Tips in CCD Solar Imaging.
by
Gordon Garcia
ALPO Solar Section Ass't. Coordinator
(copyright 1999)

On April 10, 1993 at 18:59 UT, Gordon Garcia of ALPO's Solar Section
and CCD camera designer R. Radhakrishnan took a CCD image of the Sun's
west limb using Radhakrishnan's homemade camera with a Texas Instrument
TC-211 chip. The image was significant as it marked the first time a
CCD image of the Sun was to be sent to the ALPO Solar Section. Since
that time there has been a proliferation of electronic images of the
Sun by observers. During 2000 electronic images in the Solar Rotation
Reports outnumbered both photographs and drawings.

The attraction of electronic imaging is simple. Once the image is
secured on a disk or hard drive there is no darkroom needed to develop
and print the image. A computer and printer makes processing and
printing an image much easier. Electronic images can also be sent
across the Internet and copied a multitude of times without loss of
image quality.

The purpose of this section is to outline some information about
imaging the Sun with a CCD camera. These are not the only means of
electronic imaging. Virtually all video cameras and digital still
cameras today are based on CCD technology. Images from these cameras
can be transferred by various means into a computer. Prints and
negatives can also be scanned and converted to electronic format for
image processing or transmission on the Internet.

The CCD or Charged Coupled Device is a small solid-state sensor
consisting of silicon. Each CCD chip is made up of tiny photo sites
called pixels. Each pixel reads the incoming photons of light and
converts them into an electrical charge that is amplified and converted
into a numeric or digital format that can be read and processed by a
computer. As in lunar or planetary observing it is important to match
the aperture and focal length of the telescope to the pixel size of the
sensor.  The formula for determining the correct pixel size for a
particular focal length is:  F (focal length in millimeters) = 206 x P
(pixel size in microns)/S (arc seconds per pixel). To achieve one arc
second per pixel resolution with a TI-255 sensor (10 microns) the focal
length of the optical system must be at 2,060 mm. However, the Nyquist
theorem requires that for an image to be properly sampled, the airy
disc must occupy at least two pixels. This would require that each
pixel sample 0.5 arc seconds, or a minimum focal length of 4,120 mm.
Localized seeing conditions may not allow long focal lengths. To
achieve higher resolutions longer focal lengths and possibly additional
aperture must be employed. Another consideration is the actual size of
the sensor being used. To achieve maximum resolution on large sunspot
groups (larger than 1500/millionths of the solar hemisphere) and frame
the entire sunspot group  a large CCD sensor must be used. The TC-211
and TC- 255 chips contain  relatively small imaging areas. When
considering the purchase or construction of a camera one must consider
the aperture, focal length and field of view to be achieved. It is
possible to take a series of images with a small CCD chip and then
digitally stitch them together to form a mosaic that displays the
entire active region.

The CCD sensor should be capable of some relatively fast exposures to
freeze seeing and wind induced vibration. The fastest commercially
produced astronomy cameras can transfer a full frame with an exposure
of about 1/100 second. It would be better if an exposure of 1/1000
second could be achieved. An enterprising amateur could combine a
mechanical shutter in front of the CCD chip to attain a quicker
exposure. Newer cameras and digital SLR and fixed lens cameras have
faster shutter speeds. To achieve the best results the camera exposure
must be triggered remotely.  Although at 1/100 second electronic noise
is greatly reduced, it is still advisable to take a dark frame as well
as a flat field frame and bias frame, especially if solar photometry
work is the goal of the observer. A flat field frame can be achieved by
removing the solar filter and imaging the blue sky away from the Sun.
The camera position and focus must not be changed while taking a flat
field frame. Some camera routines allow for a bias frame. Otherwise,  a
zero-second exposure can be taken for bias correction. Bias should also
be removed from both the flat field frame and dark frames before
calibration of the image frame.

CCD cameras can be used to image the continuum and various wavelengths
of the solar spectrum, the two most popular being Hydrogen-alpha (6563
Angstroms) and Calcium-K (3934 Angstroms). With narrow wavelengths of
light interference fringe patterns can be set up between the CCD chip
itself and the protective window over the chip. The best way to avoid
this problem is to initially purchase a camera that will not produce
this fringing. You can contact the camera's manufacturer to determine
if this problem exists. If you have already purchased the camera, there
are a couple of things that may eliminate or lessen the problem. The
first thing to try is tilting the camera slightly in relationship to
the filter. If you are unable to tilt the camera, inserting a linear
polarizing filter between the filter and camera appears to help
somewhat in H-alpha imaging.  Lastly, you may try either to flat field
the interference pattern out of the image, or take several images so
the fringes are averaged across the entire image, then adjust the
histogram to alleviate the darker fringe pattern.

In order to capture and process a CCD image a computer is needed.
Although older 386 and 486 machines are capable of capturing and
processing a CCD image, more modern Pentium and Power Macintosh
computers make the job much easier and faster. Whether you are using a
notebook or desktop machine you will have to provide shade in order to
see the monitor in the bright sunlight and some way of keeping the
computer from overheating on hot days. If possible, the computer should
be kept inside and connected to the camera and scope by remote cable.

