Astronomical Seeing, How much can it write off the evening?

[Joebob]

Hi all,

It appears that the clouds may part long enough this evening to actually see some stars in my neck of the woods, for the first time in quite a while! Looking at the weather forecast in Telescopius seeing will be "Bad", having first acquainted myself with what astronomical seeing actually is, I am trying to gauge what this means for any AP undertakings tonight. 

Is it generally considered not worth it to go out and take a big amount of photos if the quality is going to be terrible? I don't suppose there any precautions you can take to reduce the effects of seeing, e.g. shorter subs? My completely uneducated guess would be that any atmospheric pertubations etc would be on much smaller timescales than would be useful for taking subs so that seems unlikely, but as I said, that's a completely uneducated guess.

If it is generally not worth it, do you have anything that you generally do when the clouds clear but seeing is less than favourable?  I am still new to all of this, so I may just set it all up and have a bit of a play around even if not to take any serious shots for the night.

Thanks!

[drjolo]

That also depends on the pixel scale of your setup. Simplifying a bit - seeing blurs point image (star) to some finite diameter (FWHM, HFD). At my location average seeing is about 2.3-2.4". Good seeing is 2.0", best is 1.8" sometimes. That is for exposures 10s or longer. 

My Samyang 135 travel setup has 6"/px scale, and I never care about seeing conditions.

My 80 mm scope setup has 2"/px scale, and I almost never care about seeing here.

My 10" telescope has 0.45"/px scale, and I can always tell what the seeing is  Once it is over 2.6-2.7" I give up on imaging with this scope. Sometimes there is a light fog and the atmosphere is stable, although the transparency is poor. But these conditions are often connected with very good seeing and I am capturing then Ha narrowband data in the targets that have features in this band. Like for this M16 image 

The nebula was quite low at my location, and the transparency was also poor, but atmosphere was stable and I managed to collect 115 minutes of good quality subframes. 

Edited January 4, 2022 by drjolo

[vlaiv]

Yes, you can't do much about seeing as it "averages" out on 2s scales and "freezes" on 5-6ms scales.

Planetary astrophotographers utilize shorter exposures than say 10ms in order to capture image while seeing is frozen (distortion does not change) and then select subs with the least amount of distortion.

How much will seeing impact your final image depends on your working resolution and performance of the rest of your system.

If you use small aperture scope or lens and going for wide field with low resolution and using mount that is not very precise (your tracking / guiding is already in 1.5-2" RMS range) - then seeing won't be issue most times. But if you want to work at medium to high resolution then poor seeing will be issue.

If working with LRGB - you can capture color in nights of poor seeing - work with R and G as these are less affected by seeing than say blue.

[Joebob]

Thanks both, some great advice.

Sounds like with my setup it shouldn't be too much of a worry as it sounds like I am more limited my the performance of my setup. All the excuse I need to get out there tonight!

[ONIKKINEN]

I dont care about seeing when planning a night. If its clear i will go out regardless of seeing conditions, but i also expect to not get all that great results.

On the worst nights i could end up throwing most of the subs away, but im still left with some decent subs at the end so the night is not really fully lost.

[Astro Noodles]

I find transparency to be more of an issue than seeing. 

[Grant93]

As far as I understand, Transparency is more of an issue for DSO imaging, as @Astro Noodles said above. You will notice this when you go outside but can only see the brighter stars of the constellations, and it looks quite hazy, reflecting more light pollution also. But bad seeing effects planetry imaging, this is the wobbling effect you notice at high magnifications, much higher than is used for MOST DSO imaging. You can also see yourself if its bad seeing with the naked eye, if the stars are twinkling more than usual, its likely bad seeing.

Hope this helps

[Fegato]

I must admit I haven't really got a handle on seeing conditions, beyond a quick look for how twinkly the stars are!  Whenever I look at an astro forecast, the seeing forecast always seems to be terrible here if it's clear, and very good if it's cloudy.

My normal imaging scale of 1.98 arc secs per pixel probably means it doesn't matter too much.  But I guess I should take more notice when going for 0.67 at 1422mm focal length. But anyway, clear nights are so scarce that I just don't want to waste any, so will have a try anyway.

