 IMPORTANT NOTICE! Never use a telescope or spotting scope to look at the Sun! Observing the Sun, even for the shortest fraction of a second, will cause irreversible damage to your eye as well as physical damage to the telescope or spotting scope itself. |
Introduction
This section will discuss the principles behind CCD technology, its advantages
and disadvantages, and the things needed to take good quality images. This
is not meant as a comprehensive CCD reference, but rather a quick introduction
to the basics for the beginning imager.
If you are new to CCD imaging...
This section is written for those who are either new to astronomical imaging
or would consider themselves "casual" CCD imagers.
If you consider
yourself to be accomplished or expert, you can skip this section and go
to Section 2.
This section will guide you through each step necessary to get an image
using the Pictor CCD Imaging System and your telescope.
Before you try to image actual celestial objects, you should practice setting
up your telescope with the Pictor and your PC and make some daytime images.
In this way, you will become very familiar with the basic operation of the
camera and the software. Familiarity with your equipment and consistency
in working with your system are necessary to get good images you will be
proud of. For your convenience, two daytime exposure setups are provided.
Now, let's get set up...
Making Your First Image; a Daytime Shot
1. Properly set up your telescope as described in your telescope user's
manual.
2. If you have a large aperture telescope, you will need to make a cardboard
cover for the objective lens with a 0.5" - 1" diameter hole halfway
between the center and the rim. This limits the amount of light falling
on the CCD chip and is necessary for daylight images.
3. Focus the telescope for use with the camera. Focusing a telescope with
a CCD camera attached is one of the most difficult tasks in CCD imaging.
Once a rough focus is attained, the focus may be easily fine tuned using
the Focus Mode, described in step 10 of this section. The following step
details how to obtain the first approximate focus. Your telescope should
be pointed at a large, relatively bright object that is not too close (200feet or more if possible).
4. Take the focusing ring and slide it over the guiding barrel. You will
need to unscrew the black back of the barrel, then slide the ring on, and
reattach the black back. Slide the focusing knob until it is against the
black end of the barrel (the end that screws on to the Pictor). Now tighten
the ring by using the thumbscrew.
Next, attach a piece of scotch tape to the open end of the imaging barrel
where the Pictor would attach. Insert the barrel into the eyepiece holder
of your telescope as far as it will go (it should stop against the focusing
ring), and lock the barrel in place. With your eye about 1 foot back from
the tape, you will see a projection of the image on the scotch tape. Turn
the focuser until this image is sharply focused. Remove the tape and imaging
barrel. Take the focusing ring off the guiding barrel and reassemble the
barrel. You may see page A6 of the Appendix and make a parfocal eyepiece
at this point. You should have an approximately focused telescope. You will
need to repeat this procedure at night the first time, since the focus will
change when moving from a relatively near terrestrial object to a celestial
object such as the Moon.
5. Connect and turn on the Pictor and your PC as described in Section 1of the Users' Manual.
6. Start the PictorView XT software. Select Set User Preferences from the
File Menu. Select the COMM port the camera is connected to. For most users,
this will be COM1 or COM2. Also select the baud rate. If you have Windows95,this can be up to 56K, for Windows 3.1 you must start in standard mode to
connect at any baud rate above 19.2K. If you have Windows for Workgroups3.11, you cannot select anything above 19.2K unless you have a custom high
speed serial driver (i.e. from Digi or RocketPort). If you are not sure,
select 19.2K. Select OK. Note that this only needs to be done the first
time you run PictorView XT.
7. Click the X button on the far right of the toolbar. After a couple of
seconds, the right of the status bar at the bottom of the screen should
read
"On Com 1 @ 19.2k" where 1 is the comm port and 19.2kis the baud rate. You are now ready to take an image. If you receive an
error message, check the cable connections and make sure you have selected
the correct COMM port in Step 6. You may need to switch to COM2, for example.
8. Now select the Exposure dropdown box in the center of the toolbar. There
will be a listing of predefined exposure setups to select. Select the one
called Daytime1. Now click the camera icon on the toolbar. You will see
a message
"Taking Exposure", then another message
"Preparingimage for download, please wait". This may appear for a few seconds.
Then a percent box will appear showing the download status. The percentages
may not increase smoothly and may hang for several seconds; this is normal.
