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For those who are new to planetary and lunar imaging, we
hope that the following article will assist you in
getting started in this highly rewarding hobby. Only
modest equipment is needed to get excellent images.
Given the short exposure times required to image these
bright targets, it is more technique and practice that
gets results instead of expensive equipment. Listed are
a few tips to avoid some common pitfalls that have
newbies ripping their hair out in frustration. We will
delve into some detail here, but it can be summed up
with a few simple words:
COLLIMATION, FOCUS, EXPOSURE, PROPER SAMPLING and
PROCESSING.
What Type of Camera and Telescope Combination is Best
for Planetary Imaging?
The most important feature of a planetary camera is
small pixel size. This is the physical size of each
individual pixel, usually measured in microns, and not
the size of the array. A good range is 5-10 microns.
This will allow small details planet to cover more
pixels, thereby allowing you to record the small details
more accurately. This is known in the astrophotography
world as proper sampling.
Without a doubt, any camera capable of recording AVI
movies on your laptop is best for reasons we'll get into
later. SAC imagers, like the SAC7 series, webcams and
even video taped images can be used.
You can also shoot still frames from either the SAC,
webcam, LPI or digital camera and process the images
manually. Many astrophotographers have done stunning
work with modest digital cameras. If you go that route,
we recommend that you use Scopetronix's Digi-T adaptor
to connect your camera to the eyepiece. This adaptor
will work with most common eyepieces and get your camera
as close as possible to the eyepiece to reduce
vignetting effects. The SAC camera comes with a 1.25"
nosepiece adaptor so it'll plug straight into your
telescope. Adaptors can be purchased from Scopetronix
and other companies to allow you place a 1.25" nosepiece
on your web cams. If you are a do-it-yourself, you can
make a webcam adaptor using a plastic 35mm film
canister. As it turns out, the diameter of that canister
is 1.25 inches. Just make sure that the canister is
square to the ccd chip when it is mounted.
As far as the telescope is concerned, the larger the
aperture the better. This will give you more light to
resolve a highly magnified image. The combination of
large aperture telescope will a small pixel size camera
is a formidable weapon for this type of imaging.
Typically, a telescope of eight inches or larger is
best, but smaller ones can be used quite successfully.
Stack Your Images
Those great planetary and lunar images you see
everywhere are not single shot images. They usually
consist of many images that are mathematically averaged
(stacked) together. This is done for two reasons; noise
reduction and detail enhancement. Lets talk about noise
reduction first. The CCD sensor in your camera generates
a certain amount of electrical noise. In your images, it
takes the appearance of a graininess, or snowiness. This
noise chews into your image and obscures details. The
amount of noise that a camera generates is directly
related to the temperature that the camera is operating
at. The warmer the temperature, the more noise it will
produce. When you mathematically average two images
together you increase its signal to noise ratio (SNR).
In other words the signal (detail) increases and the
noise decreases. The more images that you average
together the higher the SNR will become. It is not
uncommon to average hundreds of images together to get
superior results. Luckily, there are free software
packages that will perform this averaging function but
we will get to that in a minute.
The main problem with averaging images is that the
pictures must be perfectly aligned (or registered) in
order for the process to work. Manually aligning
planetary images is a difficult and tedious process.
Even the auto alignment functions of the leading image
processing programs have difficulty registering
planetary images accurately. Enter Registax! Registax is
a free program available on the Internet that will
automatically align, average (stack) and process your
raw images your images. You can download it for free at
the link indicated near the end of this article.
Shoot Movies Instead of Still Images
Given the fact that you must use a high number of raw
images in your stack, it is highly feasible to shoot
movies instead of single images. Movies? Yes! Some
cameras, like the SAC imagers or web cams can record
short movies and store them on your hard drive in AVI
format. A standard video recorder and some sort of video
capture device on your computer can also be used to
record AVI movies. An AVI movie is nothing more than a
series of still frames shown to you in rapid succession.
If you could split up the movie into still frames, that
would be ideal. Well, Registax will do exactly that. It
will take a raw AVI movie, split it into individual
frames, rearrange the frames in order of quality,
automatically drop the frames that are below a certain
quality, align the remaining frames, stack them and then
drop you into a screen for final processing. It couldn't
possibly get easier.
Shooting movies has another significant advantage. When
you look at a planet visually, you know that for brief
times the image will snap into a sharp focus and then
back out as seeing conditions fluctuate. Recording
movies allows you to capture those moments of great
seeing, and Registax will automatically pick those
images out for you.
How do I Configure the Basic Setup?
I recommend a prime focus setup, where the eyepiece is
removed and a camera used in its place. It is less
complicated and will give you better initial results
than if you used eyepiece projection. Planetary and high
resolution lunar images require that you cast a large
image of the planet across the CCD chip. This can be
thought of as being highly magnified, but in reality
there is no magnification in the camera. You are simply
casting an image across the chip. I have coined the
phrase "apparent magnification" to help describe this.
