SBIG ST-402 Operating Manual
Page 16
Dark Frames
– For most objects you won’t need to take dark frames in
Focus mode to frame and focus the object because the exposures
tend to be short and the build up of dark current minimal. On
some dim objects though, checking this item, which uses
dark
subtracted
images for
Focus
mode can help to pull the object out
of the noise. It’s slightly slower when using dark frames so only
use this when you need to.
Update Mode
– Sometimes you want to tweak the focus, look at an image,
tweak it again, etc. Setting this to
Manual
causes the Focus mode
to pause between images until the
Space Bar
is hit or you click the
Pause/Resume
button.
Peak
– This shows the position and brightness of the brightest pixel in the
Focus mode image. We’ll use this later to get the best focus by
maximizing the
Peak
on a
star
.
Pause/Resume
– Click this button at any time to pause or resume the focus
mode. This can be handy for going back and forth between the
telescope and the computer or for studying the images.
You can read more about the other items in the
Help
but we now know
enough for basic operation. Exit
Focus Mode
by closing the
Focus Mode
dialog or closing the image window.
You’ve come a long way in this brief tutorial. If you want to spend
some time reading the
Help
file or the
CCDOPs Manual
, which is included
on the CD-ROM that would be useful. Otherwise it’s time for trying it out
at the telescope.
SBIG ST-402 Operating Manual
Page 17
4.
At the Telescope
Connect the camera to the computer and start
CCDOps
. Establish a link to
the camera and run the
Camera Setup
command. Enable the cooling to 20
degrees below ambient and then watch the
Link Status
field to see the
temperature come down. Once the temperature reaches the Setpoint it will
jump back and forth by about 0.5 degrees. This is normal. The ST-402
regulates the CCD temperature by dithering back and forth across the
Setpoint. At this point we’re ready to hook up to the telescope.
Focusing a CCD camera at the telescope can be a tedious operation, so
a few hints should be followed. Before using the software to focus the
camera the first time you should place a diffuser (such as scotch tape or
ground glass) at the approximate location of the CCD's sensitive surface
behind the eyepiece tube and focus the telescope on the moon, a bright
planet or a distant street lamp. This preliminary step will save you much
time in initially finding focus. The approximate distance behind the
eyepiece tube for the ST-402
is
0.690 inches
(
backfocus
).
To achieve fine focus, first center a bright star then insert the CCD head
into the eyepiece tube, taking care to seat it, and then enter the CCDOps
Focus Mode
. Do not initially try to focus on the moon or planet – they are
harder. As we learned in the tutorial, the
Focus Mode
automatically
displays successive images on the screen as well as the peak brightness
value of the brightest object in the field of view. Using the telescope
controls, center the star image in the CCD, and adjust the focus until the star
image is a small as can be discerned. Next, shift the telescope to fainter
stars so the CCD is not saturated. Further adjust the focus to maximize the
displayed star brightness in counts and minimize the star diameter. This can
be tedious. It helps considerably if a pointer or marker is affixed to the
focus knob so you can rapidly return to the best focus once you've gone
through it.
An exposure of 1 to 3 seconds is recommended to smooth out some of
the atmospheric effects. While you can use the
Full frame
mode to focus,
the frame rate or screen update rate can be increased significantly by using
Planet mode
. In Planet mode the Focus command takes a full image and
