DSLR Photometry Experiments. A dark curiosity
While examining images acquired as part of an on-going DSLR photometry campaign of the nova V1369 Cen (Nova Cen 2013), the author noticed a puzzling feature of the dark frames: their file sizes decreased in a regular fashion.
The dark frames were taken with a Canon EOS 60D (ISO 1600, 9-s exposure time) and recorded in Canon raw format (CR2). On-camera noise reduction was disabled.
The series was taken immediately after several series of light frames of V1369 Cen were recorded (see "DSLR Photometry Experiments. 4. How many darks?" for full details.)
The camera remained outdoors, the lens cap was put in place, and a dark cloth draped over the camera, covering the lens and view finder. An intervalometer was programmed to take the images with 16 seconds between the start of each frame. The series was started at 2014 January 27 04:51:59 SAST and ended at 05:02:23 SAST.
Figure 1 shows the file sizes of the 40 dark frames, in the order in which they were recorded.
Figure 1. File size of a sequential series of 40 dark frames taken with a Canon EOS 60D (ISO 1600, 9-s exposure time, CR2 raw format).
Figure 2 (left panel) shows the standard deviation of the pixel values, as reported by the stat routine of IRIS (version 5.59), while the right panel shows the mean values of the frames.
Figure 2. Descriptive statistics of the 40 dark frames, showing the standard deviation of the pixel values (left panel) and the mean pixel values (right panel) of each frame.
The author is at an utter loss to explain these relationships, which is also seen in other series of dark frames taken during the present campaign (see Appendix). At least one question that is raised is: are all darks created equally?
Depite still being lost in the dark, the author would like to thank Evan Knox-Davies, Mark Blackford, Jerome Jooste, Dieter Willasch and Brett du Preez, for valuable comments.
Several series of lights and darks were obtained, and their standard deviations (pixel value) were determined.
Series A – light frames – ISO 1600 – exposure time 5 s
Series B – light frames – ISO 1600 – exposure time 7 s
Series C – light frames – ISO 3200 – exposure time 5 s
Series D – light frames – ISO 6400 – exposure time 5 s
Series E – dark frames – ISO 1600 – exposure time 5 s
Series F – dark frames – ISO 3200 – exposure time 5 s
Series G – dark frames – ISO 6400 – exposure time 5 s
nothing more to see. please move along.