[ap-ug] Re: 'seeing cells'


Aug 29, 1999

 


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

#311 Aug 29, 1999

See the entries on Kolmogorov Theory and r0 at

> op.ph.ic.ac.uk/ao/glossary.html

>There is also a good text & diagrams in S & T, June 1994, p21. The

>authors, Wild & Fugate, use the term "seeing cell" in quotes, so they seem

>to be using it as a model.







Thanks Peter. I'll read the web URL, the Sky & Tel reference. That should

get me started. -john



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

#314 Aug 29, 1999

Peter,



Thanks for the references. I can accept that the concept might be a useful

model, but find it hard to believe that the cells actually exist. And if

they did exist, it would seem that different layers in the atmosphere would

behave differently and the odds of only looking though single cells of one

size would seem very low.



Clear skies, Alan

>[SNIP]

>But if cells are a working model, they are a very successful one. Adaptive

>optics uses the model extensively. A telescope of small aperture will show

>little or no seeing effects, increasing the aperture will cause increased

>seeing effects, and there is a threshold diameter where seeing effects

>increase sharply. Whether you take the term 'cell' literally or not, the

>effects of turbulence over an aperture are dependent on the diameter of

>that aperture, and the effects mimic a cell closely enough that I think

>their actuality is easily possible under some conditions. I wish I had an

>atmospheric scientist to ask. [SNIP]



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

#316 Aug 29, 1999

If seeing affects larger scopes, a different explanation needs to be used.

The idea of discrete cells of a given size existing flies in the face of

common sense. In all the different layers of the atmosphere turbulence

exists in an infinite range of "sizes," from the very large to the

microscopic. To think that "cells" of the same size line up in each layer to

form a long 4" (6", etc.) column of steady air extending all the way from

the upper reaches of our atmosphere all the way down to one's telescope

objective--it doesn't pass the smell test. Todd--what do you think?



Regards,

Paul Gustafson

> Thanks for the references. I can accept that the concept might

> be a useful

> model, but find it hard to believe that the cells actually exist. And if

> they did exist, it would seem that different layers in the

> atmosphere would

> behave differently and the odds of only looking though single cells of one

> size would seem very low.



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

#328 Aug 30, 1999

In a message dated 8/29/99 9:21:14 PM Central Daylight Time,

Sue_and_Alan_French@... writes:



< Thanks for the references. I can accept that the concept might be a useful

model, but find it hard to believe that the cells actually exist. And if

they did exist, it would seem that different layers in the atmosphere would

behave differently and the odds of only looking though single cells of one

size would seem very low.



Paul Gustafson writes:

If seeing affects larger scopes, a different explanation needs to be used.

The idea of discrete cells of a given size existing flies in the face of

common sense. In all the different layers of the atmosphere turbulence

exists in an infinite range of "sizes," from the very large to the

microscopic. To think that "cells" of the same size line up in each layer to

form a long 4" (6", etc.) column of steady air extending all the way from

the upper reaches of our atmosphere all the way down to one's telescope

objective--it doesn't pass the smell test. Todd--what do you think? >>



Cells affect scopes in three size bands. Cells which are significantly larger

than the aperture, cells which are about the same size as the aperture, and

cells which are significantly smaller thant he aperture.



In any slice through the atmosphere, ther are bound to be plenty of cells.

These cells are expected to have a Gausian (Normal) distribution at any

layer in the atmosphere, Since there are many layers in the atmosphere,

and layers farther away are less dense, the disruption



When a scope becomes really large (ala Keck) a difraction limited images

never forms, what forms is a pile of blobs where each blob is the focused

airy disk from one section of the objective,and another blob is from a

different

section of the objective. All the blobs added together form an atmosphere

limited image. (Texerauex)By examining many of these blobs, information

On-the-order-of difraction limited can be extracted from the blobs. This

is known as speckle interferometry. And has been used to image the

surface of Betleguess (sp).



As the scope becomes smaller, fewer and fewer cells exist between the

vaccuum of spaceand the atmosphere of the ground. Thereby, fewer blobs

form, and the imagemay have fleeting moments where the blobs all line

up and form a good image (but not difraction limited).



