Is there a way to measure seeing?

andrea tasselli · Jan 25, 2026, 08:19 AM

Yes, that is the way to measure instrumental seeing, compounding both the atmospheric seeing and your own locale seeing convolved with the instrumental seeing (tracking precision, collimation, thermal stability). The atmospheric seeing and the locale seeing, these you can't measure unless you have very specialized instrumentation.

Andrea, I have a question: do the numbers you get from the script already tell (instrumental) seeing in arcseconds? Or do they have to be “translated”?

andrea tasselli · Jan 25, 2026, 08:19 AM

The atmospheric seeing and the locale seeing, these you can't measure unless you have very specialized instrumentation.

And i even if you could, as you suggest, what matters is what your rig is capable of. What I call my seeing is the fwhm of a 4s sub after running auto-focus, but when seeing is bad, that number can change from sub to sub by an arcsecond or more….. In addition to wide variances night to night, my seeing can change significantly through the night and even according to where I’m pointing. Winter is the worst, in part because of denser air, but probably more so because of increased thermal turbulence.

Where things get even mushier, is converting NINA’s HFR measurement (while imaging) to FWHM. There isn’t a direct conversion, but I’ve found, on average, HFD in arcsec is about 1.2x FWHM in arcsec.

For forecasting, MeteoBlue is the common goto. If you ignore the numbers, I find the forecasted trend is fairly acurate — unfortunately, in my case, for tonight:
📷 Meteoblue.pngHere’s a link to how they calculate seeing, and also a way to lend your own data to the effort, though via visual observation, not images. http://www.meteosurf.com/spastro/seeing/index.html

Cheers,
Scott

Brian Puhl · Jan 25, 2026, 04:19 PM

Study your topography, if you have mountains around you, those disturb laminar airflow and can cause poor seeing on the leeward side of the mountain. If you have relatively flat topography, you’ll want to see the air flow coming from those directions for the potential indication of a good nights seeing. You don’t have to be on top of a mountain to get good seeing. In my case, it’s flat in 3 directions and based on my scope performance I’m pretty sure I’m below 1 arc second seeing on my best nights here, even at sea level.

Agreed, and a large factor in my own situation. On the side of a mountain, facing a ridge across a narrow valley, and the site of “the worst weather in the world” (and highest ever wind speed recording), about 8 miles away at the end of the valley. Additionally, until the evening temperature gradients neutralize, there’s often a cloud directly overhead, as warm ground air is pushed up the side of my mountain into colder air. Even when there’s literally no other cloud in the sky….. Can be frustrating.

Cheers,
Scott

Yes.

I go outside. If I see stars, I get to work.

If I don’t see stars, I go watch YouTube or Netflix.

Around 8 or 9 I go to bed. Tomorrow will have another night.

Sometimes I will shoot anyway. Then cull out the crappy images and stack what’s left.

Afterall, I didn’t get into this to let my equipment set in the house.

Every day has a new night, and another chance to do good.

To accurately measure your local seeing conditions, I would rely on Siril. Capture a subexposure near the zenith (to minimize atmospheric thickness) and load it into Siril.

1. Draw a selection box over a central area of the frame that contains a dense field of unsaturated stars.

2. Navigate to Analysis → Dynamic PSF (or use the Ctrl+L shortcut).

3. If your image scale is correctly defined in the FITS header or Preferences (Focal Length and Pixel Size), clicking the Sigma (Sum) icon will calculate the average Full Width at Half Maximum (FWHM) for the selected stars.

4. The resulting FWHM, measured in arcseconds, represents the "limit" the atmosphere is imposing on your resolution. Compare your result to these standard benchmarks (many books provide these):

• < 1.5" (Excellent): Exceptional stability; ideal for high-resolution planetary or small-galaxy imaging.

• 1.5" - 2.5" (Good): Typical for good inland sites; supports fine detail in most deep-sky objects.

• 2.5" - 4.0" (Fair to Poor): Average conditions; fine for wide-field targets, but fine details will be softened.

• > 4.0" (Very Poor): Heavy atmospheric turbulence. At this level, the "seeing disk" is so large that long-focal-length imaging is frustrating. It’s a better night for visual observation with binoculars.

I would point out that this is a relative measure, not a true representation of your seeing. Much will depend on how longs your subs are as there can be a noticeable difference between 15 sec. subs and 190 sec. subs. The longer the seeing and guiding works on your stars there will be some bloat. in a 60 sec. exposure a blast of really bad seeing for 10 sec. can really change the results.

using an SQM about $150 cost.

Sky Quality measurement device

Rich Sky · Jan 28, 2026, 12:48 AM

using an SQM about $150 cost.

Sky Quality measurement device

As far as I know, an SQM meter measures sky brightness, not seeing.

Seeing can be measured in real time on any imaging rig using PHD2 if one disables guiding in DEC, disables dither, then observes the reported error in DEC as the mount tracks an object. All of the reported DEC direction guiding error will be due to atmospheric movement of the guide star in that coordinate.

One can assume that similar RAM jitter is occurring in RA over the same interval so total jitter is SQRT(2) times the measured DEC jitter.

— David F.