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eta Crb & gammaCrb

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#26 WRAK

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Posted 23 May 2013 - 01:38 AM

With a wavelength of 550nm the size of the Airy disk would be 1.38 arcsec. The CO of 0.227 would reduce this to 1.3 arcsec. The size of the spurious disk for a ~+5.5mag star should be about 60% of the Airy disk - this gives 0.78 arcsec for the diameter of the spurious disk or a radius of close to 0.4 arcsec. With a separation of 0.6" for Eta CrB it would seem impossible to see dark space between the spurious disks as they have to overlap according to this calculation.
Mirceo - while I do not doubt your observation this means that there is need for explanation. Maybe my calculation or the advertised separation is wrong.
Wilfried

#27 fred1871

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Posted 23 May 2013 - 03:56 AM

Wilfried, I think Mircea was claiming a split (with dark sky between components) on Eta CrB with the 8-inch (203mm) not the 5-inch (127mm) telescope he has. With the 8-inch I don't think there's a problem as the ephemeris gives about 0.67" for Eta CrB at present, and the Dawes limit for an 8-inch scope is 0.57". The components are near-enough of equal brightness.

#28 WRAK

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Posted 23 May 2013 - 02:14 PM

Fred, don't see your point - Dawes or even Rayleigh means overlapping spurious disks.
Wilfried

#29 fred1871

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Posted 23 May 2013 - 06:55 PM

Dawes or Rayleigh mean overlapping diffraction discs, but because the discs fade towards the edges the observer doesn't see it that way - the spurious discs look smaller. There's a neat little diagram in Sidgwick's Amateur Astronomer's Handbook illustrating the point; it relates also to why dimmer stars appear to have smaller diffraction discs, and is a phenomenon that George Airy long ago commented on in his study of the diffraction image.

EdZ has remarked here some time ago that with higher magnifications one can see something of this effect, of overlapping discs; whereas at moderate magnification, one sees an apparently clean split. Based on my own observing I'm not sure how much of this effect is from scattered light, including diffuse spreading of light due to various factors (atmosphere, optical issues), and how much might be what EdZ takes it to be.

I do know that my recent observation of Zeta Bootis did not appear to have overlapping discs - given the large CO of my SCT (36%) that would reduce the diffraction discs' size a little, so the effect I saw, at what would be the Dawes Limit for an unobstructed telescope, was a good fit for what I expected - two discs almost touching. As I had reached 470x to get this view I didn't try higher powers to check for an appearance of disc overlap.

So I'm inclined to think the suggestion that we don't usually see the outer parts of the diffraction disc because it falls below the eye's detection threshold is correct, which is why we see an appearance of separate discs with pairs closer than optical theory allows. The theory is correct in describing what's there; but our eyes don't see the whole story. Spurious discs look smaller than the true diffraction discs.

#30 fred1871

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Posted 23 May 2013 - 07:03 PM

I've now found the quote from Airy in 1835:

The rapid decrease of light in the successive rings will sufficiently explain the visibility of two or three rings with a very bright star and the non-visibility of rings with a faint star. The difference of the diameters of the central spots (or spurious disks) of different stars ... is also fully explained. 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].

I'm suggesting that a similar threshold effect applies to all diffraction discs in terms of how we see them. There is this differentiation between brighter and fainter stars; but even the brighter stars don't allow the full size of the diffraction disc to be seen.

#31 WRAK

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Posted 24 May 2013 - 03:24 AM

Fred, I am speaking about the spurious disk which is the part of the Airy disk we can actually see as it is above the visual threshold. For brighter stars up to +6mag we can according to for example EdZ as well as other sources expect the spurious disk to be about 50-60% of the Airy disk (maximum to first minimum). Even if we take the lower value of 50% we can expect kissing disks at Rayleigh and overlapping disks at Dawes.
Wilfried

#32 ziridava

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Posted 24 May 2013 - 04:49 AM

As pointed out by fred1871 ,I was succesful in splitting Eta CrB with the 8 inch F/6 Dobsonian,having a 22.7% CO.
The black space between the stars was really thin and fluctuating but it was there at 320x ,without any doubts.
With my smaller Dobsonian of 125mm F/7, I was not able to go deeper than 1''.This have 28% CO but generally provide much nicer images on double stars.
Mircea

#33 fred1871

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Posted 24 May 2013 - 08:41 AM

My viewing of Zeta Bootis was similar in appearance to Mircea's of Eta CrB - I think a factor here is a contrast effect, that masks the visibility of the outer parts of the diffraction disc, even when that area might be expected to be visible. Because the outer disc areas are much less visible than the central parts the eye usually does not see light there - rather a contrast effect of darkness between the discs. The theory is solid but the eye frequently doesn't see it that way.

I'll experiment with some Dawes Limit pairs, and try much higher powers to find out if I can see more of the diffraction disc by using more than the minimum power needed, as some have suggested. Distinguishing that from scattered light from atmosphere effects and optical limitations may not prove easy.

#34 WRAK

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Posted 24 May 2013 - 11:44 AM

... I'll experiment with some Dawes Limit pairs...


