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