FWHM, what do you get ?

[Somnium]

after shutting down the observatory because of the horrible seeing conditions, it got me thinking what FWHM do people get on average. tonight i was getting close to 6 arc seconds, horrible (i have had 9"!!!). on average i say i would get 4" (one of the main reasons i am moving the obs. the best, absolute best i have ever gotten is 2.8". i know that it is dificult to top those conditions :P but i thought i would throw it out there, what do you get, average, best, worst

[LewisM]

Best I ever got was a pretty darned steady 2.4. Otherwise, usual is 3.5 up

[Shiraz]

best was under 1.5, average is about 2.5, worst up to 6 - all arcsec. all measured without elevation compensation, so actual seeing is/was better.

writeoff in winter - located by the sea and, during cold weather, the water acts as a huge heat source to keep the air boiling all night, unless it is windy.

can vary a lot during the night. http://www.astrobin.com/full/280358/0/?nc= shows how the seeing fell in a hole over a half hour, before a a storm front moved in from the west - transparency remained, but seeing went ape.

if you are having consistently bad seeing, this might be worth a read if you haven't come across it before. http://www.eso.org/gen-fac/pubs/astclim/papers/lz-thesis/node51.html

[Somnium]

nice, i could only dream about 1.5 ! the issue is with the site not the obs, i have had the system set up on the property in different locations when it was a portable rig and the same problem with the seeing. it is the mountain behind the property that has air tumbling off it, you can actually see it on spring mornings, the cloud rolls down the mountain, quite a site actually



that is what i would expect. it is great on those steady days.

[Camelopardalis]

With my Esprit 100, the best I've captured is 2"...and arguably I was undersampling :screwy:

[Atmos]

I routinely get about 2.4" in Melbourne although lately it has been nearer to 3.5-4" so I haven't even bothered setting up... that and the 20°C+ overnight temperatures haven't helped.

[ericwbenson]

In my garden in Adelaide, 2" is very rare night, 3-4" is common and worse is not uncommon. Looking at a defocused star at high power one can see the very fast wave moving across the disk, from what I read it's from higher altitude currents, I wonder if it's heat coming off the city mixing with higher wind currents.
However in Arkaroola I can generally bang out 20-30hr images with average 1.6", worst case 1.8" especially if the target is a bit lower. The good nights are usually 1.2"-1.5", really good is 1.0"-1.2", though that is pretty rare especially away from zenith. I usually throw away any L frame > 2", color I'll keep up to 2.4" ish if I have too since it has less impact to the result.

EB

[Somnium]

Impressive :eyepop:

[JA]

Hi Eric,

It's interesting that you should mention Arkaroola, as I just visited a web page about the observatory there, as a result of web-browsing seeing conditions based on interest created in this thread.

Anyway, I thought you all might be interested in this comment, although based on 50 year old data....

"For those of you with a technical mind, the seeing conditions at Mt. Searle (only 47 km to the south-west) were measured in 1965 by the Australian National University (ANU) and reported as being a mean of 0.45 arc-seconds, with seeing better than 0.35 arc-seconds on 36% of nights. Our night skies are truly spectacular! "

Here is the page: https://www.arkaroola.com.au/astronomy.php

Can anyone vouch for the nearby Mt. Searle ?

Maybe it's a small world and someone here, is from there.

Best
JA

[strongmanmike]

Hmm?..I would be doubtful of those figures (but happy to stand corrected :)), that would make it the best observing site in the World. The only genuine study I have seen that compared Siding Spring with a site in SA, while site testing for the AAT, had the averages around 1.3" I believe? (and seeing down at 0.5" a rare occurrence) with the site in SA slightly better slightly more often than Siding Spring?

Mike

[Atmos]

From memory the best site in the world in respect to seeing is Dome B/C on the Antarctic plateau, I don't think it gets below 0.5" except on rare occasions.

[strongmanmike]

There are several sites around the globe that go lower than that but not consistently. I remember reading a paper/summary on the seeing at La Palma at I recall it averaged about 0.7-0.8" with the odd 0.3" which is excellent, several of the Chilean sites average around this also, as does Mauna Kea I believe?

...aah sigh and we are gleeful with 2" :sadeyes:

Mike

[Somnium]

I jump for joy when I get 3" and I am not even kidding

[el_draco]

I get 0.2 on occasion... but then I wake up :(

[LewisM]

I recall now a night or two in Sippy Downs, QLD where the seeing was 1.7.

Tonight, surprisingly despite the heat, is very steady and wonderfully transparent for a change (Maroochy River). Watched Venus set over the across the road apartments (not even noticeable shimmer from their roof), and there was no dancing even using a 7mm EP - the crescent was very distinct and clear, and not a hint of CA in the Tak fluorite doublet - just solid views.

[Somnium]

mine too, FWHM of 4.5" tonight ...

[Spookyer]

How do you guys measure the seeing?

[ericwbenson]

I think those numbers are incorrect. I dug out the original report from Hogg 1965 (see attached PDF), and he talks about a CARSO Automatic Seeing Monitor and a Danjon 'to' parameter (the latter needs to be multiplied by 2 to be somewhat equivalent to the former). I don't think either can be interpreted as the FWHM we measure with our modern sensors.

On my webpage (http://www.faintgalaxy.com/observatory.htm) I reproduce the graph from Wood et al 1995 that is mentioned elsewhere in this thread. That data is fits in with my observations.
I think I could improve my local seeing if I could cool my dome late afternoon before nightfall (sorta like the CFHT obs in the link Shiraz points to), but running on solar power+batteries only means a very small AC unit :(

A project I have been thinking about is adding more fans to the CDK20 to cool the mirror better/faster. The later model CDKs (17/24) have extra fans on the side of the tube, alas mine predates that innovation, I wonder if it makes a big difference.
EB

[ericwbenson]

In my case I use Maxim DL's information window, you hover the mouse over a star and it displays the FWHM in pixels. You can calibrate the output to arcsec by setting the focal length and pixel size.
EB

[JA]

Hi Eric,
Thank you for taking the time to look in to that, it was right up your alley - Arkaroola. I enjoyed looking at your webpages and reading of your remote observatory development.

Best
JA

[Shiraz]

my understanding is that Danjon related the observed diffraction pattern in a 160mm refractor with properly measured seeing - presumably as a way to obtain remote seeing estimates with only a single portable scope and an eyeball. As you point out, it is a half angle measure and not really directly comparable to our FWHM.

CDK front fans will cool the mirror quicker, but they should also help to break up the boundary layer that forms on the front surface of a warm mirror - discussed in some detail in the earlier linked paper. Haven't yet got up enough courage to let the hole saw loose on my CF OTA and put in some front fans, but I am pretty sure it will be worth doing. Your scope represents about 50x my investment, so I guess you would be even more reticent to cut holes in it :P