When the aperture becomes similar in size to the smallest cell, the blob

changes form and instead of an image formed by piles of blobs, we have

an image formed in a difraction limmited sense that moves to and fro

with the cells. In essence,instead of seeing an entire wavefront (distorted

by a cell(s)) we have a wave front tipped or tilted wrt the undisturbed

wavefront. The aperture in question does not have the resolution required

to resolve the cell. First order adaptive optics (tilt/tip) can steady the

images

formed by cells of this size.



Since the entiretly of the follow a Gausian distribution, but only those cells

smaller than the aperture have real disrupting effects, the aperture filters

out

those cells significantly and they do not impact image quality but continue

to effect image stability. When the cells are really small, the seeing is so

bad

we don't set up scopes anyway. So the aperture forms a spatial filter, and

the cells form a gausian filter, what image forms is based on the complex

multiplication of the two filters (low pass aperturefilter,mid-pass gausian

filter).

The only times we set up telescopes is when the aperture filter cutoff

(spatial)

is on the same order as the Gausian filter mid-point, and we (thereby) observe

a skewed cell sized distribution. This distribution is expected to peak near

the resolution of the telescope, and thereby the size of the cells "seen" are

akin to the aperture of the scope itself--regardless of the sizes of the cells

in the atmosphere.



Therefore: Cells hurt bigger instruments more than smaller ones (d'oh).

Cells smaller than the aperture hurt the image quality, while

Cells larger than the aperture hurt the image stability

Cells which are farther ways (towards zenith) cause equivalent

image damage as cells nearer, because distance

counteracts

atmospheric density (first order)

Cells which are farther away are more easily seen because they

lie close to focus (within the focal depth) of the

image.

The two filters (cells and aperture) combine to give a

(decidely)

non gausian distribution to the cells made visible.



Mitch







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

#331 Aug 30, 1999

Hi Alan and Paul,



I too didn't like the "cell" concept, but I think the fellow who mentioned

it the other day is right. From what little I read tonight, it's used to

simplify a problem that's horrendously complicated to compute. I think the

model assumes that the cells lie in a plane parallel to the earth's surface.

That plane looks something like a piece of chicken wire (i.e. the cells are

akin to the holes in the wire). I don't want to say much more about that

because I've only read one book on the subject for 20 min tonight. It'll

take me more time than that to digest the basics. I don't think this is a

hopeless endeavour.



I think we'd all agree on what causes bad seeing. Different parts of the

atmosphere move at different rates and the airflow can become turbulent. The

result is the formation of atmospheric density gradients. The change in

density causes that parcel of air's index of refraction to change. The

latter causes the otherwise undistorted wavefront from a celestial object to

undergo localized distortions. Specifically, because the velocity of an

electromagnetic wave depends on the medium's index of refraction (i.e. n),

the phase of the wave now depends on the local values n. Thus, the

wavefront is no longer planar when it strikes our scope's objective or

mirror.



I think that the important parameters are the time and length scales of the

density (or index of refraction) fluctuations. If we make reasonable guesses

about the speed of the convected and tossled air molecules, we can easily

estimate the relevant time scales. From that, we could compute the power

spectrum, S(f) (which essentially is a frequency dependent variance), of the

number of cells in the column of air above a given size telescope:



S(f) = S(0)/(1 + (f/fc)^2))



where S(0) is proportional to the variance of the number of cells in the

column, and fc is proportional to (1/Ti + 1/Tj). Ti and Tj are the mean time

a given size cell occupies the column of air atop the scope and the time the

cell takes to find and enter that column, respectively. This equation was

derived by Stephan Machlup (1954) for a system that randomly fluctuates

between two states (e.g. in our case these states are: a given cell is in

the column and the cell is not in the column). At higher frequencies (i.e.

faster time scales), the power spectrum of this quantity scales as 1/f^2

(i.e the spectrum decreases). The spectrum is not white because the

fluctuations in the number of cells in the column cannot change faster than

the fastest transport process. From what I've read, the time scale can vary

from millisec to even longer times (these numbers make sense based on simple

calculations).



