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#1 Perseus_m45

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Posted 18 February 2014 - 10:04 AM

A question,Are all you guys using the Pickering scale as the seeing standard?
http://www.damianpea...m/pickering.htm
I think we should all be on the same page with this as there are a few different scales.
mike h

#2 drollere

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Posted 18 February 2014 - 03:28 PM

as long as the range of numbers possible with the scale is identified, which by the way is good reporting style ...

" ... observed during pickering (0,10) seeing of 5 ..."

... then you usually get an approximate equivalence between any two scales.

two points of clarification. pickering apparently was the first to suggest that seeing could be calibrated on a highly magnified stellar image, but it was actually a.e. douglass who did the detailed research on the proposal and developed the final scale, which pickering also adopted. the publications on that topic are cited and summarized here:

http://www.handprint...tml#diffraction

second point, pickering and douglass never referred to their scale as the "pickering scale", but as the "scale of seeing" or simply as the "standard scale". i understand that "pickering scale" has gotten wide acceptance, but it really shortchanges the extensive contributions by douglass, and sticks a name on the scale that the authors of the scale did not sanction. at worst, it might be called the "harvard scale", since both were associated with harvard observatory at the time.

finally, the point is really not that we all use the same scale but that we are all referring to the same visual criteria. this is not possible if you are using, for example, the typical animated gif of a stellar image, which can be interpreted in different ways by different reasonable people but which often comes down to "wobbles a little" vs. "wobbles a lot". it is also important because seeing consists of low frequency and high frequency components, roughly jet stream and near ground or instrument thermals, and one degrades an image more than the other. it's possible to do very good visual work with significant low frequency turbulence; high frequency turbulence is catastrophic.

to cope with those complications i simplified the standard scale as a 5 point scale anchored on specific parts of the diffraction artifact:

http://www.handprint...ml#compactscale

which is roughly:

F = chaotic, buzzing hive of bees with no central brightness
D = airy disk or distinct bright central point persists, but merging with speckles
C = dark interval around airy disk persists, separating it from broken rings
B = first diffraction ring persists
A = completely stable pattern including (in a reflector) 2nd and higher rings.

and you can finesse with + or -, if necessary. i usually find D+/C- is the minimum condition that allows for double star work.

point is, your reporting scale is not anchored to some abstract gradation of energy in the atmosphere, or to a "looks like" comparison of what you see overall to a criterion image or animated star image online. you are looking specifically at the appearance of the artifact that you have to examine in order to resolve a double star, and using increasingly fragile aspects of that artifact to judge the amount of distortion you are experiencing.

hope this helps.

#3 brianb11213

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Posted 18 February 2014 - 06:35 PM

A question,Are all you guys using the Pickering scale as the seeing standard?

Seeing varies with aperture. The 10-point Pickering scale is strictly applicable only to scopes with (exactly) 5" aperture, observing a mag 1-2 star at the zenith with a magnification greater than x300. See Sidgwick, "Amateur Astronomer's Handbook", vol. 1, section 26.10.

#4 drollere

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Posted 18 February 2014 - 07:38 PM

Seeing varies with aperture. The 10-point Pickering scale is strictly applicable only to scopes with (exactly) 5" aperture, observing a mag 1-2 star at the zenith with a magnification greater than x300. See Sidgwick, "Amateur Astronomer's Handbook", vol. 1, section 26.10.

that's not incorrect but i suggest it is irrelevant, with apologies to both you and sidgwick ... unless you are using the seeing scale to measure atmospheric turbulence. which i don't believe it was primarily designed to do.

the seeing scale is designed to measure image quality by describing image degradation. this is pickering's real innovation, because prior to his time astronomers assessed seeing by the amount of twinkling in the stars -- which is a very poor indication of what the star image in a telescope will look like. pickering was interesting in image, not atmospherics.

the issue of aperture was thoroughly researched by douglass, who was indeed interested in atmospheric thermals and was the first person to measure the altitude at which thermal turbulence usually occurred.

