Amateur Science Getting Started in Photometry

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Amateur Science - Getting Started in Photometry

• Tom Krajci CBA New Mexico
• (formerly CBA Tashkent, Uzbekistan)
• PO Box 1351
• Cloudcroft, NM 88317
• tom_krajci_at_tularosa.net
• http//overton2.tamu.edu/aset/krajci/

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Introduction variable stars are all around us

• http//antwrp.gsfc.nasa.gov/apod/ap041012.html
• Globular cluster M3
• Single image.

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Introduction variable stars are all around us

• Time
• series!

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Overview

• Why photometry?
• Photometry basics
• Scope and mount up to the task?
• CCD image calibration and nothing else
• What targets to select?
• Data what do you do with it?
• Mewrite a scientific paper?!

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Why photometry?

• Appeal of discovery and exploration
• Opportunity to collaborate with other interested
  amateurs and professionals
• Desire to contribute to scientific knowledge
• You love a challenge

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But Im competing with the pros

• Dont compete, complement their observing
• 10 meter scopes look deep, but sky coverage is
  far short of 100
• Time is on your side as an amateur
• Its your equipment no science board tells you
  what to observe
• Easy to collaborate with amateurs and pros

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Do I put visual observers out of business?

• No
• The Future of Visual Observations in Variable
  Star Research
• http//www.aavso.org/publications/ejaavso/v33n1/65
  .shtml
• Shows how visual and CCD observations can
  complement one another

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Dont I need a sophisticated/expensive rig?

• Use what you have
• (This is backwards from the way professionals
  approach photometry, but thats OK)
• Example http//www.aavso.org/publications/ejaavs
  o/ej37.pdf
• Cookbook CCD, 50mm camera lens, stationary mount,
  urban light pollution
• 30 39 degrees declination were surveyed
• 75 new and previously suspected variables were
  detected

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What is photometry?

• Measuring the brightness of stars
• Types of photometry
• Absolute/All-Sky
• Differential (covered in this presentation)
• Filtered
• Unfiltered (covered in this presentation)
• Aperture (covered in this presentation)
• PSF (Point Spread Function) Fitting
• Differential Image Analysis (DIA)

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Problems in photometry

• The atmosphere
• Makes absolute/all-sky photometry difficult
• The sky is not completely dark
• Adds light to the star in all our images
• How do we address these problems?

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The Answer - Differential Aperture Photometry
(unfiltered)

• Compare brightness of one star to another
• Differential photometry (through an aperture)
• Solves the atmosphere problem (mostly)
• Measure sky brightness around the stars
• The annulus (donut)
• Solves the sky glow problem (mostly)

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Differential aperture photometry

• Aperture Annulus

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Photometry Basics

• Imaging basics come first
• Find, focus, track, proper sampling, proper
  exposure
• Stay within linear limits of your CCD
• Answer some important questions
• Are my CCD images properly sampled?
• Is my CCD response linear?

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Why is proper sampling important?

• Undersampled (FWHM lt 2) star brightness depends
  on where it falls on the CCD array

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Why is proper sampling important?

• Oversampled (FWHM gt 4) Too much read noise
  because stars light is spread over many pixels

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Why is proper sampling important?

• Proper sampling (2 lt FWHM lt 4) - good compromise
  between problems in under and oversampling

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Are my CCD images properly sampled?

• Full Width Half-Maximum (FWHM)
• Less than 2 Defocus
• More than 4 Add focal reducer

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Are my CCD images properly sampled?

• Focus versus photometric precision
• http//www.socastrosci.org/Files/SAS_Proceedings_2
  005.pdf (page 101)

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Is my CCD response linear?

• Linear CCD response is essential for proper
  photometric analysis
• Most CCDs are linear at lower pixel values
  40, 50, 60?
• Use a cloudy evening to measure and confirm your
  CCDs behavior indoors
• If your CCD has anti-blooming, turn it OFF

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Is my CCD response linear?

• Need a stable light source
• LED and regulated voltage supply (good to approx
  1)
• http//www.edn.com/archives/1997/071797/15di_02.ht
  m (ultra stable light source)
• More info
• AIP Handbook, chapter 8
• http//overton2.tamu.edu/aset/krajci/st-7/st-7.htm
  
• http//overton2.tamu.edu/aset/krajci/st-7-new/st-7
  -new.htm

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Is my CCD response linear?Bench test results

• Plot of avg ADU rate vs. avg pixel value
• This CCD is linear (-1) up to 62K
• (Except for the lowest light levels)
• Typical CCDs
• show a loss of
• linearity at
• higher pixel
• values

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Is my CCD response linear?

• Other bench tests and analysis will provide you
  with CCD
• Gain
• Read noise
• Dark Current
• These values are important to determine the
  uncertainty in your photometry
• Most software today makes this an easy task

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Field testing!

• Start with unfiltered photometry (KISS principle)
• Take a time-series of any star field as an
  overall system test (use non-variable stars)
• Nominal 60sec. Exposure
• Use autoguider to keep field centered
• Keep star pixel values at/below 50 of max ADU
• Later, try Landolt standard fields (especially if
  you get photometric filters)

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Field testing!

• Second set of tests turn off the autoguider!
• Series of images with star field moved about CCD
  frame
• Tests flat field calibration/vignetting issues
• Can be more problematic if you use focal reducer
• If you have a German mount, take test images in
  both hemispheres

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Field testing of CCD linearity

• This is what you should see in a time series
• A constant star minus a constant star a
  constant (differential) brightness value

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Field testing of CCD linearity

• Another kind of sampling error aperture too
  small! (Aperture 2.5 pixel radius)

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Field testing of CCD linearity

• Find your minimum aperture and never use a
  smaller value!

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What size aperture to use?