I don't yet know how to estimate the distribution of length scales (i.e. how

large or small the patches of turbulence are). I don't know, but perhaps the

values of atmospheric flow rates and the Reynold's number for a gas at the

relevant pressures and temperatures would give a first order guesstimate. I

think the article in Sky&Tel suggested that the cells can range from sub 1

cm to about 20 cm. I assume these estimates are based on measurements.



If the cells range in sizes that overlap the diameters of the objectives of

amateur telescopes (they apparently do), then what's important is a typical

histogram of cell sizes (I haven't found a plot of that yet). For example,

if the conditions were such that the cells were all identical w/20 cm

diameters, then I think small scopes would suffer less than larger scopes

because the wavefront is "undisturbed" in 20 cm wide chunks (I'm ignoring

potential complications due to bizarre and improbable times scales for the

movement of cells into and out of the column,; i.e. the cells jitter in a

regular manner above the column). However, if the cells were predominately

1mm, then I think all scopes would take a hit.



If anything above resembles nonsense, I'm sorry. I'm trying to learn this

subject from scratch and have a long way to go.



At the moment, I'm not interested in the mechanics of how people compute the

onset of turbulence. I just want to know the basic physics that gives rise

to the phenomenon and then see if the measurements of the critical

parameters account for what we see at the EPs of different size scopes side

by side.



To summarize my thoughts above, I think the important paramters include the

turbulence "cell" size distributions (i.e. the histogram of cell length

scales) and the dynamics of "cell" movement into and across the column (for

simplicity, I'm ignoring vertical cell movement and assume that there is

only one plane of cells). There are many other complications. For example,

cell movement can be caused by the real flow of a fixed size cell or the

apparent movement of a "cell" caused by the time-dependent variations in a

given cell's size (tubulence is really dynamic).



Mitch, I've got to carefully read through your post tonight. I only

skim-read it on screen. It looks very interesting and full of goodies! I

think you stated that the distribution of cell sizes is Gaussian. You're

probably right (many distributions are Gaussian or darn close to it), but

I'm not certain how to prove that for turbulent flow. Also, the convolution

of the cell size distribution and the "spatial filter" (the scope's clear

aperture) is a nice way to think about the problem.





Best,

John



Alan wrote: >Thanks for the references. I can accept that the concept might be a useful

> model, but find it hard to believe that the cells actually exist. And if

> they did exist, it would seem that different layers in the atmosphere

would > behave differently and the odds of only looking though single cells of one

> size would seem very low.



>Paul Gustafson writes:

>If seeing affects larger scopes, a different explanation needs to be used.

>The idea of discrete cells of a given size existing flies in the face of

>common sense. In all the different layers of the atmosphere turbulence

>exists in an infinite range of "sizes," from the very large to the

>microscopic. To think that "cells" of the same size line up in each layer

to >form a long 4" (6", etc.) column of steady air extending all the way from

>the upper reaches of our atmosphere all the way down to one's telescope

>objective--it doesn't pass the smell test. Todd--what do you think?







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

#345 Sep 2, 1999

The following is some info on seeing from Dan McKenna, who posts on

s.a.a. I made a tongue in cheek comment, and he wrote a nice post! His

e-mail is Dan McKenna mckenna@...> if you would like to

individually e-mail him. I will bundle any responses. BTW Suiter has

some info on Zernike polynomials. I did very minimal editing.



For those who aren't interested in this topic, I'm sorry, but you can

always change yourself to a Web interface if you don't want to read the

group in e-mail. Besides, this is on topic, since the group name is

Astro-physics, right :-)



*****

Seeing is caused by the motion of refractive index structure. For the

most part seeing is caused by temperature gradients. Because the

refractive index is a function of air density, temperature gradients at

the lowest altitude have the greatest effect. Most of the seeing is

near the telescope and is caused by radiation cooling of the ground and

telescope structure at night. Twice a day about 45 minutes before sunset

and after sunrise the near ground temperature gradient disappears and

there is a moment (1 to 20 minutes) of good boundary seeing known as the

neutral optical event. At night as you make the transition to the free

atmosphere the seeing becomes detached from the ground and forms in

intermittent bursts in thin sheets. The interaction of the topography

and the stable night time air and the surface inversion leads to gravity

waves. These waves (not gravitational) can grow to hundreds of meters in

thickness and lead to periodic variations in the seeing. For a flat

extended surface the nocturnal boundary layer forms after the neutral

optical event and grows in height as a function of about the square root

of time. For a mountain top this changes to a layer that varies in

height as a function from the upwind edge, so if you are on a small peak

and if the wind is right the layer can be thin enough to build a

telescope above this layer.



