The History of Astronomical Photography…and Stuff Lance Simms Tri-Valley Stargazers Meeting 11/18/2016 11/18/16

“You know what’s funny? I always thought that dogs, ah…., laid eggs. And I learned something today.”

A Little About My Work Characterizing/operating sensors for CubeSat payloads Firmware for JWST detectors Sensor electronics engineering for Advanced Star Trackers 11/18/16

The Old Days That star looks like a magnitude 2.3 You’re crazy! No way it’s brighter than 2.5! You just like to argue. I’m gonna go look at stars with someone else 11/18/16

BLUF: Bottom Line Up Front Why is being able to accurately quantify the brightness of an object so crucial in astronomy/astrophysics/cosmology? Why is the naked eye so bad at doing this? What are some of the key moments in the history of astronomical photography? How do CCD/CMOS imagers work and why are they so great? Where are we at now? 11/18/16

Subjective Ancient Photometry The measurement of apparent magnitudes of astronomical objects, performed through various filters, i.e. different wavelength bands. Hipparchus (64bc-24ad) • Divided stars into six magnitudes. 1-brightest …… 6-dimmest • He did it by eye!! 11/18/16

Why Is Photometry Important? Photometry gives us the brightness of a star as viewed from Earth (apparent magnitude) 11/18/16

Why Is Photometry Important? Energy If we know how far it is, we know how bright it is intrinsically (absolute magnitude) and how much energy it emits from its surface (luminosity) 11/18/16

Why Is Photometry Important? Temperature If we know how much energy it puts out (in certain colors) we can estimate its temperature 11/18/16

Why Is Photometry Important? Surface Area If we know how luminous it is and what its temperature is, we can measure its surface area (i.e. its size) 11/18/16

Why Is Photometry Important? If we see regular changes in brightness, we can infer the presence of a planet around a star! 11/18/16

Photometry with the Naked Eye Maximum sensitivity at 560 nm Cones give color vision Rods give low intensity light vision: ~5-9 photons within 100 milliseconds to signal brain Most people sensitive to 400-700 Others are sensitive to 380-780 What about Hipparchus? Limits with a dark adapted eye: With Naked eye: 6th Magnitude With 20 cm telescope: 20th Magnitude 11/18/16

So even with a great telescope the naked eye is a really bad photometric instrument!!! 11/18/16

The First Astrophotograph Taken by John William Draper in 1840 It was a daguerreotype of the Moon  Positive-image picture made by: Exposing Developing by placing cup of heated mercury underneath exposed image “Fixing” image by dipping it in solution of hyposulphite of soda silver halide silver 11/18/16

The First Astronomical Cameras Cameras used by Warren De La Rue to photograph the moon in early 1850s Used Wet Collodion plates (equivalent of film) 11/18/16 Photos taken from http://www.mhs.ox.ac.uk/cameras/index.htm?overview

Wet Collodion Pictures…Messy Improvement Used on large telescopes from 1850-1890 or so  Involves solutions of iodides and bromides smeared onto a glass plate, dipped into a solution of silver nitrate. Plate is wet and dripping while picture is taken. Must be developed within 10 minutes or image is ruined. Horse-drawn darkrooms!! Room for Improvement with Wet Collodions - Long exposure times for little reward Very weak response to red light Do away with horse manure 11/18/16

Improvements in Emulsion Photography 1871 British Chemist Richard Leach Maddox uses gelatin in place of collodion for less sensitive “dry plate”. Stellar photography is practical. 1880 Improvements in dry plates make them 60 times more sensitive than wet plates 1910s Eastman-Kodak company works with astronomical observatories to increase sensitivity and refine granularity 1978-80 David Malin invents new techniques for imaging faint objects in color using RGB filters Photograph made using dry gelatin emulsion Emulsion - A mixture of two immiscible substances like Silver halide and gelatin. Other examples include espresso and mayonnaise. 11/18/16

Pretty Emulsion Pictures 100’ Horsehead Nebula in Orion. Images on 3 different emulsions. Exposure times of 60m 60m 60m. 2.5’ Shapley 1 planetary nebula. Images on 3 different emulsions. Exposure times of 35m 30m 30m 11/18/16

Wet or Dry: Better than the Naked Eye • Human eye cannot “integrate” photons like emulsion does. • Image can be used to do objective photometry. Sending light through negative and measuring how much gets through. Emulsion Photometry Fin Fout Fin - Flux in (shine a light) Fout - Flux out (measure with diode) d - density of silver atoms E - exposure value d(x,y) t - exposure time 11/18/16