• Rules of thumb
• Aperture 5 x sigma 2 x FWHM
• Inner Annulus 7.5 x sigma 3 x FWHM
• Outer Annulus 12.5 x sigma 5 x FWHM

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What signal to noise ratio should I expect?

• http//www.tass-survey.org/richmond/signal.shtml
• Compare actual standard deviation of brightness
  (of constant stars) to predictions
• Should be close, but can never be better
• Bright stars will have higher SNR
• A plot of magnitude versus standard deviation
  will have a characteristic curve.

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What signal to noise ratio should I expect?

• Magnitude versus standard deviation
• Variable star will not fall on the characteristic
  curve
• Variable?
• Beware
• false alarms!

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What signal to noise ratio should I try to
achieve?

• Depends on your observing goals
• Eclipsing binary SNR 501
• Exoplanet transit SNR 2001
• Faint cataclysmic variable SNR 101
• Today magnitude 17.5
• In a few years magnitude 20 - 21

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Comparison stars some recommendations

• Comp stars 0.5 to 1.5 magnitudes brighter than
  target variable
• As close as reasonably possible in color
• Minimizes differential atmospheric extinction
  effects
• Often expressed as a B-V magnitude
• Large B-V object is red
• Near-zero B-V object is white
• Negative B-V object is blue
• NOTE The redder the star, the more likely its
  variable

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Comparison stars how to get color data?

• Planetarium software
• Various star catalogs
• Beware, many star catalogs are astrometric, not
  photometricprecision may suffer
• If all else fails http//www.nofs.navy.mil/data/
  FchPix/
• Examine blue and red magnitudes in the star list

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Mount and focuser up to the task?

• Polar alignment accurate
• German mount meridian flip
• Mirror flop / differential flexure
• Focus shift when temperature changes
• DSCs good enough, or use a flip mirror
• How automated is your system?

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Controlling local seeing and light pollution

• Telescope fans
• Observatory fans (or hose down)
• Long dew/light shield
• Moveable light screens?

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Baffle your CCD

• Baffle size must accommodate your telescopes
  f/ratio (steepness of light cone)

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SCT baffling may need improvement

• No direct sky light falls on CCD, but.
• Only one oblique bounce/scatter allows light to
  reach CCD
• At oblique angles flat black is not very black

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SCT baffling 2nd order baffling

• Larger central obstruction and long dew shield
• No direct rays can even reach sky tube mouth
• Now two oblique scatters are required to reach
  CCDand at less glancing angles

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Calibration frames (and nothing else)

• Master bias
• Master dark
• Master flat (sometimes master flat-dark)
• With repeatable temperature regulation, your
  master bias and dark frames are good for months
• Only need to shoot flats every night
• Use a light box, image in twilight
• No dark time wasted on calibration frames

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What target(s) to start with?

• Eclipsing binaries of fairly short period
• Compare your results to known ephemeris data
• Heliocentric Julian Day?
• http//www.rollinghillsobs.org/
• http//www.aavso.org/observing/programs/eclipser/o
  mc/nelson_omc.shtml
• http//var.astro.cz/ocgate/index.php?langen

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What target(s) to start with?

• One source of eclipse predictions

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What target(s) to start with?

• Compare your timing results to established values
• This is an O-C (observed minus calculated) plot
  of AB And

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Make a finder chart

• Saves hassle when your IQ is low at 3AM
• Keep notes of important settings
• Tailor your chart to your equipment needs and
  capabilities

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More advanced projects

• Cataclysmic variables (global collaboration)
• Asteroid light curves (moving target, determine
  their shape/spin axis)
• Exoplanet transits (low amplitude variability)
• Gravitational lensing events (collaborate with
  pros doing large surveys)
• Gamma Ray Bursts (optical transients)
• And many others!
• Often requires collaboration with other
  astronomers, pro and amateur
• Compare you data against others work
• A tremendous opportunity to learn!

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Why global collaboration is good

• Superhump feature has different period than the
  binarys orbital period
• Superhump
• Eclipse

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Why global collaboration is good
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Why global collaboration is good
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Why global collaboration is good

• Observations from a single location would not
  cover this objects behavior sufficiently

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Check you data against others

• If someone else was working the same target at
  the same time.

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Find a mentor!

• Mentors can be extremely helpful
• Photometry skills
• Source of software tools (light curve analysis)
• Ideas for valuable and interesting targets
• Establish collaborative links
• Prevents isolation and loss of morale
• http//www.aavso.org

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Data what do you do with it?

• Send it to the lead researcher
• AAVSO
• CBA
• Analyze it yourself
• Determine eclipse time of minimum/maximum
• Period analysis of time-series
• Send this analyzed data to the lead researcher
• AAVSO EB team
• or write the paper yourself

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Mewrite a scientific paper?!

• Start simple, start small
• http//www.konkoly.hu/cgi-bin/IBVS?5690

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Mewrite a scientific paper?!

• Next, become a co-author
• Easy contribute data, let the pro do the
  analysis
• Harder do some analysis
• Want to be a lead author?
• Find a mentor
• First submissions are seldom accepted without
  edits

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Conclusion

• Beginner Sounds interesting, and its not
  intimidating
• Intermediate I could do this on the next clear
  night!

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Summary

• Why photometry?
• Photometry basics
• Scope and mount up to the task?
• CCD image calibration and nothing else
• What targets to select?
• Data what do you do with it?
• Mewrite a scientific paper?!

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Questions?
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Amateur Science - Getting Started in Photometry

• Tom Krajci CBA New Mexico
• (formerly CBA Tashkent, Uzbekistan)
• PO Box 1351
• Cloudcroft, NM 88317
• tom_krajci_at_tularosa.net
• http//overton2.tamu.edu/aset/krajci/

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Its not how it looks.
Its what it looks at!