Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

General Astronomy >> General Observing and Astronomy

Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | (show all)
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: The Skinny on Seeing new [Re: Asbytec]
      #5825888 - 04/27/13 08:51 AM

Quote:


For those of us who are fortunate enough to have no thermal problems (in a MCT, no less), and when seeing is good snough to afford nearly perfect collimation, seeing becomes the dominate aberration. That's why I want to isolate it and understand it by itself.




I think it is worth understanding seeing but in general, "seeing" and clouds are very similar, they are just something that one has to live with. Coming to terms with the seeing and understanding ones particular location is good thing to do.

In my particular situation, what I understand about seeing is that if it's not good enough for a 10 inch, it's not good enough for a 6 inch either.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: The Skinny on Seeing new [Re: Jon Isaacs]
      #5826000 - 04/27/13 10:29 AM

Quote:

...it's not good enough for a 10 inch, it's not good enough for a 6 inch either.

Jon




I think that's in part what I am curious about. Again, my seeing is about 8/10 or better almost nightly. But, is that because seeing favors smaller apertures? Can seeing drag performance of a 10" down so much (assuming hi res observing) to where a 4" outperforms it? Or is it simply more pleasing in a smaller aperture? How does the Airy pattern behave at such times spoiling resolution and contrast? And why?

I know my own 6" is very "pleasing" in terms of nice Airy patterns and planetary detail (both contrast and resolution.) Once I even caught a glimpse of a very faint belt in Jupiter's NNTB below below the brownish northern polar hood. It was faint even in images. So, at least in good seeing, it seems to be offering up everything the small aperture can offer. Its more than pleasing, it's performing...and that is pleasing.

So, ya, I guess this thread is kind of exploring what it means to be pleasing and performing and under what conditions.

One thing mentioned above, that under some conditions a 5" and 10" /seem/ to offer up about 3" Airy pattern - on average. As seeing calms, both scopes improve but as Don mentioned the 5" improves slightly while the 10" really begins resolving closer to it's aperture. On the other half of average, when seeing is worse that 3", both scopes seem to be afflicted...and seeing is not good for either.

So, it would seem larger aperture is at least equal to a smaller scope (assuming good Strehl, etc) somewhat infrequently while at some points the larger aperture dominates when seeing improves.

We all know that, but why do some folks claim a smaller aperture trumps a larger aperture under some seeing conditions when it might, at best, seem to equal it infrequently. Is it a statement on optical quality or peak intensity (obstructed vs unobstructed debate)? Does it boil down to care and maintenance of induced aberrations such as focus, cooling, collimation, and what have you? (Its probably most of that, most of the time.)

Anyway, sorry for rambling...thinking out loud.


Post Extras: Print Post   Remind Me!   Notify Moderator  
JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: The Skinny on Seeing new [Re: Asbytec]
      #5826057 - 04/27/13 11:06 AM

I applaud the attempt of OP to elevate the discussion on atmospheric seeing and telescope performance above the level of anecdotal evidence.

I recently discovered that loads of relevant material (including Fried's paper) is available for free on the Internet. I started to look into those papers. These tend to considers the effect of atmospheric turbulence on perfect (diffraction-limited and aberration-free) optics (no luck, Jim). A key result that I reported on in a thread in the refractors forum is perhaps worth repeating here:

Taking into account the effects of seeing, a large aperture diffraction-limited telescope will deliver diffraction-limited snapshots only a very small fraction of the time. In other words, the time one has to wait for what is referred to as a “lucky image” increases exponentially with aperture.

Some numbers for poor seeing (at the lower end of typical seeing conditions described by optical turbulence characteristic size r0 in the range 0.1-0.2m), with the leftmost figures giving the aperture in inches, and the rightmost figures the fraction of snapshots that yield "lucky images" (diffraction-limited performance):

4 100%
8 100%
12 98%
16 46%
20 11%
24 2.1%
28 0.27%
32 0.026%

When people make remarks about "pleasing views" they probably refer to getting diffraction-limited views at the eyepiece 100% of the time. The key result to note is that achieving diffraction limited performance for larger telescope apertures resents a challenge that increases exponentially with the scope's light gathering area. That's a steep uphill battle...

