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Observing >> Deep Sky Observing

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stratocaster
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Impact of 'seeing' on DSOs
      #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 new [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
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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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Starman1
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5618876 - 01/12/13 11:34 AM

We're so used to using a contrast-enhancing filter to look at the Veil, we forget how really low contrast it really is.

But we're not in disagreement about the really faint stuff.

The original question was, "Does seeing affect the visibility of DSOs?", and I'd say the answer is a qualified yes, depending on the type of object and what you want to see in that object.


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blb
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5618901 - 01/12/13 11:44 AM

Quote:

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.




So you are saying that seeing does not mater, correct? Well I say to that Hogwash. All of us who are experanced visual deep sky observers know that this just is not so.

Quote:

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.




Oh, I see, if I would only take the exceptions to prove what I wount to prove. You know like statisticians say, you can prove any thing you wont with numbers.

The truth is that seeing does effect what we see and there are some objects that it does not effect that much and there are those objects that it does effect a good bit.


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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: blb]
      #5619543 - 01/12/13 05:54 PM

Why the fervent near denial that for the sizes of instruments used by virtually all amateurs there are more than a few objects which are little if at all affected by atmospheric seeing? In the partial list of oft observed examples below, please provide any case your are aware of where you felt that *seeing* (not transparancy) compromised the view of the object or detail within:

North America and Pelican nebulae
Witch Head nebula
Heart and Soul nebulae
California nebula
IC 434
Iris nebula
Cone nebula
Cave nebula
Gamma Cygni nebula
Mirach's Ghost galaxy
Leo dwarf galaxy near Regulus

These are all objects which have low surface brightness and hence no visibly discernible details at the seeing scale (even with contrast enhancing filters, where useable.) And there are many more objects in this class.

These are objects for which an image obtained with a 2-3" aperture f/6 (300-500mm focal length) records detail well beyond what can be visually detected in meter class apertures. Lest one think the comparison is unfair in some way, consider the 'detail' present in an image of Jupiter taken with the same 2-3" scope at prime focus. Nothing compared to what can be seen when magnified at the eyepiece in the same scope. My point is that the small scale image of the dim DSO reveals details which are well larger than the seeing scale but which are invisible through the eyepiece on even a very large scope.

Finally, I truly feel that our now long familiarity with the photographic appearance of difficult-to-see objects can all too easily implant in our subconscious a tendency to 'see' at the eyepiece some of what the image has revealed. We must be ever vigilant against bias.


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george golitzin
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5619638 - 01/12/13 06:52 PM

I'd say anything requiring substantial power in order to resolve details will suffer, including planetary nebulae, globular clusters, and open clusters, but also larger nebulous objects. The aesthetic beauty of open clusters is greatly marred by poor seeing even at lower powers (e.g., an exit pupil of 3mm); the fainter stars in clusters do disappear; small details (including small stars) within nebulae are compromised; the faintest galaxies disappear, along with faint stars. Basically, all you can do in really poor seeing is a kind of casual sight-seeing at low power--it's better than nothing, but not nearly as satisfying.

However, I am willing to concede Glenn's point regarding the detection of the fairly large, low SB targets he has listed.

-geo


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blb
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5619683 - 01/12/13 07:17 PM

Quote:

North America and Pelican nebulae
Witch Head nebula
Heart and Soul nebulae
California nebula
IC 434
Iris nebula
Cone nebula
Cave nebula
Gamma Cygni nebula
Mirach's Ghost galaxy
Leo dwarf galaxy near Regulus



I have only seen about half of this list of objects (North America and Pelican nebulae, IC 434, Iris nebula, Gamma Cygni nebula, Mirach's Ghost galaxy, Leo dwarf galaxy near Regulus) visualy through my telescopes. With the light pollution and humid skies here in the southeastern USA, I probably will not see some of the other objects not allready seen with my scopes. Even so I have seen the list of objects and detail given by Don. Then there is the great number of objects between these extreams that reveal little detail, good seeing or not. Why take such an extream position?
Quote:

...My point is that the small scale image of the dim DSO reveals details which are well larger than the seeing scale but which are invisible through the eyepiece on even a very large scope...



