What's your typical FWHM?

[codemonkey (Lee)]

I'm curious to know, especially for those that live within reasonable distance of me, what is the typical FWHM (in arcseconds please) of stars in your images?

Using broadband filters I can't seem to get below about 2.6" with 2.8" - 3" being the norm.

So now I'm wondering; is this basically the limit of seeing here, am I able to improve things by using existing equipment better, or is there some limit in my equipment that prevents anything better?

Last night the seeing seemed pretty good. I was imaging at 1"/px with a guiding RMS error of around 0.5". I was using PEC and imaging close to pole and RA was about the best I've seen it. DEC was strangely not as good as it has been, but still, 0.5" of error is definitely at the better end of what I've been able to achieve.

I've ordered a IR-pass filter to try putting over the QHY5L-II to try and reduce the amount of seeing-induced movement on guidestars so it'll be interesting to see how much impact that has.

[LewisM]

On average in Light Pollution City, using an FSQ-106ED and an OSC CCD, I am getting anything from 2.5 to 4. Around 2.9 seems usual.

The GPD2 mount delivers absolutely remarkable tracking (EXCEEDINGLY small RMS), so it all comes down to seeing more than anything for me (my exposure time for guiding is now set to 3 seconds). I VERY VERY VERY rarely reject ANY exposures (I will for significant satellite trails or aircraft, but usually never need to for tracking errors). I just have to square the orthogonality to the camera, but will wait until the threaded adapter arrives to see if that fixes it.

[Merlin66 (Ken)]

The average seeing conditions I got in Melbourne (15Km from CBD) was always around 2.5 to 3 arc sec.
Such is life.

[Paul Haese]

I spent three years going to Clayton before I bought a property there. In fact I have imaged at high resolution from a number of locations to find the best one before I found Clayton. South Australia luckily has pretty good seeing in general. At the time I was doing a lot of planetary imaging and wanted a good site with excellent regular seeing. I have had the site tested with a DIMM a couple of times and seeing has been tested being as good as 0.7 arc seconds on a good night but more regularly at around 1.4 arc second. The drought years made the seeing very good. The FWHM I regularly get now is about 1.4-1.7" these days on both the wide and narrow field systems. Still very respectable but a bit of an anomaly for sea level conditions from what I have seen discussed. I put it down the moisture content of the atmosphere and the prevailing winds on site and the topography of the location and intervening area. When the moisture content is low the seeing can be exceptional. Remembering back to my ten years on the east coast there was always a higher moisture content and general conditions were more humid, the landscape often rises high and steeply above the sea level and these factors no doubt cause turbulence in the atmosphere.

On poor nights at Clayton when the moisture content in the upper atmosphere is very high the seeing can be up around 2" FWHM. Those nights are rare though. The jet stream also seems to regularly miss my location too which is a fortunate situation.

From what I have heard and seen those numbers you list are about right for the conditions.

[Paul Haese]

I should point out that with NB imaging the FWHM tends to be a little tighter again, but not by much.

[Shiraz (Ray)]

Hi Lee. I get a median of about 2.2 arcsec, with some nights up to about 4 (eg last night). Have occasionally had seeing down to ~1.5 arcsec. None of this is corrected for elevation angle.

this is a bit worse than Racine suggests (from large scale surveys) for sea level conditions (I am at 22m ASL). "The median ground‐level seeing at zero altitude is 1.97. At 2500 m altitude and 10 m elevation—representative values for many telescopes—the model predicts a median DIMM‐scale seeing of 0.67...It is remarkable that a knowledge of altitude and elevation alone suffices to “predict” the median seeing to an accuracy of less than 0.1". So the best possible seeing can be predicted by your altitude - at Kilcoy, median seeing below 2 arcsec should be possible at the zenith - are you in a built up area with local thermal turbulence?

another possible complication is that CA in your scopes might contribute some star broadening. If you need to refocus on filter change (assuming parfocal filters) or if your NB data has smaller FWHM, then CA could be an issue.

ref:http://www.jstor.org/stable/10.1086/429307

[codemonkey (Lee)]

Thanks everyone! Sounds like I'm in the ballpark. I have noticed, given a sample set so tiny as not be even close to significant, that there's seems to be a correlation between bandpass and FWHM.

Given too few samples and too many unaccounted for variables, I can see a median of 2.8" in L, 2.7" in Ha and 3" in OIII. This may be consistent with the wavelengths captured, or it may be related to chromatic aberration as Ray suggests, I'm not sure which... maybe both!

Thanks for the link to the study Ray, always great to back things up with studies like this! One thing that does have me question the application of this study in a more general sense is that sampling sites were specifically selected for astronomy: On the other hand, sites selected for astronomical observatories are naturally free of features that may have strong negative effects on seeing. As such, they should be expected to have seeing components close to minima and to show less scatter about any model than for randomly selected sites.

Having said that, it seems likely that it'd be a good general rule.

I suspect a lot of it here has to do with moisture in the air. We get thick fog almost every day in winter. I've noticed better results on windy days where there's less moisture in the air, even though I would have expected worse results with the greater wind.

