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stratocaster
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Impact of 'seeing' on DSOs new
      #5613966 - 01/09/13 03:57 PM

We've got some skies coming up in my area where the transparency is supposed to be excellent, but the seeing is poor.

I was just wondering how this condition would impact DSO viewing at a dark sky site.

I'm thinking a turbulent atmosphere must affect DSO viewing as substantially as planetary, lunar, or solar viewing in that contrast will be lost. I suspect faint stars in clusters would be obliterated, as well as subtle details on nebula and galaxies.

Is my reasoning sound, or are DSOs minimally affected by poor seeing and are more affected by transparency?


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Tony Flanders
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Re: Impact of 'seeing' on DSOs new [Re: stratocaster]
      #5613977 - 01/09/13 04:02 PM

Seeing does affect DSOs, but dramatically less than it affects the planets. It varies depending on object type -- least for galaxies and diffuse nebulas, most for globular star clusters and -- above all -- small planetary nebulas.

Two factors are at play. Typical magnifications for viewing DSOs are lower than for viewing planets. And the eye's acuity at low light levels is very poor to start with, so bad seeing won't make it much worse.


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Astrojensen
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Re: Impact of 'seeing' on DSOs new [Re: Tony Flanders]
      #5613992 - 01/09/13 04:10 PM

On very large, dim objects, such as a large, low surface brightness galaxy, where the smallest resolvable details will appear to be several degrees across in the eyepiece, seeing effects will not be large.

But on bright nebulae, especially those with quite sharply edged details, such as the Orion Nebula, you can indeed see more details in the moments of best seeing. This was obvious to me in a 12" on a night of poor seeing. Once in a while, the seeing would get better and the trapezium would sharpen up. In the same moment, the region around the trapezium suddenly showed a lot more detail, just to fuzz out a second later. I think I've seen the same effect in M82, but it's more difficult to readily see in fainter targets. As Tony points out, it should be obvious to see in small, bright planetary nebulae.

On a very clear, but turbulent night, the right thing to do is to go low-power wide-field observing! It will make the most of the situation.


Clear skies!
Thomas, Denmark


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uniondrone
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Re: Impact of 'seeing' on DSOs new [Re: stratocaster]
      #5613999 - 01/09/13 04:14 PM

Seeing can affect DSOs, but in a different way and to a much smaller extent. In planetary viewing, very small details must be resolved in order to get the best result. The difference between a good seeing and bad seeing night might be the difference of having resolution of 0.8 arc seconds versus 2.5 arc seconds. This would be the difference between Jupiter appearing to have tiny features appearing in each of its bands, versus just seeing that there are bands.

When observing DSOs, typically the features being discerned are larger--tens of arc seconds in size at least for many of the fine features found in galaxies for example. The main exceptions are globular clusters, where resolving individual stars is desirable, or planetary nebulae, which are usually quite small.


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MikeBOKC
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Re: Impact of 'seeing' on DSOs new [Re: uniondrone]
      #5614162 - 01/09/13 05:53 PM

On DSOs my experiemce is that poor seeing has the greatest negative impact on resolving double stars, the least on globs and open clusters.

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helpwanted
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Re: Impact of 'seeing' on DSOs new [Re: MikeBOKC]
      #5614452 - 01/09/13 08:21 PM

Remember, when looking at planets, you are looking at details that are seconds of arc in size, while looking at DSOs, most of the time, you are looking at minutes of arc detail. So seeing effects DSOs less.

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azure1961p
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Re: Impact of 'seeing' on DSOs new [Re: helpwanted]
      #5614712 - 01/09/13 11:42 PM

On galaxies poor seeing really softens would could otherwise be a very small nucleus or one appearing stellar. This is lousy when it happens even if its amazingly clear. Broader sized derpsky objects fare better but ultimately everything pays some where. Just not as bad as lunar, planetary and doublestar ovservation.

Pete


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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: azure1961p]
      #5614829 - 01/10/13 02:03 AM

If your exit pupil is about 2mm and larger, only the truly bad seeing will have an impact. And even then only when the surface brightness of the target is reasonably high, such as stars, point-like galactic nuclei and nebulae bright enough to just about show color.

Dim, diffuse objects are largely unaffected by seeing even at quite small exit pupils. And at larger exit pupils, absolutely not.


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Tony Flanders
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Re: Impact of 'seeing' on DSOs new [Re: MikeBOKC]
      #5614977 - 01/10/13 07:32 AM

Quote:

On DSOs my experiemce is that poor seeing has the greatest negative impact on resolving double stars




Oh yes, of course. I wasn't even thinking of those as DSOs. Tight double stars are affected by poor seeing every bit as much as planets.