but when he presented his aperture studies as a large table of seeing values by aperture (reproduced on the page i linked), he freely used the numbers supposedly standardized on a 6 inch aperture to describe apertures as large as 24 inches. the only way those numbers can be interpreted to make sense is to accept that douglass understood that ratings can apply at any aperture. in fact, pickering expressly made that assertion when he first published the scale.

true, douglass does say that "standardized" measures require a standard aperture, preferably 6" since "most observatories have an instrument that size or can stop down a larger aperture". but that type of standardized measurement is explicitly atmospheric measurement -- for example the kind of measurement that would be done by the survey expeditions of the era in order to identify sites with high quality seeing, or to compare the conditions at one observatory to another, or at the same observatory around the year. if those are the objectives, then brian is right, and the ratings should be standardized as much as feasible.

but here's the point: the OP is interested to find a comparable way for double star astronomers, you and me, to report the relative quality (clarity, stability) of their image. (at least, i assume the OP was not interested to find a way for double star astronomers to report their local weather conditions.) he wants our ratings to mean the same as his, regardless of our apertures, cool down curve, tube currents, whatever. reporting the amount of detail visible in the diffraction artifact is the most direct and universally valid way to do that, and that type of reporting is standardized on what matters. image quality, not inches of glass, is the relevant benchmark.

the alternative is that anyone who does not have a 6 inch aperture cannot report seeing at all, and anyone who requires a six inch aperture must (like pickering and douglass) use a refractor that eliminates mirror and tube thermals (which are not atmospheric thermals per se) to report. that doesn't seem headed in the right direction.

#5 David Knisely

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Posted 19 February 2014 - 12:03 AM

A question,Are all you guys using the Pickering scale as the seeing standard?
http://www.damianpea...m/pickering.htm
I think we should all be on the same page with this as there are a few different scales.
mike h


I prefer rating the seeing in arc seconds with perhaps an additional note of the Antoniadi scale rating along with the arc-second figure to describe its temporal behavior. Clear skies to you.

#6 Perseus_m45

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Posted 19 February 2014 - 01:14 AM

But here again we are not all on the same page. That's not good. Someone with clear authority needs to set the standard don't you think? Maybe someone with a friend in the naval observatory could see how they do it??
mike h

#7 David Knisely

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Posted 19 February 2014 - 01:30 AM

But here again we are not all on the same page. That's not good. Someone with clear authority needs to set the standard don't you think? Maybe someone with a friend in the naval observatory could see how they do it??
mike h


Most professional astronomers use arc seconds when describing the seeing at a given site, but with amateurs and double stars, there is no "authority" to dictate what scale we use. I like the simplicity of the Antoniadi scale coupled with the arc-second estimates based on things like stars with known separation or the appearance of stellar diffraction patterns in various apertures. Others might prefer the Pickering scale, although Bruce's A-F scale is kind of a nice method which could almost mirror the I-V scale of Antoniadi. What one uses is basically personal preference and not some rigid standard we are being forced to comply with. Clear skies to you.

#8 WRAK

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Posted 19 February 2014 - 04:48 AM

I tend to use the Pickering scale based on the animated images available in the link mike provided combined with reference to the used scope - Pickering 7 in a 5" refractor might be Pickering 5 in a 9.25" SCT. But the Pickering scale is certainly not the complete description of seeing conditions as this scale concentrates on the atmospheric stability and does not count for haze or other conditions making central disks fuzzy to the degree of making snow balls out of diffraction patterns limiting double star resolution to wide bright pairs.
I think that good transparency giving crisp central disks is more important for close double star resolving as atmospheric stability - you have in most nights with jumping diffraction patterns some moments with stability for fractions of seconds good enough for a confident recognition of separation and position. Means you might have a night with average Pickering 4-5 for your given scope but moments of Pickering 7 but maybe also moments of Pickering 3 - so what is then the "right" number (average or peak)?
Wilfried

#9 Rich (RLTYS)

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Posted 19 February 2014 - 08:02 AM

I use the Pickering Scale 0-10.

