Sky & Telescope - April 2025 - 57

look like, you'll quickly realize what's
going on. Notice that the North Star,
Polaris, appears essentially stationary.
Indeed, stars near the north celestial
pole appear to move very little while
those closer to the celestial equator
seem to move quite far. I'll spare you
the rigors of spherical geometry - all
you have to remember is that the farther
a star is from the pole, the greater
the span of sky it traverses in a given
amount of time.
In practical terms, what all this
means is that when you take a very
wide-field image, stars in one area may
drift across enough pixels to create a
streak in 15 seconds, while in other
areas of your image it may only take
10 seconds. Trying to calculate an
optimal exposure based on your pixel
scale, where you are pointing in the
sky, and how the trailing varies across
your entire field of view is more trouble
than it's worth. A quick and easy rule of
thumb is certainly easier to apply when
you're just getting your feet wet. The
main thing to note is that the Rule of
300 may be too strict when it comes to
shooting near the pole, and perhaps too
lenient for images of the sky near the
celestial equator. That's why it's merely
a starting point.
Experimentation is Key
The approach I use and recommend is
surprisingly simple and assumes you
are using a modern camera with a
viewscreen that has a zoom feature so
you can inspect the stars in your image.
Mount your camera on a tripod and
take a quick, well-focused shot of a star
field using the Rule of 300 to calculate
your exposure and with the ISO set
to 1600. (You can read my December
2021 article to learn why I suggest
this setting as a good place to start.)
Zoom in, and if the stars are trailing,
shorten the exposure. If they're not, try
pushing it a little longer and add a few
seconds. That's not so hard, is it? You'll
also want to pan around your image
on the viewscreen to ensure that there
are no trailing stars near the edges of
the frame. Inspecting the edges is also
important to ensure that optical aberrapCONCENTRIC
CIRCLES As Earth rotates, the stars appear to arc across the sky, as illustrated
by this hour-long star-trail image. For nightscapes, the trick is to shoot as long an exposure as
possible without trailed stars. Note how the trails lengthen the farther from the pole the stars are.
tions in your lens aren't distorting the
stars. You may wish to stop down the
lens if you notice problems, bearing in
mind that the more you do the longer
your exposure needs to be to retain the
same image brightness. Some creative
cropping in post-processing might be
the better approach.
There's one more reason to carefully
inspect the stars in your images.
Sometimes a shot may look impressively
bright on the viewscreen because
the stars are trailed. More illuminated
pixels mean a brighter image that could
trick you into thinking you've gotten a
fantastic shot. That is, until you zoom
in and discover that what looked like
a brilliant Milky Way nightscape is
actually just bright stars streaked across
many pixels. You've sacrificed all that
glorious detail for an image composed
of small dashes of light.
It really just takes a few minutes of
experimentation to discover how long
you can typically go with your particular
camera and favorite lens. And the
STELLAR LENS This Sigma 20-mm f/1.4
Art lens was the author's favorite for wide-field
Milky Way photography for some years. Its fast
focal ratio makes tripod nightscape photography
fun and easy. He now favors a Tamron
35-mm model (also f/1.4) for its superior optics.
sk yand tele scope .o r g * APRIL 2025 57
results for this modest effort can be
immensely gratifying.
Now get out there and give basic
nightscape photography a try!
¢Contributing Editor RICHARD S.
WRIGHT, JR. has been shooting nightscape
scenes with just a tripod since
2003 when he got his first DSLR, a
Canon Digital Rebel.

http://skyandtelescope.org

Sky & Telescope - April 2025

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