There are many software packages that are provided by commercial
vendors. Unfortunately, it is often necessary to purchase several
programs in order to achieve the desired results. Software programs
that are produced solely to process astronomical images are often the
best choice.  However,  a second program geared toward the graphic arts
industry is needed to add text, composite more than one image on a
page, etc.  Internet astronomy discussion groups are often an excellent
place to hear from other astronomers the merits and faults of various
software products.

When imaging in the field, the first thing is to consider is how to
organize the raw data on your computer. One easy way is to create a new
folder for each imaging session by date (i.e., 991130, year, day,
month). The most important thing in the field is to get as many images
as possible.  Dark frames and bias frames can be taken ahead of time if
you can control the shutter speed and temperature of the camera. It is
a good idea to run the camera at a cooler temperature than the ambient
air to avoid thermal noise. Be careful to bring down the temperature
slowly or dew can form on the CCD chip's protective window. The first
thing is to orient your camera so north, south, east and west is
squared onto the CCD chip position. Cameras usually have a decal or
marking on the back of the camera to delineate orientation. Also,
inspect the chip with a magnifying glass and make sure the CCD is free
of dust particles. Dust can be removed with a hand held air bulb and
some Q-tips. One way to avoid dust is to keep a minus-UV type filter
over the CCD chip. Make sure your computer clock & calendar are
accurately set using a WWV time source. There are also software
programs available that set the computer's clock in reference to the
U.S.N.O. master clock. In order to find the area of interest on the Sun
either a flip-mirror previewer or separate guide/finder scope with safe
solar filtration is very helpful. Otherwise, you will first need to
locate the active region with an eyepiece and center it in the field of
view. Once you have located the area to be imaged, you can place the
camera on the telescope. Most cameras will have a focus mode where you
can image only a portion of the sunspot and the image will be refreshed
on your monitor rapidly so you can tweak the final focus. In order to
start at a point close to focus an eyepiece parfocal with the camera
can be used to achieve an approximate focus. A final focus can then be
achieved by watching the image on the computer.  Focusing screens
(off-axis dual or triple hole masks placed in front of the solar
filter) are also helpful. Once your camera is in focus proceed to take
your reference flat field images. Be sure to remove a dark frame and
bias frame for each flat field frame taken. If you change the
orientation of the camera (or move it at all) you will need to take a
new flat field frame. Be sure to title these reference images so you
can identify them later (Flatf1, Dark1, etc.). You can then begin your
actual imaging of the Sun. The key is to take as many images as
possible during your observing session. Don't worry about processing
the images until you are done. Be sure to check the histogram as you
experiment with different exposures. Make sure that pixels are not
being saturated. The properly exposed image should have a peak
approximately 75% of saturation.

When processing the images the best rule is that the best image is the
one that requires the least amount of processing. First begin by
examining each image from the ones you will ultimately process. Select
those images that appear the sharpest and are properly exposed. Start
by calibrating the images. Be sure to save both your raw images and
calibrated, unprocessed ones as they may be needed at a later date if a
request is received by the professional community. Also be careful
where the zero-point is set on the image. Some photometry programs
create a wrap-around problem whereby umbrae values may end up being
higher than the surrounding photosphere. The first step in processing
is to perform a histogram stretch. The next step is to run an unsharp
mask routine. Most image processing programs have at least one unsharp
mask.  Start with a pixel radius of at least three pixels. Experiment
and keep notes. After you are satisfied with the image adjust
brightness & contrast and final "tweaking" such as a slight sharpening
routine.  The final image should have more detail, appear sharper, but
not look over processed. There are no hard, fast rules for processing
solar CCD images. Experiment and see what works and what doesn't.

The final step is to add the necessary information to the image. With
the great amount of digital images being received by the Solar Section
they are easily separated if the identifying information is not
embedded in the image. The necessary information can be added as text
above or below the image or embedded in the FITS header. The extension
to the file name should clearly indicate the format the image is in
(TIF, JPG, FIT, etc.). The minimum information should include the date
and time the image was taken. The time should be indicated in Universal
Time. The name, address and E-Mail address of the observer must also be
included. Other information that is needed with each image is: the
telescope used (150 mm f/8 reflector, etc.), the effective aperture,
type of solar filter used, seeing (in arc seconds), Carrington Rotation
Number, Central Meridian, Active Region Number (NOAA SEC)(if
applicable) and approximate heliographic coordinates. Two cardinal
points should also be indicated (North - up, West - right, etc.). When
sending your images to the ALPO Solar Section we strongly recommend
that you send compressed JPEG images, with the file name as the date of
the image (year, month, day). An example would be 010517a.JPG,
010517b.JPG, etc. This is due to size constraints of our Internet
server and speed in uploading files on the Recent ALPOSS Observations.