Rich (RLTYS)

#10 Ed Wiley

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Posted 19 February 2014 - 12:16 PM

I assume seeing scales are important for (1) visual observing comparing their ability to split doubles and (2) optical measuring (micrometers) for comparing the reliability of the measures obtained (or even if you should attempt a measure on a particular pair on a particular night). Is this correct?

For those of is who use CCDs or webcams, our seeing is recorded. Bad night, snow balls. Good night, airy discs. It would be interesting to see if someone has quantified seeing when photographic techniques are used. One of the reasons we imagers use lucky imaging and optical interferometric capture techniques is precisely to "beat the seeing." Sometimes it actually works.

Ed

#11 drollere

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Posted 19 February 2014 - 01:03 PM

But here again we are not all on the same page. That's not good. Someone with clear authority needs to set the standard don't you think? Maybe someone with a friend in the naval observatory could see how they do it??

as knisely says, astrophotographers and professional astronomers use the arcsecond rating. this can be calibrated to high precision with image sensors and software. most astrophotographic software automatically computes the arcsecond seeing as part of the image calibration or setup.

there is no fixed equivalence, but the input i've gotten from several people (including damien peach) is that arcsecond values of 2"-3" are "average", values below 1.5" are unusual, and values below 1" ("subarcsecond seeing") are extraordinarily good.

however, there is a problem with the arcsecond metric: it's a *sensor* metric, because it averages the image over a time interval. the arcsecond determination does not necessarily correlate with the visual quality, which means the typical visual astronomer, working with eyes and eyepieces, has no reliable way to determine it.

for example, under jet stream turbulence the star image bloats into a speckle hive of very consistent poor quality that sits in a remarkably fixed location, but under low frequency (ground heat) turbulence the image mostly jumps around with occasional glimpses of high image quality. if you look for example at the lunar surface, it is still possible to discern very fine detail under low frequency turbulence, but impossible under high frequency turbulence. to the astrophotographer it doesn't matter, because a star image bloated to 5" and a star image that just jumps around within a 5" area produce the same time lapse image. the eye sees differently and can perform better. dave jurasevich suggested to me that the time integrated CCD value for arcsecond seeing is about 3 times the visual value; in other words, if the arcsecond seeing is 3" then you can still visually split an equal magnitude binary separated by 1". this is why estimates based on binary star separations are not equivalent to estimates made by software.

again, all ratings of seeing are first of all a rating of the *image quality* and only become ratings of the *atmosphere quality* when standard methods are employed. (to ed wiley's comment, "lucky imaging" works very well under low frequency turbulence, but is no help under jet stream turbulence.)

you seem discouraged that there is no authority or single accepted standard. here's two reasons why that is not really possible. first, whatever your experience, you are misguided about seeing -- you think it is better than it really is. the mt. wilson seeing scale was developed because astronomers would come out from the turbulent east coast, observe in the west coast laminar flows, and make uselessly high estimates of the seeing. "OMG, this is amazing seeing!" "no, actually, it's pretty average." whatever you think good seeing is, the longer you observe, the more likely it is that you will encounter even better seeing than you have ever experienced before. this makes you realize all your prior ratings were too optimistic. in psychology this is called a rating bias, and there's nothing for it except to observe alongside an astronomer who has worked at the location for 10 or more years, and trust what he or she tells you. "wow, dude, this is great seeing, isn't it?" "HA HA HA!"

the other reason is: what's the point? if you use the antoniadi scale, and i use my simple scale, and somebody else uses the pickering scale, what is the reason any of us use a scale? it's to *communicate a fact*. so the main thing is not to use the same scale, but each of us to use our preferred scales with experience and especially consistently. it doesn't matter whether the scale says the seeing is 5 or 10, if that's as high as it goes then it means your socks were melting and tears were streaming down your cheeks. that's sufficient communication. the rest is my judgment of how naively you are applying that number, that is, how easy it is to melt your socks and make you cry, which is my understanding of whether you *truly* have experienced level 5 or level 10 seeing. if you haven't, then it really doesn't matter which scale you use.

