|
photonovore
Moonatic
Reged: 12/24/04
Posts: 2475
Loc: tacoma wa
|
|
The following examination was germinated from the discussions relating to 'smallest details seen' and 'most useful magnification' threads. Seeing is mentioned a lot---I also find it a major factor in relation to not just the magnification I use but also what detail I can see.
This discussion encourged me to look into this subject further for my own edification. I'll share what I believe I have learned with ya'll... 
I wanted to know how much an observer can expect to be able to 'see through' the prevailing seeing levels (nominal seeing). I could find nothing on this specifically so I thought i'd try to see if I could arrive at some sort of a quantification through examination of Lucky Imaging techniques and data combined with examination of the differences in image aquisition frequencies between the human eye and the CCDs the Lucky Imaging Group at Cambridge University uses...and were so kind as to detail on their extensive website.
What I found was that the CCD enables them to see through prevailing seeeing by a factor of five. This however is due to the high recording frequency of the CCD imager which operates at a frame rate of 100hrz or 1/100th second per frame captured. The human eye is disadvantaged here with an aquisition/recognition/cognitive minimum frequency of about 30hrz--IOW to absorb the information that a clearing in seeing provides, it must be at least 1/30 of a second in length--a reduction of the ability of a CCD by a factor of three, which leaves the human eye's ability to see through the prevailing seeing to be a positive factor of 1.6 vs. the factor of five of a CCD.
I examined a few dozen observing reports and a similiar number of CCD images of various resolution subjects (lunar and multiple stars primarily) and found that the data seems to follow along with the 1.6 factor quite consistently.
I quantified the results of all this in the following graph:
This chart is based upon the following assumptions:
*Best visual 'seeing windows' exceed nominal seeing by a factor of 1.6.
*Minimum separation necessary for visual diffetrentiation is 180" or 3 minutes of arc.
*Resolution per aperture based upon 140/aperture (mm) ; Raleigh's Limit.
The factor for best visual window per nominal seeeing was calculated as follows:
Lucky Imaging data support an increase in resolution obtainable per nominal seeing by a factor of five. Since the CCD imagers used operate at a frequency of 100hrz (images obtained at 1/100th second intervals) and the human eye operates at approximately 50hrz threshold for flicker response and about 30 hrz (1/30th second minimum time to register a change in image), the human eye is therefore less sensitive to seeing windows than a CCD by a factor of three, leaving a remainder factor of 1.6 to nominal seeing levels.
-------------------------------------------------
The next issue was to find a way I could objectively quantify my local seeing with some other method than measuring CCD star images as is done by professional observatories and some advanced amateurs. The Moon provides a good setting for objective quantification as it offers a plethora of various sized measured objects with which to calibrate one's resolution and thus one's absolute seeing in arc-seconds. By establishing absolute (maximum 'see through' resolution) one can make a consistent estimate of the average seeing by using the next table:
So how is your seeing...really? Just for interest's sake & for some comparisons I surveyed some observatories that actually make an effort to measure and track their seeing. Here's a sampling of what I found:
*Cloudbait Observatory, 40km W Pike's Peak, CO, winter seeing averages 4-5"arc, summer slightly better. (amateur)
*Van Vleck Observatory, Middletown, Connecticut, median 2.5"arc (professiona)
*High Energy Astro, Rockville, MD--2.8"arc seeing summer nominal. (amateur)
*Hume Observatory, Santa Rosa, CA Summer nominal 3" arc. (professional)
*Vedeler Obseervatory, Catalina, AZ nominal annual range 1.8-2.4"arc (amateur)
*Apache Point Observatory, Sacremento Peak, NM nominal 1.5"arc (professional; see graph)
*Stony Ridge Observatory, Angeles National Forest, CA 2-3"arc nominal annual average. (advanced amateur)
*BTA telescope, Caucasus Mountains, Russia annula 90%>1.5"arc seeing. (professional)
*MRO Observatory, Magdalena Mountains, SW Socorro, NM reports 1"arc annual average (<1.0" 49%; professional)
NCO Lu-Lin Observatory, Yu-Shan National Park, China, 1.39"arc nominal annual (professional)
*Keck Observatory, Mauna Kea, HI 0.55"arc median seeing. (Professional)
*Dome C, Antarctica, nominal 0.27"arc seeing (professional)
I think it's possible that amateurs frequently overestinmate their local seeing conditions. The frequency I see 1"arc seeing reported just doesn't jive with the objective data, especially for suburban observing sites. I feel that the *very best* I get at my suburban site is on the order of 1.5"arc in midsummer within 20degrees or so of the meridian. The average, is definately >2.0"arc; probably around 3"arc. I feel this is probably typical suburban seeing in the US generally.
