Canon 60Da Without Darks

After discussing dark frame requirements and in-camera dark suppression (not long exposure noise reduction) with Roger Clark on the Images Plus Yahoo group I decided to give his technique a try. It seems that when the clean sensor now function is selected, the camera cleans the sensor then takes a short dark to map hot pixels. This is stored in the camera and used to correct  images for hot pixels. This, along with the dark current suppression built into the hardware, removes the need for dark frames. The image below is a  20 minute exposure of M27 taken from an urban site. The image was processed using flats and bias frames, but no darks. The result fully supports Roger's assertion that in camera dark current suppression removes the requirement for dark frames ( I have to admit I was skeptical). Click on the image to see a 84 minute version taken from a dark sky site, click again on the image to return to the original. The short urban image compares very well considering the light pollution and short exposure of the suburban shot.
Object M27, RA 19:59.6 Dec  22:43
Exposure 20 minutes (10 X 2 minutes)
ISO 1600
Camera Canon 60Da DSLR
Optics 200mm SkyWatcher f/5 Newtonian reflector with Paracorr for a total  focal length of 1150 mm
Location

Bedford, Nova Scotia
Processing Calibration and stacking  Images Plus . Arcsinh stretched followed by contrast layers and a final curve adjustment. Details of the dark sky image here.


In addition to the above image I did some more quantitative measurements.  The image was calibrated with flats and bias for the no darks version and with flats, darks and bias frames for the with darks version. Both calibrated images were then multiplied by three on a pixel by pixel basis to linearly increase the brightness and contrast, no additional processing was applied. Finally the mean level and standard deviation of a 29 by 29 square was measured at three spots on the image and used to calculate the SNR of the background. There were no stars or nebula in any of the measurement areas. As you can see from the data, the SNR for each image is pretty much a wash so it would seem that the in-camera dark suppression works quite well. So much for my dark library!

Image Point (x, y) Mean Standard Deviation SNR
With darks 110, 300 14430.696 341.511 42.26
2900, 1590 14026.830 336.941 41.63
2750, 3010 14346.093 343.348 41.78
Without darks 110, 300 14493.589 355.071 40.82
2900, 1590 14045.083 331.520 42.37
2750, 3010 14376.046 341.903 42.05

The image below is a 100 percent crop of the M27 portion of the minimally processed image with no darks used in the calibration. Click on the image to see the version with darks used in the calibration. Click on the image again to return to the original.

 

No darks used in calibration