assa imaging group dslr photometry talk 27 nov 2015

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DSLR Photometry of Variable Stars David Benn, Nov 2015

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Page 1: ASSA imaging group DSLR Photometry Talk 27 Nov 2015

DSLR Photometryof Variable StarsDavid Benn, Nov 2015

Page 2: ASSA imaging group DSLR Photometry Talk 27 Nov 2015

Outline

Brief overview of variable stars The varieties of variable star observing What’s the goal? Getting started with DSLR photometry

My setup Procedure

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Introduction to Variables

Source: Society for popular astronomy

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Introduction to Variables

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Introduction to Variables

https://en.wikipedia.org/wiki/File:Eclipsing_binary_star_animation_2.gif

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Introduction to Variableshttp://www.citizensky.org

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Introduction to Variableshttp://www.astrosurf.com/aras/novae/Nova2013Del.html

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Variable Star Observing Varieties

Visual variable star estimation since 2009 Mostly binocular Simple (technique, equipment) & quick Great for long period variables, novae ~0.1 magnitude precision

Wide-field DSLR photometry since late 2014 ~0.01 magnitude precision Can quantify error Not as precise as CCD or PEP but 10x lower cost Repeatability; data re-use in future, e.g. for B, R

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+ CHOICE courses

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What’s the goal?

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What’s the goal?

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What’s the goal?

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finder chart of W Sgr to describe technique

Visual

http://www.citizensky.org, Southern Gems tutorial

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DSLR Photometry: My Setup

Overall motivation low cost, time suburban backyard

Canon 1100D with 200mm lens on LX-90 or tripod Card table and light shield; light box for flats Free or readily available software

IRIS (free; WINE on Mac), AstroImageJ, Muniwin, … Spreadsheet (Excel, OpenOffice) Sky Safari Pro (telescope control, with SkyFi)

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DSLR Photometry: My Setup

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DSLR Photometry: My Setup

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DSLR Photometry: My Setup

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My Setup

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DSLR Bayer Array

https://en.wikipedia.org/wiki/Bayer_filter

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Some Considerations

B, V, R possible from a single image May be more than one variable per image ISO 100 or 200 or 400 for high dynamic

range Exposure: depends upon tripod vs tracked Turn off auto features

noise reduction lens cleaning

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Procedure

Finder chart & photometry table Locate field, take test frame to verify

Make notes, e.g. of conditions! Deliberately under-focus for photometry Convert RAW to software’s format, e.g. FITS Calibration: lights, biases, flats, darks (maybe) Stellar registration, for automatic photometry

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Procedure

Extract green (or blue or red) channels Determine photometry radii Local coordinates of target, reference stars

Some software may automate via plate solve Photometry on individual or stacked images Look at error, residuals; change one or more

reference (comparison, check) stars if too high Ensemble-based transformation to V (or B or R)

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Photometry Table from Finder

Have to consider:

• Magnitude• B-V

when choosing reference stars.

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Deliberately under-focus

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DSLR: Bayer Array

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Calibration

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CFA Conversion

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Stellar Registration

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Extract R, G, B channels

green channel: img-gN

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Target Region

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Target Region

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Aperture Photometry

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Aperture Photometry

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Aperture Photometry

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Aperture Photometry

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Differential Photometry

Source: AAVSO DSLR manual

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