this is, again, the pickering/douglass innovation. they anchored their scale on *specific and concrete* aspects of the diffraction artifact that you should be able to see in order to give a rating. and you're helped by the fact that you know you can see more than the detail described at a lower rating, and you know you can't see the detail described at a higher rating. this was their insight: look at the image, tell me what you see. that makes the communication explicit and reduces the bias as much as practicable.

and that's why many of us prefer the Standard Scale (to call it by its true, author baptized, sidgwick endorsed name).

footnote: i believe brianb misquoted sidgwick's advice (pp. 464-67 of "Handbook") to say you should use a "magnification of at least 300x". actually sidgwick says you should use a magnification "from 60D to 100D", where D is your aperture in inches. but any magnification stated in terms of aperture is actually a specification of your exit pupil or *diffraction magnification*. be clear, that if you want to see the image quality you have to magnify the *image*, which is what exit pupil determines. you have to use an exit pupil below 0.5 in order to see the star image clearly. exit pupil has nothing to do with object magnification, which determines the angular size of the object you're looking at. apples and oranges.

#12 Bonco

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Posted 19 February 2014 - 03:53 PM

You'll know good seeing...when you see it.
Bill

#13 cildastun

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Posted 20 February 2014 - 03:54 AM

A similar question to this was raised on another astro forum; plenty of views, but no replies.

I think we were all hoping that someone would come up with the definitive, objective scale - preferably metric. However, it's clear to me from the above expert replies that there is no simple answer, so I will have to go on using Antoniadi.....

#14 Perseus_m45

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Posted 20 February 2014 - 05:54 AM

Well guys I have to give it up for Bruce here due to the fact of he knows his stuff. Just have a look at his web page.I use his web page for reference all the time.
I do like the D Peach simulation but I think simpler would be more efficient so I will use the 1 thru 5 scale myself.

#15 WRAK

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Posted 20 February 2014 - 07:23 AM

Best might be to describe with a few words what you see when you look at a bright star with a magnification large enough to see at least potentially the diffraction pattern or may be even a corresponding image - with or without reference to a more or less well known seeing scale.
Wilfried

#16 Asbytec

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Posted 20 February 2014 - 09:06 AM

I use the Pickering Scale 0-10.

Rich (RLTYS)

As do I when dealing with point sources such as double stars, and if necessary deep sky. There is still some variation in that good moments come and go. However, it seems to give a descent baseline for observing point sources as it describes their image. And those images convey a thousand words.

Antionadi is nice because it seems to deal with the behavior of bright extended objects, specifically planets and the moon. But, it still does not tell the whole story. It's hard to imagine, "perfect seeing without quiver." Even in diffraction limited seeing, standard seeing scale of 8/10 or better and what might be Antionadi I, there seems to be some quiver in planetary detail almost all the time, IME. A little rougher and it becomes Antionadi II with "moments of calm lasting several seconds." The latter appears more prevalent even in excellent seeing, even when 10/10 becomes 9/10 however briefly.

And using Ant I and Ant II together seems redundant since both involve moments of calm - one lasting presumably throughout the observation "without quiver." If it quivers at all, and even in excellent seeing it will...at least the low contrast features will do so frequently...then can it be said to be Antionadi I - ever? To call Antionadi I, I permit a tiny bit of low contrast "quiver" as long as the image and major features are generally very stable.

There may even be some micro seeing where the limb is nice and solid, but softer contrasts roll in and out of view. And maybe not all across the planet at once.

...the arcsecond determination does not necessarily correlate with the visual quality, which means the typical visual astronomer, working with eyes and eyepieces, has no reliable way to determine it.

Yea, it's always been a mystery how one computes arc second seeing. Maybe it's derived from long exposure behavior. As a visual observer, I'm not really interested in long exposures. Rather one might prefer reporting "moments of calm lasting several seconds" to describe a baseline for an observation.

So, is there a perfect scale? Can here be one? Or do we just speak the language as we understand it like "seeing was Pickering 7 to 8/10 than night?" Or, "It was Antionadi 1", even though tiny contrast features quivered a tiny bit seemingly all the time...the entire image was unusually stable otherwise and probably not Antionadi II.