In the upper atmosphere the seeing is formed in thin sheets near

inversions. Balloon data from thermalsondes display very fine layers on

the order of a few meters thick. At one time there can be many tens of

layers present. If you look at the layers that cause most of the upper

air seeing you will find only two to four layers in this group. These

layers are in wind shear zones near inversions.



The effect of seeing on a telescope is a function of D/ ro where

D is the telescope optical diameter and

r0 is the so called freid parameter.

In the visible (0.5 microns) r0 is 10 cm for 1 arc second seeing , 5 cm

for 2 arc seconds etc.



It is this r0 where the concept of cells come from. These are not

physical cells but the results of propagation over the entire optically

active path. If we start with a point source at infinity outside the

atmosphere and watch the wave front distort in space and time we will

notice that as the distortion grows by traveling deeper in to the

atmosphere the correlation between the wave front measured at two point

perpendicular to the propagation decreases. Also, at some distance the

two measurements of wave front phase becomes unrelated. This is the idea

behind r0 and the cell size.



Along with the concept of ro there is the outer and inner scale of the

seeing. Someone once wrote "big whorls make smaller whorls that feed on

their velocity, little whorls make smaller swirls and so on to

viscosity." This is the production of turbulence in the inertial sub

range. The outer scale is the trigger and it cascades to the point

where it dissipates due to air viscosity and that is known as the inner

scale.



For a large outer scale i.e. several km the idea of a infinite outer

scale was used to describe seeing early on but now the smaller outer

scale is being used to compute real world effects.



The effect of seeing on optical performance can be viewed as the

introduction of time varying aberrations. One way of thinking of these

is tip, tilt, defocus, astigmatism, coma, etc. In adaptive optics we

like to talk about Zernikes. Zernike polynomials are a way of fitting

surfaces . The relationship between Zernike number and classical

aberrations is:



2) tip

3) tilt

4) defocus

5) 3rd order astig X

6) 3rd order astig Y

7) 3rd order coma X

8) 3rd order coma Y



and up to however high you want to go although above Z 13 (spherical

astig Y) people don't usually name them, they just use the numbers.



The effect of seeing on the image quality is a function of D/ro. To

understand this imagine a wiggly line that is a cut through a distorted

wave front. As you zoom in on this line there will a time when you can

describe the distortion as a line segment or tilt. As you include more

of the wiggle it becomes a curve and now its a tilt plus a defocus and

so on.



So, if we have a fixed telescope diameter and have very weak seeing the

image is close to diffraction limited and moves around in tip and tilt.

There are components of defocus and higher Zernikes but it is mostly tip

and tilt (this is in the D/r0 range of up to about 2). For one arc

second seeing this is a telescope of 20 cm in diameter.



If the seeing is really bad then tip tilt almost vanishes and the image

is composed of high order Zernikes causing the image to break up into

speckles. Each speckle has the information of the diffraction limited

performance of the telescope but they are all dancing around so you must

freeze the image with a high speed detector and reconstruct the image by

using image processing.



The outer scale comes into play when the telescope become about 10

percent (or more) larger than the outer scale. The effect is to remove

the lower Zernike power and move it in to the higher order terms.



Dan

*****







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

#346 Sep 2, 1999

In a message dated 09/02/1999 6:30:47 PM Pacific Daylight Time,

dawong@... writes:

> The effect of seeing on a telescope is a function of D/ ro where

> D is the telescope optical diameter and

> r0 is the so called freid parameter.

> In the visible (0.5 microns) r0 is 10 cm for 1 arc second seeing , 5 cm

> for 2 arc seconds etc.



