1. Variable Star Photometry with a DSLR Camera Mark Blackford

2. Outline Where does DSLR photometry fit? Limitations Advantages Which camera and lens? Tripod configuration Tracking mount Camera settings Software control of camera and mount Software for calibration and measurement Transformation Extinction correction Image acquisition and analysis example Spreadsheets for data reduction and observation reporting

3. Why Bother with DSLR Photometry? Drawbacks: – Never designed as a scientific instrument – Lower cadence than visual observers – 14 bit ADC (12 bit for older models) – Non-photometric filters – Defocused image – Small image scale (close stars overlap in images) – Fewer colours than CCD observers

4. Why Bother with DSLR Photometry? Benefits: – Simultaneous RGB (transform to BVR c ) – Don’t need filter wheel and filters – Precision <0.01 magnitude in V – Accuracy <0.01 magnitude in V (with care) – Wide FOV (several target and comparison stars) – Images can be archived – Relatively cheap camera that is also useful for everyday photography – Can do useful science

5. Which Camera? Compact point & shoot cameras – Small lens aperture, limited zoom, small sensor (bright stars only) DSLR – Interchangeable lenses, larger lens aperture & sensor – Canon and Nikon – Canon 450D and later have 14bit ADC

6. Which Lens? Zoom lens – Range of focal lengths – More glass (lower transmission) – Distortions greater – Zoom creep Prime lens Focal length 85mm to 200mm (or longer) Focal ratio as small as possible

7. Choice of Camera Mount Fixed tripod – 6 sec exposures with 85mm f1.8 lens, mag 8 in V – 2.5 sec exposures with 200mm f2.8 lens, mag 10 in V

8. Choice of Camera Mount Manual/motorised barn door mount – 30 sec or longer exposures – Limited tracking time for time series

9. Choice of Camera Mount Alt Azimuth GOTO mount – Easy centring of target – Longer exposure times – Field rotation issues

10. Right Angle Viewfinder

11. Choice of Camera Mount Equatorial GOTO mount – Exposures up to several minutes – Long time series (8 hrs) – Meridian flip issues

12. Attaching Camera to Telescope Mounting rings for alignment and to lock focus

13. Dew Heater Necessary for long time series DIY or commercial product

14. Camera Power

15. Image Capture to Memory Card Remote release cable Camera time and date set manually Camera parameters set manually Must be present during entire session to initiate each exposure Intervalometer allows automatic exposures

16. Camera Configuration RAW format only Manual mode Internal noise reduction (dark subtraction) disabled Exposure times >20sec (scintillation) ISO 400 or lower Lens stopped down one or two stops Defocused to avoid saturation

17. Bayer Filter Array Reasons for defocus: – Adequate sampling of all three colours – Avoid saturation – Increase total photon count

18. Under focus/Over focus Red green1 green 2 blue

19. Camera

20. Image Capture to Computer USB connection to computer – mount and camera control Dimension4 – computer time synchronisation BackyardEOS – framing & focusing MaxIm DL – image capture sequencing – FITS image format

21. Image Calibration & Measurement MaxIm DL – bias, dark & flat field corrections – separate into R, G1, G2 and B images AIP4Win Magnitude Measurement Tool – aperture photometry

22. Master Calibration Frames Master Bias Frames (at least 64 frames) – easy and fast, 1/4000 sec in total darkness Master Dark Frames (at least 32 frames) – easy but not so fast, same exposure as light frames but in total darkness Master Flat Frames (at least 64 frames) – not easy but fairly fast – twilight flats no good, need evenly illuminated screen (light box, EL panel, projection screen, etc)