Edited by JKoelman (04/27/13 04:05 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Cotts
Just Wondering
*****

Reged: 10/10/05

Loc: Toronto, Ontario
Re: The Skinny on Seeing new [Re: JKoelman]
      #5826372 - 04/27/13 01:47 PM

With regard to the size of the "cells" as they relate to aperture, I doubt very much one can simply say, "the cells are 4" tonight....." These are natural phenomena which will form a normal distribution around a mean size, which may be 4" but the "cells" may have a huge range in size...

Dave


Post Extras: Print Post   Remind Me!   Notify Moderator  
JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: The Skinny on Seeing new [Re: Cotts]
      #5826436 - 04/27/13 02:21 PM

Quote:

With regard to the size of the "cells" as they relate to aperture, I doubt very much one can simply say, "the cells are 4" tonight....." These are natural phenomena which will form a normal distribution around a mean size, which may be 4" but the "cells" may have a huge range in size...

Dave



Absolutely. That would be naive, wrong and misleading. One should not describe the Fried parameter r0 as a cell size. The Fried parameter characterizes the size distribution of the turbulent refractive index variations in the atmosphere. It is the "huge range in sizes" that renders possible the 'lucky images' for large aperture scopes.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: The Skinny on Seeing new [Re: JKoelman]
      #5826773 - 04/27/13 05:24 PM

Quote:

I applaud the attempt of OP to elevate the discussion on atmospheric seeing and telescope performance above the level of anecdotal evidence.

I recently discovered that loads of relevant material (including Fried's paper) is available for free on the Internet. I started to look into those papers. These tend to considers the effect of atmospheric turbulence on perfect (diffraction-limited and aberration-free) optics (no luck, Jim). A key result that I reported on in a thread in the refractors forum is perhaps worth repeating here:

Taking into account the effects of seeing, a large aperture diffraction-limited telescope will deliver diffraction-limited snapshots only a very small fraction of the time. In other words, the time one has to wait for what is referred to as a “lucky image” increases exponentially with aperture.

Some numbers for poor seeing (at the lower end of typical seeing conditions described by optical turbulence characteristic size r0 in the range 0.1-0.2m), with the leftmost figures giving the aperture in inches, and the rightmost figures the fraction of snapshots that yield "lucky images" (diffraction-limited performance):

4 100%
8 100%
12 98%
16 46%
20 11%
24 2.1%
28 0.27%
32 0.026%

When people make remarks about "pleasing views" they probably refer to getting diffraction-limited views at the eyepiece 100% of the time. The key result to note is that achieving diffraction limited performance for larger telescope apertures resents a challenge that increases exponentially with the scope's light gathering area. That's a steep uphill battle...




You posted these numbers in another thread. I hope others have inspected them.

I will simply note here that according to this modelling, "poor seeing" means that 98% of the time, a 12 inch scope will provide diffraction limited performance. Diffraction limited performance for a 12 inch scope is under 1/2 arc-seconds. I am quite sure many if not all would consider those views "pleasing."

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: The Skinny on Seeing new [Re: Jon Isaacs]
      #5827168 - 04/27/13 07:37 PM

Quote:

The key result to note is that achieving diffraction limited performance for larger telescope apertures resents a challenge that increases exponentially with the scope's light gathering area.




The point on seeing cells being non uniform is an important one. They are not, to my knowledge, nor is there any reason they should be. Still, seeing does seem to fluxuate around a given level on a given night. It's usually "pretty good" or "pretty poor" or some other description on any given night.

So, as Johannes said above, it does seem it is an uphill battle for larger apertures to be diffraction limited. But, is that really so important in terms of resolution and contrast? Silly question?

The Airy pattern does shrink with aperture, so while the view might not be "pleasing" in that it's not diffraction limited, it still might be good enough to render some finer details a smaller aperture is not capable of under those same conditions. Point being, there might still be a reason to have some aperture for higher resolution observing.

Thiking out loud, if my 6" is in 8/10 seeing conditions, I know first hand I am "pleased." Now, my own scope has a reasonably good Strehl, and thats probably not uncommon. So in 8/10 seeing with the Airy pattern pretty much intact, I can still pretty much enjoy diffraction limited performance - I'd think - and be "pleased." However, in this case, the 6" is still putting up pretty much the hi res and contrast it is capable of - most of the time - since the Airy pattern remains pretty much intact. So, such conditions are both "pleasing" and performing at that aperture.

Now, a 12" in those same conditions might be experiencing something near 5/10 seeing. Here the Airy pattern is near the breaking point and not as well defined. So, the hi res a 12" is capable of is washed out. However, the Airy pattern itself is half the size. So, it seems to me, some hi res work is still posible, at least higher than what might be seen in a smaller aperture.