The total download time will be 1min - 2min at the most. If you get a message
saying
"Communication Error; Retry, Cancel", with Retry
and Cancel buttons, press Retry. If the message reappears without any increase
in download status, press Cancel. Note: this message can be caused by interference
from electrical appliances, or poor serial connections.
9. You should now have an image on your screen. If the image is out of focus,
you may try the Focus Mode. This is entered by clicking on the Focus icon
on the toolbar (it looks like a target). Focus mode will download an image,
then ask you to draw a box around a subframe. This is so that a part of
the image (the part you want to focus on) is downloaded each time, for quicker
response. Draw the subframe by holding down the left mouse button and moving
the mouse to from a rectangle around the object you want to focus on. When
you release the left button, a dialog will appear asking if this subframe
is OK. After clicking YES (you can click no and redraw another subframe),a window will appear that has two rectangles in it. After a few seconds,
an image will appear in the left window (the image should match the subframe
you drew).
Every few seconds, the image will update, and the previous image will be
moved to the right window. This is so that you can see if the focus is getting
better or worse. To focus, wait until the image appears, the move the focus
knob on your telescope. Wait for one or two updates of the focusing window,
and you will see if the focus is getting better or worse. It is important
to not move the focusing knob too much at each turn, or you may go past
focus. This will depend on how sensitive the focuser is on your telescope.
When the image is focused, press the Done button. The focusing window will
disappear within a few seconds (it may be in the middle of downloading,
which can take a few seconds). If the object is too far away or the day
is hot or windy, perfect focus may not be obtainable due to atmospheric
conditions.
If you have a Meade LX200 with an electric focuser, you will not need to
manually focus the telescope as long as you have set up your LX200 options
in Set User Preferences under the File Menu and you have a second serial
port on your PC to connect to the RS-232 port on the LX200.
10. You now have a focused Pictor. When repeating this step at night, you
can make a small mark on the focusing tube of your telescope if you wish;
this will enable you to come closer to focus before entering the Focus Mode.
Alternatively, you can adjust the position of the ring on your parfocal
eyepiece.
11. Now select the
Exposure Setups dropdown
list and select Daytime binned. Click the Camera icon. You will notice the
download time is much quicker. When the image appears, it is one quarter
the size of a full image. This is a process called binning, which has several
advantage:
1. Downloads are much faster.
2. The image is sharper.
3. The signal to noise ratio is improved.
Any of the Exposure Setups may be binned by selecting them from the list
box, then pressing the Edit Setup tool (on the toolbar to the right of the
Setups list box). Then click Binning On, then Save. That Exposure Setup
will now produce a binned image. Note that you cannot change the name of
the Default setup. For more information, see Edit Setups under the Camera
Menu.
12. Experiment with other options in the Edit Setups window; Number of frames,
Prescale, Antiblooming, Gain, and Shift and Combine. The number of frames
option will take multiple images without any further intervention, for example,
type 3 in the entry field, then click Save. Now, anytime you select that
Exposure Setup and click the Camera icon, the camera will take 3 identical
images. Up to 50 shots can be taken this way.
Prescale is important for night shots, which most of your images will be.
This option will automatically set the background and range of the image
to display your shot in the best possible manner without processing. To
further adjust the background and range, use the arrow keys on the keyboard.(Background and range is analogous to brightness and contrast). The antiblooming
feature is also most useful for night time shots. This will help prevent
a bright star from blooming, or saturating part of the image.
Gain is the amount of amplification applied to the camera signal. A setting
of 1 is acceptable for most images, but faint nebula may need a setting
of 2. Settings higher than 2 should rarely be used. Presets are provided
for all these options which will cover most situations. For more information
on other options such as Automatic New Dark Frame, and Flat Field, see the
Camera Menu section.
The Shift and Combine feature allows the camera to take several exposures
and add them together to produce a single image. This is desirable because
the Pictor cannot effectively expose for more than 25 minutes or so, but
some faint objects can benefit from long exposures. Also, many telescopes
cannot track an object accurately for more than a few minutes without being
manually guided or attached to an autoguider. Finally, note that all Pictor
cameras can autoguide as well as image.
