Sky & Telescope - October 2023 - 63

you can simply find a way to hold the filter material a few
inches away so that it blocks the Sun in the frame during
each shot of the sequence.
The specific choice of filter material will affect your outcome.
Typical visual filters and eclipse glasses have an optical
density (OD) of 5, filtering all but 0.99999% of incoming light.
Such material delivers a solar image that's safe to look at, but
the solar disk may be too dark for photography while the unfiltered
foreground is properly exposed. A more transmissive
filter, such as the Baader Planetarium AstroSolar Photo Film
with an OD of 3.8, will produce a well-exposed solar disk with
a usable landscape below. It transmits 10× more light compared
to OD 5 filters, which means it should be used and stored with
caution since it isn't safe for visual use.
Since a solar filter has to be used during the partial phases
of the eclipse, you'll also need an unfiltered shot of the sky
and foreground to use as the base image that all the other
exposures will be composited onto. During this month's
annular (or partial) eclipse, you can shoot a separate picture
of the foreground before the sequence begins or after it ends,
preferably before the Sun enters the frame or after it departs.
If you're planning to photograph next April's total solar
eclipse, you can capture your base image during totality.
Choosing Intervals Between Photos
The interval between each exposure in a sequence needs to be
consistent. While you can manually trip the shutter for each
image, timing errors and distractions may lead to gaps and
inconsistent spacings in your final composition. This is where
a programmable timer or intervalometer helps. You can set it
up beforehand to fire the camera at your chosen interval. If
you don't have one, you can try setting a series of alarms on
your phone to signal when you need to take a photo.
Given that the Sun moves its full diameter every 2 minutes,
most sequence photographers use an interval of 4 or
5 minutes. An interval of 2½ minutes, on the other hand,
CUTTING With the Sun selected, use Select > Inverse from the dropdown
menu to select everything you don't want in the layer, then use the
backspace key or Edit > Cut to delete the selection.
delivers a series of solar disks that nearly touch each other,
like pearls on a necklace. The 2½-minute interval is best for
partial and annular eclipses but isn't recommended for total
eclipses as the phases will interfere with the image of the
extended corona in your composite.
Determining when to start the image series is a bit tricky
- you need to calculate backwards from the mid-point of
totality so that the partial phases are evenly spaced around
the totally eclipsed Sun.
When the Moon completely covers the Sun, remove the
filter and lengthen your exposure up to a second or two to
record the corona. You can also take a few pictures with
different exposures in order to make a high-dynamic-range
totality frame. Once totality ends, be sure to put the filter
back over the top half of the lens to shoot the last hour of
partial phases.
On eclipse day, you'll be taking around 30 to 40 photographs
at intervals of 4 or 5 minutes and upwards of 60 frames
if you're shooting an annular eclipse at 2½-minute intervals. If
the eclipse happens when there's considerable cloud cover, your
image sequence might be interrupted, or clouds in your master
background frame won't match those in the other frames. In
that situation, you may want to photograph a clear sky from
the same spot later for your master frame.
You don't need to wait for an eclipse to practice this technique,
either - any sunny day will do.
Assembling the Sequence
Putting together the sequence composite is easiest with Adobe
Photoshop. I suggest first opening the master
frame and adjusting it for the desired color
balance, brightness, and contrast. Once you're
happy with how it looks, save the picture as a
TIFF file. Next, open each of the partial-phase
photos and process them all in a similar way.
With all the phase images open, copy each one
NATURAL APPEARANCE Change the Layer Blending
Mode from Difference to Lighten and the first image
in your sequence will be completed and in place in the sky.
sk yand tele scope .o r g * OCTOBER 2023 63
https://skyandtelescope.org/
Sky & Telescope - October 2023

Table of Contents for the Digital Edition of Sky & Telescope - October 2023
