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ziridava
super member


Reged: 08/17/12

Loc: Arad,Romania,Eastern Europe
Re: Delta Cyg reloaded new [Re: WRAK]
      #5469377 - 10/14/12 07:43 AM

I used Delta Cyg to get the right focus before attempting to split 23 Aql on the 6-th of October,as reported here:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/double/Number/4734189/page/0/view/collapsed/sb/5/o/all/fpart/2

I understand my 125mm F/7 reflector is falling somewhere in the middle of the aperture range required to split Delta Cyg, computed with the ''rule of thumb''.
My magnifications for double stars are between 116x and 236x.
On that night I used 144x/Radian 6mm for Delta Cyg,it was very nice and clean split.
For me,the use of diffraction rings to measure separations is a bit confusing because the CO on my reflector is 28% and I know I don't see the first diffraction ring.
However ,the separation of the companion of 23 Aql seemed to be 10-15 % bigger than that of Delta Cyg. Still,23 Aql is a much more difficult double star than Delta Cyg.
Last year I had very hard time until I split Delta Cyg:I was ''young'' as double star observer.
This is my personal list of things important for succses-so they are important only to me.I come up with this list after two years since I fall in love with double stars:
-seeing
-quality of the objective
-personal skills
-aperture
-telescope type
-eyepiece,Barlow,mounting stability,ergonomics of observation-sitting or standing.

Please note this is a very personal list.
The aperture is ranking on my list where is ranking because I'm speaking about objects within the resolving power of the smaller telescope.
My biggest telescope yet is an 8 inch F/6 Dobsonian.

Mircea


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: Delta Cyg reloaded new [Re: ziridava]
      #5469392 - 10/14/12 08:16 AM

Quote:

... Still,23 Aql is a much more difficult double star than Delta Cyg...Mircea



Depends certainly on a lot of factors - last time I visited 23 Aql I could go down to 80mm aperture and still split it. Delta Cyg limit for this was about 100mm so in terms of required aperture for splitting 23 Aql seems easier.
Wilfried


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drollere
Pooh-Bah
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Reged: 02/02/10

Loc: sebastopol, california
Re: Delta Cyg reloaded new [Re: Cotts]
      #5470141 - 10/14/12 04:43 PM

Quote:

You and I both agree that there is no formula for relating magnitude and the radius of the spurious disc as defined above. There most certainly is a 'formula' relating magnitude and the radius of the Airy Disc - specifically, the Airy disc size is constant for a given aperture without regard to magnitude or telescope type.




dave, i certainly understood you, and in my original post i clearly respected the dependence of the theoretical airy disk on aperture only. i am pushing back on your spurious distinction (pun intended).

the easiest route is for you to cite an authoritative modern source where your usage is respected. suiter (p.12) cites the rayleigh formula to calculate "the visual radius of the Airy disk". couteau (p.31ff) refers to "the Airy disk" as an *image* attribute. warren smith (p.192) speaks of point sources "imaged as an Airy disk." rutten & van venrooij (p.213) call "the central light spot the Airy disk." i can continue enumerating examples, but nowhere is the airy disk termed "spurious".

your may be referring to sidgwick (p.38ff), who applies the term "spurious disk" as a synonym for "Airy disk" -- "termed the spurious disk OR Airy disk" (emphasis mine) -- but sidgwick *never* (so far as i can find) contrasts the Airy disk (theoretical) with the spurious disk (visible). he simply acknowledges john herschel's coinage, adopted by airy, that signals the disk is not a *physical* or stellar disk (as herschel and other ancestors believed) but an optical one (as described, e.g., http://arxiv.org/pdf/1003.4918.pdf ... a nice historical summary). in fact, if you google "airy spurious disk" you can come up with several 19th century sources where the distinction is pounded into the ground.

otherwise sidgwick talks about the "theoretical appearance" of "the disk" and all its various attributes "in theory", which i accept as modern usage.

this usage prevents any ambiguity: texereau (p.5) explicitly equates the *theoretical airy disk* with the *visual airy disk* when he says (in translation): "one should be familiar with this "ideal" star image, consisting of the "false disk" and surrounding rings ..." (quotes in original). here "ideal" and "false" have the same status because the "false" is one component of the "ideal"!

so i'm not confused about your usage; i'm asserting it is antiquated or obsolete, and a neologism to the extent that you use "spurious" as a contrast to "theoretical", when the original contrast was with "physical". there is no good reason not to follow sidgwick's and general modern usage when the "apparent" Airy disk is contrasted with its "theoretical" figure.

you tip your rhetorical intention, dear cotts, when you refer to the "imaginary" dark rings, which are patently not imaginary but perfectly visible -- and in certain situations reproducible in a camera. if you go down that path, then you must refer to the "spurious diffraction rings" and the "spurious diffraction artifact", since all parts of the artifact arise from the same physical process described by the same singular "theoretical function" ... they deserve the same name.