You'll know good seeing...when you see it.
Bill

Yep. I like the jaw drop scale for communicating it.

#17 cildastun

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Posted 20 February 2014 - 10:26 AM

As a PhD spectroscopist of 40 years standing, I like things to be objective, with no room for doubt..... Couldn't we use a scale based on eg multiples of the Dawes limit for the specific scope, eg 0.7, 1, 2, 5, 10, using a small group of specific doubles. At least it would be "numerical" based on true visual seeing and not too opinion based.

#18 Perseus_m45

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Posted 20 February 2014 - 12:19 PM

As a PhD spectroscopist of 40 years standing, I like things to be objective, with no room for doubt..... Couldn't we use a scale based on eg multiples of the Dawes limit for the specific scope, eg 0.7, 1, 2, 5, 10, using a small group of specific doubles. At least it would be "numerical" based on true visual seeing and not too opinion based.

I like this idea but who and how would do this? Or should I say who and how could do this?
mike h

#19 WRAK

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Posted 20 February 2014 - 05:39 PM

...using a small group of specific doubles...


This reminds me on my favorite seeing scale: http://www.carbonar....ting-seeing.htm
Wilfried

#20 drollere

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Posted 20 February 2014 - 07:49 PM

As a PhD spectroscopist of 40 years standing, I like things to be objective, with no room for doubt..... Couldn't we use a scale based on eg multiples of the Dawes limit for the specific scope, eg 0.7, 1, 2, 5, 10, using a small group of specific doubles. At least it would be "numerical" based on true visual seeing and not too opinion based.

yes, this has been done. see:

http://www.handprint...2.html#tombaugh

and note the problems with the scale described there. star interval scales are not commonly used.

#21 3c_273

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Posted 20 February 2014 - 08:34 PM

As a PhD spectroscopist of 40 years standing, I like things to be objective, with no room for doubt..... Couldn't we use a scale based on eg multiples of the Dawes limit for the specific scope, eg 0.7, 1, 2, 5, 10, using a small group of specific doubles. At least it would be "numerical" based on true visual seeing and not too opinion based.


I'm reminded of an old quote:

The government [is] extremely fond of amassing great quantities of statistics. These are raised to the nth degree, the cube roots are extracted, and the results are arranged into elaborate and impressive displays. What must be kept ever in mind, however, is that in every case, the figures are first put down by a village watchman, and he puts down anything he damn well pleases.
-- Attributed to Sir Josiah Stamp, 1840-1941, H.M. collector of inland revenue.

We must keep in mind that we are that night watchman, peering through our eyepieces and subject to a plethora of variables that we distill into our estimate of seeing. The Pickering ( / Douglass, as bruce points out) scale,is, if anything too precise, as there will be a variety of estimates from various observers under identical skies, as I've noticed at various star parties I've attended over the years.

Also, Pickering / Douglass, for all it's faults, is very well embedded in the astronomical community, which is noted for it's conservatism with respect to measurement (magnitudes, solar masses, parsecs, OBAFGKM... <Barf!>). We can no more adopt a better metric that dump the poorly designed QWERTY keyboard.

<Sigh!>

#22 cildastun

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Posted 21 February 2014 - 04:11 AM

As a PhD spectroscopist of 40 years standing, I like things to be objective, with no room for doubt..... Couldn't we use a scale based on eg multiples of the Dawes limit for the specific scope, eg 0.7, 1, 2, 5, 10, using a small group of specific doubles. At least it would be "numerical" based on true visual seeing and not too opinion based.


I'm reminded of an old quote:

The government [is] extremely fond of amassing great quantities of statistics. These are raised to the nth degree, the cube roots are extracted, and the results are arranged into elaborate and impressive displays. What must be kept ever in mind, however, is that in every case, the figures are first put down by a village watchman, and he puts down anything he damn well pleases.
-- Attributed to Sir Josiah Stamp, 1840-1941, H.M. collector of inland revenue.

