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Flat
Fielding, OR Ghosts & Gremlins Exposed
Below is a flat field
image taken with my ST-10XME w/H-alpha and OIII filters on my 10" LX200R at a 3
o'clock position. This image has been stretched to the maximum in order to
show the many artifacts that are inherent with CCD flat field images. This
is why we need to process with flats.
CLICK on the
image for full resolution and looking closely you can see:
- The actual pixel
grid layout
- Optical
vignetting, or at the very least Cos4 illumination drop off on the left side
(probably due to CCD/reducer mis-alignment)
- Large dust mote
in upper center
- Smaller dust
motes (small donuts)
- Hot pixels in the
flat field that need to be dark-subtracted before being used in processing
- Diagonal "moiré
banding" that is quite possibly a result of the misregistration of
the microlensing over the pixel grid. Also note that this banding is
opposite between the Ha and the OIII flats. In other words, bands that
are bright in Ha are dark in the OIII flats. Look closely at the very
tip of the bottom right corners of both images to see this effect.
- Mottling possibly
due to variations in the filter glass, cover slip, filter wheel window.
Sometimes this shows up as a color/filter specific gradient in a final
image.
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