I would agree but surely you really don't think a CCD camea, even in a 2 to 3-inch telescope, is comparable to a visual view in a medium sized or larger telescope. Yes there are details that are larger than the seeing scale that can be photographed but that doesn't mean that we can't see detail that is effected by seeing. A CCD camera does not see the same way that our eye do,Correct? So even if there is only one DSO that is affected by the seeing, then seeing affects our viewing of DSO's. You must agree with that, right?
Quote:

I'd say anything requiring substantial power in order to resolve details will suffer, including planetary nebulae, globular clusters, and open clusters, but also larger nebulous objects. The aesthetic beauty of open clusters is greatly marred by poor seeing even at lower powers (e.g., an exit pupil of 3mm); the fainter stars in clusters do disappear; small details (including small stars) within nebulae are compromised; the faintest galaxies disappear, along with faint stars. Basically, all you can do in really poor seeing is a kind of casual sight-seeing at low power--it's better than nothing, but not nearly as satisfying.



EXACTLY! Just because there are objects that do not appear to be affected by seeing does not mean that seeing does not affect our viewing of DSO's.


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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: blb]
      #5619889 - 01/12/13 10:02 PM

I used 'extreme' examples because it's easier to obtain consensus on their being little if any affected by seeing. Once that has been conceded, then at least there is the appreciation that there is a continuum over which surface brightness impacts the amount of detail possible to see.

I knew the example of a CCD image (even through a rather small instrument) would cause some to exclaim, "Unfair comparison!" which is why I took pains (unsuccessfully ) to point out what a prime focus image of a planet would record. Or not record, really.

At prime focus, the image of a bright but small object such as Jupiter comes nowhere close to revealing the same detail visible through the same scope with an eyepiece working at a 1-2mm exit pupil. If the camera is sampling at, say, 3 arcseconds/pixel, it's resolving details at closer to double that sampling frequency, or near to 6 arcseconds. But the scope visually resolves to 1-2 arcseconds. This disparity carries on to ever larger apertures, until seeing eventually imposes its ultimate limit.) And so we see that for prime focus imaging, a telescope does not resolve to its real limit; the image is undersampled. That's why fir planetary imaging, the f/ratio must be increased to the f/30 range in order to exploit the available resolving power of the aperture.

Back again to the small aperture scope and its image of a dim nebula. We know that this image is undersampled. Yet you can appreciate how the detail seen with that scope through the eyepiece under the best of conditions is probably two orders of magnitude (100 times) coarser. For example, a 300mm telephoto image will reveal striations (due to intervening dust clouds aligned by magnetic fields) in the North America/Pelican field which are utterly invisible in even a meter class telescope. Yet many of these striations are of order an arcminute in spatial frequency, or the size of one of the larger planetary nebulae, or Jupiter. If we could magically pump up the surface brightness of these nebulae so as to approach that of a bright planetary nebula (from 24-24.5 MPSAS to about 15 MPSAS; a brightness increase of 5,000 times, and where color would be seen), those striations would be well seen via small-ish apertures, provided contrast is sufficient.

I suspect many backyard observers haven't yet fully internalized just how atrocious is the eye's resolving power at low illumination and low contrast. In the photopic regime the retina resolves to 2 arcminutes--as low as 1 arcminute for the eagle-eyed. But at the dim limit, the resolving power is as poor as 5 degrees or more. That's a ratio of 150, and even as much as 300 for some.

Let's suppose that at some moderate magnification of, say, 100X you are just detecting the effects of seeing on Jupiter's disk features . In other words, the 1.2 arcsecond seeing is magnified 100X so that it subtends 2 arcminutes on the retina. For a dim object at the limit of detection, it would require something like a further 150-fold increase in magnification--to 15,000X--in order for that 1.2 arcsecond seeing subtend the requisite 5 degrees for perception. And even if such a dim feature need only subtend a mere 1 degree, the magnification would still have to be 3,000X. And for worse seeing of 2.4 arcseconds, a magnification of 1,500X.

To obtain such high magnification while preserving what surface brightness there is requires larger exit pupils and hence *large* apertures. If it is accepted that at lower surface brightness and contrast an object or feature must subtend the better part of a degree or more on the retina in order to be detected at all, then it immediately becomes apparent that rather massive apertures are required to sample down to the seeing scale on the light starved and poorly resolving retina.


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galaxyman
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5620077 - 01/13/13 12:49 AM

I don't think anyone is denying that certain DSO's are much more effected by transparency than seeing, but at the same time there are a number of DSO's (including galaxies) that certainly benefit from good seeing along with good transparency, as I now pointed out in two prior post.

Even compared to planets, there are far more DSO's in this category of the benefits of good seeing. In reality for most amateur scopes only three planets show any quality detail as in Jupiter, Saturn, and Mars. Even here both Jupiter and Saturn can still give quality views on nights of fair seeing, whereas tiny Mars is more of a "bugger" to see any detail. We can point to a number of DSO's that seeing isn't a big deal, though the same can be said with some planets like Venus for example.