So, while such seeing might not be pleasing in terms of good looking, steady Airy patterns resembling 8/10 Pickering, there might still be some relative performance happening - on average, and certainly in calmer "snapshots" - with smaller Airy patterns resembling 5/10 Pickering but offering some resolution (just shy of 12" aperture.) Advantage: aperture. Right?

Still thinking out loud, once speckeling begins in a larger aperture, though, I would think all bets are off. The seeing gets ugly and I would not hazard any quess about pleasing or perforring.

Edited by Asbytec (04/27/13 07:45 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: The Skinny on Seeing new [Re: Asbytec]
      #5827222 - 04/27/13 07:59 PM

Having said that, Vlad does some work on optical quality. He shows adjusted RMS and Strehl values for various aberrations. What I'd be interested in is understanding seeing induced RMS in aggregate (and random, on average.) Seeing is an aberration and it does increase RMS. Seems to me, once total RMS - Strehl plus seeing - rises much above 0.070 then a scope will fall below diffraction limited. This might be seen in the Airy pattern. Dunno.

But, still, if a very good scope with 0.020 RMS of error finds itself in something less that 0.050 RMS of seeing it should hold the diffraction limit. A lesser corrected scope might more easily rise over 0.070 RMS and loose it.

Then there is the question of peak intensity in obstructed scopes vs unobstructed scopes. Say an obstructed CAT has a peak intensity of 0.80 (really a diffraction limited peak intensity.) Any seeing at all should knock that scope below diffraction limited 80% (normalized) light in the central disc and more into the rings. In other words, loss of contrast and resolution.

Here, it seems a refractor will have the advantage since they are unobstructed and ofer peak intentisy equal to the Strehl. It would take plenty of seeing to knock a .98 Strehl APO down to less than 0.80. It seems lesser adverse seeing would drive a 0.98 Strehl obstructed scope with a obstructed peak intensity in the 80's percent to something less that 0.80 peak intensity with brighter rings.

So, are better corrected, unobstructed scopes less prone to seeing? I would think so, but that's just a working hypothesis. I have not yet run across anything that says so or says otherwise.

Edited by Asbytec (04/27/13 08:35 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: The Skinny on Seeing new [Re: Jon Isaacs]
      #5827505 - 04/27/13 11:28 PM

Quote:

according to this modelling, "poor seeing" means that 98% of the time, a 12 inch scope will provide diffraction limited performance. Diffraction limited performance for a 12 inch scope is under 1/2 arc-seconds. I am quite sure many if not all would consider those views "pleasing."




Jon -- there is two things to note here:

1) For apertures three or more times Fried's seeing parameter r0, the figures are extremely dependent on the precise value for r0. Drop r0 from 0.10 m to 0.07 m and a 12" aperture plummets in performance by delivering diffraction-limited views no more than 32% of the time.

2) Fried defines diffraction-limited “lucky exposures” as those with atmospherically-induced phase errors across the telescope aperture having an RMS error of <1 rad (roughly 1/6 wavelength). These errors come on top of the lens errors we tolerate for optics we refer to as "diffraction limited". There is little doubt in my mind that for a view to be referred to with the subjective term "pleasing" it will need to be a "lucky exposure" as defined by Fried. However, not all "lucky exposures" will be termed "pleasing" by all observers.


Post Extras: Print Post   Remind Me!   Notify Moderator  
JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: The Skinny on Seeing new [Re: Asbytec]
      #5827528 - 04/27/13 11:50 PM

Quote:

So, are better corrected, unobstructed scopes less prone to seeing? I would think so, but that's just a working hypothesis. I have not yet run across anything that says so or says otherwise.



If we take two diffraction-limited scopes with the same light gathering area, one obstructed and one unobstructed, then the unobstructed one will be less prone to seeing.

I have not come across simulations who yield this result, but it is perfectly clear from the math behind this all (the so-called Kolmogorov theory) that being 'less prone to seeing' translates into a more 'compact' light gathering area.

To simulate the effect of seeing on telescope optics, all one would need is software to compute spot diagrams that allows the introduction of phase errors over the aperture area. If someone can direct me to such software, I would be very interested to do these simulations.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: The Skinny on Seeing new [Re: Asbytec]
      #5827536 - 04/27/13 11:56 PM

Quote:


So, as Johannes said above, it does seem it is an uphill battle for larger apertures to be diffraction limited. But, is that really so important in terms of resolution and contrast? Silly question?

