|
drollere
Pooh-Bah
Reged: 02/02/10
Loc: sebastopol, california
|
Re: Simple resolution acuity test
[Re: azure1961p]
#5271334 - 06/14/12 02:01 PM
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
Quote:
And of what practical use is your calculated personal R value of 83 arc seconds - does this mean that you can separate 2.2" eps2 lyra with a magnification of x38? Certainly not (else you would have an entry in the guiness book of records)
your skepticism alerted me that there might be a problem with my old records ... and you're right, my resolution appears at around 120 arcseconds or a magnification of about 260 times.
Quote:
Resolution does not come to a hault at the rayleigh. We see detail all the time that is well beyond the Rayleigh in terms of elongated doubles, Dawes, Cassinis Division through a 70mm refractor as well as crater chains onle one mile in width.
you are right, and i wasn't specific because i assumed we are still talking about an acuity test relevant to double star astronomy, not lunar or planetary astronomy. for resolution of extended detail within the 20:1 to 100:1 contrast limits of reflective shadowed surfaces, a different acuity test would be necessary.
again, the point is that the diffraction pattern is a spatial frequency stimulus, at the same *very* small angular scale as a double star, and appears as a light in darkness rather than a surface, and at the dark adaptation, stimulus illuminance and exit pupil that characterize double star observations, while being fairly robust to variations in the magnitude of the target star and to atmospheric turbulence.
Quote:
Normally visual acuity refers to the ability to resolve two separated points or lines, but there are other measures of the ability of the visual system to discern spatial differences.
as i have been saying. (although it is always creepy to discover that wikipedia agrees.) "acuity" normally means text acuity, measured by optometrists as recognition of textlike dark characters printed on a white surface viewed at short distances under somewhat dim illumination levels. this resembles in no way the visual stimulus presented by a telescopic image.
the history of visual psychometrics over the past century and a half has been among other things the realization of the importance of context and stimulus attributes in visual tasks and the very powerful capabilities of the eye to adapt to different stimulus challenges in ways that produce a very scattered picture of visual capabilities.
|
azure1961p
Postmaster
Reged: 01/17/09
Loc: USA
|
Re: Simple resolution acuity test
[Re: David Knisely]
#5272120 - 06/14/12 11:28 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
Quote:
Quote:
The Harvard refractor did tests where they could resolve a human hair at some wildly far distance... a quarter mile or there abouts, which worked out to be 1/15th its Dawes Limit.
Im off work, but thanks for the post - gotta go.
Pete
They didn't actually resolve the hair; they merely detected its presence. We routinely "detect" the presence of things that are way below the resolution limits like stars (or, in the case of dark features, Cassini's division in a 2.4 inch refractor) without actually resolving them. Clear skies to you.
If I gave the idea that the hair at 1/15 the Dawes Limit was angular resolution, then I wasn't clear enough. It is resolved to a degree, but not in any angular sense. The same could be said of Cassinis division through my 70mm or stars [through anything].
There is satisfaction in detecting though. Not as defined as angular resolution, but its a nice consolation prize ;P
Pete
|
EdZ
Professor EdZ
Reged: 02/15/02
Loc: Cumberland, R I , USA42N71.4W
|
Re: Simple resolution acuity test
[Re: azure1961p]
#5272360 - 06/15/12 07:24 AM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
over the years this topic has been beaten to death. Chapters of discussion on this topic reside in both this forum and the binocular forum and on the Astromart forum dating back 8 to 10 years.
There's a few things that I would add.
the reference to 60 arcseconds resolution if I'm not mistaken, comes from the Snellen tests. The capital letter E at 20/20 vision is said to be 1 arcminute of resolution (actually about 58 arcseconds, all documented in the binocular forum). BUT, that refers to the thickness of ONE LINE. Look at the E. It has three lines and two spaces from top to bottom. It actually measures 5 arcminutes tall when seen at 20/20 vision. However, it has lines and spaces that are one arcminute wide. For eye exam visual acuity that represents 60 arcseconds resolution.
Line resolution, line pairs resolution (USAF 1951 resolution targets), curved lines with cutout spaces(lanctot), black dots on a white ground (craters), white spots on a black ground (double stars) and thin lines (Cassini and power lines) ALL HAVE DIFFERENT LIMITS OF RESOLUTION.
Dawes does not represent a split. It is a measure that results in about a 10-15% overlap of a pair.
Scopes can exceed Rayleigh resolution limit in daylight on line pairs targets. (See the eyepiece forum for study of resolution plots of Megrez 80, TMB 80 and TV102).
It has been shown repeatedly by testing that thin line resolution can exceed point source resolution by factors of perhaps 5x to 10x, possibly more. That is because the black line extends continuous in one direction across our receptors and activates adjactent receptors.
Daylight tests will vary depending on what type of target and shape you use and how brightly lit the target is.