[ONIKKINEN]

15 minutes ago, Fegato said:

I must admit I haven't really got a handle on seeing conditions, beyond a quick look for how twinkly the stars are!  Whenever I look at an astro forecast, the seeing forecast always seems to be terrible here if it's clear, and very good if it's cloudy.

My normal imaging scale of 1.98 arc secs per pixel probably means it doesn't matter too much.  But I guess I should take more notice when going for 0.67 at 1422mm focal length. But anyway, clear nights are so scarce that I just don't want to waste any, so will have a try anyway.

If you use NINA to capture you can set it to measure and show HFR per sub. HFR depends on your telescope and tracking too but seeing has the biggest effect and i find that on good nights it can be under 3 while on terrible nights its 4+. Most or even all of the HFR 4+ go straight to the bin, and this is with 1.84 arcsec/pixel.

I think youll need to bin2 with the 1400mm fl scope unless imaging from a mountain top in a desert/doing planetary as seeing doesnt really support that kind of resolution 99% of the time.

[Fegato]

23 minutes ago, ONIKKINEN said:

If you use NINA to capture you can set it to measure and show HFR per sub. HFR depends on your telescope and tracking too but seeing has the biggest effect and i find that on good nights it can be under 3 while on terrible nights its 4+. Most or even all of the HFR 4+ go straight to the bin, and this is with 1.84 arcsec/pixel.

I think youll need to bin2 with the 1400mm fl scope unless imaging from a mountain top in a desert/doing planetary as seeing doesnt really support that kind of resolution 99% of the time.

Thanks, yes I do use NINA, and I do take note of the HFR readings.  I guess I'm more worrying about focus (and backfocus / tilt with the RASA) when I look at these, so perhaps when I'm really confident I've got that sorted I will take more note of these with regard to seeing. I think my norm is about 2.7, but HFR is pixels so can't directly compare?

re: working at 0.67 - I tend to software bin (due CMOS camera), but haven't done too much imaging at this scale. Having two scopes and only one mount is faintly ridiculous with the weather here!

[ONIKKINEN]

2 minutes ago, Fegato said:

Thanks, yes I do use NINA, and I do take note of the HFR readings.  I guess I'm more worrying about focus (and backfocus / tilt with the RASA) when I look at these, so perhaps when I'm really confident I've got that sorted I will take more note of these with regard to seeing. I think my norm is about 2.7, but HFR is pixels so can't directly compare?

re: working at 0.67 - I tend to software bin (due CMOS camera), but haven't done too much imaging at this scale. Having two scopes and only one mount is faintly ridiculous with the weather here!

I think HFR values are a bit arbitrary and difficult to compare, since it depends on the number of stars NINA has decided to include in the calculation, tracking accuracy and field flatness, so seeing is just one part of the equation. But you should be able to produce an average value from your own data to see how much seeing has an effect. My HFR values are a bit higher than one should expect from an 8 inch newtonian, since my coma corrector is not that great (TS maxfield 0.95x), so mine should be much closer to yours on average since the imaging resolutions are quite similar.

Im not 100% sure on this, but i think NINA HFR values are in arcseconds based on the focal length you have input in the settings tab.

[Fegato]

6 minutes ago, ONIKKINEN said:

I think HFR values are a bit arbitrary and difficult to compare, since it depends on the number of stars NINA has decided to include in the calculation, tracking accuracy and field flatness, so seeing is just one part of the equation. But you should be able to produce an average value from your own data to see how much seeing has an effect. My HFR values are a bit higher than one should expect from an 8 inch newtonian, since my coma corrector is not that great (TS maxfield 0.95x), so mine should be much closer to yours on average since the imaging resolutions are quite similar.

Im not 100% sure on this, but i think NINA HFR values are in arcseconds based on the focal length you have input in the settings tab.

Thanks, yes, I'll keep a closer eye on it I think.  NB Just checked, and the NINA online doc glossary says HFR is pixels.