The Airy pattern does shrink with aperture, so while the view might not be "pleasing" in that it's not diffraction limited, it still might be good enough to render some finer details a smaller aperture is not capable of under those same conditions. Point being, there might still be a reason to have some aperture for higher resolution observing.




There is no doubt in my mind that there are good reasons "having some aperture for higher resolution observing." This is borne out at the eyepiece and Vlad's simulations show this.

One way to think about this is that while it gets more and more difficult for a larger and larger scopes to operate in a diffraction limited mode, that does not necessarily mean that it is particularly difficult for a larger scope meet or exceed the capabilities of a smaller aperture telescope. The comparisons need to be made at equivalent resolutions rather than this sliding scale that penalizes greater resolution and contrast transfer.

I think one has to question the validity of the diffraction limited = pleasing views assumption. One has to include the eye's resolution and response at the working magnification. At 250x, a 25 inch scope is providing a 2.5mm exit pupil. The image is bright and filled with contrast and detail but this is not sufficient magnification to for the eye to resolve the airy disk structure of a 25 inch scope so the fact that it is not "diffraction limited" does not necessarily mean that the view is not "pleasing." In terms of resolving the seeing, this would be comparable to operating a 6 inch scope at 60x, you cannot yet resolve the seeing unless it is rather poor.

The question that Rutten and Venrooij asked was not how often a scopes of differing apertures could reach diffraction limited performance but rather how the different apertures compared on an absolute scale as a function of the seeing conditions.

To my mind, that is the right question to ask.

---------------

Here's a question worth thinking about: What is your observing strategy? How do you do you approach the evening, particular in regards to seeing?

As I said previously, I am an opportunistic observer. If it is clear, I will be out there. I keep an eye on the weather conditions, I keep an eye on the Clear Sky Chart and if the seeing looks like it will be solid and I am feeling that there is time for an extended evening, then I will choose either my 10 inch Dob or one of my 12.5 inch Newtonians, set them out at least an hour prior to sunset and start the fans running.

Once night has fallen, I tend to just back and forth between the planets, double stars and deep sky objects, checking on the thermal status of the scope, catching a quick view of the planet/double and then moving on.

When everything seems optimal, that's when I will pay more attention close doubles and the planets. My expectation is steady, stable views, if seeing is an issue, I spend my time observing objects that do not require stable seeing.

Jon

Edited by Jon Isaacs (04/28/13 12:30 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
JKoelman
professor emeritus


Reged: 05/16/11

Loc: Bangalore, India
Re: The Skinny on Seeing new [Re: Jon Isaacs]
      #5827588 - 04/28/13 12:53 AM

Quote:

The question that Rutten and Venrooij asked was not how often a scopes of differing apertures could reach diffraction limited performance but rather how the different apertures compared on an absolute scale as a function of the seeing conditions.

To my mind, that is the right question to ask.




Correct. But the problem is that Rutten and Van Venrooij don't fully answer that question. They limit their discussion to time-averaged images without considering the occurrence of "lucky images".

I think what we all want to see is plots that tell us for given atmospheric conditions (i.e. given r0) what fraction of the time scopes operating at different apertures yield a resolution better than a specified value.

Edited by JKoelman (04/28/13 01:30 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: The Skinny on Seeing new [Re: JKoelman]
      #5827738 - 04/28/13 03:34 AM

Quote:

One should not describe the Fried parameter r0 as a cell size.




I'm not sure where you are getting this since Fried himself, and Goodman, and countless others do refer to r0 in this manner - as the diameter of an isoplanatic patch of the wavefront arriving at the entrance pupil. It can indeed be measured at a given site at a given time as an indication of seeing. The number of speckles in a star image will roughly correspond to the area of the pupil divided by the cell area - consistent with the number of isoplanatic patches present.

Although its definition is rooted in turbulence, its purpose is to provide a single parameter that characterizes the phase errors across a wavefront, and its impact on the image. To do something like that with a single parameter involves simplification, but the whole point is that the wavefront can be considered to comprise independent isoplanatic patches with diameter roughly given by r0.

As a side note - keep in mind that most of the literature on topics such as seeing and lucky imaging refer to actual imaging situations rather than visual observation. I consider the role of perception important, difficult, and subjective in discussions such as this one - and there will be a gap between models based on static images with linear detectors, vs. perceived resolution derived from a stream of fluctuating images captured by the eye and transmitted to the brain.

