QHYCCD QHY8L - Manual (Page 21)

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Partial Cropped DARK Image (D)
DARK image which be calibrated by BIAS Field （D-B）
Capture FLAT Image
FLAT image can be used to rectify the problem of
the center light but the edge dark which caused by
the vignette of optical system. It also can be
corrected the problem due to some small dust on
the CCD sensor glass surface.
Flat-field calibration method is this: you need
uniform luminous lightbox. The lightbox will be
placed in front of the telescope. You would set the
appropriate exposure time, then continuous shoot
10-50 frame flat-field images. Save and overlay
these images to form a FLAT Master image, save
the image.
To obtain an accurate flat-field calibration, Please
note the following matters:
You have to take immediately flat-field images
before the shooting or after finishing shooting.
Like do this in order to avoid dismantling the
device, dust on CCD surface and vibration of CCD
cause the change of location. The scope of
exposure is better for 30% of the maximum
dynamic range , which means you have gotten
20000-30000 pixel of flat-field image.
Images of BIAS Master, DARK Master and FLAT
Master will be calibration data as image post-
processing of the captured images .
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