I wasn't measuring FWHM until recently so it'll be interesting to see what I can get when it starts warming up.

[Paul Haese]

Originally Posted by Shiraz

Hi Lee. I get a median of about 2.2 arcsec, with some nights up to about 4 (eg last night). Have occasionally had seeing down to ~1.5 arcsec.

That is vastly different from what I got last night and demonstrates how different seeing can be from region to region. While you got 4 last night I was getting 1.86. Did you have north westerlies last night Ray? I did not have any wind until about 6 am. Air stream and cloud flow was moving from that direction. As I understand it seeing is good at your site when you get south easterlies, am I right in thinking that. That would mean a high pressure must be located in the centre of the Bight for you to get 1.5" seeing. Is this right? I thought the seeing whilst pretty good was influenced by the air stream coming down from the north.

[Shiraz (Ray)]

Originally Posted by codemonkey

Thanks everyone! Sounds like I'm in the ballpark. I have noticed, given a sample set so tiny as not be even close to significant, that there's seems to be a correlation between bandpass and FWHM.

Given too few samples and too many unaccounted for variables, I can see a median of 2.8" in L, 2.7" in Ha and 3" in OIII. This may be consistent with the wavelengths captured, or it may be related to chromatic aberration as Ray suggests, I'm not sure which... maybe both!

Thanks for the link to the study Ray, always great to back things up with studies like this! One thing that does have me question the application of this study in a more general sense is that sampling sites were specifically selected for astronomy: On the other hand, sites selected for astronomical observatories are naturally free of features that may have strong negative effects on seeing. As such, they should be expected to have seeing components close to minima and to show less scatter about any model than for randomly selected sites.

Having said that, it seems likely that it'd be a good general rule.

I suspect a lot of it here has to do with moisture in the air. We get thick fog almost every day in winter. I've noticed better results on windy days where there's less moisture in the air, even though I would have expected worse results with the greater wind.

I wasn't measuring FWHM until recently so it'll be interesting to see what I can get when it starts warming up.

that sounds to me like normal seeing variation with wavelength, not CA.

Yes, the paper provides a way of estimating the best possible seeing for a given altitude - a median of 2 arcsec would seem to be a good estimate for most of Australia and if we are not getting that it may be worth trying to find out what is stuffing things up.

I also get better results with wind - seems to break up the turbulent columns of air that (I assume) develop if it is still.

Originally Posted by Paul Haese

That is vastly different from what I got last night and demonstrates how different seeing can be from region to region. While you got 4 last night I was getting 1.86. Did you have north westerlies last night Ray? I did not have any wind until about 6 am. Air stream and cloud flow was moving from that direction. As I understand it seeing is good at your site when you get south easterlies, am I right in thinking that. That would mean a high pressure must be located in the centre of the Bight for you to get 1.5" seeing. Is this right? I thought the seeing whilst pretty good was influenced by the air stream coming down from the north.

It was still here, but after a sunny day, this site gives poor results in still air. I put that down to thermal columns developing over the roadways and houses in the town, but that is no more than a hunch. With onshore sea breezes, everything cleans up and seeing can get below 2 arcsec. But sea breezes around here can be 20+ knots, which brings another set of problems.

[Slawomir (Suavi)]

Hi Lee,

Below are the results for the subs from my current project, measured in PixInsight.

Equipment: 4" doublet, 3nm narrowband filters, 15 minute subs and AZ-EQ6 (on average 0.5-0.8 arc seconds total tracking error), imaging at 1.33 arc seconds per pixel.

Site: Paddington in Brisbane

Ha: FWHM 2.5-3.3
S2: FWHM 2.1-3.2
O3: FWHM 2.4-2.7 (all in arc seconds)

Hope it helps

EDIT: these results are for best/worst subs for each filter collected over 6 different nights.

[Shiraz (Ray)]

lee, further to Slawomir's post, if this is with your 100mm scope, scope diffraction from the relatively small aperture will broaden the stars a little - eg, if you measure 2.8 arc sec, the underlying seeing is about 2.5 FWHM and the scope will have bumped it up to 2.8.

[alpal]

Originally Posted by Merlin66

The average seeing conditions I got in Melbourne (15Km from CBD) was always around 2.5 to 3 arc sec.
Such is life.

The best I have ever measured in Melbourne is 2.2 arc seconds.
Many nights it's as bad as 6 or more arc seconds.
If the stars are twinkling - I don't set up my telescope equipment.

It can also change drastically over the night -
it can start off at 4 arc seconds at 9pm & be down to 2.5 arc seconds by 1 am.
Often it's the best at about 1 to 4 am in the morning.

cheers
Allan

[Paul Haese]

Originally Posted by Shiraz

I also get better results with wind - seems to break up the turbulent columns of air that (I assume) develop if it is still.



It was still here, but after a sunny day, this site gives poor results in still air. I put that down to thermal columns developing over the roadways and houses in the town, but that is no more than a hunch. With onshore sea breezes, everything cleans up and seeing can get below 2 arcsec. But sea breezes around here can be 20+ knots, which brings another set of problems.