What is strikingly obvious is that in any case it requires exceptional suburban seeing to realize the resolution potential of apertures larger than 8". As resolution is the primary paradigm upon which lunar telescopic performance is based, this information gives a useful guide to what size of telescope is cost effective for a given area.
Comments, more data etc, welcome!  This is all a premiminary effort, so I don't consider anything here 'cut in stone'.
--------------------
Mardi
4" achromat, ETX-70.
Whitepeak Lunar Observatory Website
|
Tim2723
The Moon Guy
Reged: 02/19/04
Posts: 5121
Loc: Northern New Jersey
|
|
Thanks for the report, Mardi. Interestingly, when I was searching for my best choice dedicated lunar scope, I settled on a 6" aperture. My personal experience from my observing site suggested to me that I seldom achieved 1"arc seeing, but instead most of the time it was probably closer to 1.5" to 2" arc. I had nothing to back that up but my own estimates from observing small craters and linear features over a long period.
I chose a 6" aperture because I felt it would allow me to see 95% of what I could see 95% of the time, and I decided to invest in high quality 6" optics rather than brute aperture from which I would seldom reap a benefit.
Leaving aside the arguments of central obstruction, which I have come to believe represent a special case in lunar observing, I still feel my choice of a 6" Maksutov has been best for me. If I'm reading your chart correctly, it suggests that, under nominal seeing at 200x, I'm probably getting close to the best performance I should hope for from my site, which is more rural than suburban.
PS. Although I'm sure your first chart is within the image size guidelines, would you mind reducing it just a bit, as it runs off the screen by about 20%. A little hard to read. Thanks.
--------------------
The crwth will set you free!
|
photonovore
Moonatic
Reged: 12/24/04
Posts: 2475
Loc: tacoma wa
|
|
Tim, thanks for the mention on the size; i reduced it, hope that is sized more appropriately now. I forget about my 1280x1024 screen settings sometimes...please forgive me. 
My seeing is much like yours--and my reasoning also parallels yours in choice of aperture size. In fact, if not for the fact that I couldn't afford one at the time, I too would have chosen a 6" mak instead of the refractor I have--and ended up with a much smaller observatory to build in the bargain!  Ah, hindsight! :sigh:
Anyway, what i think is kind of neat about a craterlet size method of evaluating seeing (besides measuring seieng on the lunar surface instead of a unrelated star) is that it is related directly to the actual angular size of the diffraction disk and thus translates to an objective arc-second seeing figure more directly than with the arbitrarily numbered scales (like the Pickering and Antoniadi scales) that depend upon a star diffraction pattern for their quantification. The flip-side is that it isn't possible to measure seeing at arc" less than the resolution of one's scope--so an 80mm scope wouldn't be able to quantify seeing better than 1.7 arc" seeing or so. (at least not without using this!) Technically, I cannot measure sub-arc second seeing even if I got some as my 5" is too small.  Well, based on the number of good nights I get that I can measure, the ones *better* than that would be rare indeed, if they happen at all.
--------------------
Mardi
4" achromat, ETX-70.
Whitepeak Lunar Observatory Website
|
Matt Looby
scholastic sledgehammer
Reged: 07/09/03
Posts: 793
Loc: Wadhams, NY
|
|
Hello Mardi,
Thanks for your work regarding seeing and effective aperture.