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drollere
Pooh-Bah
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Reged: 02/02/10

Loc: sebastopol, california
Re: Delta Cyg reloaded new [Re: WRAK]
      #5470154 - 10/14/12 04:52 PM

Quote:

Thus the radius of the spurious disk of a faint star, where light of less than half the intensity of the central light makes no impression on the eye, is determined by [s = 1.17/a], whereas the radius of the spurious disk of a bright star, where light of 1/10 the intensity of the central light is sensible, is determined by [s=1.97/a]."




excellent catch, wilfried, i had forgotten he put numbers on his visual threshold explanation.

in part i forgot it because i dismiss it as invalid. we've hashed this out in another topic, but the flaw is that you can see rings around the airy disk at magnitudes where a threshold explanation, applied to the *log* form of the bessel function (which yields perceived brightness; see the paper cited in my previous), falls above the peak intensity of the rings and therefore predicts they will not be visible. or, as sidgwick (p.39) says, "the rings are brighter than theory would indicate, the first ring being not much fainter than the disc itself."

this implies a fundamental problem with airy's "threshold" explanation, which makes it unreliable as a basis for correction.


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Cotts
Just Wondering
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Reged: 10/10/05

Loc: Toronto, Ontario
Re: Delta Cyg reloaded new [Re: drollere]
      #5470794 - 10/14/12 11:44 PM

"Do not confuse the definition of the Airy disk as the bright central dot in the diffraction pattern. This this term is very often confused in much of the literature in print. The Airy disk is measured out to the minimum of the first diffraction interspace. The central dot may be referred to as the spurious disk and is more easily understood described as the visible disk. The true measurement of the spurious disk itself is difficult. The measurement 5.45/D based on the wavelength of light (and specific only to yellow light at 550 nanometers) is out to the first minima. The edges of the central visible disk usually cannot be seen as the light falls off to zero towards the first minima as we move from the center of the central disk out into the first diffraction interspace where the minima occurs. "

The above is from Ed Z. It makes sense, it removes ambiguity from the nomenclature of these phenomena. He's done as much research on this topic as anyone and I, for one, support his view.

i haven't even read any of the books you mention. Allthose authorities, though, throw these terms arond in a very mixed up manner as you have pointed out with the difference between 'modern' and older experts.

And the spaces between the rings for virtually every star in the sky are spaces. Thus invisible. What star can you look at where the diffraction pattern is a continuum of light, visible to your eye with the rings brighter and the 'not-rings' are visible but less bright? Maybe Sirius - maybe. In the thousands of double star observations I've done there is blackness between the diffraction rings and between the spurious disc and the first ring, not just less light.

and don't call me 'dear'.

Dave


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RobDob
sage


Reged: 04/10/10

Loc: Brentwood (East Bay Area), CA
Re: Delta Cyg reloaded new [Re: WRAK]
      #5470847 - 10/15/12 12:26 AM

Wow, you guys are way over my head . Suffice it to say that Delta Cygni looks to me like a summer/fall version of a mini Rigel.

Blessed with good seeing last night and tonight the pair was spectacular through my Z12 and 6mm Z-planetary!

