These are the slides for a talk I gave to the imaging group of the Astronomical Society of South Australia about DSLR photometry of variable stars on November 27 2015.

1. DSLR
Photometry
of Variable Stars
David Benn, Nov 2015

2. Outline
 Brief overview of variable stars
 The varieties of variable star observing
 What’s the goal?
 Getting started with DSLR photometry
 My setup
 Procedure

3. Introduction to Variables
Source: Society for popular astronomy

4. Introduction to Variables

5. Introduction to Variables
https://en.wikipedia.org/wiki/File:Eclipsing_binary_star_animation_2.gif

6. Introduction to Variables
http://www.citizensky.org

7. Introduction to Variables
http://www.astrosurf.com/aras/novae/Nova2013Del.html

8. 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

9. + CHOICE courses

10. What’s the goal?

11. What’s the goal?

12. What’s the goal?

13.  finder chart of W Sgr to describe technique
Visual
http://www.citizensky.org, Southern Gems tutorial

14. 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)

15. DSLR Photometry: My Setup

16. DSLR Photometry: My Setup

17. DSLR Photometry: My Setup

18. My Setup

19. DSLR Bayer Array
https://en.wikipedia.org/wiki/Bayer_filter

20. 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

21. 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

22. 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)

24. Photometry Table from Finder
Have to consider:
• Magnitude
• B-V
when choosing reference stars.

25. Deliberately under-focus

26. DSLR: Bayer Array

27. Calibration

28. CFA Conversion

29. Stellar Registration

30. Extract R, G, B channels
green channel: img-gN

31. Target Region

32. Target Region

33. Aperture Photometry

34. Aperture Photometry

35. Aperture Photometry

36. Aperture Photometry

37. Differential Photometry
Source: AAVSO DSLR manual

40. 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_nw=174
 https://www.aavso.org/observed-minima-timings-eclipsing-
binaries
 https://en.wikipedia.org/wiki/Color_filter_array