Daylight tests can be correlated to nighttime tests, once you've tested both and find your own correlation.
My bright daylight acuitity on line pairs is about 80 arcseconds unaided. It is about 75 arcseconds aided, regardless of power. My night acuity on point source pairs is about 120-130 arcseconds. BUT, that is ONLY for line pairs ans star pairs that have not approached the resolution limit of the optics. For instance a 3 arcsec pair in a 5" scope at 40x-50x or a 14 arcsecond pair in a 10x50 binocular.
As soon as you approach the resolution limits of the optics, the power needed to resolve increases dramatically. A 1.2 arcsecond pair in a 5" scope might require 200x for an apparent resolution of 240 arcseconds.
Acuity falls of for fainter stars. So your night time acuity of 120-140 arcseconds won't hold up if you are attempting to view a pair of mag 8 - mag 9 stars. That requires higher power. Likewise the same occurs for stars that vary in magnitude by a significant amount.
edz
|
azure1961p
Postmaster
Reged: 01/17/09
Loc: USA
|
Re: Simple resolution acuity test
[Re: EdZ]
#5272469 - 06/15/12 09:17 AM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
I think that capped it excellently Ed.
Thanks.
Pete
|
drollere
Pooh-Bah
Reged: 02/02/10
Loc: sebastopol, california
|
Re: Simple resolution acuity test
[Re: azure1961p]
#5273090 - 06/15/12 03:27 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
Quote:
If I gave the idea that the hair at 1/15 the Dawes Limit was angular resolution, then I wasn't clear enough. It is resolved to a degree, but not in any angular sense. The same could be said of Cassinis division through my 70mm or stars [through anything].
this only points up the fact that you have to be very clear about what "acuity" means, and whether or not your specific visual test matches that conception.
in my ignorance, i have not seen acuity applied to low contrast details on extended image areas such as planets or the moon; the illuminance levels in an astronomical image would make daylight acuity tests, even those made through a telescope, irrelevant in any case.
usually acuity is invoked in relation to the spot size that produces a visibly noticeable optical aberration in an off axis star image or the necessary separation that produces the rayleigh "resolution" of two matched magnitude stars. in both situations the image illuminance is very low and the eye is typically in a fully dark adaptated condition.
it's interesting to me to see how difficult it is to walk away from the idea of visual acuity as the grounding perceptual attribute. the fact (as i've said repeatedly) that it varies with the task doesn't get you to the sensible discussion of an acuity test. you have to identify the attributes of the task that matter to the acuity that concerns you.
the basic issue is not that the eye is enormously adaptive, but whether your test of acuity matches the visual demands placed on the eye in the situation where acuity is relevant -- double star astronomy, i've assumed all along. that is the starting and end point, and the main problem with the test posed in the OP.
|
WRAK
sage
Reged: 02/18/12
|
Re: Simple resolution acuity test
[Re: EdZ]
#5273241 - 06/15/12 04:51 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
Quote:
over the years this topic has been beaten to death. ...edz
Ed, the topic acuity may have been beaten to death by some people and it would certainly be good if your Chapter about acuity in your "Understanding Resolution" would have a separate link in your meta post "LINKS to Double Star Resources, posts and articles".
What I did not find so far was a simple test for your own specific double star splitting or resolution acuity - besides looking through your telescope at doubles with different magnification and doing then some calculations but here are always side effects to consider so this it not such a good approach in my opinion. So I tried developping an easy to do objective test with comparable results myself - if others can do it better I would be glad to hear about it.
Wilfried
|
WRAK
sage
Reged: 02/18/12
|
Re: Simple resolution acuity test
[Re: Cotts]
#5275920 - 06/17/12 05:10 PM
|
|
Edit
|
Reply
|
Quote |
Quick Reply
|
|
|
Quote:
This test, being done in ordinary daylight, is probably not nearly as stringent as trying to 'split' two illuminated pinpoints with a dark background and dark-adapted eyes.
...
I would think that if you tried this test in the dark with illuminated pinpoints you wouldn't get nearly as good resolution.
Can you 'split' the double-double in Lyra naked eye? They are 210" apart. According to your test you should easily be able to do so.
Dave
Dave, I tried now the test with inverse setup means a flashlight with a mask with two 5mm holes in 10mm distance from center to center. The result was basically the same with inital problems with too much glare from the flashlight (a smaller diameter of the holes in the mask but with same distance would probably even give "better" results) - this is already the problem with such a setup as you would need to specify the light source exactly making the test no longer a simple one.
Last night I was in a somewhat darker location so I could at least see eps lyra with naked eye. But eps lyra was still rather dim (seeing was not very good) and I was therefore far from splitting eps lyra with naked eye - but I doubt better seeing would make much difference here.
Wilfried
|
|
5 registered and 5 anonymous users are browsing this forum.
Moderator: PJ Anway, RLTYS
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 2407
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