Rob


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drollere
Pooh-Bah
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Reged: 02/02/10

Loc: sebastopol, california
Re: Delta Cyg reloaded new [Re: Cotts]
      #5472680 - 10/16/12 02:20 AM

Quote:

i haven't even read any of the books you mention. All those authorities, though, throw these terms arond in a very mixed up manner as you have pointed out with the difference between 'modern' and older experts. ... and don't call me 'dear'.




yes sir, mr. cotts sir! ;-)

well sir, i believe you quoted herr edzel to exactly my point: "the central dot [he means disk] ... is more easily understood [if] described as the visible disk." or how about the visible airy disk?

you never doubted that i might confuse the airy disk with a stellar diameter, so your insistence that *i* use the term "spurious" seems to have some other intent ... something more akin to those "imagined" dark rings.

why would anyone prefer the terminology of herschel junior and apply it to an intention he never endorsed? airy adopted the term merely as received, as published, in deference. nothing is gained, made clear or established as fact by it in usage today.

as for "the airy disk is measured out to the minimum of the first diffraction interspace ..." and so on: i am certainly interested to hear established what exactly the diffraction artifact consists of. but there is no "disk" in a complex intensity function, the fact that airy had to go to variable *visual* thresholds to make the disk larger or smaller makes that point exactly.

of course the modern sources are confused -- airy was confused, and john herschel was baffled. i cannot find a person who is not confused about this topic. the disk changes size with magnitude ... but actually it doesn't? the rings are as bright as the disk ... but actually they're much fainter? this is gibberish. you have on the one hand a mathematical explanation worked out by airy, and then on the other hand you have the several (six, by my count) ways that sidgwick and i point out that the mathematical explanation does not match the visual facts.

sidgwick uses the term "theoretical" to refer to the diffraction flux profile, and "visible" otherwise. that is modern, perspicuous, precise usage, and i endorse it. if "spurious" works for you, then work it. i'm sure we can leave it at that, since it is off the OP.


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fred1871
scholastic sledgehammer


Reged: 03/22/09

Loc: Australia
Re: Delta Cyg reloaded new [Re: drollere]
      #5474470 - 10/17/12 03:44 AM

The issue, I think, relates to how one connects what is seen with Airy and others' understanding of diffraction. That one does not see exactly what theory suggests does not in itself indicate a problem with theory. It may well be that perception (the eye/brain combination) isn't up to the job of fully perceiving the exact form of the image.

I can see Dave saying, with Hamlet, "we must speak by the card, or equivocation will undo us". Frankly, I see no basis for harsh words or attitudes here.

There's some good material and interesting discussions in this thread. Let's keep the comments in that direction. Please.


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drollere
Pooh-Bah
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Reged: 02/02/10

Loc: sebastopol, california
Re: Delta Cyg reloaded new [Re: fred1871]
      #5475256 - 10/17/12 03:00 PM

Quote:

The issue, I think, relates to how one connects what is seen with Airy and others' understanding of diffraction. That one does not see exactly what theory suggests does not in itself indicate a problem with theory. It may well be that perception (the eye/brain combination) isn't up to the job of fully perceiving the exact form of the image.




yes, well, how exactly is "eye/brain" not up to the job? it seems to rest on airy's "threshold" explanation of why the image and theory diverge, or why the image is "spurious". on that point, i assert that the "threshold conjecture" fails to describe the facts. referring to a log luminance diagram of the diffraction artifact:



you'll see that the "disk" diameter remains effectively constant (vertical) across all luminances where any rings would appear as parts of the profile (above any arbitrarily placed horizontal line or "threshold"). yet the visible airy disk contracts by as much as 50% in diameter in refractor views of stars that continue to show rings; the diagram argues that is impossible.

if you refer to the paper i cited two posts above, in the diagrams at the end of the paper, you'll find airy's argument illustrated in detail ... but note well, the rings are omitted from the diagram.

i speak no harsh words, fred, and i regret any impression that my critique disparages individuals instead of ideas ... always. otherwise, i decline the privilege of having other people put their words in my mouth. as i have already politely requested ... let's leave it at that, please ... with apologies to wilfried.


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Bonco
Post Laureate
*****

Reged: 04/17/06

Loc: Florida
Re: Delta Cyg reloaded new [Re: WRAK]
      #5484055 - 10/22/12 05:33 PM

Last night put my 60mm f/15 on Delta. Seeing about a 7 Pickering. Tough but detected in the SW PA quadrant. I'm sure I've had clearer views with a 60 mm years ago. This view was difficult.
Bill


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: Delta Cyg reloaded new [Re: Bonco]
      #5484775 - 10/23/12 04:30 AM

f/15 scopes seem to be excellent for observing doubles. 2.7" is near the Rayleigh limit of 2.3" for a 60mm scope but this is meant for doubles of similar magnitude and here we have a delta-m of 3.4. I consider the Dawes "valley" where separation in arcsec is equal to delta-m as split horizon under best conditions (and only if you consider an egg as split) and this observation is certainly beyond. Congratulations Bill.
Wilfried


