How to Gauge 'Seeing'

[Pat Curran]

Hi List,

Here in Ireland the clarity of the night sky varies enormously from night to night, but can anyone tell me how to quantify the level of 'seeing' from one night to another?

Is it possible to score it using definitive feedback from software such as NINA?

Thanks,

Pat

 

 

Edited May 27 by Pat Curran

[Pat Curran]

...should I be comparing the HFR figures in PHD2?

[TiffsAndAstro]

I'm a noob so bare that in mind.

I'm ok at focusing but hfr in Nina seems higher during bad seeing. I realise that's nebulous and not much help, but I'm also interested in the answers to this  

There is a column on meteoblu website lists your seeing by the hour in arc seconds.

However, while I'm noob I'm in the UK so that prediction might be as accurate as weather forecast are here ;(

Mine will be just 1.6" at midnight tonight. Apparently.

Can't wait

Edited May 27 by TiffsAndAstro

[dweller25]

You could use an eyepiece…..

 

[Clarkey]

Technically you can use figures from software to give you an idea, but this is affected by guiding and focus. For me it is simple. 95% of the time my seeing is OK, the rest is poor except one night per year when it is good. I should add that I do not image from May to August so I can't comment on summer figures.

[Elp]

I've found when fine focusing and the seeing is decent, you should get fairly consistent FWHM updates on your focusing star at short exposures say 0.5-1s and it shouldn't impact your ability to judge the best focal point. The other day I was trying for focus and the seeing was awful FWHM figures all over the place, short exposures weren't working so had to extend to 2s in the hope they'd average out. Over time you'll know when you see it, also don't use bright stars to focus off, use medium ones.

[ONIKKINEN]

If you use your kit for long enough and pay attention to the reported HFR you get an idea what the typical HFR values are and can make some estimates on how good the seeing was. If guiding works well and you shoot high enough in the sky the only variable will be seeing, so abnormally high HFR values will be just due to seeing.

Regarding guiding, it can also give hints about seeing since with good seeing also comes clean and stable guiding. Bad seeing can lead to a seismograph in the PHD2 graphs, which doesn't happen under better seeing. Looping short exposures back to back also gives hints about the seeing, if HFR values fluctuate by more than lets say 0.2 pixels between back to back 3 second exposures then seeing is probably quite unstable. Under good seeing the HFR values stay more or less the same between exposures.

These values wont translate to your kit, but with my setup (8'' newtonian, TeleVue Paracorr at around 1018mm focal length with 3.76 micron pixels) the typical HFR in pixels is somewhere around 3.5-3.7. Bad seeing can be more than 4, even up to 5 on the worst nights if there was jet stream above. Good seeing is a bit above 3 and the best nights have been approaching 2.5 which is very rare from my usual imaging location at sea level (handful of nights in the past 4 years).

By the way i recommend you let NINA write the HFR value into the file name of the saved images, so you can quickly inspect the stats at a glance later. My file naming scheme below includes the name of the sequence, timestamp, exposure time, number of image in the sequence, guiding error during just this one exposure, HFR in pixels and finally star count used to determine that HFR.

[Pat Curran]

Thanks to All for valuable feedback.

ONIKKINEN, I Will certainly make that change to naming convention in NINA to include HFR values

Appreciated.

Pat

[TiffsAndAstro]

4 hours ago, Pat Curran said:

Thanks to All for valuable feedback.

ONIKKINEN, I Will certainly make that change to naming convention in NINA to include HFR values

Appreciated.

Pat

Me too