Uh huh, right. Now everything is much clearer now. Whew! I was almost afraid

I would be lost with the above explaination. Now lets see, if I can just get

it Polar aligned........... ;-) Jeff Vickers



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

#347 Sep 2, 1999

-----Original Message-----

From: Derek Wong dawong@...>

To: ap-ug@egroups.com ap-ug@egroups.com>

Date: Thursday, September 02, 1999 6:29 PM

Subject: [ap-ug] Re: 'seeing cells'



>The following is some info on seeing from Dan McKenna, who posts on

>s.a.a. I made a tongue in cheek comment, and he wrote a nice post! His

>e-mail is Dan McKenna mckenna@...> if you would like to

>individually e-mail him. I will bundle any responses. BTW Suiter has

>some info on Zernike polynomials. I did very minimal editing.

>

>For those who aren't interested in this topic, I'm sorry, but you can

>always change yourself to a Web interface if you don't want to read the

>group in e-mail. Besides, this is on topic, since the group name is

>Astro-physics, right :-)

>

>*****

>Seeing is caused by the motion of refractive index structure. For the

>most part seeing is caused by temperature gradients. Because the

>refractive index is a function of air density, temperature gradients at

>the lowest altitude have the greatest effect. Most of the seeing is

>near the telescope and is caused by radiation cooling of the ground and

>telescope structure at night. Twice a day about 45 minutes before sunset

>and after sunrise the near ground temperature gradient disappears and

>there is a moment (1 to 20 minutes) of good boundary seeing known as the

>neutral optical event. At night as you make the transition to the free

>atmosphere the seeing becomes detached from the ground and forms in

>intermittent bursts in thin sheets. The interaction of the topography

>and the stable night time air and the surface inversion leads to gravity

>waves. These waves (not gravitational) can grow to hundreds of meters in

>thickness and lead to periodic variations in the seeing. For a flat

>extended surface the nocturnal boundary layer forms after the neutral

>optical event and grows in height as a function of about the square root

>of time. For a mountain top this changes to a layer that varies in

>height as a function from the upwind edge, so if you are on a small peak

>and if the wind is right the layer can be thin enough to build a

>telescope above this layer.

>

>In the upper atmosphere the seeing is formed in thin sheets near

>inversions. Balloon data from thermalsondes display very fine layers on

>the order of a few meters thick. At one time there can be many tens of

>layers present. If you look at the layers that cause most of the upper

>air seeing you will find only two to four layers in this group. These

>layers are in wind shear zones near inversions.

>

>The effect of seeing on a telescope is a function of D/ ro where

> D is the telescope optical diameter and

> r0 is the so called freid parameter.

>In the visible (0.5 microns) r0 is 10 cm for 1 arc second seeing , 5 cm

>for 2 arc seconds etc.

>

>It is this r0 where the concept of cells come from. These are not

>physical cells but the results of propagation over the entire optically

>active path. If we start with a point source at infinity outside the

>atmosphere and watch the wave front distort in space and time we will

>notice that as the distortion grows by traveling deeper in to the

>atmosphere the correlation between the wave front measured at two point

>perpendicular to the propagation decreases. Also, at some distance the

>two measurements of wave front phase becomes unrelated. This is the idea

>behind r0 and the cell size.

>

>Along with the concept of ro there is the outer and inner scale of the

>seeing. Someone once wrote "big whorls make smaller whorls that feed on

>their velocity, little whorls make smaller swirls and so on to

>viscosity." This is the production of turbulence in the inertial sub

>range. The outer scale is the trigger and it cascades to the point

>where it dissipates due to air viscosity and that is known as the inner

>scale.

>

>For a large outer scale i.e. several km the idea of a infinite outer

>scale was used to describe seeing early on but now the smaller outer

>scale is being used to compute real world effects.

>

>The effect of seeing on optical performance can be viewed as the

>introduction of time varying aberrations. One way of thinking of these

>is tip, tilt, defocus, astigmatism, coma, etc. In adaptive optics we

>like to talk about Zernikes. Zernike polynomials are a way of fitting

>surfaces . The relationship between Zernike number and classical

>aberrations is:

>

> 2) tip

> 3) tilt

> 4) defocus

> 5) 3rd order astig X

> 6) 3rd order astig Y

> 7) 3rd order coma X

> 8) 3rd order coma Y

>

>and up to however high you want to go although above Z 13 (spherical

>astig Y) people don't usually name them, they just use the numbers.