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fred1871
scholastic sledgehammer


Reged: 03/22/09

Loc: Australia
Re: Delta Cyg reloaded new [Re: WRAK]
      #5485987 - 10/23/12 08:20 PM

Yes, it is remarkable that some of the very small and long f-ratio scopes do so well, at least on bright objects where there's enough light - seeing Delta Cygni split with 60mm is way ahead of what could be expected, yet there are enough reports by experienced observers such as Bill to show it can be done.

I noticed a similar "does better than predicted" pattern in my own observing long ago when I had a 75mm f/15 refractor.

Normally, I'd think seeing Delta Cygni with a 125mm Newtonian was pretty good, as reported earlier in this thread by Mircea. I've seen it with my 140mm refractor, but at low altitude from where I am so it required unusually steady air conditions to barely make it out.

One thing it does show is that the RoT doesn't always predict limiting conditions with small telescopes. It still seems to me a fair estimate for mid-size amateur telescopes - say, those in the 12-25cm range.


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Bonco
Post Laureate
*****

Reged: 04/17/06

Loc: Florida
Re: Delta Cyg reloaded new [Re: fred1871]
      #5487308 - 10/24/12 04:42 PM

A follow up on my 60mm report. Last night I thought conditions were about the same. But I absolutely could not split or view the double with my excellent 75mm. Shows the importance of seeing conditions. I checked my notes and found in the past I've viewed delta Cyg with an 80mm, my 75mm and the same 60mm Unitron. Notes on the 80mm and 75mm views indicate it was rather easy. Using a Nagler 4.8mm with the 60mm scope gave a solid view.

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Bonco
Post Laureate
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Reged: 04/17/06

Loc: Florida
Re: Delta Cyg reloaded new [Re: Bonco]
      #5487751 - 10/24/12 10:15 PM

Follow up #2.
Florida is known for very steady skies. On occasion, usually with clear skies and high humidity, we get excellent seeing. Tonight I'd guess it was an 8 or 9 Pickering but I'm not a great judge of the scale.
Put the 75mm on Delta and with 9 mm eyepiece got a perfectly clean split. Next I used a 4.8mm Nagler and it was a better view. So I bring out my 60mm f/15 using a Unihex and original .965 eyepieces. 7mm easily showed the double not like the other night where it was extremely difficult. The 6mm was not as good as the secondary seemed dimmer. No question in my mind that a 60mm telescope will show this pair in good seeing conditions.
Bill


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: Delta Cyg reloaded new [Re: Bonco]
      #5488020 - 10/25/12 03:05 AM

Bill, thanks for your interesting reports. Stopping down the aperture of my refractor to about 65mm I would get too an f/15 ratio and the quality of my refractor should be good enough to get the same results. My drawback is the light pollution I cannot avoid in my location - naked eye visual loss is about 3 magnitudes and with the telescope I estimate 2 magnitudes. I assume this works like observing stars 2 mags fainter giving in this case +4.9/8.3mag for Delta Cyg - this would certainly be a bit harder to split. I looked for a double with such values in the same field of view but found only 1.7" BU248 +5.4/8.8 and this is probably too close to be comparable.
Wilfried


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Bonco
Post Laureate
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Reged: 04/17/06

Loc: Florida
Re: Delta Cyg reloaded new [Re: WRAK]
      #5489345 - 10/25/12 10:05 PM

Wilfried,
My skies are terribly light polluted. Typically I can barely detect 4th mag stars with my eyes. Also my recent observations that I reported had a 1st quarter moon illuminating the sky where I could only detect 3rd mag stars. Still this didn't seem to affect viewing Delta Cyg. The terrible light pollution is one reason I mostly observe double stars and planets from my home site.
Best wishes, Bill


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WRAK
Pooh-Bah


Reged: 02/18/12

Loc: Vienna, Austria, Europe
Re: Delta Cyg reloaded [Re: Bonco]
      #5491209 - 10/27/12 05:12 AM

Mmmh, this would give you an advantage of about 1 mag in terms of light pollution - seems not this much.
May be we are in a different league of acuity - mine is certainly in best case average.
Wilfried


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