Q S I 5 0 0 S E R I E S U S E R G U I D E
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The above numbers represent the minimum of each type of frame you should use. Taking
more of each type of frame will further reduce the noise present in each calibration master
image and improve your final results.
Why
you take different types of image calibration frames was discussed in some depth in
the CCD Imaging Overview section. This section covers
how
to take the required
calibration frames.
In the previous section, we showed how to use the Sequence tab to automatically take a
sequence of images. Adding the required dark frames to an automated sequence is often
conven
many
dark fra
r.
Taking
environ
ight
frames
An exc
me during the night is reserved for taking light
frames. Dark frames can be taken after the sun starts to light up the sky as long as no stray
g
, take at
exposure. Taking more dark frames reduces the
r, producing higher quality images.
e
n general you should plan on always taking at least one
set of bia
aging session.
To take
Sequen
ring
the imag
r light
and dar
Fla i
Flat eld
rities in your optical system, such as vignetting or dust motes,
and
t sensitivity of each pixel on the CCD. Some astroimagers
con
onal. While calibrating images with flat fields does not have as
dramatic an effect on CCD images as applying dark frames, flat fields are nevertheless
importa
Most a
tely
dark or
oaches. If you’re taking twilight flats the timing of those
images is critical as you have only a few minutes when the sky is appropriate for taking
twili
rames and bias frames it is important that flat field frames are
Dark Frames
Dark frames are used to subtract the build-up of dark current from raw CCD images.
Subtracting dark current is generally considered the most important image calibration step.
ient because taking dark frames consumes a lot of time. You need at least as
mes as you take for each type of light frame, twice as many darks is even bette
the dark frames in the same sequence as the light frames also ensures that the
mental conditions are as close as possible to the conditions when you took the l
. This ensures the closest match when subtracting dark frames from light frames.
eption to this rule is if all the dark ti
light reaches the CCD and the camera can maintain the same CCD temperature as the
morning air starts to heat up.
To take a series of dark frames, either add them as an additional group in the Setup
Sequence window or setup a new sequence of just dark frames at a convenient time durin
the imaging session. If you’re taking 5 exposures each with the R, G and B filters
least 10 dark frames using the same
noise in the combined dark maste
Bias Frames
Bias Frames record just the subtle variations in the zero level of each pixel plus any nois
introduced when reading the image from the CCD. Taking a complete set of bias frames
takes only a couple of minutes so i
s frames during each im
a series of bias frames, either add them as an additional group in the Setup
ce window, or setup a new sequence of just bias frames at a convenient time du
ing session. Take at least 16 bias frames at the same temperature as you
k frames.
t F elds
fi
s record any irregula
the slightly different ligh
sider flat fields to be opti
nt for producing the best images with the highest possible signal to noise ratio.
stronomers take their flat fields either in the evening before the sky gets comple
in the morning as dawn appr
ght flats. As with dark f