Seeing and what it can do to your images

[bartman (Bart)]

sorry Mike,
I just was trying to be funny in my response but failed....
I just did not understand the reply you made to Greg.......It's a bit beyond my scope of knowledge.....
Thanks anyway

Bart

[marc4darkskies (Marcus)]

I initially thought ... What tha? ... Mike (and you know I loves ya mate) is stating the bleeding obvious. But then I realised that we (people in this forum) are not all experienced imagers and seeing seeing (hehe) illustrated like this is beneficial.

My advice to any up & coming imager who doesn't already do so is to monitor FWHM on all their subs!! While FWHM does depend on the star field (number of bright stars vs fainter stars & exposure duration), it is an excellent indicator of seeing. I check every sub using CCDInspector and I've come to know the best and worst seeing conditions at my site. Typically I expect to see FMHM ranging from 1.5 to 3.75 arcsecs (and much worse sometimes), with the average "good" night around 2.5. I automatically discard ANY sub greater than 4 (even at F5.5) and if most subs are significantly less than that I lower that threshold because I strive for fairly uniform FWHM in my image stacks. If I'm imaging at F11.7 (1.06 arcsec/pix) then the threshold is also lower.

As Mike's example shows, and considering I obsess over detail, there's simply no point imaging beyond a certain threshold - especially for Lum and Ha. Of course this threshold will vary depending on your site and your image ratio so if your rig has a ratio of say greater than 4 arcsec/pix (the super WFers among us) then lucky you - you'll be able to image even when the seeing sucks!!!

Cheers, Marcus

[Peter Ward]

Originally Posted by strongmanmike

Greg this is purely an illusion of course, the detail in the image is affected exactly the same by the seeing and it is really only the choice of file size you choose to display that causes this apparent difference. When you have a wide field compared to a narrower field at the same arc sec/pix and then display them at the same file resolution the affects of the seeing on the two images will look identical.

Mike

Well, yes...... but..... unless the sampling is adequate you can never get very high resolution images with a short focal length.

Under-sampling will easily mask any seeing blur....with images looking pretty much the same from night to night, no matter how good or bad the seeing is.

At longer FL's you end up in the over-sampled region, hence seeing makes obvious differences to star sizes and resolution.

[strongmanmike (Michael)]

Originally Posted by marc4darkskies

I initially thought ... What tha? ... Mike (and you know I loves ya mate) is stating the bleeding obvious. But then I realised that we (people in this forum) are not all experienced imagers and seeing seeing (hehe) illustrated like this is beneficial.

Cheers, Marcus

And that was my point exactly

Love you too

Mike

[jjjnettie (Jeanette)]

ATM any clear sky is good sky.

[Peter.M]

I think I understand what Mike is saying here about focal legnth. If you take the same camera say my QHY9 with 5.4nm pixels and put it on my scope at 800mm focal legnth I get an image scale of 1.39 arc seconds per pixel.

Now If I were to take the same camera and put it on a 2000mm focal legnth scope the image scale would be 0.55 arc seconds per pixel.

now say the seeing is around 2 arc seconds. Because the sensor size has not changed the same star with the same seeing will appear as more pixels in the 2000mm scope (4 pixels as apposed to around 1.6 with perfect guiding)

[Peter Ward]

Ok, this is probably cracking a nut with a steam-rolller, but the mouse-over below....illustrates my under-sampling point pretty well...

No amount of bad seeing is going to phase a 10 arc sec per pixel image, but it certainly will blur-out a 1 arc sec per pixel image.

the link is here

[Paul Haese]

Yes know all about good seeing versus bad seeing. Except at native puny focal lengths seeing is nothing. When you get some focal length then seeing is everything.

Good seeing

Bad seeing.

Good seeing makes great planetary imagers. Bad seeing and no one wants to know you. Know all about this good seeing bad seeing thing Mikie.

That's about 0.1 arc second per pixel.

[RobF (Rob)]

Yeah, I figured you had education in mind Mike.
I found the seeing simulation this page helpful early on, particularly from point of view of visual observing, but applies to imaging too of course.

http://www.ayton.id.au/gary/Science/...st_viewing.htm

[multiweb (Marc)]

Originally Posted by RobF

Yeah, I figured you had education in mind Mike.
I found the seeing simulation this page helpful early on, particularly from point of view of visual observing, but applies to imaging too of course.

http://www.ayton.id.au/gary/Science/...st_viewing.htm

Very cool link - plenty of info there.

[strongmanmike (Michael)]

Originally Posted by Paul Haese

Yes know all about good seeing versus bad seeing. Except at native puny focal lengths seeing is nothing. When you get some focal length then seeing is everything.

Good seeing

Bad seeing.

Good seeing makes great planetary imagers. Bad seeing and no one wants to know you. Know all about this good seeing bad seeing thing Mikie.

That's about 0.1 arc second per pixel.

It's all about degrees Paul. To my eye at least, the effects of the seeing on your planetary images looks no more damaging from an aesthetics comparative viewpoint than it was with the Cen A shots like I linked too . Specifically comparing your two Saturns in fact suggests less of a detrimental effect even..? This would likely be because you were still able to remove the worst seeing affected fraction of a second subs from the poor seeing session but I wasn't able to do this because my subs were much longer.