I have stopped using arbitrary scales of seeing because none are
accurate. I have plans to purchase a graduated reticle eyepiece in an attempt to measure my local seeing conditions.
Regarding lunar formations, do you have a list of the size of craters or other formations that would help me assess
the seeing/effective aperture?
Thanks,
Matt Looby
--------------------
CN GALLERY
|
Thunderhead
professor emeritus
Reged: 08/27/05
Posts: 562
Loc: Melbourne, Australia
|
|
just had a good view of the moon. My 4.5" newtonian showed 3 craterlets in Plato as white spots at 120x and in moments of steady seeing as tiny bowls at 242x. The smallest craters I could resolve would be around 2.5-3km, esp near the terminator where there is shadow relief. I counted about 20 craterlets in clavius at 242x although this figure is only more of an estimate as there was mid and high level cloud interrupitng my observing. Seeing was around 7/10 and thin cloud so there is room for improvement. But otherwise I can be satisfied wth my scope's performance.
--------------------
SAB - Melbourne, Australia
GSO 10" F5 Dob with flocking & dewshield
12" (304mm) F4.6 truss dob with premium optics
|
Tim2723
The Moon Guy
Reged: 02/19/04
Posts: 5121
Loc: Northern New Jersey
|
|
Hi Matt,
You can find lists of small craters throughout Rukl's atlas on the appropriate charts (examples: charts 11, 13, and 20). Use the labeled small craters to judge the size of the unlabeled craterlets that also appear on the charts. There are lots of these, but try to pick craters that are nearer the equator so you don't have libration complicating things. You can also use any small labeled crater, for example, Artemis or Verne on Chart 20. You have to pick test craters for yourself in any region of the moon that are appropriately sized for your aperture, for instance, if one cannot see Collins with a 60mm scope, it doesn't mean anything about the seeing conditions. That's the drawback of this method.
--------------------
The crwth will set you free!
|
ragebot
Pooh-Bah
Reged: 08/26/05
Posts: 1207
Loc: Tallahassee, FL, USA
|
|
What an interesting post. I frequently take a very modest telescope (Meade ETX 90) and a very modest EP (Scopetronix 40mm) to my clubs sidewalk astronomy sessions. What is interesting is that I screw my Nikon Coolpix 4500 to the EP, and connect it to a 13 inch television ($US99 from Best Buys).
This set up has the best wow factor of any thing that has ever been at the sidewalk astronomy sessions (including a 20 in dob). I partially attribute this to your comments about how a ccd works at 100mz while the human eye works at 30mz.
But keep in mind that most of the folks who read posts in this forum have spend many hours looking thru one eye into an EP that may not have all the eye relief we might like. The folks at the sidewalk astronomy sessions my never have looked thru an EP in their life, while they have probably spent plenty of hours looking at a TV.
There is also an advantage to this setup when doing astrophotography. I find it much easier to focus using a TV, than the EVF on my cameras, or the mirror on my dslr.
I have seen the electronic EPs that are available from various sources, but the quality IMHO is not close to my Nikon (or a decent Canon, or Oly, or whatever mostlikely).
With this setup I am able to use a much smaller EP, and boost the power even more with the cameras zoom than other club members who have much better telescopes, diagonals, and EPs.
Just as an example here is a link to a recent image I posted here
http://www.pbase.com/tommy2guns/image/49179572/large
the origional (big but worth a glance)
http://www.pbase.com/tommy2guns/image/49179572/original
If you look at the images at these links and the one attached I think you will see a noticible difference in color, as well as more blurry, but usable detail in the origional.
Would someone care to speculate on the quality of seeing based on the origional image?
