Capture the Sun with Your PST

In this magazine's February 2005 issue, page 96, I
reviewed Coronado's Personal Solar Telescope (PST) — a
groundbreaking instrument that brings hydrogen-alpha
observing to amateur astronomers at an affordable price.
For the review, I took a few images through the instrument
in an attempt to approximate the view through the eyepiece.

Photography with the Coronado Personal Solar Telescope (PST) and digital cameras is relatively easy once some hurdles are crossed. Sky & Telescope assistant editor Sean Walker has developed techniques that take advantage of the resolution of inexpensive digital cameras to capture breathtaking images of the entire solar disk. The views here were obtained with a Canon PowerShot A85 digital camera aimed into a Tele Vue 20-millimeter Plössl eyepiece.

Sean Walker

While my images were successful, they highlighted
the PST's relatively small "sweet spot" that shows Hα features
at maximum contrast. Thus the entire solar disk isn't
evenly illuminated, and prominences are visible on one
side of the Sun while not on the other until the PST's tuning
ring is turned to display that area properly. Due to this
shortcoming, I originally deemed the PST to have limited
use as an imaging platform. Nevertheless, taking those images
lit a fire of interest, and I decided to try pushing this
little scope to the limits of its potential.

What I discovered is that excellent photographs of the
Sun through the PST are possible once a few obstacles
are overcome. In addition to the uneven illumination, the
PST has limited back focus. While not a problem for visual
use, the focal plane is very close to the top of the
1¼-inch eyepiece holder, and there is not enough back focus for
an SLR camera. I had to shoot my photos using either eyepiece
projection or a focal imaging (aiming a camera's lens
into an eyepiece).

My first afocal images were taken with a borrowed Nikon
Coolpix 990 and a Tele Vue 19-millimeter Plössl eyepiece.
Soon, however, I purchased a Canon PowerShot A85 point-and-shoot camera. While I can't offer opinions on other
camera models for use with the PST, the Canon turned out
to be extremely well suited for solar imaging. I also purchased
a commercial adapter to connect this camera to a
Tele Vue 20-mm Plössl eyepiece.

By experimenting, I found that my best exposures were
1/160 second, with focus set at infinity and the camera's
built-in optical zoom set to maximum (3x). Focusing was
done with the higher-magnification 11x digital zoom, and
critical adjustments were made with the PST's focus knob.
Sunspots are the easiest to focus on, but filaments and
prominences are useful when no significant spots are visible.
I found that a small video monitor attached to the camera
made focusing easier, because the larger image made
it easier to see when the best focus point was established.
When a monitor isn't available, I use the small screen on
the back of the camera, placing a dark cloth over my head
to shield the screen from bright sunlight and help me see
low-contrast features.

The PST does not show maximum contrast of Hα features across the entire solar disk at one time — features visible on one side of the disk will disappear when the scope is turned to show details on the other side. By shooting a series of exposures that record features at different tunings, you can combine them to create a portrait showing detail across the entire Sun. The three images shown above have been contrast stretched to better display the differences between them.

Sean Walker

To overcome the PST's uneven illumination, I capture
several exposures, turning the scope's tuning ring slightly
between each one. I then combine the pictures to average
the detail captured in each exposure, thus creating an
evenly illuminated photograph of the entire solar disk. I use
from 3 to 20 frames, but I tend to capture at least three images
at each setting of the tuning ring in an effort to get at
least one during a moment of good seeing. Similar to what
planetary and deep-sky astrophotographers experience, the
more frames I stack, the smoother the resulting image and
the more it can withstand subsequent image processing.

I use my Canon PowerShot A85 in black-and-white mode.
Other cameras without this feature can still capture good
Hα images in color mode, as James M. Weightman explained
in his article in the July 2004 issue, page 137.

Image Processing

Once I've captured my exposures, I download them to my
computer. The easiest way to align and stack a series of
images is to bring them into the freeware program RegiStax. However, because my
frames differ from one another due to the PST's uneven
illumination, the same solar features don't always show on
every frame, so RegiStax may not have a consistent alignment
feature. In such cases, I manually align each image
in Photoshop. I start by opening all the frames, discarding
the blurriest ones, and pasting the best ones over the first
image in the series. Using keyboard shortcuts, I can make
quick work of selecting the image (Control+A), copying (Control+C), pasting (Control+V), and closing the now-copied
image (Control+W). When my image stack is complete,
I save the file as a Photoshop document (PSD).

The original combined image is processed two times (and saved separately each time) to produce two views: one that shows features across the solar disk, and another that shows prominences on the limb. This is done with the Curves function in Photoshop. Both views can be sharpened using Unsharp Mask, High Pass filtering, or other routines that use advanced sharpening algorithms. (Click for larger image).

Sean Walker

To align each image (each "layer" in the jargon of Photoshop)
to the base image (called "Background" in the Layers
palette), I first hide every layer except the Background and
Layer 1 by clicking the small eye icon to the left of each
layer I want to hide. I then change the blending mode of
Layer 1 from Normal to Difference. This causes the two
layers to cancel out when detail is aligned, and to highlight
non-overlapping detail. I use the Move tool to roughly
slide Layer 1 around until it cancels out most of the Background,
and then switch to the arrow keys on the keyboard
to make the final, small adjustments. Once these layers
are aligned, I change the blending mode of Layer 1 back
to Normal. I repeat these steps until all layers have been
aligned to the Background, then save the file.

