first and the updated integrated 2 frame composite will be shown. Then a 3-frame
composite, then a 4-frame composite, then a 5-frame composite. After that, the first frame
will be discarded from the composite and the 6
th
captured frame will be added to the
composite. So every new frame will be added to the composite and every oldest frame will
be discarded from the composite. This has the effect of preserving the live preview frame
rate at the expense of showing time-delayed trails of moving objects in the scene.
Note that this is integration, not averaging, so the final result will therefore get lighter
overall (assuming each individual frame is not completely black with zero pixels all over –
as may occur if you are using a preview master dark frame as described below). Because
of this, lighter pixels in each individual frame can get saturated in the integrated result. To
give you some control over this there is a ‘
Preview Bias
’ control. The number you select
here is added to the integral result prior to display and so can shift the overall brightness
of the resulting integrated frame. If you use a negative number here then very bright
regions that would otherwise display as saturated white regions can be brought down
back into the 0 – 255 greyscale display range to reduce or eliminate saturation. You can
also use a positive number here to lighten the overall result. The available range for
Preview Bias is from -4845 to +512.
Note that preview frame integration and bias calculations are all performed in integer
arrays in the program (that is, the ‘int’ data type in the C programming language) so can
handle very large resulting negative and positive intensity values (at least
±
32768 for
computers with 2-byte integers but many modern computers have an ‘int’ that is 4 bytes
long so the range is
±
2147483648). The resulting integrated and bias-shifted intensity
values in the frame buffer are cropped into the range 0-255 for display only after all
corrections have been applied.
Preview dark current correction and preview flat field correction multifunction
button.
The button shown in figure 6.2a labelled as ‘
Load P.Dark
’ is actually a multi-function
button used to load and eject optional master dark and master flat correction images for
the live preview. This button can be cycled into three modes: ‘Load P. Dark’, ‘Load P.Flat’
and ‘Eject All’. The latter option (‘Eject All’) will not be available unless a custom preview
master dark or master flat image is currently loaded. How to cycle the button will be
described shortly, but first I’ll give an explanation of these modes.
An internal preview master dark image and an internal preview master flat image is
always present and actively applied when previewing in frame integration mode (even if
only 1 frame is being ‘integrated’). The internal master dark is always subtracted from
each live preview frame first and then the resulting dark-frame corrected image is divided
OptArc AF51 Camera Page 67 of 99 User Guide v1.02
