Digital Image Processing WorkShop'06
Where : Korea Astronomy and Space Science Institute.
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About of Calibration of CCD images ...
Calibration (or PRE-PROCESSING) of an astronomical CCD deep-sky image consists of removing the bias and thermal contribution (DARK FRAME) and dividing the resultant image by the FLAT-FIELD in order to standardize the response of each image pixel.
Calibrated = (Raw - Bias - Thermal) / Flat = (Raw - Dark) / Flat
The DARK frame should be removed firstly from the RAW image then the result should be divided by the FLAT-FIELD.
DARK FRAME
A dark frame is an integration in which no light strikes the CCD. It records the BIAS noise and THERMAL noise for a specific CCD temperature and integration time:
DARK FRAME = BIAS FRAME + THERMAL FRAME
To take a dark frame you can simply cap your telescope and integrate using the same time that you are using for acquiring the raw image. If you are not able to control the chip temperature (+/- 0.1 °C), dark frames should be taken immediately before or after taking a raw image.
For better results shoot lots of dark frames and AVERAGE or MEDIAN SUM them. Averaging frames reduces the random components by the square root of the number of frames averaged: although it takes more time, you will obtain better images.
BIAS FRAME
The bias frame is an image made with an integration of zero seconds and shutter closed. It contains the amplifier zero-point offset, the random readout noise from the amplifier and the noise from camera electronics.
CCD cameras made for scientific imaging usually include the ability to read out a bias frame but for the simple purpose of subtracting THERMAL FRAME and BIAS, a DARK FRAME serves equally well (DARK FRAME = BIAS + THERMAL FRAME). The examination of a bias frame tells you if your camera is working properly: if you see wavy lines or patterns, your camera may not be functioning well.
FLAT-FIELD
The FLAT-FIELD frame is a photosite-by-photosite map of a CCD's sensitivity to light. It is an image of a uniform object such as twilight sky or a sheet of opal glass attached to the inside of the observatory dome. Chip sensitivity, vignetting and dust all appear as variations in the sensitivity of the CCD itself: division by FLAT-FIELD will remove these defects.
When you make a raw FLAT-FIELD, then you must subtract the THERMAL and the BIAS frame from it (or the DARK). For the lowest possible noise, AVERAGE more then one FLAT-FIELD to obtain a Master Flat-Field.
Three different methods of calibration can be used:
* Basic calibration - requires a master dark with the same integration time as the raw image;
* Standard calibration – requires a master dark that has the same integration time as the raw image and a master flat;
* Advanced calibration – requires a master flat, a master dark (same or longer integration time) and a master bias.
Basic calibration is adequate for simple observing tasks. Standard calibration corrects for vignetting and CCD nonuniformity, but constrains you to the same integration timr for your images and darks. Advanced calibration gives you the freedom to use different integration times for your images and provides superior dark subtraction via dark-frame matching.
Individual dark and flat fielded frames or calibrated images can be added or averaged for better results.