: like getting well away from the coast, getting altitude, getting away from population centres - Alan's place fits that description.

Whoops, just saw your last post, sounds good.

Edit. Get a good set of Astro binoculars, a nice reclining camp chair, and a pictorial print out of the Pickering Scale, and a bottle of Port. Should be all you need.

[Somnium (Aidan)]

so i was thinking of using the method outlined in this doc

http://www.eaae-astronomy.org/catcha...ure_seeing.pdf

but my image scale is 2.22" per pixel for my portable set up, any thoughts on whether this could discern seeing as low as 1" ?

[Atmos (Colin)]

At 2.2" you are probably a bit under sampled, just won't be accurate unless you have really bad seeing (for calculating FWHM).
What is your mobile setup?

[Somnium (Aidan)]

Originally Posted by Atmos

At 2.2" you are probably a bit under sampled, just won't be accurate unless you have really bad seeing (for calculating FWHM).
What is your mobile setup?

i was thinking of using a 400mm ed refractor with a canon 600d. if i had to lug my 1000mm newt around, that might be difficult

[rally]

Colin is correct - its unlikely that you will be able to accurately quantify seeing (FWHM) that is under the image scale, much better to be slightly over sampled.
So probably better to have an image scale of lets say 0.5 to 0.7 arcsec/pix

Some larger stars are likely to be spread across multiple pixels so you would potentially get some insight but its not ideal, and these arent typically the stars you would choose to use anyway.
2x converter maybe ? - trouble is that might introduce further aberration into your imaging system which will simply inflate the real FWHM you are trying to measure - Catch22

And who knows maybe the site is capable of sub arc second seeing at times !

But light gradients from the two nearby major cities will be a constraint for lower alt, wide field imaging.
Just take a 360 degree panorama of long exposures with a wide angle lens and see what you get, but under 25kms from a couple of 40,000+ population cities will produce quite a glow !