Now we can say the same on many DSO's, that even on less than average seeing, quality views still can be had. To really push both the scopes and object for more detail, higher magnification at times needs or can be used for seeing fleeting or small detail within the DSO (as I pointed out before).

I guess the point is that some planetary observers want optimum views that require great seeing, but at the same time some DSO observers want both great transparency and good seeing to push the observing limits (like me).


Karl
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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: galaxyman]
      #5620132 - 01/13/13 02:18 AM

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.

For some, the application of mathematics is anathema, and a relinance upon 'impressions' and 'feelings' carries more weight. In spite of the demonstrated unreliability of our senses, or more properly, the interpretation derived therefrom.

If a case study using established facts as derived from an understanding of the operation of the eye is dusputed, I would like to see a rebuttal based upon quantitative analysis, not qualitative 'feelings', in which bias is likely to be present.


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Astrojensen
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5620181 - 01/13/13 04:49 AM

Quote:

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.




For visual amateur observers, this is true. But perhaps we ought to think more about what instrument we are likely to be using. A 10x50 binocular will give essentially perfectly sharp views on any night, regardless of the seeing. I've yet to see seeing bad enough that it obscured a deep-sky object in my binoculars. The resolution of a 10x50 binocular on even quite bright DSOs is on the order of something like one arc minute, larger if the object is dimmer.

But if I observe with a 12" scope, I don't think the claim that *all* objects are to some degree affected should be rejected out of hand. Maybe the effect is subtle, but if the seeing is bad enough (and it can get pretty bad out in the deserts, or so I've heard), then I think that the detection or details of even large nebulae and galaxies might suffer, even at low magnifications. A dim planetary nebula, say, some ten arcminutes across, but with a relatively sharp edge, may well get fuzzed out enough that the edge becomes indistinct and a deep-sky object with a sharp edge is far easier to detect than one with a indistinct one. If the object is on the threshold of detection in stable seeing, it might well become invisible in bad seeing, even if all other factors remain identical.

I think that for each instrument, deep-sky objects will be spread across a line, with some being almost invulnerable to bad seeing, some that are in some degree affected and some that are highly affected. The smaller the instrument, the larger the number of nearly invulnerable objects, and the larger the scope, the larger the number of highly seeing dependent objects. But each instrument will have some of each category.

Perhaps this is just a summary of what everybody's been saying, but at least I've now proved that I understand it - maybe!


Clear skies!
Thomas, Denmark


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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: Astrojensen]
      #5620255 - 01/13/13 07:53 AM

Thomas,
You have divined the essence of the matter; there is a continuum wherein object size, surface brightness, contrast, aperture and exit pupil diameter determine the degree of resolution.

I invoke mathematical characterization of these and other problems, not to 'bamboozle with statistics' or 'obfuscate with numbers', but to advance the argument with science. I find it astounding that some amateur *astronomers* should be so averse to even the simplest--and fundamental--quantifications via mathematics.

To possess even a first order understanding of the range of values occupied by such variables as object surface brightness, contrast ratio as a function of sky brightness, visual system resolving power, image surface brightness as a function of exit pupil diameter, contrast gains via filtration, and much else besides, is of great benefit in understanding the impact of same on object visibility. To be content to proceed merely by relying on 'impressions' is to remain lacking in true understanding. Science progresses via numbers, not feelings.


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galaxyman
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5620741 - 01/13/13 12:56 PM

Quote:

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.

For some, the application of mathematics is anathema, and a relinance upon 'impressions' and 'feelings' carries more weight. In spite of the demonstrated unreliability of our senses, or more properly, the interpretation derived therefrom.

If a case study using established facts as derived from an understanding of the operation of the eye is dusputed, I would like to see a rebuttal based upon quantitative analysis, not qualitative 'feelings', in which bias is likely to be present.




Glenn I do agree with you, though in certain particular DSO's of all types, better seeing enhances the view. Same can be said about planets, for it's mostly about what you want to get out of your personal observing expierence.


I agree there are certain DSO's that seeing has minimal effect on the view, and transparency is of course the key factor.

I do as you may have known, the extreme end of DSO observing, so I want everything I can get what the sky can give me.


Karl
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blb
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Re: Impact of 'seeing' on DSOs new [Re: galaxyman]
      #5621367 - 01/13/13 06:11 PM

Quote:

Quote:

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.

