Eye as a Photometer

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Eye as a Photometer

• By Mike Linnolt (LMK)
• Advanced Visual Observing Workshop
• 94th Annual Meeting of the AAVSO
• Oct. 14, 2005

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(No Transcript)
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Contents

• History of Astronomical Optical Photometry
• Biology of vision
• Physics of vision
• Comparisons of optical photometers
• Optimal visual techniques

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History of Optical Photometry

• 130,000 years ago first modern man Homo sapiens
  sapiens observes the sky.
• 3000 B.C. First systematic recorded astronomy.
  (Egypt, India, China)
• 1608 First telescope. (Lipperhey, Galileo )

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History

• 1849 First Astronomical photograph. (Full Moon by
  Canandaigua, NY amateur)
• 1850s daguerrotypes at HCO.
• 1880s photography overtakes the eye as primary
  technique. (Henry Draper/Barnard pioneers)

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History

• Prior to 1932 telescopic photometry was done
  visually using diaphragm and wedge photometer.
  Accuracy 0.1 mag.
• Subsequently, Vyssotskys development of
  thermoelectric microphotometer made plate
  photometry the principal technique.

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History

• ca. 1950 Photomultiplier tubes (PMT) took over as
  main tool for photometry. (RCA 1P21 / Hammamatsu)
• ca. 1980-present CCD became primary photometric
  technology.
• Visual photometry was primary method for 99.9
  human history!

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Biology of Vision - pioneers

• 1684 First microscopic study of retina
  (Leeuwenhoek)
• 1825 Purkinje spectral shift in dark.
• 1832 Webers Law contrast thresholds.
• 1853 Grassman Laws of trichromacy.
• 1866 Schultz rods and cones.
• 1878 Kuehne isolates rhodopsin.

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Pioneers

• 1893 Cajal complete anatomy of retina.
• 1918 Holmes first map of visual field in striate
  cortex.
• 1942 Hecht, et. al. rods respond 1 quanta.
• 1952 Kuffler center-surround receptive field of
  ganglion cell.
• 1971 Lands Retinex theory of color constancy vs.
  luminance.

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Biology of vision - system

• Retinal ganglion cells send axons
  temporal/ipsilateral nasal/contralateral to half
  brain.
• Ganglion cells connect to LGN, sup. colliculi,
  NOT.
• V1 primary visual cortex field in topographic
  order.
• 60 of cortex for vision.

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Biology of vision - eye

• Cornea aqueous humor provide 2/3 of refraction,
  lens 1/3.
• Ciliary muscles control lens shape for
  accommodation (focus).
• Focus ability is lost past 5th decade.
• Iris has muscles to control pupil size.

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Biology of vision retina

• Retina is 0.4mm thick of 7 functional layers.
• 3 dark layers of cell bodies.
• 3 light plexiform layers of axons and synapses.
• Photoreceptor layer at the back (light must
  pass thru all layers 1st )

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Retina photoreceptor cell

• In outermost cell layer - transduces light signal
  to nerve impulse.
• Outer segment stacked photosensitive discs.
• Inner segment in ONL has the cell bodies.
• 2 types rods, cones.
• Rods monochromatic, very sensitive.
• Cones color vision, bright light.
• Leaky pacemaker type cells constant influx Na

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Rods

• Far outnumber the cones.
• Just one photopigment rhodopsin, peak _at_ 505nm
• Can respond to single quanta!
• Many-to-one connection with bipolar cell.
• Outer segment a modified cilia.
• Discs constantly regenerated from plasma
  membrane.

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Cones

• 6 million in retina
• 50µ x 2µ
• S,M,L cones trichromacy
• Each has different photopsin (pigment filters
  incoming light in discs)
• S cones about 1/10 as numerous as M,L.
• S are UV sensitive.
• 1-to-1 conx with bipolar.

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Rod vs. Cone distribution

• Rods are far more numerous than cones.
• Peak concentration of rods at 15º eccentricity.
  (averted vision)
• Note asymmetry in rods medial vs. lateral - not
  axially symmetric.

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Retina horizontal cell

• Processes in OPL.
• Receives input from PR.
• Inhibitory sends processes laterally to
  adjacent bipolar cells.

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Retina bipolar cell

• Cell body INL.
• Leaky pacemaker like photoreceptor.
• Dark-adapted bipolar continuously inhibited by
  PR. (Na channels closed cell is off due to
  constant pacemaker input from PR)
• Synapses (again inhibitory) with Ganglion cell in
  IPL.
• First response to contrast detection.

