SkyTools
3 Sub-Exposure Times
It's
All About Better Images Some
images simply look better than others. "Better" also
means being able to detect fainter objects. A better image
starts with maximizing the signal from the astronomical object
and at the same time minimizing the noise. We can assign a
number to an image that is the ratio of the signal to the noise,
or SNR. A high SNR means a better image. To maximize the signal
from the object we simply expose longer. But longer exposures
maximize the noise as well. So we use a trick: by stacking many
"sub exposures" together using a median filter combine
we can remove some of the noise, increasing the SNR to make a
better final image.
Why
Not Just Use What The Masters Recommend? Maximizing
your SNR used to be much simpler. There was a time when nearly
everyone used traditional CCD cameras attached to long focal
length scopes at dark sites. Many books were written assuming
these three truths and many rules of thumb resulted. But we have
seen a revolution in astronomical imaging. Today wide fields of
view and light polluted locations are common even with the more
traditional CCD cameras. The introduction of DSLRs has
taken these things to an even greater extreme. The three most
important factors that determine the noise reduction that can be
achieved for a given sub-exposure time are the readout noise,
dark signal and sky background. Today readout
noise and dark signal vary greatly from one camera to another.
And the sky brightness is not only a function of light pollution
but field of view as well; wide fields of view magnify the sky
background. The astrophotographer has a lot to gain by tailoring
the sub-exposure to the telescope, camera, and sky conditions. A
rule of thumb that says to use, (as an example) 5 minute sub
exposures, created for a traditional CCD camera at a dark site
with a small field of view simply doesn't cut it. By taking the
various factors into account better images can be made with less
time at the scope. SkyTools
uses a scientific model that includes all these factors in
its recommended sub exposure. This gives the astrophotographer a
big advantage. Once you have SkyTools dialed-in (see below) you
can use it to help you make decisions under different conditions.
What if you go on exposing after the moon rises? Do you need to
go to shorter sub exposures? And if so, how short? What if you
take your scope to a dark site? What if you use a certain
filter?
How
It Works The
optimum sub-exposure time is computed by comparing the noise
that comes about when reading out each sub exposure to the noise
generated during the exposure. On the one side longer exposures
are better and on the other shorter exposures rule. A sweet spot
is found in between that minimizes the total noise.
For most astronomical CCD cameras the dominant noise during the
exposure comes about from photon noise in the sky signal. The
random component of this noise can be removed by stacking the
images. For very dark sites the noise is low, for brighter skies
the noise is high. Wide fields of view will also increase the
sky signal. So if this were the only consideration short
exposures would always be better than long ones in the presence
of a significant sky background.
Some cameras (mostly DSLRs) are limited by their high dark
signal rather than the sky brightness. This works the same way
as the sky brightness--arguing for shorter exposures--except
that it doesn't matter what the conditions are or what the field
of view is.
The read noise associated with reading-out the exposure is
contributed each time the chip is read out. This noise compounds
with each sub exposure. If this were the only source of
noise, longer exposures would always be superior to shorter
ones.
So the typical game is to estimate the noise generated by the
sky + dark noise on the one hand and the read noise on the
other, looking for the sweet spot in between that minimizes the
total noise. The sub-exposure time that coincides with this
sweet spot is what SkyTools reports.
Dialing-In
Your SkyTools for Best Results
The age old adage of garbage-in = garbage-out applies here. It
is important that your camera be set up appropriately. In
particular the readout noise and dark signal are important to
the result. It can be useful to measure your actual values from
test images rather than use the manufacturers numbers. How this
is done is outside of our scope here but there is software
available and/or instructions that can be found on the web to do
this.
Often
the most critical input is your sky brightness, particularly if
you are combining a light polluted sky with a wide field of
view. A brighter sky will tend to drive the algorithm to short
sub-exposures. It is very important to make sure you have an
accurate value entered for your light pollution at your
location. If the sky brightness is overestimated very short
sub exposure time recommendations often result. Short of a Sky
Quality Meter the best way to measure the sky brightness is
to find the faintest naked eye star you can see overhead
on the darkest night. For best results you need to be as
dark adapted as possible. It is very easy to overestimate your
light pollution particularly when there are nearby lights; a
casual glance at a constellation often won't suffice.
For the most accurate results measure the sky signal on one of
your images and adjust the naked eye limit for your observing
site such that SkyTools will compute the same sky background. To
do this make sure to set up the SkyTools Exposure Calculator to
exactly match the test exposure in every way (date/time,
location, filter, seeing, temperature, humidity, etc). Set the
sub-exposure manually to the length of the actual exposure. The
predicted sky signal will be displayed. When you compare this to
the actual image make sure the image is dark subtracted first,
and beware that most software adds an arbitrary offset (usually
100 ADU) that you need to subtract when you measure the sky
signal. If you measure a typical sky signal on your image of 220
ADU, subtract the offset (often 100) to get 120 ADU. Experiment
with changing the sky brightness for your location until
SkyTools reports the same sky brightness in ADU. One final thing
to be aware of in this process is that the gain value for your
camera is used to compute the ADU so be sure it is as accurate
as possible.
Once
SkyTools is dialed-in it will automatically account for changing
conditions such as twilight and moonlight. For even more
accurate results enter the current (or expected) values for
temperature, seeing and humidity each time you observe.
Once
again the critical inputs are:
-
Sky
Brightness (set for your observing site)
-
Readout
noise (set in your camera specs)
-
Dark
signal (set in your camera specs)
How
Critical Is the Sub Exposure Time for Me?
SkyTools
will always compute an optimum sub exposure time but it
doesn't tell you how much difference the sub exposure time
actually makes. For some the improvement in following the
SkyTools recommendation can be significant. For others the
sub-exposure time makes little difference in practice. Use the
Exposure Calculator to answer this question. Set up a test
exposure on a typical target for a typical night. Set the sub
exposure time to manual. Enter various values for the sub
exposure and look at how this affects the final SNR. If the
final SNR varies very little then you can go ahead and use any
sub exposure time that is convenient--at least for these
observing conditions. If it does vary then it will be beneficial
to use the SkyTools recommendation.
There
have been a few users for whom SkyTools always recommended very
short sub exposure times. In these cases there was typically
light pollution combined with a wide field of view. When we
tried the test above we found that although SkyTools technically
claimed very short sub exposures were always better, in fact
they made little or no difference. So they were free to use any
sub exposure time that was practical for them regardless of what
SkyTools displays. But it is important to make sure that
SkyTools is dialed-in--that all the inputs are accurate--before
coming to this conclusion!
Related
Topics:
Using
the SkyTools Exposure Calculator Planning
Imaging Sessions Adding
Cameras Manually to SkyTools |