--------------------
Meade ETX 90, Meade AR5, Orion ED80, Atlas GT, 8 in Newt, Coronado DS SM40, Garrett 10.5X70, Sigma SD10, SD14, Canon 1D2, Xti, Nikon CP4500, C-14
|
photonovore
Moonatic
Reged: 12/24/04
Posts: 2475
Loc: tacoma wa
|
|
I revised the "Min. Craterlet Size" in the second table for seeing estimation. I previously used the Moon's km diameter/arc" diameter to calculate the angular size of a feature--I should have used the Moon's (km circumference/2)/arc" dia for this figure. The new value is (zero meridian, zero libration) 1arc" = 2.92km or (3467km*3.14)/2 divided by the median diameter in arc" of 1865". So, it's 2.92 km per "arc on the actual lunar surface.
I did say it was a work in progress! Sorry I didn't catch this error earlier.
It is true (as Tim mentioned) that the best accuracy is obtained only at the closest point upon the moon to the earth (zero lib). That said it really doesn't matter where on the surface you measure as a practical matter; this method will give good results anywhere you can discern a craterlet.
Here's why. Foreshortening: Since the vast majority of these small craterlets are essentially circular in (absolute) form, therefore only one axis will be radically affected by foreshortening; that axis which lies along a line from the center of the moon, through the feature, to the limb. However a foreshortened feature's other axis (the optically unaltered axis perpendicular to the other) *will* be further away from the observer the closer it lies towards the Moon's limb (remembering the Moon is a 3 dimentional ball). So, of course the angular size of both axis, normal & foreshortened, will then also decrease as a function of the distance increase from the observer.
This decrease in angular size due to increased distance, though quite real, is really negligable for our purposes-- you see, the Moon's angular diameter only varies 0.005"arc per km of distance varience.* So, for example, the decrease in apparant size of a 2km dia. crater as measured (0.68"arc) at the closest point on the globe compared to it's apparant size on the absolute limb would be only 15meters less or an error of 0.007%, which is well within the error range for the whole method (+ or - 0.2"arc.) Put another way, there is only 0.02"arc difference in the angular measurement of the axis (unaltered by foreshortening) of a 1km diameter feature between the meridian and the limb. (half the Moon's diameter in km times 0.005" gives the figure of 8.69" for the entire lunar disk) So the smallest crater pit anywhere you can see on the surface should be sufficiently accurate for seeing estimation.
Actually, a function more relevant to measurement accuracy is the Moon's actual distance varience between apogee and perigee. That introduces a potential error of +- 0.7%. But again, I don't see that as substantial enough to affect the seeing estimates made using the chart.
* derived as follows: (apogee angular diameter-perigee angular diameter)/(apogee distance-perihelion distance)= angular variation, or 247"arc/50,000km=0.005km angular change per km distance change. (base figures taken from Rukl's.)
_______________________________________________
"Would someone care to speculate on the quality of seeing based on the origional image?"
What you do sounds like a blast! I bet people do get worked up over seeing such images...on the tv especially. The Moon is a sure way to impress, for sure.
As to the seeing evidenced by your image: i'll take a crack at it.
The original (in the link) appears to have a resolution of about 10km based on the smallest feature that I can say "That's a crater!" about.
Now based on a 10km best resolved feature, that works out to a best angular resolution of 3.42"arc. That would indicate the seeing limit based on this image at that one point in time when it was taken. (assuming it isn't a composite but a single image). If this was the average view that night, that would be the nominal seeing figure as well. If it was one of the better images culled from a group of single images taken that night, then the nominal seeing would be a higher number. Anyway, assuming this image is representative of the quality of view you had that night, then 3-4"arc seeing on the side of the road in an urban area (i assume) wouldn't seem too unusual.
--------------------
Mardi
4" achromat, ETX-70.
Whitepeak Lunar Observatory Website
|
ragebot
Pooh-Bah
Reged: 08/26/05
Posts: 1207
Loc: Tallahassee, FL, USA
|
|
Quote:
As to the seeing evidenced by your image: i'll take a crack at it.
The original (in the link) appears to have a resolution of about 10km based on the smallest feature that I can say "That's a crater!" about.
