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# Size of spurious disk

51 replies to this topic

### #51 fred1871

fred1871

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Posted 05 February 2013 - 12:16 AM

A comment regarding Taylor - yes, he mentions using very large CO ratios, but indicates that none of the results he reports made use of that technique. Rather, it was with the 12.5-inch reflector in normal mode, with its normal small CO.

Quote:
But may be the size of the spurious disk lies in the eye of the observer?
I've been saying this for some time now, in various versions, and I've noticed some others expressing a similar view.

How the eye sees what's there is interactive with the diffraction image, affected by seeing, CO effects, optical aberrations, and the quality of the eye along with the observer's ability to see (a skill improved by experience/practise).

### #52 WRAK

WRAK

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Posted 25 May 2013 - 01:39 PM

... in Lord's table ... edge of disk center is another point that needs discussion. He seems to have chosen an arbitrary point, that at which the light is 50% or one half of the peak intensity.
... Are we to assume the disk edge cannot be seen beyond a point where the light intensity drop below 50% of peak intensity? Or is this simply an arbitrary point? I think it is just an easily identified arbitrary point used forr consistency
...
edz

Had to reactivate this thread for good reasons. Any approach for calculating the size of the spurious disk has to make assumptions regarding the visual theshold or use such a parameter as part of the calculation.
50% of the intensity seems an assumption very far away from reasonable to me - as we can see with good enough seeing conditions the first diffraction ring (or even more) of bright stars and as we know according to diffraction theory the energy resting in the first diffraction ring the visual threshold has to be less than 10% for scopes without CO.
But if you have a look at graphs of the diffraction pattern (like for example from http://micro.magnet....ionfigure3.jpg) you see instantly that this small gap between spurious disk and first ring is certainly not what you see in the scope where the dark space between spurious disk and first ring seems much larger. But despite this I still think that the size of the spurious disk is at least for refractors much larger than 50% of the Airy disk (for brighter stars).
Have also searched my observation notes for dark space at Rayleigh for my 140mm refractor means 1" separation - found only one for the equal faint pair HO22 1" +8.5/8.64mag. All other observations at this separation range (not many as I have not this many nights with fair seeing) were notched rods at best and all positive obervations below 1" were with overlapping spurious disks from rods to eggs (considered positive resolutions when still allowing successfully to estimate the position of the companion).
Wilfried

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