>

>The effect of seeing on the image quality is a function of D/ro. To

>understand this imagine a wiggly line that is a cut through a distorted

>wave front. As you zoom in on this line there will a time when you can

>describe the distortion as a line segment or tilt. As you include more

>of the wiggle it becomes a curve and now its a tilt plus a defocus and

>so on.

>

>So, if we have a fixed telescope diameter and have very weak seeing the

>image is close to diffraction limited and moves around in tip and tilt.

>There are components of defocus and higher Zernikes but it is mostly tip

>and tilt (this is in the D/r0 range of up to about 2). For one arc

>second seeing this is a telescope of 20 cm in diameter.

>

>If the seeing is really bad then tip tilt almost vanishes and the image

>is composed of high order Zernikes causing the image to break up into

>speckles. Each speckle has the information of the diffraction limited

>performance of the telescope but they are all dancing around so you must

>freeze the image with a high speed detector and reconstruct the image by

>using image processing.

>

>The outer scale comes into play when the telescope become about 10

>percent (or more) larger than the outer scale. The effect is to remove

>the lower Zernike power and move it in to the higher order terms.

>

>Dan

>*****

>

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

>MyPoints-Free Rewards When You're Online.

>Start with up to 150 Points for joining!

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>

>

>







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

#349 Sep 2, 1999

In a message dated 09/02/1999 7:24:54 PM Pacific Daylight Time,

rnapo@... writes:

> I see it as a failing in his ability to translate his mathamatical

> models into English, at least if his intent it to have a diaglog with

> a non math oriented person.

>

> Rich



Rich, face it, someone with his intellect trying to explain his concept to us

is like us talking to a tree. I am not sure if he can't help him self or we

can't help our selves. I am sure we have more fun though. ;-) Jeff



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

#350 Sep 2, 1999

> I see it as a failing in his ability to translate his mathamatical

>> models into English, at least if his intent it to have a diaglog with

>> a non math oriented person.

>>

>> Rich

>

>Rich, face it, someone with his intellect trying to explain his concept to

us >is like us talking to a tree. I am not sure if he can't help him self or we

>can't help our selves. I am sure we have more fun though. ;-) Jeff





I don't find it fun. I don't assume that because someone is good in

math they have a better intellect. I don't assume they are right (or

wrong).

You can use math to describe something very precisely but if your premis

is wrong you can be wrong very precisely . We (he and I) are having a

communication problem.



Rich

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

>MyPoints-Free Rewards When You're Online.

>Start with up to 150 Points for joining!

>clickhere.egroups.com/click/854

>

>

>eGroups.com home: www.egroups.com/group/ap-ug

>www.egroups.com - Simplifying group communications

>

>

>

>



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

#353 Sep 2, 1999

>> I see it as a failing in his ability to translate his mathamatical

>>> models into English, at least if his intent it to have a diaglog with

>>> a non math oriented person.

>>>

>>> Rich

>>

>>Rich, face it, someone with his intellect trying to explain his concept to

>us

>>is like us talking to a tree. I am not sure if he can't help him self or we

>>can't help our selves. I am sure we have more fun though. ;-) Jeff

>

>

>I don't find it fun. I don't assume that because someone is good in

>math they have a better intellect. I don't assume they are right (or

>wrong).

>You can use math to describe something very precisely but if your premis

>is wrong you can be wrong very precisely . We (he and I) are having a

>communication problem.

>

>Rich





Well, Rich, I have to disagree on this one. So often people post with

rudimentary, nonexistent, or obviously incorrect scientific or mathematical

positions. Whenever that happens, people love to indulge in name-calling or

even mocking. But when someone posts with information that demands a

stretch by the amateur cognescenti, and requires some actual head

scratching and parsing and learning to understand, I think a bit more

respect is in order, less we all just keep ourselves dumbed down to the

lowest common denominator.



