Let there be light: and there was light.
Correct setup of planetary imaging cameras
Do camera parameters really matter?
In
planetary
imaging,
it
is
often
said,
high
frame
rates
with
the
benefit
of
capturing
as
many
frames
as
possible
is
the
one
and
only
key
to
succesful
lucky
imaging.
In
this
article,
we
want
to
dive
a
bit
deeper
and
evaluate
the
effect of different camera settings on the final image result.
For
that
series
of
experiments,
Mars
was
captured
in
several
consecutive
runs
at
the
same
night
without
any
change
of
focus
or
image
scale
within
a
time
frame
of
less
than
ten
minutes
from
the
preconditioned
dome
at
Juniper
Hill
Observatory,
assuring,
that
both
the
dome
climate
and
the
imaging
train
were
in
complete
thermal
equilibrium.
Optical setup
The
optical
setup
was
a
190mm
(7.5“)
Maksutov
Newtonian
combined
with
a
Televue
5x
powermate.
In
addition,
a
Baader
IR/UV
block
filter
was
used
to
limit
the
captured
spectrum
to
VIS
only,
avoiding
blur
due
to
miisfocused
NIR.
As
imaging
camera,
a
ZWO
ASI
224
MC
color
camera
was
used
to
avoid
any
influence
caused
by
derotation
processes
necessary
in
LRGB-imaging
sequences
using
a
monochrome
camera.
The
effective
focal
length
of
this
setup
was
about
6600mm
or
f/35
for
the
given
aperture.
The
images
were
later
stacked
with
a
drizzle
factor
of
1.5x
to
increase
the
visibility
of
small
albedo
structures.
Note,
that
the
apparent
diameter
of
Mars during capture was 14.2 arcsecs. Capture interval was Jan-03-2023 from 18:10 to 18:16 UTC.
Test scenario
The
test
scenario
should
answer
one
basic
question:
Is
it
better,
to
go
with
a
minimum
exposure
time
and
highest
possible
fps
at
the
cost
of
high
gain,
higher
noise
and
lower
dynamic
range
or
go
the
oppsite
way
with
low
gain
for
a
maximum
dynamic
range
with
lowest
noise,
but
high
exposure
time
and
low
fps.
Let‘s
first
look
at
the camera charactersitics, so that we can later understand the third capture scenario of this test:
Source: https://astronomy-imaging-camera.com/
We have chosen the following scenarios for high and low gain:
1
.
High gain:
Gain = 325, exposure time = 6.675msec, fps = 149, total frames = 18000 (120sec)
2
.
Low Gain:
Gain = 153, exposure time = 44.51msec, fps = 22, total frames = 2700 (120sec)
As
the
camera
is
-
as
is
typical
for
modern
CMOS
planetary
imagers
-
operated
in
High
Speed
mode,
the
ADC
runs
on
10bit
resolution
instead
of
12bit.
If
you
check
the
blue
curve
above
(DR
stops),
you
can
see,
that
with
the
high
gain
settings
of
Gain=325,
you
have
a
dynamic
range
of
9bits,
whereas
in
our
low
gain
scenario,
at
Gain=153,
you
haver
nearly
12bits
of
dynamic
range.
So
in
low
gain,
your
noise
is
below
the
sampling
threshold,
whereas
in
the
high
gain
scenario,
the
noise
affects
the
2
LSBs
of
the
sampling
process.
An
optimum
gain
could
be
around
a
value
of
250
with
a
DR
of
10bits,
where
the
SNR
perfectly
matches
the
ADC‘s
resolution.
So
we
have
a
third
scenario:
3. DR10 gain:
Gain = 250, exposure time = 16.13msec, fps = 61, total frames = 7500 (120sec)
Let‘s first have a look at the quality graphs of the three scenarios:
1
.
High Gain:
2. Low Gain:
What
can
clearly
be
seen,
is,
that
in
the
high
gain
scenario
you
are
facing
a
quite
steep
quality
graph
with
lots
of
frames
with
poor
quality,
but
with
much
more
frame
than
in
the
low
gain
scenario.
The
low
gain
scenario
shows
a
relatively
flat
quality
graph
with
lots
of
frames
being
about
a
certain
quality
threshold:
this
is
mainly
due
to
the
fact,
that
the
associated
long
exposure
times
average
out
high
frequency
seeing
effects,
resulting
in
less
sharp,
but more consistent frames.
Let‘s now see the compromise scenario, whre SNR is matched to the ADC resolution:
3. 10DR Gain:
As to expect, the quality graph lies somewhere in between the scenarios 1 and 2.
As
a
next
step,
each
of
the
three
scenarios
has
been
stacked
with
a
frame
selection
rate
of
15%,
and
then
sharpened using Registax wavelets. The result can be seen in the following image. Klick for a 100% view:
The
clear
winner
here
is
the
10DR
Gain
scenario,
with
the
Gain
adjusted
to
match
the
10bit
resolution
of
the
ADC.
It
shows
at
least
the
fine
details
of
the
High
Gain
scenario,
but
with
the
smoothness
and
clarity
of
the
Low
Gain
scenario, where the latter is missing the finest detail due to long exposure times.
To
be
honest,
the
differences
are
quite
subtle,
so
neither
imaging
scenario
is
a
real
show
stopper
and
it
might
make
sense,
to
chose
the
parameters
in
a
way
to
best
meet
the
circumstances
(seeing,
focla
length,
pixel
size
etc.)
of
your
imaging
setup.
But
in
general,
reducing
the
gain
to
a
value
of
10bits
DR
drastically
reduces
disk
space
and
processing
time
and
yields
cleaner
and
more
detailled
results
than
sampling
too
fast
using
high
gain
values.
What
definitely
should
be
avoided,
is
to
shorten
exposure
times
below
the
frame
rate
that
your
camera
can achieve with the selcted ROI.
For
example,
if
your
camera
supports
100fps
at
an
ROI
of
800x600
pixels,
it
makes
no
sense
to
push
your
exposure
below
10msec:
you
do
not
get
more
frames
but
you
get
considerably
more
noise
and
lower
dynamic
range.
So
take
1/max{fps}
as
your
minimum
exposure
time
and
try
to
limit
your
gain
to
a
value
giving
you
10
stops of dynamic range.
If
your
planetary
camera
supports
high
conversion
gain
mode
(this
is,
where
the
readout
noise
drops
at
a
certain
gain value) NEVER do planetary imaging with gain values below the HCG mode.
With
that
points
considered,
one
can
achieve
a
maximum
detail
level
in
planetary
imaging
without
generating
excessive amounts of data.