But in the end, it is true, seeing can be a pain for all but the shortest of focal lengths....viva la Marc Aragnou

Mike

[Ross G]

Thank you Mike.

To people new at this like myself, a post like this is very informative and helpful.... a real "thousand words" of information in your 2 photos.

Ross.

[Ken]

Interesting post Mike,as I image at 2000mm with a small pixel camera seeing affects me every night. I often start a imaging session with poor seeing then shift to better then it may deteriorate then improve again. A quick glance at the dec. guiding graph tells all if the corrections are greater than R.A. If the seeing is poor but not very poor I switch to a colour filter and reduce the star sizes in image processing the RGB. On those great seeing nights take all the Luminance I can.
Clear skies Ken.

[Paul Haese]

Originally Posted by strongmanmike

It's all about degrees Paul. To my eye at least, the effects of the seeing on your planetary images looks no more damaging from an aesthetics comparative viewpoint than it was with the Cen A shots like I linked too . Specifically comparing your two Saturns in fact suggests less of a detrimental effect even..? This would likely be because you were still able to remove the worst seeing affected fraction of a second subs from the poor seeing session but I wasn't able to do this because my subs were much longer.


Mike

You're right I can remove a lot of the problems with frame selection and with gathering high speed frames. A better example of what I am trying to say is the following images. One is 8.5/10 and the other just barely 5/10. That is when it can be chalk and cheese. Remembering of course there are many nights I look and the seeing is 5/10 so I don't bother setting up. At least with nights of average seeing one can get reasonable data for DSO imaging. Maybe not luminence but certainly the RGB.

Great seeing

Average seeing.

Long and the short of it though and for those wanting to learn about seeing, Mikes point is well made. Luminence should only be gathered near the meridian and during good seeing. Likewise planetary imaging should not be conducted in poor seeing. If you are using one shot colour, well image when the seeing is best if you can, but if you have constant cloud, image whether the seeing is good or bad.

[dugnsuz (Doug)]

Mike - Thanks for taking the time to clearly illustrate this difference - amazing to see the number of stars lost to bad seeing.
Have to disagree with the 'puny' focal length comments (Paul's) though - good/bad seeing is good/bad seeing at any focal length...its differences are obviously marked as FL increases as stated.
Doug

[strongmanmike (Michael)]

Originally Posted by Ross G

Thank you Mike.

To people new at this like myself, a post like this is very informative and helpful.... a real "thousand words" of information in your 2 photos.

Ross.

Yeh I thought so too, no need to complicate it, most people don't get to see a direct comparison like this, plus given many people have seen my high res deep sky work, here is proof that it isn't always possible...as I am discovering with my new AG12, not because of the scope but the seeing

Mike

[strongmanmike (Michael)]

Originally Posted by Paul Haese

You're right I can remove a lot of the problems with frame selection and with gathering high speed frames. A better example of what I am trying to say is the following images. One is 8.5/10 and the other just barely 5/10. That is when it can be chalk and cheese. Remembering of course there are many nights I look and the seeing is 5/10 so I don't bother setting up. At least with nights of average seeing one can get reasonable data for DSO imaging. Maybe not luminence but certainly the RGB.

Great seeing

Average seeing.

Long and the short of it though and for those wanting to learn about seeing, Mikes point is well made. Luminence should only be gathered near the meridian and during good seeing. Likewise planetary imaging should not be conducted in poor seeing. If you are using one shot colour, well image when the seeing is best if you can, but if you have constant cloud, image whether the seeing is good or bad.

Yeh! that's a hell of a difference mate

We are all in the same boat when the seeing doesn't cooperate

I hope the comparison was helpful to people anyway

Mike

[dugnsuz (Doug)]

Originally Posted by Paul Haese

You're right I can remove a lot of the problems with frame selection and with gathering high speed frames. A better example of what I am trying to say is the following images. One is 8.5/10 and the other just barely 5/10. That is when it can be chalk and cheese. Remembering of course there are many nights I look and the seeing is 5/10 so I don't bother setting up. At least with nights of average seeing one can get reasonable data for DSO imaging. Maybe not luminence but certainly the RGB.

Great seeing

Average seeing.

Long and the short of it though and for those wanting to learn about seeing, Mikes point is well made. Luminence should only be gathered near the meridian and during good seeing. Likewise planetary imaging should not be conducted in poor seeing. If you are using one shot colour, well image when the seeing is best if you can, but if you have constant cloud, image whether the seeing is good or bad.

Another great comparison - the difference is jaw dropping!

[mill (Martin)]

Good seeing or bad seeing, getting out and making some pictures is the whole point of astronomy to me.
When the seeing is bad then you can make pictures that are not great but still good and when the seeing is excellent, the picture just jumps out and is good.
The comparison is very good to show what seeing does to a picture.

[stardust steve (Steve)]

great info for me. Good to see the side by side comparison. As a not quite there beginner yet, i try and soak in all this info i read.
And what a great image that is of Centaurus A Ultra Deep Field - Multi Award Winner. Images like this inspire me to sell everything thats not bolted down just to get a modest little setup under way.