For some, the application of mathematics is anathema, and a relinance upon 'impressions' and 'feelings' carries more weight. In spite of the demonstrated unreliability of our senses, or more properly, the interpretation derived therefrom.

If a case study using established facts as derived from an understanding of the operation of the eye is dusputed, I would like to see a rebuttal based upon quantitative analysis, not qualitative 'feelings', in which bias is likely to be present.




Glenn I do agree with you, though in certain particular DSO's of all types, better seeing enhances the view. Same can be said about planets, for it's mostly about what you want to get out of your personal observing expierence.


I agree there are certain DSO's that seeing has minimal effect on the view, and transparency is of course the key factor.

I do as you may have known, the extreme end of DSO observing, so I want everything I can get what the sky can give me.


Karl
E.O.H.




My point exactly Karl. We must have similar experences. Seeing does not effect all DSO's the same. Poor seing hurts a number of the objects we view and is hardly noticeable in others. This has more to do with the type of object being viewed than the seeing. Still seeing effects what we see, it is just that some objects do not show bad seeing like other objects but it still effects what we see. So giving the impression that it does not is wrong in my book.


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blb
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Re: Impact of 'seeing' on DSOs new [Re: blb]
      #5621384 - 01/13/13 06:20 PM

Quote:

I used 'extreme' examples because it's easier to obtain consensus on their being little if any affected by seeing. Once that has been conceded, then at least there is the appreciation that there is a continuum over which surface brightness impacts the amount of detail possible to see.




Quote:

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.




Yes, I agree completely. I guess it only sounded like you were saying that seeing didn't matter. Seeing does matter, it's effects just are not noticeable on some objects due to the nature of the object being observed.


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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: blb]
      #5621479 - 01/13/13 07:17 PM

Here's a neat experiment Thomas Jensen performed not too long ago. He observed the Moon through a solar filter, and saw indeed just how poorly the eye resolves at 'DSO brightness levels.'

I'm sure he'll remind us of his specific results, but in the meantime I'll add these notes...

Assuming one has a Baader solar film filter of density of 5.0 (dims light to 1/100,000), the dimming amounts to 12.5 magnitudes. (Interestingly, the ratio 1:100,000 is very nearly equal to the ratio of the solar disk's area to the area of the 180 degree hemisphere of the sky.) This brings the -10.58 MPSAS Sun down to a rather more comfortable +1.92 MPSAS, but still not much less bright than sunlit snow.

Through the solar filter, the first quarter Moon's +5 MPSAS surface brightness would be brought down to +17.5 MPSAS, which is not far brighter than the color detection threshold of 18-19 MPSAS. The Moon would now have a surface brightness of reasonably bright planetary nebula which might show a greenish hue.

Observing the familiar lunar terminator, where one has the benefit of very high contrast afforded by shadows, estimate how much poorer is the eye's resolving power.

And this is nowhere near the lower limit of brightness the eye can work with, it being about +27 MPSAS, or near 10,000 times dimmer. To appreciate what further dimming does, you could install a variable polarizing filter, which at maximum dimming should get you perhaps another 4 magnitudes fainter, or about 21.5 MPSAS (a decent night sky brightness level.)

But even at the fairly bright +17.5 MPSAS for a quarter Moon, can one resolve even awful seeing? I wonder. I hope someone who has a large-ish solar filter (I have none, myself) and matching scope will try this experiment. The bigger the better, for then one can obtain sufficient magnification without having to go to a small exit pupil, thus retaining image brightness (in order to make the test as much a best case scenario as possible.) Naturally, the full magnification regime is to be explored nonetheless, as one might do for such a bright DSO having a surface brightness as high as 17.5 MPSAS. (The brightness range of nebulae and galaxies ranges from 14 MPSAS down to the contrast limit of about 26 or thereabouts.)


If you have a different filter whose density is known, you can calculate its dimming in magnitudes very easily. If the density is given as the logarithm of the attenuation, merely multiply this by 2.5.

For example, the Baader film having a density of 5 means it attenuates by 10^5, or a factor of 100,000. The logarithm of the attenuation is 5, which when multiplied by 2.5 yields a dimming of 12.5 magnitudes.

A filter of density 6 attenuates by 10^6, or 1,000,000. 6 * 2.5 = 15 magnitudes of attenuation.

Incidentally, do not confuse this with the units of filtration as employed by photographers. These filters are rated either by the fraction of light transmitted or the number of f/stops of attenuation.


