This is because variations in AF51 exposure setting will also impact on the gain setting
and this in turn may alter the variation in light sensitivity across the field due to non-
linearity of the sensor gain characteristics.
I also advise you make use of the ‘Save as raw doubles?’ option to capture all the
enhanced radiometric information afforded by the multi-frame averaging procedure.
Figure 7.1
Left
: A master flat field image made using the AF51 with PARD Capture. This was done using the
Y-only signal from the camera and multi-frame averaging (only 64 frames were used in this example). The
actual image is darker than shown here because I was making this to do low light level epi-fluorescence
imaging but the grey levels have been stretched (contrast enhanced) for the purposes of this illustration to
show you the blurry dust spots and field curvature which this master flat will be correcting for.
Right
: The
corrections mask used when making this master flat. This accounts for the artificial sharp cut-off seen in the
master flat image. I use a corrections mask in this case because the periphery of the camera field is not
contributing to the imaging of the region of interest (the central round eyepiece field of view).
Quantisation artefacts and random noise
The raw output of each frame from the AF51 camera can distinguish 256 brightness
levels per channel from 0 to 255 (maximum). This 8-bit depth of intensity quantisation
causes ‘chunky’ steps in colour or grey levels in regions where the intensity varies only
slightly. This is quantisation artefact – the actual intensity varies smoothly in real life but
can only be digitised into discrete steps.
Such ‘chunky steps’ in variation of grey scale or colour will also be made worse by the use
of compressed video streams such as MJPEG or compressed image save-as formats
such as JPEG. This form of chunky or blocky image regions will affect spatial resolution
variation and not only intensity variation. These are compression artefacts.
Examples of both quantisation and compression artefacts are shown in figure 7.2 which is
a small crop from a master dark frame image taken with different compression and multi-
frame averaging settings as described in the figure legend.
Using the YUYV camera stream will avoid (M)JPEG compression artefacts but not the
quantisation artefacts. Both these artefacts (but especially grey level quantisation
artefacts) can be greatly reduced by the use of multi-frame averaging instead of taking
just a single frame (figure 7.2). Multi-frame averaging also reduces random noise that
contaminates all camera signals – and especially so in dark conditions.
OptArc AF51 Camera Page 89 of 99 User Guide v1.02
