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Astronomical Cameras

Lance SimmsMass 1/17/07

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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.

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Subjective Ancient Photometry

PhotometryThe 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!!

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Observations with the Naked Eye

• Maximum sensitivity at 560 560 nmnm

• Cones give color vision• Rods give low intensity

light vision: ~5-9 photons within 100 milliseconds to signal brain

• Most people sensitive to 400400--700700

• Others are sensitive to 380380--780780

• What about Hipparchus? Limits with a dark adapted eye: With Naked eye: 6th MagnitudeWith 20 cm telescope: 20th Magnitude

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The First Astrophotograph

• Taken by John William Draper in 1840• It was a daguerreotype of the Moon

silver

silver halide

1) Exposing2) Developing by

placing cup of heated mercury underneath exposed image

3) “Fixing” image by dipping it in solution of hyposuphite of soda

Positive-image picture made by:

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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)

Photos taken from http://www.mhs.ox.ac.uk/cameras/index.htm?overview

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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

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Improvements in Emulsion Photography

British Chemist Richard Leach Maddox uses gelatin in place of collodion for less sensitive “dry plate”. Stellar photography is practical.

1871

1880 Improvements in dry plates make them 60 times more sensitive than wet plates

1978-80

David Malin invents new techniques for imaging faint objects in color using RGB filters

Photograph made using dry gelatin emulsion

1910s Eastman-Kodak company works with astronomical observatories to increase sensitivity and refine granularity

Emulsion - A mixture of two immiscible substances like Silver halide and gelatin. Other examples include espresso and mayonnaise.

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Pretty Emulsion Pictures

2.5’ Shapley 1 planetary nebula. Images on 3 different emulsions.Exposure times of 35m 30m 30m

100’ Horsehead Nebula in Orion. Images on 3 different emulsions. Exposure times of 60m 60m 60m.

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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.

Fin Fout

d(x,y)

Emulsion Photometry

Fin - Flux in (shine a light)Fout - Flux out (measure with diode)

d - density of silver atoms

€

d = log10(Fin /Fout )

E - exposure valuet - exposure time

€

Intensity = E(d) / tVery Nonlinear!

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Enter the CCD!

- 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

Charge Coupled Devices

Photos:Top: A plethora of CCDsBottom: Kodak KAI-1301E

1024x1280 (1.3 Mpixels) array with 4 micron pixels.

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The Overall Structure

Photo taken from Electronic and Computer-Aided Astronomy: from Eyes to Electronic Sensors

• 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.

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Buried channel

Depletion Region

Field-Free Region

VB

N-type material

P-type material

A Cross Section

Silicon Dioxide

Region Name Material A Single Pixel

Insulating Region

P-type material

VGMetal Gate

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Collecting Charge in an Exposure

VB

A Single Pixel

VG

1) Photon Comes in and enters material

e-

hole2) 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.

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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”Photo taken from Electronic and Computer-Aided Astronomy: from Eyes to Electronic Sensors

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More Clocking

1 of 3 electrodes on a pixel is high

1 1

22 Adjacent electrode is

brought high. Lets charge leak over.

3Adjacent 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

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QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

ReadoutSlow Clock

Fast Clock

Output amplfier gives analog voltage. Digitizing that gives Analog Data Unit (ADU)

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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).

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Sensitivity -- Quantum Efficiency

Quantum EfficiencyA measure of how many photons incident on the detector are converted to charge (0-100%) at a given wavelength.

-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.

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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.

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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.