Though I couldn't follow all the math, I could ride the concepts, and I

learned from how the subject was structured. If I wanted to go deeper, I'd

ask some specific questions about the math - or look it up myself. But just

because I wasn't familiar with the math, doesn't mean the level of math was

inappropriate. If you have some specific questions, ask them! But I don't

see how you can justify trashing someone just because you feel he's talking

over your particular head, and want to bother with stretching your mind a

little to understand what he's saying. After all, a lot of newbies must

feel towards your level of explanations what you're feeling towards this

mathematical model. But you'd hardly accept gripes from them to dumb

yourself down to their level. You'd tell them to rise to yours, or ask

questions!



And as for being on topic for the group, understanding "seeing" is one of

the most frustrating things about this hobby, over which many hours, and

lots of money, is risked to try to get past it. And I still haven't heard

an explanation that settles the debate, or tells me which scope is the best

approach.



John



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

#354 Sep 2, 1999

>> I see it as a failing in his ability to translate his mathamatical

>>>> models into English, at least if his intent it to have a diaglog with

>>>> a non math oriented person.

>>>>

>>>> Rich

>>>

>>>Rich, face it, someone with his intellect trying to explain his concept

to >>us

>>>is like us talking to a tree. I am not sure if he can't help him self or

we >>>can't help our selves. I am sure we have more fun though. ;-) Jeff

>>

>>

>>I don't find it fun. I don't assume that because someone is good in

>>math they have a better intellect. I don't assume they are right (or

>>wrong).

>>You can use math to describe something very precisely but if your premis

>>is wrong you can be wrong very precisely . We (he and I) are having a

>>communication problem.

>>

>>Rich

>

>

>Well, Rich, I have to disagree on this one. So often people post with

>rudimentary, nonexistent, or obviously incorrect scientific or mathematical

>positions. Whenever that happens, people love to indulge in name-calling or

>even mocking. But when someone posts with information that demands a

>stretch by the amateur cognescenti, and requires some actual head

>scratching and parsing and learning to understand, I think a bit more

>respect is in order, less we all just keep ourselves dumbed down to the

>lowest common denominator.

>

>Though I couldn't follow all the math, I could ride the concepts, and I

>learned from how the subject was structured. If I wanted to go deeper, I'd

>ask some specific questions about the math - or look it up myself.



I did.

> But just

>because I wasn't familiar with the math, doesn't mean the level of math was

>inappropriate. If you have some specific questions, ask them!



I did.

>But I don't

>see how you can justify trashing someone



What exactly do you think I said that was "trashing" someone?



>just because you feel he's talking

>over your particular head, and want to bother with stretching your mind a

>little to understand what he's saying. After all, a lot of newbies must

>feel towards your level of explanations what you're feeling towards this

>mathematical model. But you'd hardly accept gripes from them to dumb

>yourself down to their level. You'd tell them to rise to yours, or ask

>questions!



No, I do not.

>

>And as for being on topic for the group, understanding "seeing" is one of

>the most frustrating things about this hobby, over which many hours, and

>lots of money, is risked to try to get past it. And I still haven't heard

>an explanation that settles the debate, or tells me which scope is the best

>approach.



I don't expect one explanation since seeing comes in many different

forms. My quesition was about poor seeing and how it affects long

vs short focal ratio telesocpes.



Rich

>

>John

>

>

>

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>







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

#356 Sep 3, 1999

Rich wrote:

>What exactly do you think I said that was "trashing" someone?



The following is what I call trashing him - you previously wrote:

>> I see it as a failing in his ability to translate his mathamatical

>> models into English, at least if his intent it to have a dialog with

>> a non math oriented person.



The reason I see this as trashing him is that he spent quite a bit of

effort translating his mathematical models. Sure, the math was referenced,

but I found his explanations painstaking on his part. What I see as

trashing is dismissing his extensive efforts to do just what you're

claiming he didn't do, which is just not true. He might not have done it to

your satisfaction, but that doesn't mean that he failed, or that his intent

to effectively communicate was lacking. Judging by the depth of his

explanation, the whole purpose of his post was to explain himself "in

English". Dismissing the existence of that significant effort is exactly

what the word trashing means.



John



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

#357 Sep 2, 1999

Rich wrote:

>

>>What exactly do you think I said that was "trashing" someone?

>

>The following is what I call trashing him - you previously wrote:

>

>>> I see it as a failing in his ability to translate his mathamatical

>>> models into English, at least if his intent it to have a dialog with

>>> a non math oriented person.

