The main difference between finding objects with your telescope visually and
finding them with a CCD is that the camera takes the place of your eye.
Instead of seeing the telescope image in an eyepiece you see it on a computer
screen. The advantage of this system is that CCDs are far more sensitive
than the human eye, so you can more easily see faint objects. There are
still some tricks, however, to finding targets and then framing them properly in
the CCD's field of view. This section will guide you through the process
of using MaxIm DL to find and center imaging targets.
The first step is to begin taking a continuous sequence of exposures.
This will allow you to see what the telescope "sees". With smaller CCD
cameras or those using high-speed connections, such as USB 2.0, there will be only a
short delay between each exposure. With some CCDs there will be a longer
delay, but this need not be a hindrance to finding objects and tips are given
below to make using large-format CCDs easier.
Select the Focus tab in the MaxIm CCD camera control window. Select
for Binning. This will let the camera operate in low-resolution mode for
faster exposures and download times.
The proper exposure time will depend on the CCD camera and telescope being
used, and the object being imaged. An 8" f/1.8 HyperStar SCT might need only a
1 second exposure to detect M51, whereas a 12.5" f/9 Ritchey-Chrétien might need
a 10-second exposure to find a faint galaxy cluster. Remember that at this
point you don't need a great picture, you only need to be able to vaguely make
out the target to get it into view. In the Focus window, select an
exposure time in the Seconds box. 3 seconds is probably a good starting point unless you know you are
imaging something especially bright like the Orion Nebula, in which case a
1-second exposure should suffice for just about any system.
Be sure the Subframe option is not selected so that the entire field of view
of the CCD is displayed. The Continuous box should be selected in all
cases except older large-format, parallel-based CCDs which may require a while to
download even in binned mode. In this case it is desirable to use the manual update mode
allowing you to take another image only after making an adjustment to the
telescope (if necessary). The Dark box should also be checked so that a
dark frame is subtracted making finding your target easier. Unless the
temperature changes significantly during the centering process only one dark
frame will be taken and then reused.
The "Frame" Nebula
So, suppose you have decided to image NGC2024, the Flame Nebula in Orion.
This is a good example of finding and framing an object. Finding is easy,
even without a goto telescope as the Flame Nebula sits right next to Alnitak,
the easternmost star in Orion's belt. Due to its faintness, finding it
requires a longer exposure than, say, the Orion Nebula, but it is easier than
something very faint like the nearby Horsehead Nebula (see following section).
The key to imaging the Flame Nebula, however, is proper framing. What
makes the Flame a challenging target is the same thing that made it easy to
locate: Alnitak. This 2nd-magnitude star is so close to the Flame
Nebula that keeping it in the image can cause its brilliance to drown out the
faint glow of the Flame.
Suppose that your goto telescope has plopped NGC2024 right in the middle of
CCD field. While normally this would be great, in this case it means that
Alnitak is glaring away from one side of the image.
You can move the telescope either with the hand controller or by using the
direction buttons in the Telescope Control window of MaxIm DL (which requires
having the telescope control cable hooked up from the computer to the telescope
mount). Which way the buttons move the object in the image is dependent on
the orientation of the CCD, the type of telescope, the type of mount, the
position of the mount (eastern sky or western sky) if the mount is a German
equatorial, whether it is the second Tuesday of the month, and the whims of the
Fates. Okay, actually there is a way to figure it out, but your best bet
is to push a button and see what happens. Remember (or better yet, write
down) which way each button moves the image; assuming your imaging setup is
basically the same each time, you will pretty quickly learn what does what.
In the case of the Flame Nebula, move Alnitak out of the field of view while
leaving the nebula itself in sight. You are now ready to image.
Similar techniques apply for framing objects which are either off-center due to
the telescope's pointing (or your pointing, if you have a non-goto scope) or
which are extended more in one direction and needs to be intentionally offset.
The Orion Nebula is a good example of a necessary-offset object. Goto
scopes will generally center the bright core of the nebula, the region around
the Trapezium star cluster, but much of the nebula actually sits off to the
southwest of this core.
Okay, What About the Really, Really Faint Stuff?
The Horsehead Nebula. You looked for it in your first 6" Dobsonian,
couldn't see it, bought an 8" telescope, no luck, moved up to a 10", nothin'
doin', tried a 12" belonging to a friend, not a chance, looked through a 20"
monster scope at a star party in the darkest place on Earth, using special filters,
and finally -- barely -- glimpsed the stupid thing. Now, realizing how much
more efficient CCD imaging is than the eye, you wish to capture this elusive
target with your new camera. Finding the Orion Nebula was a cinch to find with a
1-second exposure, and the Flame Nebula was clear in 3 seconds, even with Alnitak in the way. But the Horsehead
might not appear until you've
taken a 15-second shot. And it is in the lower right hand corner of the
field of view. How to center it up without taking all night doing it?
Take a single exposure using either the Focus mode (in manual update mode so
only a single shot is taken) or using the Expose function. Set the exposure
long enough to see the faint subject you are looking for.
Above: A 30-second exposure clearly shows the Horsehead
Nebula. Note the bright stars which can be used to center the object in a
Now, go back to the Focus window, set the exposure to something of more
tolerable duration, say, a second or so, and begin taking a continuous sequence
of exposures as you would to center a bright target (make sure Continuous is
checked again). By moving the stars you noted above to the
appropriate positions in the image, you can easily frame a dim subject without
even seeing it.
Above: Move the bright stars noted in the original long
exposure to their appropriate positions.
Now a long exposure should show the target in the correct location.
Next, Basic Imaging...