The next step is to average these layers to combine the
features recorded in them, as well as to increase the signal-to-noise ratio. (By stacking images, random noise is reduced,
while detail that repeats is reinforced.) In Photoshop,
the best way to average exposures is to change the opacity
of each layer sequentially from the bottom to the top of the
stack. The rule for achieving a proper average is to have
each layer’s opacity set according to its location in the stack
divided into 100. For example, the Background layer will be
100%, Layer 1 (the second image in the stack) will be 50%,
Layer 2 will be 33%, and so on. I find a practical limit for
this technique to be about 20 layers, with anything more
offering no visible gain. At this point I flatten the image
stack and save it as a new file, preferably in a format such
as TIFF that doesn’t compress the data.

As explained in the text, the separately processed views are combined after the one showing disk detail is carefully selected to avoid overlapping prominence detail. (Click for larger image.)

Sean Walker

Once I have a stacked image with roughly
even illumination across the entire solar disk,
it’s time to enhance the contrast and sharpen
the detail. I start by opening the Curves palette
and adjusting the image to display the
surface features in high contrast. This usually
subdues prominences, so I save the file
with a new name, such as "disk curve.tif." I
then open the original file again and apply
another curve that enhances the contrast of
the prominences, this time at the expense of
disk detail. I save this image as "prominence
curve.tif." Next, I'll bring these processed
files into RegiStax separately and use the
program's excellent wavelet filter to sharpen
them. I prefer this technique to Photoshop's
Unsharp Mask because of the extended control RegiStax provides. Once they’re sharpened,
I save the images as bitmap (BMP)
files and bring them back into Photoshop to
combine.

Because hydrogen-alpha telescopes block all light except a narrow slice of the red spectrum, many solar astrophotographers prefer to colorize their images with a palette that includes yellow, orange, and red to better display different features. This can be done with the Curves function by adjusting the red and green channels and reducing the blue channel to zero. This should be done separately for the disk and prominence views so that different color contrasts can be maintained, allowing prominences to appear pure red. (Click for larger image.)

Sean Walker

I start with the disk image and open
Color Range from the Select menu. Using
the Eyedropper tool and holding the
Shift key, I click on different parts of the
area outside the Sun's disk until the Selection
Preview displays a view with the
background completely white and the
solar disk blacked out. I then click OK.
I now have a selection line around the
solar disk, but slightly farther away from
the disk than I want. I then use the drop-down
menu Select > Modify > Expand,
and increase the area of my selection by
about 4 pixels, which reduces the radius
of the circle around the solar disk. I also
use Select > Feather and soften my selection
by 2 pixels. I copy this and paste it
onto my prominence image. Usually I
need to align the disk and prominence images,
using the same procedure as before.

The final processing step evens out the brightness gradient that makes the center of the solar disk appear brighter than the edge. (Click for larger image.)

Sean Walker

I now have a decent image of the Sun,
but there are still a few steps remaining.
I like my solar images to be in color, but
images through any solar Hα telescope
are only red. I prefer a color palette of
yellow, orange, and red, which highlights
contrasting features better than a single
red palette. To colorize the picture, I
convert the image from Grayscale to RGB
color and adjust the color channels using
the Curves function. I first select the
prominence layer. Using the drop-down
menu Image > Adjust > Curves, I reduce
the blue channel to zero, lower the green
curve’s midpoint, and boost the red channel
so that my prominences have a fiery
red appearance. I then select the disk
layer and adjust the curves slightly differently,
reducing the blue channel to zero,
raising the red midpoint, and adjusting
the green curve until my disk is an orange
ball with reddish filaments.

Once I'm satisfied with the color adjustments,
my final step is to reduce the
limb darkening on the solar disk. I make
a duplicate layer of the disk image by
selecting the drop-down menu Layer >
Duplicate Layer. I carefully use the Clone-
Stamp tool to replace prominent features
with similarly shaded areas around them
until I have an image of the disk devoid of
large-scale features. I then apply a Gaussian
Blur filter set to a radius of 25, which
eradicates small features. I'm left with a
soft layer that shows mainly the disk illumination
I wish to correct. With this
layer selected, I now use the drop-down
menu Image > Adjust > Invert to make a
negative image of the disk illumination.
By changing the layer-blending mode to
Overlay and lowering the opacity to 50%, I
get a view with edge darkening reduced to
an acceptable level. I can now flatten the
image layers and save the final result.

Some of the techniques I use in Photoshop
can be accomplished with other software,
and I often jump between several
programs. But Photoshop tends to be my
"do everything" program when all else
fails, and all the steps I describe here usually
take me less than an hour from image
capture to final product.

By following these steps, you too can
produce excellent images of the Hα Sun
on a small budget. And unlike nighttime
astrophotography, you won't lose any
sleep over it!

On clear days, assistant editor Sean Walker spends lunchtime outside the Sky & Telescope offices with a dark cloth over his head.