Shutting Down the Camera
To end your imaging session, disconnect from the Pictor by pressing the
X button on the toolbar. You can select Perform Safe Shutdown to ensure
that your Pictor does not warm up too abruptly, which can cause hot pixels
to appear. Hot pixels are pixels that will always read brightly no matter
what you are imaging. If you forget to do a controlled shutdown once in
a while don't worry; it takes many times for the hot pixel problem to show
up.
The First Nighttime Image
1. After getting familiar with the software, you are now ready to begin
night imaging. For the first session, it is probably best to pick a target
such as the Moon. Set up the telescope and camera as you did for daytime
imaging. Remember to repeat Steps 4 and 5 to refocus your telescope and
readjust your parfocal eyepiece. Select one of the Lunar Exposure Setups
from the dropdown list, then connect to the camera by pressing the X icon.
Click the Camera icon. You should now have a picture of the Moon! If the
image is too dim or bright, try varying the background and range using the
arrow keys. If it is still not correct, you may need to change the exposure
time in the Exposure Setup.
2. To save an image, press F2. A default path and suggested file name will
appear. You can change the file name and format by retyping them in the
area provided.
3. Continue imaging the Moon or planets if you wish, or you can move onto deep sky imaging.
4. Deep Sky imaging: this is taking images of objects such as nebulae, double
stars, clusters, and galaxies. For good deep sky images, good calibration
frames are essential. See the section following this introduction and the
section on Imaging With the PictorView Software in Section 2 of the User's
Manual (or the Viewing Images section) for detailed information about calibration
frames. Exposure times for deep sky objects will vary widely depending on
the objects magnitude, seeing conditions, telescope size, etc. A sampling
of DeepSky setups are preset for your convenience, but you will surely want
to fine tune them as you gain experience.
Customizing and Adding Exposure Setups
As you gain experience, you will want to create custom Exposure Setups or
change the options on the presets. PictorView XT allows you to do this easily.
There are a total of 16 possible Exposure Setups, including the Default.
Ten of these are predefined when you receive your camera, the other 6 are
called User1 to User6. To create a custom setup, select one of the user
setups in the dropdown list on the toolbar, then click the Edit Setup icon.
Make the changes you want in the Edit Setup window and give the setup a
name, such as MyDeepSky or Binned Moon, or whatever is meaningful to you.
Click Save, and the new setup appears in the dropdown list. The same procedure
can be applied to renaming or modifying the preset Setups. There is a special
setup, called Default, that is always loaded. This setup may be modified
just like any other, but its name cannot be changed. Use this to enter your
most common options. Also, on the Edit Setup window there is a button called
Shoot Now. This button allows you to take an exposure immediately. This
is the same as clicking Save, then clicking the Camera icon.
CCD Technology
A CCD (Charge Coupled Device) is a semiconductor device similar to the integrated
circuits found in televisions and computers. It's usually a small (less
than 1" square) silicon chip subdivided into as many as 4 million picture
elements, commonly known as pixels.
The pixels are arranged in a matrix, or rectangle. The figure below displays
a simplified arrangement of pixels on a chip, with an individual pixel highlighted.
When light (in the form of a photon) hits a particular pixel, an electron
is generated. The CCD chip will store this electron at the location of the
pixel; for example, if a photon of light hits a pixel at the upper right
of the CCD chip, the electron will be stored at the upper right of the CCD
chip. More than one electron can be stored at a particular pixel location;
if a number of photons hit a specific pixel, a corresponding number of electrons
will be stored at that location.
By tracking the number of electrons stored at a particular location on the
CCD chip, the Pictor can monitor any changes (movement of stars) in its
field of view. For example, if a large number of photons have hit one pixel,
this probably indicates the location of a particular celestial object. As
these electrons move across the CCD chip, the Pictor 208 is alerted to that
object's movement in the sky. Then the Pictor can send the commands to the
telescope to keep it centered on that object.
Likewise, the Pictor can transmit the pixel data to the PictorView XT software
which takes the intensities (the number of electrons that have struck each
pixel), and converts that into a useable, visible picture.
Patience is needed because sometimes it can take several tries to make an
excellent image.
A final tip is do not delete any raw frames. Images that look terrible can
look excellent after being cleaned up. Images always look worst right out
of the camera.