Now based on a 10km best resolved feature, that works out to a best angular resolution of 3.42"arc. That would indicate the seeing limit based on this image at that one point in time when it was taken. (assuming it isn't a composite but a single image). If this was the average view that night, that would be the nominal seeing figure as well. If it was one of the better images culled from a group of single images taken that night, then the nominal seeing would be a higher number. Anyway, assuming this image is representative of the quality of view you had that night, then 3-4"arc seeing on the side of the road in an urban area (i assume) wouldn't seem too unusual.
One of my neighbors was interested in seeing my dslr, so I set up my Orion ED80 (80MM aperature 600mm fl) on a Gitzo tripod and put my Sigma SD10 and a 1.4 TC on the OTA and carried the whole thing about a block to his back patio. We both live in the same condo complex and not only was their security lighting, but his portch light was on when I took the image.
He was very impressed when I uploaded the image to my laptop and showed him the result. The light polution was so bad the moon was the only thing in the sky that was visible.
To me that is one of the great advantages of observing the moon, light polution actually makes things a little better, not worse as with observing almost anything else.
Thanks for your analysis.
--------------------
Meade ETX 90, Meade AR5, Orion ED80, Atlas GT, 8 in Newt, Coronado DS SM40, Garrett 10.5X70, Sigma SD10, SD14, Canon 1D2, Xti, Nikon CP4500, C-14
|
Pirx
sage
Reged: 05/11/05
Posts: 374
Loc: Calgary, AB, Canada
|
|
A good test for seeing the smallest details is crater Clavius. It's a big feature with many details in it. There is a string of craters inside of decreasing size, so you can follow them and find out how many you can see. If you can see all of them all (6 of them, though 6th is really small, some people count 5 of them). Now if you can see them all, there is much more of them. Go and try to pick as many as you can. That's a lot of fun.
Am I a loonie yet?
--------------------
Pirx
Modified Orion XT10i
SkyWatcher 80ED on AZ3
Antares 20x80 binoculars
|
desertstars
Deja moo
Reged: 11/05/03
Posts: 30057
Loc: Tucson, AZ
|
|
--------------------
Tom W.
SVP8 'She turned me into a 3-legged Newt' EQ
Ralph, the All-Purpose 102mm Refractor
Under the Desert Stars
Alcohol and calculus do not mix. Please don't drink and derive.
|
kraterkid
Post Laureate
Reged: 03/07/05
Posts: 3881
Loc: Poway, California
|
|
, and is that a dome I see to the left of the second largest crater? You may have noticed I'm a bit 
Rich 
--------------------
Rich
[image]http://www.cloudynights.com/stars/CNS0606.jpg" border="0[/image]
My CN Gallery
[image]http://cleardarksky.com/c/JBObCAcs0.gif" border="0[/image]
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
|
|
mardi,
Can measuring one's seeing be done using point sources? Or is this better done using extended objects, such as craters?
The reason I ask is I often reach nights of 1.5 arcseconds. And sometimes when I make the attempts, I can reach 1.0 arcsec and 0.9 arcsec. My large scopes are a 5" C5 SCT and a 6" CR150 refractor. I best I've ever reached with the 5" is 1.33".
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
desertstars
Deja moo
Reged: 11/05/03
Posts: 30057
Loc: Tucson, AZ
|
|
Pardon the interuption:
This thread has been added to the "Best of the Lunie Bin."
--------------------
Tom W.
SVP8 'She turned me into a 3-legged Newt' EQ
Ralph, the All-Purpose 102mm Refractor
Under the Desert Stars
Alcohol and calculus do not mix. Please don't drink and derive.
|
Tim2723
The Moon Guy
Reged: 02/19/04
Posts: 5121
Loc: Northern New Jersey
|
|
Hi Professor Ed, great to have you in the Bin once again!
Would you kindly expand on your comment for us please? What do you mean when you mention 'making an attempt' to gain better resolution? Do you have some technique to share?
Concerning point sources on the Moon, my personal experience suggests that small craterletts, near the lunar equator, provide the more substantial test for seeing with this method. There are, in my experience, certain very small but exceedingly bright features that, under ideal illumination, might mislead the observer, as point sources and extended sources tend to respond differently in the telecope. While one might detect the existance of a brilliant point source, one might not resolve the feature itself to any detail. This of course points to the basic limitation of the method: That any given instrument cannot judge seeing conditions beyond its own resolution.
