Caption: "Slow-motion film of the star ε Aquilae taken with the Nordic Optical Telescope on the morning of 2000 May13 for testing lucky imaging (which is one form of speckle imaging)." (Slightly edited.)

Features:

1. Unfortunately, the caption that came with this flick is remarkably uninformative despite having been written by an expert.

   Yours truly will do their best to explicate, but caveat lector.

2. First of all it is a negative-image film---so dark is light and light is dark.

3. The repetition time of the film seems to be about 12 s.

4. The frequency of imaging is to order-of-magnitude 10 hz (i.e., frame rate 10) which is marginally noticeable by the human eye (see Wikipedia: Frame rate: Background).

5. You are NOT seeing the Airy diffraction pattern for the star because of turbulence in the Earth's atmosphere.

   The turbulent motions constantly distort the wavefronts of light or, from geometrical-optics perspective, the paths of the light rays.

6. The distortion from the ideal Airy diffraction pattern is called astronomical seeing (or seeing for short).

   The seeing is good if the distortion is small, bad if it's NOT.

7. The fuzzy blobs in the film which for short exposure-time images (like images that make up this film) are probably approximately the bright central fringe of the Airy diffraction pattern.

   In longer exposure-time images, all one would see is a single larger fuzzy blob called the seeing disk which is the time-averge of the individual ones.

   The human eye actually can marginally detect the individual blobs which we call twinkling (AKA scintillation). Nevertheless, we tend to judge the apparent size of star by region in which we see twinkling occurs.

8. We see multiple blobs at one instant because the turbulent motions provide a small range of different paths for the light rays to reach the telescope.

9. Assuming the blobs are approximately the bright central fringe of the Airy diffraction pattern, one can estimate their angular diameters using the Rayleigh criterion.

   Setting λ=0.5 μm (just guessing that the film effective wavelength is somewhere in the middle of the visible band (fiducial range 0.4--0.7 μm)) and using D_in=101 for Nordic Optical Telescope, we obtain an angular diameter for the blobs of about 0.05 arcseconds ('').

10. Given the blob angular diameter, length of a frame side to be approximately 2.5''.

11. For non-short exposure-time astronomical imaging and visual astronomy (aside from twinkling), all one can really observe well is the seeing disk for effective point light sources (e.g., stars).

12. The angular diameter seeing disk (which is also the approximately the angular resolution) is set by the available seeing and is a common measure of seeing.

13. Great seeing is ∼ 0.4'' (which is available at high-altitude mountaintop observatories) and good seeing is ∼ 1'' (see Wikipedia: Astronomical Seeing: The full width at half maximum (FWHM) of the seeing disc).

14. In cities, seeing is often much poorer (i.e., larger) than ∼ 1''.

    It has been claimed---by Diane Pypher Smith?---that ∼ 4'' seeing is available near the Las Vegas Strip. Yours truly will believe it when yours truly resolves Castor AB which currently have angular separation of 3.8''.

15. Because of seeing, the practical angular resolution of telescopes without special techniques is limited by the size of the seeing disk rather than Rayleigh criterion above really very modest primary diameters.

16. We can inverse the Rayleigh criterion to get a formula for the primary diameter above which practical resolution (without special techniques) saturates at about the size of the seeing disk:

      D_in(saturation) = (4.952'')*[(λ_μ/0.5 μm)/θ_seeing_disk('')]  , 

    where D_in(saturation) is the saturation diameter in inches, λ_μ is wavelength in microns (μm), and θ_seeing_disk('') is the seeing disk diameter in arcseconds.

    From this saturation formula, it's clear that with good seeing ≅ 1'', the saturation diameter is about a measly 5 inches. With great seeing ≅ 0.4'', the saturation diameter is about a measly 12 inches.

17. As far as angular resolution goes (without special techniques), giant telescopes are no better than relatively small ones (e.g., the Celestron C8 telescope).

    However, light-gathering power grows as the square of the primary diameter, and so yes, the bigger the telescope, the better for going deep (i.e., for observing faint objects).

    That's why we keep making bigger telescopes (e.g., extremely large telescopes (ELTs) such as TMT, GMT, and EELT).

18. What about the human eye?

    Its angular resolution is typically about 1 arcminute (') = 60'', but some sharp-eyed people may be able to do better (see Wikipedia: Naked-eye astronomy).

    Taking the human pupil as the circular aperture, one can use the Dawes limit to estimate human eye angular resolution.

    There is considerable variation between individuals, but typically for scotopic vision the pupil diameter is in the range 4--9 mm ≅ 0.15--0.35 in (see Wikipedia: Pupil: Optic effects). With these values, the Dawes limit formula,

      θ_DL('') = (4.56'')/D_in  , 

    suggests that human eye angular resolution as small as 13'' is possible.

    However, the Dawes limit and the Rayleigh criterion are probably only part of the story in determining human eye angular resolution.

    In any case, human eye angular resolution is too big for seeing (if it isn't awful) to be a limitation in naked-eye astronomy.

19. The seeing limit to angular resolution can be evaded with special techniques as aforementioned.

    These techniques include lucky imaging, speckle imaging, and, creme de la creme adaptive optics.

    None of these are currently available in UNLV's intro astronomy labs.

20. Another way to evade the seeing limit is to observe from space using space observatories (e.g., Hubble Space Telescope (HST)).

    Actually, adaptive optics have gone a long way to reduce the need for space astronomy, but there are still many observations that can only be done from space.

    There is no immediate replacement for the HST when it dies sometime after 2020 (see Wikipedia: Hubble Space Telescope: Orbital decay).