Our study of detectors begins with the most important Detectors 1 — astronomical detector of all: the human eye the human eye The eye is more than just a detector: Observational Astronomy 2019 it is a complete instrument package! Part 6 it includes a remarkable optical system and an Prof. S.C. Trager impressive detector

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Elements of the eye Elements of the eye

The iris is an adjusting pupil that dilates and contracts changes the speed of the The lens is the objective eye from ~f/3 to ~f/8 and focuses light onto the back of the eye can also squeeze the lens to change the focal f≈24 mm (focal length) length of the eye (“accommodation”) When dark adapted, the pupil has diameter~7 mm

3 4 Elements of the eye Elements of the eye

The retina is the detector, consisting of light-sensitive neurons on the (remarkable!) curved focal plane of the eye The optic nerve is the Curvature allows 180º “electronic cabling” that field of view! carries the signals from the retinal neurons to the The fovea is center of the brain retina Retina: from Latin rete, “net”

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The retina The retina

Typical spectral response of human eye Typical spectral response of human eye

The optics of the eye are very interesting... but here we’ll focus on the The “readout time” of the eye is quite fast, ≈30 Hz detector: the retina This is why video uses 25–30 fps (frames per second) typically The human eye can detect typically from 3900 Å—7800 Å, with some people able to see as blue as 3100 Å (the atmospheric cutoff!) and some as Gives the eye the ability to detect rapid motions red as 1µm(!) The eye is also sensitive to a wide range of light levels: 9–10 UV transmission set by the transmission of the crystalline lens orders of magnitude!

7 8 Retinal cells Retinal cells

There are about 125 million Like a frontside-illuminated (1.25×108) photoreceptor CCD (as we’ll see), light cells (“pixels”) over the reaches the photosensitive part of the nerve cells after focal surface of the eye passing through the These sense light by “electronic cabling” of the chemical charges induced nerve fibers leading to the by photon absorption optic nerve Also like a CCD, a smaller The retina — like a number of nerve cells are photographic plate — is multiplexed to read out a chemical detector multiple receptors

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Retinal cells Retinal cells

Rods (2 µm diameter): There are two types of nerve endings: The majority (95%) of retinal cells Rods Panchromatic but not Cones frequency sensitive

11 12-1 Retinal cells Retinal cells

Rods (2 µm diameter): The majority (95%) of retinal cells Rods: Panchromatic but not frequency sensitive Active chemical is rhodopsin, a complex protein weighing about 4×105 amu When a photon is absorbed, rhodopsin splits off a 264 amu fragment (chromophore) called retinaldehyde

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Retinal cells Retinal cells

Rods: It requires 1–10 photons to trigger a particular rod Many rods are bundled together by a single nerve

Rods: fiber and work together Instantaneously with this splitting one of the double bonds changes from a cis to a Regeneration of the full rhodopsin molecule takes trans bond ~30 minutes This splitting appears to cause the neuron’s membrane to become permeable to Na ions, changing the electrical potential of the cell This in turn propagates through nerve cells to transmit the signal to the brain

14 15 Retinal cells Retinal cells

Rods: Rods are concentrated to the outer part of the retina Cones (6 µm diameter) They are completely About 5% of retinal missing from the 0.3 mm- cells diameter fovea centralis, in the center of the yellow Probably work similarly patch called the macula to the rods: active Note that the image of chemical is iodopsin the full moon on the retina is 0.2 mm...

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Retinal cells Retinal cells

Cones:

Provide color sensitivity through three different Cones: kinds: β (blue), γ (green), and ρ (red) Cones are 1:4:8 by number concentrated in the macula at the center of The combination of relative the retina excitation and transmission of signals gives the color sensitivity of the eye Rough sensitivity of cones

18 19 Photopic vision: Dark adaptation In bright light, the rods are not sensitive because the rhodopsin has already been split Occurs as the rods “turn Thus the cones are mostly at work on” Scotopic vision: As illumination decreases, The rods are active in dim light three process occur to allow you to continue to So are the cones, but they have only 1% of the maximum see: sensitivity of the rods and do not provide much information Pupil dilation, taking The retina is most sensitive when the rods are “on”, but there is no ~1.5 seconds color response

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Dark adaptation Dark adaptation

Neural adaptation Photochemical synaptic interactions adaptation effectively bundle photoreceptors together Rhodopsin is similar to “binning” on a secreted and CCD, and like in that case, regenerated results in loss of resolution Takes ~30 minutes In daylight, resolution is ~1′ but can be as bad for full sensitivity, but as 1º in darkness most dramatic effect is in first ~3 minutes Takes ~200 ms

22 23 Dark adaptation Resolution of the eye

In naked-eye astronomy, we use averted vision to see faint In principle, set by sources, because rods are absent at the fovea centralis diffraction: θ=1.22λ/d Don’t stare directly at your source! d is the diameter of the Look to one side but avoid the pupil blind spot where the optic nerve passes out of/into the eye Therefore resolution of Because rhodopsin can the eye depends on become depleted, it’s also best pupil size to keep moving your eye around Image of the retina of the eye showing optic nerve “blind spot” on left where optic nerves collect. The highest sensitivity spot on the retina is symmetrically opposite, near the dark spot in this image. Image from www.croydonastro.org.uk/Vision7-mod3.pdf.

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Resolution of the eye Resolution of the eye

To distinguish two point- At maximum diameter – sources from each other, 5–7 mm – this limit is they need to be received by separate cones and the smaller (θ≈20″) than the signals must be sent down minimum spacing of separate nerve fibers cones in the eye In the fovea, cones are At minimum diameter, the separated by ≈30″ and limit is ≈40″ and the eye is there is a one-to-one diffraction-limited connection with nerve fibers

26 27 So the effective resolution of the eye at the fovea is This is still larger than the “spot size” in the large-pupil ~1′–2′ limit! The eye can overcome this in two ways: In this case, the eye is undersampled the eye supersamples the image by rapid, 10″ However, in this limit we’re usually in the scotopic vision oscillations on the order of ~few Hz, scanning the regime and not using the cones image across the finest foveal cells

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and cones are actually rods don’t have this property direction-sensitive light pipes outside the fovea, cones are actually broader they are elongated and most of the iodopsin is in rods are multiply-connected to single nerve fibers the bottom of the cell more sensitive to light results in resolution as bad as 1º in dark coming from the conditions center of the pupil than the periphery (the Stiles-Crawford effect)

30 31 All of this means that the point spread function of the eye is quite different from that of, say, a CCD camera exaggerated by the random placement of the “pixels” in the eye

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