Variable Star Photometry with a DSLR Camera Mark

Variable Star Photometry with a DSLR Camera Mark Blackford

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

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

Why Bother with DSLR Photometry? Benefits: – Simultaneous RGB (transform to BVRc) – 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

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 450 D and later have 14 bit ADC

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

Choice of Camera Mount • Fixed tripod – 6 sec exposures with 85 mm f 1. 8 lens, mag 8 in V – 2. 5 sec exposures with 200 mm f 2. 8 lens, mag 10 in V

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

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

Right Angle Viewfinder

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

Attaching Camera to Telescope • Mounting rings for alignment and to lock focus

Dew Heater • Necessary for long time series • DIY or commercial product

Camera Power

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

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

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

Under focus/Over focus Red green 1 green 2 blue

Camera

Image Capture to Computer • USB connection to computer – mount and camera control • Dimension 4 – computer time synchronisation • Backyard. EOS – framing & focusing • Max. Im DL – image capture sequencing – FITS image format

Image Calibration & Measurement • Max. Im DL – bias, dark & flat field corrections – separate into R, G 1, G 2 and B images • AIP 4 Win Magnitude Measurement Tool – aperture photometry

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)

Lightbox

Lightbox

Master Flat Frame 200 mm f 2. 8 lens fully open ~25% vignetting at corners

Master Flat Frame 200 mm f 2. 8 lens at f 3. 2 ~9% vignetting at corners

Master Flat Frame Highly stretched image of evenly illuminated light box

Master Flat Frame Highly stretched image of evenly illuminated light box

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

Image Analysis Spreadsheets • Extract raw instrumental magnitudes from AIP 4 Win 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

Case Studies 1. Standard Stars – E 1 Region 2. Nova – V 1369 Cen (nova Cen 2013) 3. Cepheid (pulsating) variable – R Crucis 4. Eclipsing binaries – eta Muscae – AE Phoenicis

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

E 1 Region in Phoenix

E 1 Region analysis delta = measured magnitude minus catalogue magnitude

E 102 (HD 8382) Light Curve

Canon 1100 D tests ISO 100 200 400 800 Gain 3. 41 1. 67 0. 84 0. 40 FWC 46376 25603 12787 Read Noise 24. 2 12. 5 7. 0 4. 2 electrons Max ADU 13584 15303 15304 ADU 14 bit 16384 e/ADU 6088 electrons http: //www. stark-labs. com/craig/articles/assets/Canon. Linearity. pdf (PHD Guiding, Nebulosity)

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

Case Studies 1. Standard Stars – E 1 Region 2. Nova – V 1369 Cen (nova Cen 2013) 3. Cepheid (pulsating) variable – R Crucis 4. Eclipsing binaries – eta Muscae – AE Phoenicis

V 1369 Cen AAVSO Light Curve

How Does DSLR Compare With CCD?

BMGA and BSM South

V 1369 Cen spectrum 28/12/13 4250 4700 5160 5600 6050 6500 6950 Spectroscopy provided by Terry Bohlsen wavelength (Å)

Spectral Response Curves BVRc H-beta H-alpha DSLR

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

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

Atmospheric Extinction Primary eclipse of RS Sagittari

Acquisition and Analysis Example • Image capture • Bias, Dark, Flat correction and separation of R, G 1, G 2 and B colour channels • Measurement of variable, check and comp stars • Analysis spreadsheet

Image Analysis Spreadsheets • Extract raw instrumental magnitudes from AIP 4 Win 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

Problems To Look Out For • Canon 450 D had a number of hot pixels • Canon 450 D USB connector mounted on motherboard, very expensive to fix if broken • Laptop stolen – regular backup recommended • Canon 600 D 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)

600 D Instability

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/