23. Lightbox

25. Master Flat Frame 200mm f2.8 lens fully open ~25% vignetting at corners

26. Master Flat Frame 200mm f2.8 lens at f3.2 ~9% vignetting at corners

27. Master Flat Frame Highly stretched image of evenly illuminated light box

28. Master Flat Frame Highly stretched image of evenly illuminated light box

29. Master Bias Frame Master bias showing fixed pattern noise (a few ADUs)

30. Image Analysis Spreadsheets Extract raw instrumental magnitudes from AIP4Win MMT report Calculate extinction and transformation coefficients then apply them to get transformed magnitudes Ensemble differential photometry Record and plot observations Present observations in format ready for submission to AAVSO database

31. Case Studies 1.Standard Stars – E1 Region 2.Nova – V1369 Cen (nova Cen 2013) 3.Cepheid (pulsating) variable – R Crucis 4.Eclipsing binaries – eta Muscae – AE Phoenicis

32. Photometric Standard Stars Carefully measured many times in many photometric filter bands – Landolt equatorial standard stars – SAAO (Cousins) standard star fields: Nine E Regions at -45 Dec Three F Regions at -75 Dec Magellanic Clouds – Transformation procedure discussed in VSS Newsletter May 2011

33. E1 Region in Phoenix

34. E1 Region analysis delta = measured magnitude minus catalogue magnitude

35. E102 (HD8382) Light Curve

36. Canon 1100D tests http://www.stark-labs.com/craig/articles/assets/CanonLinearity.pdf (PHD Guiding, Nebulosity)

37. Flat Field Check Line profiles from unevenly and evenly illuminated master flats diagonal 1 (blue) 1% systematic variation diagonal 2 (red) 0.2% systematic variation diagonal 1 (blue) 0.1% systematic variation diagonal 2 (red) 0.3% systematic variation

38. Case Studies 1.Standard Stars – E1 Region 2.Nova – V1369 Cen (nova Cen 2013) 3.Cepheid (pulsating) variable – R Crucis 4.Eclipsing binaries – eta Muscae – AE Phoenicis

39. V1369 Cen AAVSO Light Curve

40. How Does DSLR Compare With CCD?

41. BMGA and BSM South

42. V1369 Cen spectrum 28/12/13 4250 4700 5160 5600 6050 6500 6950 wavelength (Å) Spectroscopy provided by Terry Bohlsen

43. Spectral Response Curves BVR c DSLR H-alphaH-beta

45. Lessons for DSLR photometry Transformation of DSLR instrumental magnitudes is possible for normal stars with approximately black-body spectra Instrumental magnitudes of targets with strong emission and/or absorption features cannot be reliably transformed Useful measurements can still be made of these targets, e.g. time of minimum or maximum light

46. Atmospheric Extinction Observing at low elevations can lead to significant airmass differences across a wide FOV Differential extinction distorts light curves See VSS Newsletter February 2012 Avoid observing below 40 degree elevation Choose comparison stars close to target colour and position

47. Atmospheric Extinction Primary eclipse of RS Sagittari

48. Acquisition and Analysis Example Image capture Bias, Dark, Flat correction and separation of R, G1, G2 and B colour channels Measurement of variable, check and comp stars Analysis spreadsheet

49. Image Analysis Spreadsheets Extract raw instrumental magnitudes from AIP4Win MMT report Calculate extinction and transformation coefficients then apply them to get transformed magnitudes Ensemble differential photometry Record and plot observations Present observations in format ready for submission to AAVSO database

50. Problems To Look Out For Canon 450D had a number of hot pixels Canon 450D USB connector mounted on motherboard, very expensive to fix if broken Laptop stolen – regular backup recommended Canon 600D developed an instability that caused red and blue channels to exhibit slow oscillation in ADU level Make sure image calibration is done properly – check FITS header history Try to stick with the same camera settings as much as possible (ISO, f ratio, exposure time)

51. 600D Instability

52. Conclusions DSLR photometry can be both accurate and precise when suitable targets are selected Targets with strong emission or absorption spectra are not suitable for transformed magnitudes but useful observations are still possible (e.g. novae and supernovae) Cepheid variables and eclipsing binaries are suitable and scientifically worthy targets Variable Stars South can give your (night) life purpose http://www.variablestarssouth.org/