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Retina amacrine cell

• Another inhibitory cell with horizontal processes
  in IPL.
• Interconnects ganglion cells.
• Limiting factor for scotopic resolution.
• Density peaks 5-15º of eccentricity.
• Corresponds to maximum scotopic resolution (not
  sensitivity).

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Retina ganglion cell

• Innermost cell layer which sends final composite
  retinal signal via optic nerve to brain. (axons
  go direct to LGN)
• Also leaky pacemaker normally on in absence of
  input from bipolar cell.
• Basis of Center-surround receptive field.
• Small central area signal summation.
• Larger surround region subtracts.
• 4 types ON, OFF, midget, parasol.

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Retina - evolution

• Vertebrate retinas are inverted
  photoreceptors at the back.
• Cephalopods, the highest invertebrates
  (Octopus/Squid) have everted receptors in
  front, not outgrowth of brain.
• Evidence both evolved in parallel and not of
  homologous ancestry.

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If Owls could do CVs

• Nocturnal animals evolved structures to maximize
  light sensitivity.
• Tapetum lucidum reflects quanta that miss PR
  back to be detected.
• Larger retinas, larger pupils, more rods.
• 5 magnitude advantage over humans!
• If we had such eyes, Visual would have Respect!

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Where vision begins - opsins

• Belong to the family of GPCR (G-protein coupled
  receptors).
• One of 4 main classes of receptors.
• Spans lipid bilayer of cell membranes.
• 7-transmembrane a-helix structure.
• Modification of ligand by light causes
  conformational change, initiates intracellular
  signaling cascade.
• Many hormones (eg. Insulin) use GPCR.

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Opsins in the membrane

• This image shows the opsin molecule (yellow)
  embedded in the lipid bilayer of outer segment
  discs.
• The light-sensitive ligand 11-cis-retinal
  (orange) is covalently bound within it.

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Light-sensitive retinal

• First step in vision is absorption of photon by
  chromophore (ligand) 11-cis-retinal.
• Fast isomerization to all-trans retinal in 200fs.
• Slow reversion (enzyme mediated) back to cis.
  (dark adaptation)

?
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Signaling cascade

• The isomerization to all-trans retinal forces a
  conformational change in the opsin.
• This activates the cytoplasmic G-protein
  transducin.
• Which leads to hydrolysis of cGMP.
• This causes closing of Na channels and
  hyperpolarization of the photoreceptor cell.

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Unusual signaling

• Typical nerve cells are polarized -60mV (Na
  channels closed) in resting state.
• Photoreceptor cell relatively depolarized (Na
  channels open) in dark.
• Cell is on sending continuous signals to
  bipolar cells.
• But the synapse (neurotransmitter glutamate) is
  inhibitory.

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Retinal is form of vitamin A

• 3 forms of vitamin A retinal, retinol, retinoic
  acid depending on redox state of terminal C15
• Precursor is ß-carotene from dietary vegetables
  which is cleaved in half by intestinal enzyme to
  retinal .

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4 opsins 1 chromophore

• Rods scotopsin 11-cis-retinal rhodopsin.
• Cones photopsins I,II,III 11-cis-retinal
• Human opsins 350 aa proteins.
• Sequence variations between individuals.
• Congenital color/dark vision differences between
  people.
• Why are women rarely color blind? Ans...

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Evolution

• Photo-sensitive and photo-transducing macro
  molecules have been around a long long time.
• Light responding transmembrane proteins found in
  the earliest forms of life (3.5 Gya Precambrian
  era)
• Photosynthesizing cyanobacteria/Archaea.
• Multi-cellular complex life only in last 0.6 Gya.
  (Cambrian explosion Burgess shale)
• Recent research maybe a slower radiation?

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Evolution of opsins

• Opsins appear to be descendents of
  bacteriorhodopsin (BR) of Archaea.
• Structural, functional and sequence homology.
• Rhodopsin (rods) most highly conserved.
• Cone opsins arose by gene duplication and
  subsequent mutation.
• Natural selection dictates which opsin
  sequence/spectral sensitivity is best for the
  species.

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Evolution

• Photosensitive responses in early life (motile
  eubacteria and Halobacteria salinarium).
• Contain additional sensory rhodopsins and
  phototransducers besides BR.
• High sequence homology (30) with BR.
• Strong evidence for evolutionary descent.

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Bacteriorhodopsin

• Found in Archaea the earliest life forms in
  extreme environments. (H. salinarium)
• A 7-transmembrane proton pump.
• Converts light energy (568nm) into pH gradient.
• Electrochemical gradient used by ATP synthase to
  power the cell.

