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#26 RobDob

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Posted 14 October 2012 - 11:26 PM

Wow, you guys are way over my head :bow:. 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

#27 drollere

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Posted 16 October 2012 - 01:20 AM

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.

#28 fred1871

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Posted 17 October 2012 - 02:44 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.

#29 drollere

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Posted 17 October 2012 - 02:00 PM

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:

Posted Image

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.

#30 Bonco

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Posted 22 October 2012 - 04:33 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

#31 WRAK

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Posted 23 October 2012 - 03:30 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

#32 fred1871

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Posted 23 October 2012 - 07:20 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.

#33 Bonco

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Posted 24 October 2012 - 03:42 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.

#34 Bonco

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Posted 24 October 2012 - 09:15 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

#35 WRAK

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Posted 25 October 2012 - 02:05 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

#36 Bonco

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Posted 25 October 2012 - 09:05 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

#37 WRAK

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Posted 27 October 2012 - 04:12 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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