Frank


Post Extras: Print Post   Remind Me!   Notify Moderator  
EddWen
Pooh-Bah


Reged: 04/26/08

Re: The Skinny on Seeing [Re: freestar8n]
      #5827742 - 04/28/13 04:16 AM Attachment (14 downloads)

Folks, inherent in any discussion of 'seeing' are the dynamics of the atmosphere. Here is a picture of the atmosphere which illustrates the dynamics. I took this over central Arizona at 17,500' above mean sea level. You may say, yes, those are clouds and I won't be taking the scope out tonight.

Well, yes they are clouds, and being visible, they allow us to see what the atmosphere is doing. In this case, there is enough water vapor in this air so the water condenses when the temperature reaches the dew point. The visible droplets are not the cause of what you see, but they allow us to see what the air is doing.

You may note that the dew point is reached lower than the level of the plane, I estimate to be 10,000' AGL, the base of the cloud layer. There is an abundance of what I would call 'roiling' visible in the clouds. Below the base of the clouds none can be seen. But it is there. The atmosphere is not tranquil between the ground and the cloud base. So, if this air carried too little water vapor to condense, the roiling would not be visible, except by making star images jiggle in our eyepieces.

Note there is a scattered layer of stratus clouds, remnants of an earlier build-up that has dissipated. There is some very light cirrus activity and a few contrails. The whole air mass is moving eastward at varying speeds. Take what you see here, decrease the humidity only and all the dynamics are there, but not visible.

So, can you model such a thing and its effects on a 4"or 12"scope? I think not.

Edited by EddWen (04/28/13 04:24 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: The Skinny on Seeing new [Re: EddWen]
      #5827942 - 04/28/13 09:22 AM

Johannes, thank you for chiming in.

Jon, excellent point relating diffraction limited to "pleasing." Off the cuff, while we might not be fully resolving diffraction limited Airy patterns, the tremendous view we enjoy results from it...maybe. Well, maybe as long as the 'seeing pattern' remains very small compared to the aberration free FWHM and to the eye? Hmmmm...

Fascinating discussion, thank all of you for chiming it. Lot to absorb.

I am interested in the amount of time we spend observing "lucky images" as well as the average image. It seems both, under moderate to excellent seeing, favor aperture to some degree. Contrast that with the addage seeing favors "pleasing" smaller apertures...and well, my snapse just shorted out.

Quote:

It (Fried parameter) can indeed be measured at a given site at a given time as an indication of seeing.




Frank, yes, those were my thoughts in the wee hours last night. Sidgwick and Vlad both discuss this time averaged R0, Sidgwick mentions some evidence and studies that determine a rough diameter of the, "isoplanatic patch of the wavefront arriving at the entrance pupil."

More comments to digest...again, thank you for the discussion.

Edit: Edd, beautiful description of the sky. I guess the theory tends to break down the atmosphere into boundary layer and higher level stuff. The boundary layer, for the most part, is the stuff closer to the ground. The idea is, they say, seeing tends to be best when the higher level atmosphere is steady. I think it has something to do with the larger ray angles from higher altitudes. Gotta add your imput to the discussion, though. Thanks.

Edited by Asbytec (04/28/13 09:53 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: The Skinny on Seeing new [Re: Asbytec]
      #5828111 - 04/28/13 11:13 AM

It has often been pointed out that the seeing is best for us not when the air is completely still, but when the air movement is completely laminar. Laminar air flow cannot occur when the air movement is too fast, so we will not have good seeing when directly under the jet stream.
But completely stagnant air never really occurs. There is always movement at some level in the atmosphere, whether high or low.

We have a circumstance that occurs here in Los Angeles that may occur elsewhere, too:
The basin, which is ringed by mountains, gets warm, developing a high pressure "bubble" sitting on the city. A few thousand feet up, there is a transition to cooler air, and the cooler air is flowing over the top of the hot air bubble below it. Very little mixing is occurring at the transition. The air below is not moving (and may be a little smoggy) and the air above is moving smoothly sideways relative to the bubble.
And seeing is magnificent, with virtually no visible air movement.