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Bacteriorhodopsin

• 7 transmembrane helices retinal just like
  rhodopsin.
• Absorption of light causes chain of structural
  changes to facilitate proton transport outwards.
• Significant sequence homology with rhodopsin if
  allow for exon shuffling/duplication.

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Physics - Acuity vs. luminance

• Cone branch linear range of 3 log units.
• Asymptotes at high photopic levels.
• Scotopic quantal capture more probable in
  periphery where greater spatial summation, so
  loss of resolution.

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Physics - resolution

• Maximum foveal cone density spacing 2.5µm 30
  arcsec.
• Rayleigh diffraction limit a 1.22 x ?/d 27.7
  arcsec for 5mm pupil.
• Good evolutionary match.
• True foveal vision only in central 1-2º.
• Involuntary visual saccades (4 Hz) give illusion
  of foveal resolution over wider field.

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Physics Blochs law

• Stimuli below 100ms exposure intensity are
  reciprocal (like film).
• Only applies when no background noise.
• Once background light exceeds signal,
  detectability decreases more rapidly.
• Vision is complex, not a single channel
  integrator like CCD!

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Blochs Law

• From ganglion cells to cortical cells, two
  parallel systems M, P.
• M fast, response to motion, contrast, but poor
  acuity.
• P slow, high acuity color. More P?

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Riccos Law

• Spatial summation ability of eye to sum quanta
  over a critical diameter(area).
• L x An k
• Threshold of detection when total luminous energy
  equals constant k.
• Critical diam 30 arcmin parafoveal, 2º at high
  eccentricity.

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Webers Law

• Visual system is evolutionarily selected to
  detect contrast. (object vs. background)
• NOT a simple photon counter!
• Contrast is independent of illumination.
  (contrast invariance ?L/L k)
• K Webers constant 0.14 for rods, .025 for
  cones.
• A major disadvantage vs. PMT/CCD ?

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Physics - Purkinje

• Shift of maximal spectral response to the blue,
  from 550 to 505nm as illumination decreases.
• Due to rods/rhodopsin dominate response at bluer
  peak spectral sensitivity.
• Result Dim red stars appear substantially
  fainter than dim white stars.
• The redder the star, the worse the problem

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Threshold of detection

• Limit is internal neural noise in the retina.
• Spontaneous (thermal) isomerizations of
  chromophore 11-cis-retinal.
• Random opening of cell membrane ion channels.
  (Poisson process/drift-diffusion)
• Spontaneous neurotransmitter releases.

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Physics threshold/QE from obs.

• Dark sky LM16.3 in 37cm reflector (LMK). QE?
• Vega f?(555nm) 3.44x10-8 erg/s/cm2/nm
• F? x (0.1s) x (960cm2) x (300nm) 9.9x10-4 ergs
  incident on eye in 100ms from Vega.
• Mag 16.3 star (3x106 fainter) 3x10-10 ergs
• 1 quanta_at_ 505nm h? 3.9x10-12 erg/q
• 3x10-10 ergs/ 3.9x10-12 ergs/q 77 quanta
• QE 1 response/77 quanta 1.3

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QE of detectors
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Eye vs CCD
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Eye vs CCD
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Eye vs CCD
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Optimal visual techniques

• Place comp and target in same relative position
  in visual field.
• Observe them for identical times.
• Dont fixate exactly the same spot.
• Use comps of similar color as target.
• Very red (B-Vgt2.0) stars are issue.
• Interpolate using comps lt0.5 mag apart.

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Optimal techniques

• Reduce background light as much as possible
  highest magnification, etc.
• Defocus to compare discs not points.
• Allow 30-40 mins for full dark adaptation if
  observing near limits.
• Realize astrometry is poor at scotopic levels.

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Epilogue

• Billions of eyes all over the earth.
• With minimal training and basic equipment the
  potential for scientific contribution is
  enormous.
• For same simple large aperture visual scope
  mag. limit approaches complex CCD setup.
• Eg. Robert Evans visual SN hunter can match
  performance of automated surveys efficiency!

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Take Home message

• Take advantage of the eyes unique properties and
  nurture its potential for scientific
  contribution! (Rapid response)
• Monitoring objects for TOO, satellite,
  ground-based studies - Pro-am collaborations!
• Observe targets without or large gaps in
  coverage. (AAVSO in need of)
• Conduct surveys/searches, keep vigilant for
  unusual behavior or new objects.