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Tony Flanders
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5614979 - 01/10/13 07:35 AM

Quote:

Dim, diffuse objects are largely unaffected by seeing even at quite small exit pupils. And at larger exit pupils, absolutely not.




That depends on the size of your scope. In my 7-inch Dob, I have never noticed any effect whatsoever from poor seeing when viewing galaxies. In my 12.5-inch, where a 2-mm exit pupil is 150X, there definitely is an effect, albeit not huge. And in a 30-inch Dob, where powers below 250X are nearly unuseable, seeing is a very big deal indeed for faint fuzzies.


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C_Moon
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Re: Impact of 'seeing' on DSOs new [Re: MikeBOKC]
      #5615416 - 01/10/13 12:18 PM

Quote:

On DSOs my experiemce is that poor seeing has the greatest negative impact on resolving double stars, the least on globs and open clusters.




I disagree on the globs & open clusters. Both tend to look quite mushy when you throw any power at them which is quite common to do for globs, and occurs often enough for open clusters.


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la200o
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Re: Impact of 'seeing' on DSOs new [Re: C_Moon]
      #5615429 - 01/10/13 12:25 PM

The stars in open clusters will "smear" (for want of a better word)if the seeing is poor.

Bill


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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: la200o]
      #5615741 - 01/10/13 03:36 PM

Tony,
I should have been more explicit. When I say "dim and diffuse", I mean low surface brightness/low contrast, and hence necessarily large in order to be detected. You know, stuff like the Cave nebula.

Certainly, small galaxies of the type the monster Dob users often hunt down, where sizes are often in the arcminute range and even smaller, are affected by seeing. But these objects typically have at least central parts of moderately high surface brightness.

What is the smallest object detectable in a 30-incher which has the surface brightness of, say, IC 434 (the Horsehead's background), and how does this size compare to the scale of atmospheric seeing? At *least* an arcminute, vs a few arcseconds. Being the smallest detectable, it will show no detail, and so the concern now is whether the seeing will cause the object to "wink" in and out. I posit that it will be quite steadily visible, for the same reason the to the unaided eye a 30" planet's disk hardly twinkles.

My point is that over the huge range of object surface brightness observable, the eye's own resolving power has a very significant impact on how large the 'distortion' must appear on the retina in order to be detectable as a degradation.

Thomas Jensen (Astrojensen) performed a fascinating experiment recently. He observed the Moon and Jupiter through a solar filter, which dims them down to a DSO level of surface brightness. If anyone wishes to experience just how awful is the visual system's resolving power at such low light levels, do this. Then you'll appreciate how minimal is the impact of atmospheric seeing on dimmer nebulosities.


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galaxyman
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Re: Impact of 'seeing' on DSOs [Re: Tony Flanders]
      #5616424 - 01/10/13 10:40 PM

Yes, most definitely and I disagree with many of my DSO colleagues here.

Galaxies do suffer in bad seeing when looking for detail. Many or most times when viewing particular galaxies and waiting for moments of good seeing can make a difference. This includes spiral arms (particularly smaller galaxies), small HII regions, thin dark lanes on edge-on's, faint stellar nuclei and other detail, particularly when the use of higher magnifications is needed. Also viewing small faint galaxies in the numerous galaxy clusters, good seeing can make a difference in just the visibility of them. You may not be able to see much detail in each or any galaxies in these far away clusters, BUT seeing numerous members (like the fainter ones) of a galaxy cluster, such as many of the Abell clusters is in a sense an "object" in itself.

Many ARP galaxies can show amazing detail when using higher magnification and a bit of patience for those moments of good seeing.

I would also recommend using very high magnification on a number of bright galaxies with very large (over 18") scopes, particularly something like M-51, whereas small detail within the spiral arms can be seen in nights of good seeing. M-101 and M-33 are other great spirals to do this.

When seeing steadies, detail can "snap" into focus.

Many planetary nebulas (small ones included) have incredible detail (more than most planets) that can be seen when extremely high power and good seeing is brought together. Look at something like the Eskimo nebula at over x1000 with a large dob, it's awesome.

So good seeing with of course good transparency can make a big difference in DSO detail.

The era of "just faint fuzzies" is long gone.


