How to identify "seeing"?

There’s always the star twinkle factor….though my stars always twinkle. Maybe cuz my seeing always sucks!…ha! I’ve had the same experience with MB so I never really know until the first auto-focus run. And then it can double or halve in an hours time…. Meteorically, it’s not so much about air movement but dynamic thermic conditions. Wind can sometimes even help by keeping things mixed but then is it’s own problem without an observatory.

Cheers,
Scott

Wei-Hao Wang:
First you need to understand your instrument well.  Then measure the star FWHM of every image you took.  The FWHM is determined by both your instrument (optical aberration, diffraction, and tracking error) and seeing.  Taking out the instrumental part, you are left with seeing.

This is where discussions of seeing can be confusing. There is the potential seeing based on meteorological conditions that MB purports to show, and then there's the resolution we each can realize of that potential with our own equipment, but when seeing conditions are talked about, I'm never sure which is meant. Note that there are also other seeing factors neither meteorological, nor mechanical, like imaging a target that happens to be just above a warm house on a cold night.... Anyhow, as Wei-Hao Wang said, over time you establish what your own very best fwhm is, and then your current seeing is measured off that based on images taken. The challenge then is determining if gear is optimized (best focus, collimation, backfocus, etc.) or if there's anything you can do to close the gap between potential and realized, and more challenging when there isn't any accurate current seeing conditions report to reference.

Seeing can also change dramatically through the night, so if I get started and find it's not worth imaging (about 75% of the time during the winter), I'll leave everything set up and then get up periodically to check if it's improved.

Cheers,
Scott

My seeing is so bad and variable, I use my guiding as an indicator. If my guiding is below 0.65 and steady then it’s a great night and try to optimize everything. Between 0.7 and 1, keep imaging. Seeing is between 2 & 2.5. Above 1, I switch to bigger pixels or go to bed. For me , the guider is the simplest system to get optimized and doesn’t change much so it is a reliable indicator that is constant between different equipment combos. I also monitor my average ADU. This doesn’t give me an absolute repeatable reference like guiding but does give me an indication of the changes through the night.

BTW, I don't think we can predict seeing as well as we predict temperature, wind, and precipitation. Seeing is a much more complicated and more poorly understood. It's also highly related to microclimate in your neighborhood, which cannot be included in the large-scale model used by systems like meteoblue.  So I never take meteorblue's seeing forecast seriously.

In my experience, it’s thermal turbulence much more than air movement, which is why wind can sometimes actually help. A couple weeks ago I was imaging in gusts up to 25mph+ and guiding sub 0.7”. FWHM’s we’re in the mid to high 2’s, but low 2’s is the best I ever get. This winter I had many calm nights and seeing so bad ASTAP couldn’t plate solve Another correlation I saw very consistently this winter (when seeing is worst, generally) is if it was clear all night, the seeing was terrible, and if the seeing was decent at the start, I could count on two hours at best before the clouds arrived. Though, maybe that’s just the Murphy factor…..

Cheers,
Scott

I haven’t used it either, but maybe it monitors FWHM variability and that way doesn’t have to take into account local factors, like environmental and equipment, or change in focus?

Cheers,
Scott