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Before I began imaging I read two books
which proved very helpful in getting me started. In the
following webpages I have summarized the equipment and techniques
that I currently use to take my images. Hopefully this
information will prove useful to you. The imaging techniques
and equipment you need to use vary slightly depending on the type of
imaging you want to do. The two main types of imaging are Deep
Sky and Planetary,
Lunar and Solar imaging
- You need to choose a focal length
appropriate for the object you wish to image.
- You need to be able to achieve good
focus
on the object
- You need to be able to find and centre
the object on the CCD chip.
-
Good quality deep sky images require long
exposures (e.g. up to several hours). This is
because objects such as galaxies and nebulae are very dim and a
long exposure is needed to collect enough light to form a good
image. It is therefore important to have a good polar
alignment so that the guide corrections needed during the
exposure are minimized. Fortunately with CCDs you do not
need to take a single image lasting several hours. It is better to take
say 24 images of 10 minutes each and add
them together afterwards using image processing software.
This helps minimize the affect of any misalignment of the polar
axis and RA drive tracking errors. The other advantage, is
that combined with a technique called dithering it allows you to
use statistical rejection techniques as you combine the images
which can remove hot/cold pixels and satellite trails. The need
to take long duration exposures, rules out the use of webcams
for imaging all but the brightest of deep sky objects as they do
not have the capability to take images of more than a few
seconds at best.
-
Taking a color image
with a monochrome CCD camera requires separate images to be
taken through red, green and blue filters. These images
can be combined using image processing software to create a true
color image. Alternatively a single color image can be
taken if you use a color CCD camera such as the Starlight
Xpress MX5C, a color webcam or Digital SLR.
-
During a long exposure, noise (known as dark current) builds
up on the CCD chip. This noise would severely affect the
final image. Fortunately it can be subtracted from the
image using dark
frames. Even with this technique it is still good to
minimize the dark current generated on the chip. The dark
current generated is proportional to the temperature of the
chip. The lower the temperature of the chip the lower the
dark current. So for the best quality deep sky images it
is best to use a CCD camera with the ability to cool the the CCD
chip to around 30 degrees Celsius below ambient air
temperature. CCD cameras from Starlight Xpress and SBIG
have this ability, whereas webcams and Digital SLRs do not.
- Taking Flat Field
images is recommended to achieve the highest image quality.
- Once the images have been captured, the final step is image
processing
- You need to choose a focal length
appropriate for the object you wish to image.
- You need to be able to achieve good
focus
on the object
- You need to be able to find and centre
the object on the CCD chip.
-
Good quality planetary images require short
exposures (e.g. a few tenths of seconds or less). This
is because the planets, moon and sun are very bright compared to
deep sky objects such as nebulae and galaxies. The short
exposure has the added benefit of 'freezing' atmospheric
turbulence allowing finer detail to be resolved than in deep sky
images. Important: When taking an image of the Sun, the
use of a solar filter is essential. Looking at the Sun
through a telescope without a solar filter, even for a second,
will cause permanent blindness. Using CCD imaging
equipment to image the Sun without a solar filter will damage
the equipment beyond repair.
-
As the exposure duration is very short, a good polar
alignment is not critical as the the object will not drift
much in a few tenths of a second. However, a good polar
alignment will help keep the object on the CCD chip as you take
multiple exposures.
-
The key to good high resolution planetary, lunar and solar
images is to take as many images as possible as quickly as
possible. If you can take a large number of images you can
sort through them afterwards and keep the ones that were taken
at the best moments of seeing and so have the most detail.
These 'best' images can be added together using image processing
software to produce an image with much less noise than a single
image. This permits more detail to be enhanced using image
processing techniques such as Unsharp Mask.
-
The need to take a large number of images quickly means that
webcams are ideally suited to planetary, lunar and solar
imaging. Webcams can take tens of images per second.
It is not uncommon for an imager to take 3,000 images of an
object using this technique and then sort through them later
looking for a few hundred 'best' images that can combined.
-
Taking a
color image
with a monochrome CCD camera requires separate images to be
taken through red, green and blue filters. These images
can be combined using image processing software to create a true
color image. Alternatively a single color image can be
taken if you use a color CCD camera such as the Starlight
Xpress MX5C, a color webcam or Digital SLR.
-
Even during a short exposure, noise (known as dark current)
builds up on the CCD chip. This can affect the quality of
the image. Fortunately it can be subtracted from the
image using dark
frames.
- Taking Flat Field
images is recommended to achieve the highest image quality.
- Once the images have been captured, the final step is image
processing
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