And then we get the days when transparency is magnificent and both the dust and water vapor content of the air drops to nearly zero. I can see islands 30 miles away as if they were next door.
But, the wind movement is brisk and the air is filled with roiling, moving, streams that go a hundred different directions and rise in places and fall in others, and seeing is horrendously bad. As I think I mentioned, I have seen seeing that makes Pickering 1 look clean and sharp. I have tried to look at the Moon on nights like that and the image was never ever in focus. I could play with the focuser and the instant I thought it was getting sharper, it blurred again. Charitably, I would say the best viewing on such a night is with the naked eye.

Edd's comments are definitely apropos. You can have clear air, but it in no way means the atmosphere is not in motion.

About once a decade, though, I see a night where the seeing is as good as my first scenario, and the transparency as good as the second. I write those down in my notes and I still remember them 30+ years later.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: The Skinny on Seeing new [Re: Starman1]
      #5828384 - 04/28/13 01:32 PM

Edd, thinking more about your post above, those clouds do seem to outline huge pockets of air rising and boiling overhead. I think what is important, however, is the coherence of those "cells." The clouds are defintely warmer than the surrounding air, but they are basically coherent in temperature (density and refractive index) over a ground based aperture. So, even though they might be powerful enough to draw huge amounts of wet air aloft, they are homogenous to some degree. I am not sure where to go with that, just throwing that out there for discussion.

Don, excellent point on laminar flow. There was an aircraft based telescope that must have been looking through 600mph winds. It's not windspeed that matters, but coherence or homogenity of temperature, density, and refractive index. But, that air flow can be fast because it is probably not dirupted as much as airflow near the ground or higher airflow disrupted by mountain ranges. Your example is interesting, I can see the warm air bubble being very stable in terms of seeing. Sure, as the air moves laterally, if it has any proturbance at all, that would give rise to smaller(?) and less coherent mini air masses passing rapidly over your aperture. In other words, "poor seeing."

The airflow over head at my location does have some tilt (I can see that happening real time) and some rapidly fluxuations in density, but generally is fairly laminar...it seems (and is part fo my question) at least on scales "favoring" 6" apertures. Living at the base of some 5000' hills on the leward side of the Pacific's laminar flow does disrupt seeing a bit and reduce transparency a bit. Frequently, however, seeing is very good...at 6" aperture. It is both pleasing and performing at that aperture. How would a larger aperture perform under those same conditions? How? Why? And how imortant is optical quality and peak intensity of the spurious disc at various levels of seeing?

I am still getting to know the theory and the modeling to somehow quantify and put those questions and answers into perspective. It's something that intrigued me since I first say nearly perfect intact Airy patterns and beautiful contrast on Jupier in a 6" obstructed scope.

Sigh, man, this is a complex topic.

Edited by Asbytec (04/28/13 01:55 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: The Skinny on Seeing new [Re: Asbytec]
      #5828553 - 04/28/13 02:57 PM

You might like this weather site (relocated to your site) for seeing and clouds:
http://www.meteoblue.com/en_US/weather/charts/seeing/chula-vista-campground_u...
And I do recommend reading the 3 articles listed Here:
http://www.cloudynights.com/byauthor.php?author_id=594
about seeing and using weather maps for predictions thereof.

But, if you want to know what I've seen over the years, I'm selling my 4" refractor and 5" Mak and replacing them both with a 10" dob as my "grab'n'go" for the house.
Even planets, Moon, and double stars are better in a 10" newt than a smaller scope--regardless of seeing.


Post Extras: Print Post   Remind Me!   Notify Moderator  
acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Re: The Skinny on Seeing new [Re: Starman1]
      #5828759 - 04/28/13 05:03 PM

Don: That Meteoblue.com weather web site is very interesting! Reading it for seeing planets, the smaller the Seeing Arc Sec number the better, right?
The Mt. Pinos, CA location Arc Sec number tonight (Sunday, 4/28/13) about .35, sounds good, but jet stream number on the chart not favorable.
Tues night, Arc Sec number drops to .29 and jet stream number is in the green, so maybe a better night for planets? However, some clouds that night. Your thoughts?
Andy


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: The Skinny on Seeing new [Re: acochran]
      #5828828 - 04/28/13 05:55 PM

Getting everything to happen at the same time is rare.
You want seeing to be green and no clouds at any level (though 1-5% is minor and usually implies high cirrus or clouds on/near the horizon).


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | (show all)


Extra information
1 registered and 9 anonymous users are browsing this forum.

Moderator:  cildarith, panhard, tecmage 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 4479

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics