Astronomical Cameras Lance Simms Mass 1/17/07 1/17/07

The Old Days That star looks like a magnitude 2.3 You’s trippin! No way it’s brighter than 2.5! My head’s bigger. You know I’m right. 1/17/07

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!! 1/17/07

Observations 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 1/17/07

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 hyposuphite of soda silver halide silver 1/17/07

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) 1/17/07 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 1/17/07

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. 1/17/07

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 1/17/07

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 Very Nonlinear! 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 1/17/07

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. 1/17/07

The Overall Structure • CCD consists of a set of picture elements (pixels) • Main material is a semiconductor like Silicon or Germanium • To “read” an image, the charge is coupled from one pixel to another • Typical size of a pixel is about 10 microns on a side • It has been likened to collecting rain in buckets and putting them on a conveyor belt to be fed out. 1/17/07 Photo taken from Electronic and Computer-Aided Astronomy: from Eyes to Electronic Sensors

A Cross Section Region Name Material A Single Pixel Metal Gate VB VG Field-Free Region P-type material Depletion Region P-type material VB Buried channel N-type material Insulating Region Silicon Dioxide Metal Gate VG 1/17/07

Collecting Charge in an Exposure A Single Pixel 1) Photon Comes in and enters material 2) It knocks an electron into the conduction band so it can move through the material 3) Electrons Collect in a potential well. # Electrons is proportional to light that’s hit the pixel. hole VB e- VG 1/17/07

Moving the Charge: 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” 1/17/07 Photo taken from Electronic and Computer-Aided Astronomy: from Eyes to Electronic Sensors

More Clocking 1 1 of 3 electrodes on a pixel is high 1 2 2 3 3 Adjacent electrode is brought high. Lets charge leak over. 2 2 3 Adjacent electrode stays high while other goes low. 3 2 Sets of Clocks: 1 set for horizontal shifting (fast) 1 set for vertical shifting (slow) Transfer in perpendicualr direction blocked by channel stops 1/17/07

Readout Slow Clock Fast Clock Output amplfier gives analog voltage. Digitizing that gives Analog Data Unit (ADU) 1/17/07

Keeping It Cold CCDs are subject to dark current- charges liberated by thermal motion rather than light. The colder they are , the less dark current there is. For amateurs, a peltier module with cooling fins works well (top) Research cameras are generally put into Dewars and cooled with liquid nitrogen (bottom). 1/17/07

Sensitivity -- Quantum Efficiency CCDs are much more sensitive to light than emulsions or the eye 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. 1/17/07

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 CCD. Much time is spent calibrating the device to deal with this. 1/17/07

Nowadays … we’re closer to That star measures 1100 counts in the detector. That means it’s magnitude 2.3 You’re absolutely right! You still got a big ass head. 1/17/07