Astrophotography of faint
deep-sky objects like galaxies and nebulae is
all about signals and noise. The more signal you
collect, the better your photos will be. It's
that simple. You should consider this to be your
number one priority in astrophotography:
collect more signal!!!
In digital astrophotography, there are several
ways to collect more signal: a.)
frames longer; b.)
Combine many individual
Use a larger aperture in a
faster optical system;
Use a sensor that has a
higher quantum efficiency.
The signal from the deep-sky object recorded in
our images comes from the photons of light that
have traveled incredible distances across the
universe to hit the sensor in the camera and get
counted. This signal is recorded in a
- the actual exposure of the sky object.
In digital astrophotography, there are other
signals that also get recorded along with the
object signal, such as the thermal signal.
Thermal signal is generated by heat in the
sensor even when it is not exposed to light.
Thermal signal can be recorded by itself in a
an exposure the same length and at the same
temperature as the light frame, but with the
lens or scope capped so no light reaches the
There are also other noises present in every
image we take: BIAS and Flat field.
frames are used to address unwanted problems in
the optical path, including dirt/dust,
vignetting and internal reflections. Any change
in the optical path will require a new flat
field frame. To obtain the "best" possible flat
field frames would mean taking it at the same
conditions as the light, dark and bias frames.
To take flat field frames, first you need to get
your scope into focus and then point it to an
evenly illuminated surface.
Flat field frames can be
taken from the twilight sky.
When using a CMOS/CCD chip, not all the pixels
start out at a value of zero. The purpose of a
is to apply it to a dark and light frame to
bring all the pixels on the chip to an equal
starting value. To make a bias frame, cover the
aperture in the same fashion as the dark frame.
Capture at the lowest possible time setting your
camera will allow.
Bias frames should be taken at the same
temperature and settings as the light frames.
With these frames (dark, bias and flat field) we
can generate a
master bias frame,
a master flat filed
a master dark frame.
The master frames are called
These unwanted signals can be subtracted from
our images by correct use of calibration frames
and this is a way to calibration our light