I still maintain though that this method is quite good for lunar observers testing the performance of their telescopes, but only on the Moon itself, to be sure.
|
photonovore
Moonatic
Reged: 12/24/04
Posts: 2475
Loc: tacoma wa
|
|
Quote:
mardi,
Can measuring one's seeing be done using point sources? Or is this better done using extended objects, such as craters?
The reason I ask is I often reach nights of 1.5 arcseconds. And sometimes when I make the attempts, I can reach 1.0 arcsec and 0.9 arcsec. My large scopes are a 5" C5 SCT and a 6" CR150 refractor. I best I've ever reached with the 5" is 1.33".
edz
Ed, I suppose one could use spots but the reason I didn't try was because they are difficult to measure--the scale is too small.
Since a black spot on a white background, roughly analogous to a 'spot' craterlet on the lunar surface, can be detected at 1/3 the Dawes limit (Sidgwick) this makes for some potentially *tiny* craterlets to measure! For example, a 6" has a Dawes resolution limit of about .75"arc-- 1/3 of this is .25"arc times 2.92km/arc" on the lunar surface and that leaves a .7km craterlet! That would be an increase over Raleigh's Limit by a factor of 7! Time to get out the micrometer and the lunar orbiter atlas and I'm not sure even that would be sufficient for decently accurate measurement of such small features. Obviously we can see far smaller 'spot' craters than craters seen as dimentional pits though probably not quite as small as 1/3 of Dawes as the lunar syrface isn't strictly 'black & white' but shades thereof and that would decrease resolution somewhat, certainly, but I'd hate to try and quantify *that* over a range of shading relationships! No, I just used the latter for a seeing measurement standard because resolved craterlets are 1)easier to estimate as to size from an atlas like Rukl's and 2) Raleigh's Limit is more tested and less dependant upon finer levels of optical quality than limits representing a fraction of the Dawes limit would be. Achieving and then measuring resolutions at a fraction of the Dawes limit would be, I thought, too difficult & ultimately less precise for the quick and dirty seeing estimation technique I was after. Make sense?
--------------------
Mardi
4" achromat, ETX-70.
Whitepeak Lunar Observatory Website
|
EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
|
|
yeh, makes sense,
I myself have never attempted to resolve tiny craters. I've always used point source resolution as a measurement for my optics. But I don't find it particularly difficult to approach 1 arcsecond seeing with a 6" or 2 arcsecond seeing with an 80mm. In fact I would say I almost always have 2 arcsecond or better seeing and quite often have near to 1 arcsecond seeing. I'm quite surprised by the measures stated by some of the observers at their locations.
By attempting, I mean on those nights when I take my scopes up high enough in power and target stars close enough to get those kinds of measurements. I'm not out there ever week trying to observe 1 arcsecond doubles.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
|
Anonymous
Unregistered
|
|
Hi,
I guess you saw the image improvement plots for "lucky imaging" in:
http://www.arxiv.org/abs/astro-ph/0507299 -- particularly Figures 2 and 3.
Bob
|
revans
scholastic sledgehammer
Reged: 09/26/05
Posts: 811
Loc: Fitchburg, MA
|
|
This is a great framework for looking at resolution vs seeing conditions and has certainly made things a lot clearer for me. At last I understand why my webcam does so much better than my eye in picking up fine detail in average to poor seeing conditions.
Rick Evans
|
Centaur
Pooh-Bah
Reged: 07/12/04
Posts: 1123
Loc: Chicago
|
|
There was no need to offer a correction. Your original calculation was essentially correct. Using your figures of 3467 km (I would use 3476) and 1865" the right answer would be 1.86 km per arcsecond (ignoring changes in distance to the observer.)
|
|
2 registered and 0 anonymous users are browsing this forum.
Moderator: desertstars
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 2577
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 