Karl
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Chesmont Astronomical Society - www.chesmontastro.org
Galaxy Log - http://www.youtube.com/user/GalaxyLog4565?feature=mhee
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB/APM 8" f/9 Refractor”The Beast”. One great DEEP SKY achro
ES 6" f/6.5 achro. Good one
Celestron Omni XLT 102 refractor.
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azure1961p
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Re: Impact of 'seeing' on DSOs new [Re: galaxyman]
      #5617210 - 01/11/13 12:41 PM

Just to reiterate here, seeing in medium apertures won't necessarily kill spiral arms but if you seek challenging faint galactic cores these will easily smear out of site. Tiny challenging HII regions and such too. When the details are mere seconds in diameter as in galactic cores and such poor seeing can act like a giant cosmic eraser.

Pete


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Starman1
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Re: Impact of 'seeing' on DSOs new [Re: azure1961p]
      #5617696 - 01/11/13 06:09 PM

Seeing will definitely hurt DSOs.
In good seeing, I've seen:
--M15 resolved to the very center
--M14 covered with stars from edge to edge
--multiple stars in NGC206 star cloud (in M31 !)
--ropy tendrils surrounding the fainter sections of M27
--wave-like striae of nebulosity from side-to-side in NGC6888
--a dark lane from end to end in NGC891 that varies in width and has knots in it.
--structure in M82
--Structure in NGC3628
--the central star of M57
--IC1296 near M57
--IC4617 near M13
--G1 in M31 as a small ball of fuzz
--6 galaxies in the Stephan's Quintet area with distinct cores in each, including the pair hard to separate.
and so on.....

Almost none of those things is easily visible (or even visible at all) when the seeing is poor.

You don't have to be a lunar/planetary observer to want/need good seeing. Much detail is visible in a lot of DSOs of all kinds when the seeing is good.
When the seeing is bad, nearly everything suffers.


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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: Starman1]
      #5618282 - 01/12/13 12:21 AM

The kinds of DSOs given above as examples of those which are affected by seeing all have at least moderately high surface brightness, where the eye's resolving power is not so poor. But it's a different matter for those objects which hardly stand out from the sky. To be seen at all they have to subtend on the retina a which is much larger than the scale of seeing (except for really monster scopes, which *very* few if us have access to.)

For example, will poor seeing affect the visibility of the Horsehead, if transparency remains unchanged? Or the Helix?

A better appreciation is arrived at by considering this. Examine an image of some favorite DSO, such as M51, taken with a scope having an aperture no larger than the one you observe with. Note the detail in the image, and contrast this with the best you've seen at the eyepiece. Then consider that the image might well have been taken under less than ideal seeing conditions, and moreover integrates what seeing there was over many minutes. A better example of this disparity is afforded by the Helix. The more prominent of the small cometary features just inside the annulus can be imaged with a *very* much smaller aperture than required to see directly.

A prime focus image of dimmer extended objects reveals significantly more detail than can be glimpsed visually in even a rather larger scope. And even when seeing is not the best. This shows that below some threshold of surface brightness and contrast, the eye's horrendous resolving power is just too poor to resolve down to anywhere near the seeing scale.


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galaxyman
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5618376 - 01/12/13 01:46 AM

Yes Glenn I agree there are DSO's that require far more transparency then best seeing, but at the same time there are many DSO's that certainly benefit from both as I stated in my previous post.

My best views of Abell 2065 in the 22" came on a night with very good seeing (it was at a very good dark site), whereas the steady seeing allowed for the small fainter members to be seen at all. On previous nights from the same site and very good transparency, but with much lesser seeing conditions I did not see near as many members. Though per say these faint small galaxies do have a relative high S.B., just the poor seeing on those lesser nights smeared the light enough and reduced the ability to detect them.

Our M-51 example can be intriguing in the aspect of going beyond just the overall view of this grand spiral. Here for just viewing the spiral pattern, transparency wins out big time, though going beyond that realm is the intriguing part. As I mention before the use of very high power (1000x plus) in a large dob can bring out photographic detail in each spiral arm, including small HII regions and star clouds. Doing this on many occasions gave the best views on nights of course with better seeing.

The same can be said on the great edge-on Ngc 4565, whereas using this same extreme type of magnification brought out small details within the dark lane.

As we also mentioned is the various planetary nebula that show incredible detail when power can be pushed to the extreme. Ngc 1501 is another that shows lots of structure throughout its disc without the use of any filtration. Example http://www.starimager.com/Image%20Gallery%20Pages/Planetary%20Nebulae/ngc%201...

This is only some brief examples that concur what can be seen using magnifications that do require very good seeing, for I have tried this (of course) when seeing was poor throughout much of the night, and as you would suspect the views were just nowhere near the same.

I will agree go for the darkest skies no matter if seeing is off. I of course agree that dark skies and poor seeing trumps bright skies and excellent seeing, though I think though the overall consensus here is that the combination of good seeing and transparency is by far best

Oh, I will also say to the original post that just because it says bad seeing does not always mean through the whole night. Even so, there are moments where you can be viewing a particular object and the seeing becomes very good for a time. This is where as I describe in the galaxy log videos of having patience when viewing a particular object.