Generally, focal ratios of f20 and beyond are required
for high resolution imaging. If you use a standard SCT.
already at f10, you can increase it to f20 by simply
inserting a 2X barlow in front of the camera. For f30,
use a 3X barlow. If you are using a Newtonian which is
typically at f5, you can also use a barlow, or stacked
barlows to achieve the same effect. The limit of
"apparent magnification" you can achieve is determined
by a number of factors, not the least of which is your
current seeing conditions. I generally record my first
movies of the evening at f10, and then progressively
step up to the limit, first a 2X, then 3X barlow and
beyond. Higher f ratios will require that your mount is
polar aligned and tracking properly. If not you will
spend most of your time chasing the target instead of
gathering data.
Some Tips for Getting Great Images
Seeing Conditions
You will be limited by the current seeing conditions at
your location. You can get a general feel for it by
looking straight up to see how much the stars are
twinkling. Stars do not twinkle, they are solid point
sources of light. Any twinkling you see is due to
atmospheric disturbance.
Collimation
The collimation (the alignment of your optics) is
critical at high f ratios. It will be impossible to get
any substantial detail with an un-collimated scope.
There are many articles on the Internet dealing with
this subject and it is beyond the scope of this article.
Suffice it to say that a six inch reflector that is in
collimation will easily out perform a ten inch that is
out of collimation.
Temperature Equalization
Allow your telescope, camera and all other optics
sufficient time to cool to equalization. For a ten inch
telescope this could be over an hour, especially when
the temperature shift is greater than forty degrees.
Collimation of optics should only be performed on an
equalized telescope.
Timing and Target Location
If at all possible, wait several hours after sunset
before you start imaging. During the first few hours,
the Earth radiates most of the heat it has built up
during the day. This is true in both summer and winter.
This radiation of heat will give you a shimmering
effect. The same thing can happen if you shoot over a
heated dwelling or large body of water. A laptop
computer sitting in front of, and below the telescope
can also cause this. No kidding! Also, refrain from
using dew heaters whenever possible.
Targets are best imaged when they are sixty degrees
above the horizon or higher. Ideally the target should
be straight up. When you shoot straight up you are only
shooting through the height of the atmosphere. When
targets are down low, as Mars was last August, you are
shooting diagonally through it and are subject to much
more atmospheric dispersion and turbulence.
Atmospheric dispersion is common when targets are at low
angles. The atmosphere acts like a lens and diffracts
light at different angles depending on the color. You
can notice it easily if you see a blue halo being
projected from the top of your target and a red one
below (or vice-versa depending on the setup). The
atmosphere splits up the light like a prism. The only
way to get around this is wait for the target to get
higher. It can also be corrected for, in a large degree,
during processing. You can use Photoshop or another
program to split up the RGB image, realign the primary
colors and then recombine them into a color image. The
blue part of the image will be slightly larger because
it is slightly out of focus, so the process isn't 100%
effective. With more work on the blue frame you can get
better results. Luckily, the new version of Registax
will perform this function automatically and is quite
effective.
The effects of atmospheric dispersion are evident in
this raw Mars image. Notice the top blue, and bottom red
halos. It can also be easily seen on the polar cap. This
was common on most Mars photos during its opposition of
August 2003 as it was at a low angle, especially at the
upper latitudes.
Focus is Critical
The human eye is a remarkable device that is capable of
focusing in real time. It will adjust, to a certain
degree, to compensate for an unfocused image at the
eyepiece. The CCD is completely unforgiving and must be
exactly focused. There are no image processing tricks
that can repair an unfocused image, although many try to
cover it up with unsharp masking and other techniques.
This only introduces processing artifacts into your
image making them look unnatural. Focus is critical. Use
an electronic focuser whenever possible. If you are
shooting movies, most software will allow you to focus
and position on a live image. This is a significant
advantage in both time savings and frustration.
Dirty Optics
When specks of dirt or other debris are on the optics
they will most certainly show up in your images as dust
motes. They have a donut like appearance and their size
depends on how far they are away from the camera. Any
dust or dirt on the secondary mirror, and especially the
CCD itself will produce these nasty dust motes in your
pictures. The closer to the CCD they are the more they
affect your photographs. On the CCD itself is the worst
place to have them. If they are located on your
corrector plate, objective lens or meniscus then they
are generally so far out of focus that you wont see
them. Anything close to the camera has to be really
clean. This includes diagonal mirrors, barlows or
powermates, filters, prisms etc.
The way to remove dust motes from you images are to
shoot "flat fields". A flat field is a picture of an
evenly illuminated surface. When you examine the picture
you will see the effects of dirty optics and uneven
sensitivity in both the camera and optics. When you
correct your image using the flat field all of these
effects disappear.
If you have never experienced "dust motes" in any of
your night time pictures, you will certainly be beaten
over the head with them during any solar shoot. Your
optics need to be especially clean. Below are some
pictures taken during a solar shoot where the dust motes
ruined the image, and then were corrected out.
Raw solar picture of a basic bare sun with a group of
sunspots on the left side. The dust motes in this case
were located on the barlow lens and can be seen on the
right half of the photo.
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