Bias Frame Calibration - SBIG STF Series User Manual

3.1 Bias Frame Calibration

Bias is an offset that occurs when a pixel is read from the camera.
Unfortunately, bias can vary across the image. Also, if bias is not corrected,
then flat-field calibration will not work correctly.
A bias frame is essentially an exposure with a duration of zero (or in reality, as
close as possible to zero length) and taken with the shutter closed. Each pixel
will have a slightly different value, but except for a small amount of noise, the
value for any one pixel will be consistent from image to image. Since the bias is
consistent from image to image, it can be subtracted.
The bias frame itself contains a small amount of readout noise. This readout
noise is produced inside the electronics that read the pixels. It can be very low
in sophisticated cameras, but it is never zero. This noise can be easily
suppressed by combining a number of bias frames together.
Ideally, the other types of calibration frames should also be bias-frame
calibrated. MaxIm LT does this automatically when bias frame files are
selected.
The bias for a particular camera is generally constant over a substantial period
of time. This means that you can take bias frames just once, and use them on
all your images for many months to come. Note that some cameras may have a
small bias dependency on temperature. Small bias offsets are not important of
themselves, but they can degrade the effectiveness of flat-fielding calibration.
It should be mentioned that bias is also included in dark frames. As such, it
actually is possible to perform accurate calibration without using bias frames.
That said, you should always use bias frames if either of the following are true:
• You are using exposure scaling to match dark frames to light frames
You are using flat-field frames but are not using matching dark frames
•
("flat-darks")
Bias frames are extremely easy to acquire, so there is little reason to skip
them. You can quickly acquire 10 or 20 frames to average, so the read noise in
the bias frames will not be contributing to your overall noise level.
Taking Bias frames
1. Connect to the camera (as per section 2.1) and click the Expose tab.
2. Configure the tab's parameters as follows:
•
Click on the Camera 1 radio button.
•
Exposure Preset: irrelevant
•
Subframe: Deselect the On checkbox. MaxIm DL will extract the
appropriate subframe from the calibration full frame if required.
3. Click the Options arrow and set the following:
•
Click on No Calibration.
•
Click on Set Image Save Path and specify the desired file location.
SBIG STF SERIES - CAMERA USER'S MANUAL
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