Clear Dark "steady" Skies


Karl
E.O.H.


Chesmont Astronomical Society - www.chesmontastro.org
Galaxy Log - http://www.youtube.com/user/GalaxyLog4565?feature=mhee
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB/APM 8" f/9 Refractor”The Beast”. One great DEEP SKY achro
ES 6" f/6.5 achro. Good one
Celestron Omni XLT 102 refractor.
Celestron 10x60mm Binos

Edited by galaxyman (01/12/13 01:49 AM)


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Starman1
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5618377 - 01/12/13 01:49 AM

Quote:

The kinds of DSOs given above as examples of those which are affected by seeing all have at least moderately high surface brightness, where the eye's resolving power is not so poor. But it's a different matter for those objects which hardly stand out from the sky. To be seen at all they have to subtend on the retina a which is much larger than the scale of seeing (except for really monster scopes, which *very* few if us have access to.)

For example, will poor seeing affect the visibility of the Horsehead, if transparency remains unchanged? Or the Helix?

A better appreciation is arrived at by considering this. Examine an image of some favorite DSO, such as M51, taken with a scope having an aperture no larger than the one you observe with. Note the detail in the image, and contrast this with the best you've seen at the eyepiece. Then consider that the image might well have been taken under less than ideal seeing conditions, and moreover integrates what seeing there was over many minutes. A better example of this disparity is afforded by the Helix. The more prominent of the small cometary features just inside the annulus can be imaged with a *very* much smaller aperture than required to see directly.

A prime focus image of dimmer extended objects reveals significantly more detail than can be glimpsed visually in even a rather larger scope. And even when seeing is not the best. This shows that below some threshold of surface brightness and contrast, the eye's horrendous resolving power is just too poor to resolve down to anywhere near the seeing scale.



Glenn,
This is similar to the argument that people use to say that a large aperture won't see small details any better than a small scope if the seeing doesn't allow the big scope to resolve to a limit beyond the capabilities of the smaller scope.
Since the eye can't resolve the very smallest details does it mean it cannot resolve any? And, like faint stars, cannot bad seeing simply blur them to invisibility?

A better example is to limit the photograph to a very short duration, but long enough to expose the center or brightest section of the object. In that case, you'd say the eye can see SO much more. The eye has a hugely larger dynamic range than the camera--we can see the very faint outer parts and the bright middle (not overexposed) at the same time, and with a much shorter duration image capture. By the time the camera exposes the faint outer parts, the center is way overexposed. A good example is the Orion Nebula, where I am constantly disappointed with photographs because they don't show the faint outer parts visible to the eye in a telescope. Another example is the nebulosity visible between M16 and M17 to the eye that requires VERY long exposures to show photographically.

But, to return to the question at hand, a really good example of how seeing affects DSO details is in the Veil Nebula. In really good seeing, small knots in the ropy tendrils become visible and each braid becomes its own wisp of smoke and you definitely get a sense of moving shock waves overlapping. In bad seeing, a lot of that detail goes away and what detail is there becomes indistinct or not seen.

In a friend's 28", the wisps of nebulosity in the nose of the Horsehead become visible in good seeing. They're just not there otherwise. The resolution of our eye isn't the problem, it's the seeing.


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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: Starman1]
      #5618822 - 01/12/13 11:08 AM

Again, Don, you're invoking examples where the size (a star, being a 'point' source), and 'higher' surface brightness (the tendrils of the Veil, which I'd call bright) which allow to see stuff down to of order the seeing scale.

Instead of considering bright stuff having surface brightness of about 21 MPSAS and brighter, look at the dim stuff of closer to 25 MPSAS. Let's imagine a reflection nebula, for which filtration offers little or no improvement in visibility. Under a very dark 22 MPSAS sky, the three magnitude fainter nebula will be 6% brighter than the sky. Such low contrast requires the maximum useable exit pupil, and the object (or some detail within) would require to subtend *at least* 5 degrees on the retina. To be affected or not by seeing, we may assign a threshold size of, say, 5 arcseconds. In order to magnify a 5" feature to 5 degrees requires 3600X. At a 6mm exit pupil, the aperture is 21.6m.

If my reasoning is in error by a full order of magnitude (a factor of 10), the required aperture is still a couple of meters. One *might* argue that this low contrast target need only subtend a degree or two on the retina, and that seeing will impact features up to 30" in size, but I feel this would be artificial.


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