We must keep in mind that we are that night watchman, peering through our eyepieces and subject to a plethora of variables that we distill into our estimate of seeing. The Pickering ( / Douglass, as bruce points out) scale,is, if anything too precise, as there will be a variety of estimates from various observers under identical skies, as I've noticed at various star parties I've attended over the years.

Also, Pickering / Douglass, for all it's faults, is very well embedded in the astronomical community, which is noted for it's conservatism with respect to measurement (magnitudes, solar masses, parsecs, OBAFGKM... <Barf!>). We can no more adopt a better metric that dump the poorly designed QWERTY keyboard.

<Sigh!>



I take your point - well made - although the Stamp (b 1880) quote did refer to India at a time when accurate data on tax assessment for the colonial regime was perhaps not a precise art.....

I seem to remember that bringing in the SI system also met with many of the above arguments....

#23 drollere

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Posted 21 February 2014 - 11:11 AM

What must be kept ever in mind, however, is that in every case, the figures are first put down by a village watchman, and he puts down anything he damn well pleases.

yes, but the beauty of statistics -- i mean the maximum likelihood estimators like the average and standard deviation -- is that they punch through error. so long as all the watchmen lie, confabulate or misperceive in their own erratic individualistic ways, the average will come out the same in large samples. it's when the watchmen all lie, confabulate or misperceive in the same direction by the same amount that statistics go awry.

provided you *consistently* lie, confabulate or misperceive, it's still possible to subtract your bias and arrive at the true value. this was the point i made that i have to know how easy it is to make you cry. if i know how consistently you misperceive what you rate, for example if i know that you consistently do not use a small enough exit pupil to really see the diffraction artifact clearly, then i know how to interpret your report.

The Pickering ( / Douglass, as bruce points out) scale,is, if anything too precise, as there will be a variety of estimates from various observers under identical skies, as I've noticed at various star parties I've attended over the years. Also, Pickering / Douglass, for all it's faults, is very well embedded in the astronomical community.

pickering and douglass both referred to their scale as the Standard Scale, which is the same term that sidgwick uses to refer to it in his mid 20th century handbook. "pickering scale" is evidently a latter day rebaptism because "standard scale" begs that question -- standard of what?

the argument that a scale may be too precise because the seeing estimates of people at star parties are so varied is lowering the bar to the ignorant (and omits that they are probably using different apertures). despite the riposte that "you'll know good seeing when you see it", in fact you don't know good seeing until you actually do see it, and in fact whatever is the best seeing you have seen is the seeing that you think is the best seeing possible -- until you see better. most people at star parties are there to learn.

if ratings are imprecise, something has to be made clearer about what the rating involves: can you see the dark interspace around a clearly formed airy disk, yes or no? can you see the first diffraction ring as a complete ring, yes or no? does the star image show any motion at all, no matter how slight, yes or no? this is qualitatively different from an animated .gif that each person is free to interpret in their own way.

what exactly are "all its faults"? that it's "too precise" perhaps? here i feel i can learn something that i did not know before. so far as i know, the standard pickering/douglass pickering scale is widely used because it does the job.

#24 3c_273

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Posted 21 February 2014 - 01:03 PM

This is a laudable thread that I think had educated most of its readers, especially me.

As for what scale I use, it's the Pickering scale, when I use it. And I'm acutely aware that my estimates of seeing are dependent not only on the seeing but my mood at the time, and how well my C-8 is in thermal equilibrium with its immediate environment. So the standard deviation of my estimates is depressingly large.

#25 Perseus_m45

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Posted 21 February 2014 - 01:42 PM

[quote name="3c_273"]This is a laudable thread that I think had educated most of its readers, especially me.

As for what scale I use, it's the Pickering scale, when I use it. And I'm acutely aware that my estimates of seeing are dependent not only on the seeing but my mood at the time, and how well my C-8 is in thermal equilibrium with its immediate environment. So the standard deviation of my estimates is depressingly large. [/quote

Tom I agree with you here and I,m guessing the way to go then would be pickering with maybe a brief description of your star image at the time of doing.
What do the rest of your fellas have to say on it?
mike h






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