ASSA imaging group DSLR Photometry Talk 27 Nov 2015

DSLR Photometryof Variable StarsDavid Benn, 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

Introduction to Variables

Source: Society for popular astronomy

Introduction to Variables

Introduction to Variables

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

Introduction to Variableshttp://www.citizensky.org

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

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

+ CHOICE courses

What’s the goal?

What’s the goal?

What’s the goal?

finder chart of W Sgr to describe technique

Visual

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

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)

DSLR Photometry: My Setup

DSLR Photometry: My Setup

DSLR Photometry: My Setup

My Setup

DSLR Bayer Array

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

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

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

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)

Photometry Table from Finder

Have to consider:

• Magnitude• B-V

when choosing reference stars.

Deliberately under-focus

DSLR: Bayer Array

Calibration

CFA Conversion

Stellar Registration

Extract R, G, B channels

green channel: img-gN

Target Region

Target Region

Aperture Photometry

Aperture Photometry

Aperture Photometry

Aperture Photometry

Differential Photometry

Source: AAVSO DSLR manual

Links https://aavso.org http://www.variablestarssouth.org https://en.wikipedia.org/wiki/Binary_star https://www.cfa.harvard.edu/news/2009-23 http://astro.unl.edu/naap/ebs/animations/ebs.html http://www.popastro.com/news/newsdetail.php?id_n

w=174 https

://www.aavso.org/observed-minima-timings-eclipsing-binaries

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