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galaxyman
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5622006 - 01/14/13 12:57 AM

Glenn I don't think anyone is disagreeing with you about some of the DSO's you've mentioned. Just not sure if you are agreeing that at the same time many DSO's benefit on nights of the combo of good transparency and good seeing

I believe I have pointed this out already with some examples.

Here in Pennsylvania we have heavily varied seeing conditions. Even on nights of poor to fair seeing, there are times while observing an object that seeing will get very good for moments at a time while the object is in the eyepiece, showing a bit more detail within that object at those moments. This also happens when I'm viewing an Abell galaxy cluster when seeing the faintest members my scope (and eye) can see. Bad seeing smears the fleeting light, so of course better seeing at a particular dark site will enhance the view of a galaxy cluster by reeling in more members.

I wonder how many observers out there with large scopes realize what can be seen in many planetary nebula at very high power as I pointed out with Ngc 2392 and Ngc 1501? The same with some inner detail of some bright galaxies using very high power that good seeing helps in both cases.

I will reiterate that I will take the darkest skies over good seeing, but having both is optimum for my type of observing.




Karl
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GlennLeDrew
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Re: Impact of 'seeing' on DSOs new [Re: galaxyman]
      #5622291 - 01/14/13 09:04 AM

Karl,
I did acknowledge earlier here that those DSOs comprised of stars, are of small size and have higher surface brightness will be affected by poor seeing. My emphasis has been directed toward the lower surface brightness regime, as a way of demonstrating that *not all* DSOs are impacted. I never did say *none* are impacted.

I wish all deep-sky observers could view the Moon through a solar filter so that they could obtain a most immediate appreciation of the eye's atrocious resolving power at DSO levels of surface brightness. It's so bad that it would probably qualify as functional blindness. Imagine in your daily life wearing 'Coke bottle bottom' glasses which threw everything out of focus so that any point source or sharp edge were to be spread out to a degree--or the five(!) degrees the dimmest stuff is seen at. What a blurry view!


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blb
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5622383 - 01/14/13 10:08 AM

Quote:

Imagine in your daily life wearing 'Coke bottle bottom' glasses which threw everything out of focus so that any point source or sharp edge were to be spread out to a degree--or the five(!) degrees the dimmest stuff is seen at. What a blurry view!



We all do, It is the air above us that makes it hard to see. That's why we need nights of good seeing.


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galaxyman
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Re: Impact of 'seeing' on DSOs new [Re: GlennLeDrew]
      #5622442 - 01/14/13 10:41 AM

Agree with the low surface brightness objects which by far suffer under poor transparency, though the true test are the individual objects (galaxies and planetary nebula) themselves, and many do have a high surface brightness. Paying attention as I mentioned with moments of good seeing does bring out small details in a number of galaxies that under poor seeing does not. This is why I do say to have patience when viewing a particular object, and play with different magnifications. We would hear the same from other serious DSO observers like Barb Wilson and Alvin Huey of Faint Fuzzy fame.

Roger Clark's book Visual Astronomy of the Deep Sky talks about using more magnification on DSO's. Under a night sky with the great observers (and great scopes) of the CAS, anyone could see this demonstration of the amount of detail a number of objects can produce, using some wide ranges of magnifications including over 1000x. When using this kind of magnification, better seeing does come into play.

The original post question of if DSO's are effected by seeing. I think the answer and a better way to conclude this is that better seeing is helpful to those who are very serious about DSO observing.


Karl
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Chesmont Astronomical Society - www.chesmontastro.org
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Homemade (Parks Optics) 12.5" F/4.8 Dob
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russell23
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Re: Impact of 'seeing' on DSOs new [Re: galaxyman]
      #5631198 - 01/19/13 09:28 AM

The last time I had my scope out I had nice dark skies, but the upper air was so turbulent that I couldn't get sharp pinpoint focused stars at magnifications as low as 53x. That was the worst seeing I've seen and it made enjoying any kind of starfields, star clusters, or deep sky objects with stars that are an integral part of the aesthetics completely unenjoyable.

Dave


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Dennis_S253
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Re: Impact of 'seeing' on DSOs new [Re: russell23]
      #5633923 - 01/20/13 08:59 PM

I think they should break the reflector section down to three parts.
1. Up to 8".
2. 10" to 16".
3. Bigger than 16".
I get tired reading post about people with there 16", 22" or 30".
like I could see that in my 6". Even if the seeing was good or bad.


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Dennis_S253
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Re: Impact of 'seeing' on DSOs new [Re: Dennis_S253]
      #5633940 - 01/20/13 09:15 PM

oops, the DSO section also.

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