Hmmm interesting. At Clayton typically there is wind all day ranging from a breeze to gusts. Always coming from the southern ocean (ocean being heated). Once night falls the heating stops and winds cease. It is rare to have wind at the site at all over night. We mainly get them in summer when the high pressures are over NSW or just leaving the NSW coast. I am sure the presence of Lake Alexandrina close by also influences the conditions there too. There is no topographic features for 30km or more in any direction. If there is wind at Clayton the seeing is not at good for me. The heated air is more turbulent and it is coming from over the Mt Lofty Ranges 35km away. Far enough away to settle a bit but still enough to make the seeing degrade a bit.

[Paul Haese]

Originally Posted by alpal

The best I have ever measured in Melbourne is 2.2 arc seconds.
Many nights it's as bad as 6 or more arc seconds.

Holy crap. 6 seconds would make most high res imaging impossible. I feel for you guys.

[alpal]

Originally Posted by Paul Haese

Holy crap. 6 seconds would make most high res imaging impossible. I feel for you guys.

Thanks Paul,
I am getting sick of Melbourne.
I have a brand new 10" f4 carbon fiber Newt. with
20 mm extensions on the mirror mount to hopefully get camera focus this time &
after many months I still haven't managed to set it all up.
Hopefully the sky will clear up in spring - 3 weeks to go.

cheers
Allan

[Atmos (Colin)]

Originally Posted by alpal

Thanks Paul,
I am getting sick of Melbourne.
I have a brand new 10" f4 carbon fiber Newt. with
20 mm extensions on the mirror mount to hopefully get camera focus this time &
after many months I still haven't managed to set it all up.
Hopefully the sky will clear up in spring - 3 weeks to go.

cheers
Allan

I am sorry but I am buying a new telescope next week so the weather gods are going to be bringing more clouds to Melbourne on my behalf

[alpal]

Originally Posted by Atmos

I am sorry but I am buying a new telescope next week so the weather gods are going to be bringing more clouds to Melbourne on my behalf

I hope not -

here's hoping for lots of clear skies.

[gregbradley]

At my home (170 metres altitude) it is variable and of course it comes down to how you measure FWHM (FWHM varies considerably within a single image). Winter seeing seems better on average. I've had a good run lately for example.

A sub from tonight has a FWHM 1.9 star but average is more like 2.2 on one sub.

Typically I think 2 on a good night and 2.5 to 3 on an average night and not sure what a bad night number is but I sure know what it looks like at the focuser. You can't focus it as its always shifting.

At my dark site it is typically better than that. The best I have seen is stars of FWHM 1.2 in an AP140 subexposure. Average was 1.7 to 1.9.
Probably around 2.5 is average there perhaps lower. Stars tend not to twinkle above 30 degrees. That's at nearly 800 metres altitude.

Greg.

[codemonkey (Lee)]

Thanks everyone!

Originally Posted by Slawomir

Hi Lee,

Below are the results for the subs from my current project, measured in PixInsight.

Equipment: 4" doublet, 3nm narrowband filters, 15 minute subs and AZ-EQ6 (on average 0.5-0.8 arc seconds total tracking error), imaging at 1.33 arc seconds per pixel.

Site: Paddington in Brisbane

Ha: FWHM 2.5-3.3
S2: FWHM 2.1-3.2
O3: FWHM 2.4-2.7 (all in arc seconds)

Hope it helps

EDIT: these results are for best/worst subs for each filter collected over 6 different nights.

Definitely helps, thanks S! If it's not too much trouble (seriously, don't bother if it is), would you happen to know the median in each of those?

Originally Posted by Shiraz

lee, further to Slawomir's post, if this is with your 100mm scope, scope diffraction from the relatively small aperture will broaden the stars a little - eg, if you measure 2.8 arc sec, the underlying seeing is about 2.5 FWHM and the scope will have bumped it up to 2.8.

ha! Good point, Ray. I'd lost sight of that. Probably not a great idea to sample beyond the point of the diffraction limit, is it?

Now the question is, Dawes or Rayleigh? If we go with Dawes that's 1.16 for a 100mm 'frac.

Rayleigh I think is a better measurement since you can actually tell the points apart, rather than just identifying that there's two points.

Given a wavelength of 550nm (green), the Rayleigh limit on this scope is about 1.38". It goes up to ~1.65" in H-alpha, and drops to ~1.25" in OIII.

Either way, I'm oversampling here.... can't wait until I get that 8" newt ;-)

[SkyViking (Rolf)]

On most nights I get 2.0"-2.5" and on really good nights it's below 2" - down towards 1.7-1.8". On what I consider a poor night it's +3" and then I only shoot RGB data, or even binned 2x2.

One thing to keep in mind when measuring is that FWHM is highly dependent on collimation / alignment of the optical train. At least I find that my FWHM slowly begins to suffer when it is time to re-collimate, but identifying when that happens is difficult due to the nightly differences in seeing.