Sky & Telescope - October 2023 - 68

S&T Test Report
ing several nights of data. It wasn't until
several days later that I began looking at
the data in detail and realized something
wasn't right: These were not the 16-bit
images I was expecting.
Generally, I don't take the time
and space in reviews to write about
screwups that are my own fault (there
are too many of them). But this one is
worthy of an exception because some
of the problem stemmed from my years
of experience using MaxIm DL, and I
know there are a lot of others like me,
so my tale may serve as a cautionary
word for others.
Because I was shooting with the
Stack Pro feature, I expected my 5-minute
exposures to actually be stacks of 16
short exposures (in this case each about
19 seconds long) automatically stacked
in the camera and read out as single
images. And, as I mentioned earlier, the
stacking process happens so fast I knew
that my 5-minute exposures would
essentially take 5 minutes since there's
no noticeable overhead associated with
the stacking process.
I also knew that there are fundamental
advantages to making individual
deep-sky exposures that are as long as
sky conditions permit. This is mentioned
in the STC-7 manual, and accordingly
there's a place in the camera setup
telescope and connected the camera
to MaxIm DL, it was my experience
with the software that misled me into
thinking I'd easily understand all the
settings. But I mistakenly interpreted
one as being the minimum subexposure
length for Stack Pro. The correct place
to set this value was in another dialog
box that I skipped over (and, more
importantly, the value was set to 3,600
seconds by default).
This meant that all of my Stack
While the STC-7 does not shoot video, it can
save very rapid sequences of individual frames.
This view of the Moon was captured last June
22nd more than 90 minutes before sunset.
Thirty 0.07-second exposures through an
H-alpha filter (used to darken the sky) were recorded
in less than 45 seconds and combined
to produce the final image.
within MaxIm DL that lets you specify
the minimum length of the subexposures
used by Stack Pro even if it results
in less than 16 stacked frames in the
final image. For example, a minimum
subexposure of 30 seconds would create
a stack of 10 rather than 16 frames for
a 5-minute image. For my early tests,
however, I was simply letting the camera
make 16-frame stacks before experimenting
with longer subexposures.
In hindsight, when first I got to the
Pro images were just single exposures,
because the 3,600-second minimum
subexposure was longer than the total
exposure time I had requested for my
images. Obviously, the error was mine
and in part was due to my overconfidence
with using MaxIm DL - a new
user might have been more diligent
about exploring the dialog box that I
overlooked. And my error negated the
advantages I was expecting from using
Stack Pro. After I properly set my subexposure
duration, it was smooth sailing
on future nights.
Diffraction Limited states that the
camera's fan-assisted, two-stage thermoelectric
cooler can drop the sensor
temperature by " approximately 30°C "
from ambient air temperature, though I
found it a safer bet to not ask for more
than 25°C of cooling, especially on
warm nights. Even with minimal cooling,
however, the CMOS chip has a very
low dark current and produces clean
images with only a minor sprinkling of
hot pixels visible before calibration.
The camera's regulated cooler typically
holds the set temperature to an
accuracy of 0.1°C. As such, I was able
to make a library of dark calibration
frames that were useful over many
nights, as long as I always ran the camera
at the same temperature.
People like me with a long history of
Despite thick smoke from Canadian wildfires blanketing New England, the author recorded this
view of the galaxy M101 sporting an 11th-magnitude supernova (arrowed) on the night of last May
26th. The LRGB image was assembled from 35 minutes of total exposure through each filter.
68 OCTOBER 2023 * SKY & TELESCOPE
working with CCD cameras will find
a few changes in the procedures for
calibrating images made with the STC-7
because of its CMOS detector. The user
manual has a detailed section on image
calibration, and I suggest that even
those with extensive CCD experience
read through it.
One of the differences from CCD

Sky & Telescope - October 2023

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