>

>The reason I see this as trashing him is that he spent quite a bit of

>effort translating his mathematical models. Sure, the math was referenced,

>but I found his explanations painstaking on his part. What I see as

>trashing is dismissing his extensive efforts to do just what you're

>claiming he didn't do, which is just not true. He might not have done it to

>your satisfaction, but that doesn't mean that he failed, or that his intent

>to effectively communicate was lacking. Judging by the depth of his

>explanation, the whole purpose of his post was to explain himself "in

>English". Dismissing the existence of that significant effort is exactly

>what the word trashing means.

>

>John







I don't consider that trashing and trashing was not my intent.



You say because he didn't write to my satisfaction it doesn't mean

he failed. Maybe, but I can't get inside your mind or anyone else's

to know if he was effective. From a couple of posts it looks like others

also had trouble with his explanation. In my opinion he was ineffective.



All this said, I don't think he was writing for our group. If he had he may

have been able do dumb it down well enough for you and I to understand.



Rich ;-)



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

#358 Sep 3, 1999

"Rich N." wrote: >

> I don't expect one explanation since seeing comes in many different

> forms. My quesition was about poor seeing and how it affects long

> vs short focal ratio telesocpes.



Rich, Dan's post was in response to a question I had about seeing cells,

not about your question. I realize he is being technical, but there are

very few people who can give a crystal clear nonmathematical explanation

of a technical concept (see Jay Freeman's old s.a.a. post on MTF for a

good example of this kind of explanation).



We can try to have a conversation with Dan, and perhaps we'll be able to

dissect out the concepts without the math.



As to poor seeing in short vs. long scopes of the same aperture, I think

it was settled on s.a.a. The bottom line is that the short and long

scope are affected approximately equally (barring tube currents). The

reason is that the short scope will have less focus shift than the long

scope, but this is compensated by the short scope's decreased focus

tolerance.



Derek



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

#359 Sep 3, 1999

"Rich N." wrote:

>>

>> I don't expect one explanation since seeing comes in many different

>> forms. My quesition was about poor seeing and how it affects long

>> vs short focal ratio telesocpes.

>

>Rich, Dan's post was in response to a question I had about seeing cells,

>not about your question. I realize he is being technical, but there are

>very few people who can give a crystal clear nonmathematical explanation

>of a technical concept (see Jay Freeman's old s.a.a. post on MTF for a

>good example of this kind of explanation).

>

>We can try to have a conversation with Dan, and perhaps we'll be able to

>dissect out the concepts without the math.

>

>As to poor seeing in short vs. long scopes of the same aperture, I think

>it was settled on s.a.a. The bottom line is that the short and long

>scope are affected approximately equally (barring tube currents). The

>reason is that the short scope will have less focus shift than the long

>scope, but this is compensated by the short scope's decreased focus

>tolerance.

>

>Derek





Hi Derek,



For me the issue of poor seeing affecting long and short telescopes

isn't settled. One person was working form theory and said he had

little experience with telescopes.



I'm not saying these people are wrong but I can't argue with their logic

because I don't understand the terms they are using. What I see doesn't

seem to go along with what they are saying.



Rich







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

#361 Sep 3, 1999

I don't consider that trashing and trashing was not my intent.

>

>You say because he didn't write to my satisfaction it doesn't mean

>he failed. Maybe, but I can't get inside your mind or anyone else's

>to know if he was effective. From a couple of posts it looks like others

>also had trouble with his explanation. In my opinion he was ineffective.

>

>All this said, I don't think he was writing for our group. If he had he may

>have been able do dumb it down well enough for you and I to understand.

>

>Rich ;-)





Fair enough, I don't want to start a flame or pick a fight with you. One of

the things I like about s.a.a. and it's adjuncts (like our group) is the

pretty general effort to be precise and not shy away from technical

explanations. I agree that there is a difference between an appropriate

explanation for a given group, and "dumbing down". It just seemed to me

that he was more in the former than the latter - obviously your mileage

varied : ) And if you were following enough of the math to question some of

it's logic, that's probably why his explanation bugged you more than me - I

couldn't see the discrepencies you could.



John


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