Takahashi FC-60 60mm f/10.7 (640mm) 120 seg. 800 ISO (mode 1)
original (cropped) tiff (1.5 Mb)
Below are two consecutive raw shots of Messier 13 globular cluster taken with the noise reduction off (mode 1), and the other with noise reduction on but having turned off the camera while it was taking the dark frame (unofficial mode 3), leaving on the memory card the original unadultered CCD image before the dark subtraction and filtering.
Read the excellent article at Christian Buil's site http://astrosurf.com/buil/d70v10d/eval.htm for a full evaluation and a comparative test with Canon 10d.
The images speak for themselves - while the hot pixels are fully subtracted on "mode1", the filtering applied is imensily destructive on fainter stars, looking defocused, and sometimes simply vanish. The only survivours are pratically the two brightest stars. Notice that on both images have the same size = same focus at same exposure. In the case of nebulas and galaxies, this median filtering isn´t much noticiable, except if it contains faint undersampled stars.
By comparing the two images I've noticed that stars with sizes less than 3 or 4 pixels (somewhat undersampled) are removed , 5 to 10 pixels the centroid is smeared, above 10 pixels are unaffected.
The images were converted with exactily the same settings from the NEF format.
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Messier 13 Takahashi FC-60 60mm f/10.7 (640mm) 120 seg. 800 ISO (mode 1) original (cropped) tiff (1.5 Mb) |
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Messier 13 Takahashi FC-60 60mm f/10.7 (640mm)120 seg. 800 ISO (mode 3) original (cropped) tiff (1.6 Mb) |
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Messier 13 e NGC 6207 Takahashi FC-60 60mm f/10.7 (640mm) 10x120 seg. 800 ISO (mode3) |