Diffraction SBIG STC-7, Руководство пользователя

Страница: 21 / 47

SBIG STC-7 CAMERA
USER’S MANUAL
21
2.3 StackPro
TM
Subexposure Function
The subexposure function allows you to increase the effective dynamic range of
the camera. It works by taking a series of shorter exposures, and then
numerically summing them internally to the camera. Since the STC-7 has very
low read noise, this greatly increases the camera's dynamic range while it only
modestly increases the total read noise in your image. Due to the very fast
readout speed of the CMOS sensor, the extra time required is negligible.
As an example, when using the
Medium Gain mode the camera can record
about 10,000 photoelectrons before saturating, corresponding to a readout
value of 4,095 (12 bits). The read noise is approximately 2.5 electrons.
If you mathematically stack 16 individual
Medium Gain exposures the total
number of photoelectrons that can be recorded increases 16X, but the readout
noise only increases 4X. The read noise is now 10 electrons, and the stacked
exposure can record 160,000 electrons, corresponding to a readout value of
65,535 (16 bits).
In comparison, our previous generation STF-8300-OEM CCD camera has a
typical read noise figure of 9.3 electrons, and can record a maximum of 25,000
electrons in a single exposure. The noise performance is about the same, but
the total dynamic range achieved is over six times greater.
The STC-7 automatically performs the subframe exposures internally, so they
appear from the outside to be a single exposure. The camera also automatically
adjusts the number and duration of the subframes to try to achieve the target
Subframe Duration . The actual subexposures may be slightly longer if the
requested exposure is not an exact multiple, as explained below.
Subexposure Duration must be at least 1 second. This allows ample time for the
stacking to occur after each subframe is read out into the camera's internal
memory.
When you request an exposure, the camera automatically calculates an actual
subexposure duration and number of subexposures, based on the target
Subexposure Duration, according to the following rules:
•
If the exposure time is less than twice the Subexposure Duration , a single
exposure will be taken
•
If the exposure time is greater than 16 times the Subexposure Duration ,
then the total exposure time will be divided by 16, and 16 subexposures
will be taken and stacked.
•
If the exposure time is an exact multiple of 2X to 16X the Subexposure
Duration , then the appropriate number of subexposures of the target
duration will be taken and stacked.
•
If the exposure time is not an exact multiple, then the Subexposure
Duration will be increased by the minimum amount required to make an
even number of 2 to 16 exposures. For example, if you have a Subexposure
Duration of 10 seconds and ask for an exposure of 25 seconds, you will get
two subexposures of 12.5 seconds.