Enter the CCD! Charge Coupled Devices - Invented in 1969 by William Boyd and George Smith at Bell Laborotories in New Jersey. No plates or film or wet pastes; just good ol’ fashion circuits Superior to emulsion photographs in almost every regard Photos: Top: A plethora of CCDs Bottom: Kodak KAI-1301E 1024x1280 (1.3 Mpixels) array with 4 micron pixels. 11/18/16

CCD Array and Pixels Simplified Top Down View Simplified Cross-section View 4x4 CCD Pixel Array Single Pixel 11/18/16

Moving the Electrons: Clocking - 3 phase transfer CCD shown above - Each pixel has 3 electrodes that take on three different voltages in sequence. This process is called “clocking” 11/18/16 Photo taken from Electronic and Computer-Aided Astronomy: from Eyes to Electronic Sensors

CCDs: Key Points of Operation Exposure and readout are separate processes Some CCDs have slight variants of this Destructive readout To read a pixel you must remove its charge Not Randomly Accessible Must shift N-1 pixels to reach Nth pixel in sequence. Only one output amplifier Usually extremely low noise Readout in Charge Domain Moving charges takes power 11/18/16

CCDs: Pros and Cons Reasons why CCDs have dominated astronomy Extremely low noise (sub electron!) Excellent uniformity across pixels Low dark current Good quantum efficiency (photons detected/photons incident) Limitations of CCDs Slow operation Serial access of pixels Very high power (1-5 Watts) Very susceptible to radiation damage Destructive readout (no flux vs. time for long exposure) Blooming of pixels 11/18/16

What kind of instrument do you used to navigate through a slippery forest? A C-MOS (See-Moss) Detector

CMOS Imagers 11/18/16

CMOS Imagers In Detail 3T Single Pixel CMOS Pixel Array In Monolithic CMOS, photodiodes and readout circuitry on same piece of silicon Usually pixels have 3 or 4 transistors depending on architecture Output is a voltage, so we say that CMOS operates in Voltage Domain M. Bigas et al. / Microelectronics Journal 37 (2006) 433–451 3T Single Pixel CMOS Pixel Array

CMOS: Operation Exposure and readout are simultaneous Non-destructive readout charge in pixel is not removed during readout Randomly Accessible Individual pixels and windowed regions of detector can be read and reset while not affecting other pixels. Multiple outputs A row of pixels can be read out of multiple outputs to increase speed Readout in Voltage Domain Voltage of pixel (V=Qpix/Cpix) is buffered to output through source follower (Source Follower per Detector) 11/18/16

CMOS: Pros and Cons Reasons why astronomers have avoided CMOS Higher read noise (much improvement in last decade, though) Poor uniformity across pixels (correctable) High dark current Low fill factor (% of pixel sensitive to light) Benefits of CMOS for astronomy Fast operation Random access of pixels (windowing capability) Very low power (milliwatts) Very low susceptibility to radiation damage Non-Destructive readout (flux vs. time for long exposures) Anti-blooming capability In pixel functionality (event threshold, A/D conversion, etc.) 11/18/16

Recent Advances and Trends CMOS imagers are getting much better (and much faster)! Pixels are getting smaller (bad for astronomy in most cases) Both types of detectors are getting bigger—more pixels e2V CCD290-99 Back Illuminated Scientific CCD Sensor 9216 x 9232 Pixels Sixteen Outputs 10 micron pixels 92.2 mm × 92.4 mm Sony ASI174mm CMOS detector claims 0.8e-! 11/18/16

Keys: Quantum Efficiency + Linearity CCDs and CMOS are Much more sensitive to light than emulsions or the eye And Much more linear No threshold for number of photons per unit time to liberate a charge as in emulsions. Quantum Efficiency A measure of how many photons incident on the detector are converted to charge (0-100%) at a given wavelength. 11/18/16

Photometry Couldn’t be much easier • A little ambiguity about where you count, but digital numbers are easy to add • Also have to worry about noise in the readout amplifier and non-uniform response in pixels of the detector. Much time is spent calibrating the device to deal with this But we are much better off than we were with the naked eye! 11/18/16

Nowadays … we’re closer to That star measures 1100 counts in the detector. That means it’s magnitude 2.3 Technology has made you a lot more tolerable as an observing partner 11/18/16