Would be of interest as it could very well be that a +6mag spurious disk is less than 40% of the Airy disk because this would explain such observations.
Wilfried

#35 WRAK

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Posted 24 May 2013 - 05:47 PM

Fred, the calculation for your observation of Zeta Boo gives the following results:
Airy disk for 235mm without CO = 1.18"
Airy disk for 235mm with 0.36 CO = 1.04"
Spurious disk 60% assumed = 0.624"
With a separation of 0.6" this would mean a minor overlap and therefore no dark space between the spurious disks but
- the size of the spurious disk should be a bit smaller due to the fact that CO not only reduces the size of the Airy disk but also the amount of energy in the Airy disk and therefore in the spurious disk. Same visual theshold with less energy should result in a smaller spurious disk
- separation 0.6" does not sound very precise and could mean 0.56" to 0.64" - if the latter ...
- may be the spurious disk size for +4.5mag stars is not 60% but 55% of Airy disk. Have not yet found a precise algorithm here.
The first factor alone makes your observation of Zeta Boo with dark space between the spurious disks plausible according to the rules of diffraction theory.

Mircea, again the calculation for your observation of Eta CrB with some modifications:
Airy disk for 200mm without CO = 1.38"
Airy disk for 200mm with 0.227 CO = 1.3"
Spurious disk assumed (to be on the save side with only) 50% = 0.65"
With a separation of 0.6" this would mean a minor overlap and therefore no dark space between the spurious disks but as Fred mentioned the separation may be rather 0.67". Together with the argument that the energy loss due to CO may reduce the size of the spurious disk a bit more makes your observation plausible within the rules of the diffraction theory.

Question for me remains the actual size of the spurious disk in % of the Airy disk depending on magnitude. This 50% or 60% or less or more seems so far not more than (hopefully educated) guessing.
Second for me open question is the potential additional reduction of the size of the spurious disk due to the CO caused energy loss in the Airy disk. Further investigation needed.
Wilfried

#36 ziridava

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Posted 25 May 2013 - 03:04 PM

Wilfried
Thank you,your explanation is an eye opener for me.
Last night I was trying to duplicate the split of Eta CrB with the 8 inch Dobsonian at 200x/Radian 6mm,but I had not any chance.The turbulence was high and fast moving clouds were crossing the sky,I was trying my luck observing trough sucker holes.
When clouds covered Eta CrB ,I was jumping to clear corners of the sky.This way I revisited
Polaris,Izar,Saturn,Mizar,Albireo and Delta Cyg,all resolved,only Eta CrB remaining just a fuzzball.Then I noticed masive clouds comming from South-West.I made a final try at Eta CrB,seeing only the same annoying fuzzball.I continued to look as clouds become thicker.
The image start to fade and ,from a certain moment, to become clearer until the eight shape of the touching spurious disks become visible.After about two minutes from noticing the eight shape,Eta CrB was no more visible in the telescope.
Mircea

#37 Nucleophile

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Posted 31 May 2013 - 01:01 AM

I have observed these recently with two reflectors as detailed below.

With a 15" f/4.5 reflector fitted with a Paracorr lens on a night of very good seeing (Pickering 7):

Eta: 398x (Pentax 5mmXW): resolved; 498x (UO 4mm): barely a split with a thin black line; 569x (3.5mm Pentax XW): clearly split to two evenly matched light yellow stars--beautiful.

Gamma: [observed using a newly installed Antares Optics 1/30 wave secondary]: 220x (9T6): elongated; 398x: beautifully resolved-----and then a huge fog bank rolled into sight and its been cloudy up until tonight. I need to finish this one for the split--guessing the 4UO may do the trick or the 3.5 Pentax.

Tonight I revisited these with my 8" f/5.9 reflector (also fitted with a Paracorr lens). I wanted to make sure I could observe at high power for extended periods, so my equatorial platform was used for tracking. I really wasn't expecting much but felt the data was important nonetheless. The seeing tonight was excellent--easily an 8 on the Pickering scale. The results were amazing to me:

Eta: 395x (Pentax 3.5mm XW): definitely elongated; 460x (3 mm Radian): classic peanut shape on the border of elongated and resolved; 550x (Pentax 2.5 mm XO): resolved! no question 2 objects were seen.

Gamma: 460x: single star; 550x: clearly elongated indicating two entities.

The Dawe's limit for this 8 inch telescope is computed to be 0.57"---I think I got really close to it tonight.

#38 ziridava

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Posted 15 June 2013 - 02:23 PM

Mark
Congratulations,I'm envious on your equipment and I see you have such a good seeing there too!Those high magnifications are out of question at my location.

Last night I revisited Eta Cr B on a Pickering 6/7 sky.At 200x/Radian 6mm,the two discs were clearly visible,in an eight shape.I spent at least half an hour looking and looking again at that beautiful image.
Next I increased to 320x/Celestron ''orange'' 7.5mm Plossl+2x ''no name'' Japanese Barlow and at times,fluctuating,the black space between the components was there.But the image was not so pretty like at 200x,so after obtainig the proof of split,I returned at lower magnifications and nicer images.

I had no luck with Zeta Boo and Gamma Boo which I courted.

Mircea

#39 Nucleophile

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Posted 15 June 2013 - 07:42 PM

I spent at least half an hour looking and looking again at that beautiful image.


Many of my double star entries contain the word "beautiful". I had no idea what a satisfying endeavor double star observing was when I first forayed into astronomy. ( I am still addicted to faint fuzzies, for what it is what).

#40 WRAK

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Posted 09 July 2013 - 02:58 PM

Yesterday with NEML +3.5mag and medium seeing Pickering ~5-6 I did a session in CrB including Gamma CrB 0.6" +4.04/5.6mag according to WDS. I noticed despite the not this good seeing several times for parts or seconds an elongation in the direction of 2 o'clock - checking afterwards for my time and location confirmed this observation (or with a low probability of less than 5% a lucky hit). This should indicate that the above mentioned ~0.7" separation is probably correct as I don't think such an observation could be in combination with a delta-m of 1.56 possible with a separation of 0.6".
Wilfried






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