SBIG ST-402 Operating Manual
Page 18
then lets you position a variable sized rectangle around the star. On
subsequent images the Planet mode only digitizes, downloads, and displays
the small area you selected. The increase in frame rate is roughly
proportional to the decrease in frame size, assuming you are using a short
exposure.
The telescope focus is best achieved by maximizing the peak value of
the star image. You should be careful to move to a dimmer star if the peak
brightness causes saturation. The saturation levels of the various resolution
modes are shown in Table 3.2 below. Another point you should also be
aware of is that as you approach a good focus, the peak reading can vary by
30% or so. This is due to the fact that as the star image gets small, where an
appreciable percentage of the light is confined to a single pixel, shifting the
image a half a pixel reduces the peak brightness as the star's image is split
between the two pixels. The Kodak CCD pixels are so small that this is not
likely to be a problem.
Resolution
Saturation Counts
High Res
~35,000 forABG Cameras
65,000 for Non-ABG cameras
Med/Low Res
65,000
Once the best focus is found, the focusing operation can be greatly
shortened the second time by removing the camera, being careful not to
touch the focus knob, and inserting a high power eyepiece. Slide it back and
forth to find the best visual focus (don’t touch the focus knob), and then
scribe the outside of the eyepiece barrel. The next time the camera is used
the eyepiece should be first inserted into the tube to the scribe mark, and the
telescope visually focused and centered on the object. At f/6 the depth of
focus is only 0.005 inch, so focus is critical. An adapter may be necessary
to allow the eyepiece to be held at the proper focus position. SBIG sells
extenders for this purpose.
Finding and Centering the Object
Once best focus is achieved, we suggest using
Full-Low
mode to help
center objects. This mode gives a full field of view, but reduces resolution
in order to increase the sensitivity, and digitization and download rate. If
you have difficulty finding an object after obtaining good focus, check to be
sure that the head is seated at best focus, then remove the head and insert a
medium or low power eyepiece. Being careful not to adjust the focus knob
on the telescope, slide the eyepiece in or out until the image appears in good
focus. Then visually find and center the object, if it is visible to the eye. If
not, use your setting circles carefully. Then, re-insert the camera and use
the
Focus Mode
with an exposure time of about ten seconds, if the object is
quite dim. Center the object using the telescope hand controls.
SBIG ST-402 Operating Manual
Page 19
Note
: With a 10 second exposure, objects like M51 or the ring-nebula are
easily detected with modest amateur telescopes. The cores of most galactic
NGC objects can also be seen.
Taking an Image
Take an image of the object by selecting the
Grab
command and setting the
exposure time. Start out with the
Image size
set to
full
. The camera will
expose the CCD for the correct time, and digitize and download the image.
One can also take a
dark frame
immediately before the light image using
the Grab command by selecting the
Dark Also
option.
Because the ST-402 has regulated temperature control, you may prefer
to take and save separate
dark frames
, building up a
library
at different
temperatures and exposure times, and reusing them on successive nights.
At the start it's probably easiest to just take the dark frames when you are
taking the image. Later, as you get a feel for the types of exposures and
Setpoint temperatures you use, you may wish to build this library of dark
frames. Fresh darks seem to work better, so it is not recommended to build
a huge dark library.
Saving Images
Save the images using the Save command in the File menu. We strongly
recommend you save images in the SBIG Compressed format as it is the
most compatible. Use the TIFF and JPEG formats only when you are done
with the image as you won’t be able to reopen it with CCDOps. Those
formats are useful for export to photo programs.
Further Adventures
At this point we refer you to the
CCDOps Manual
that can be found on the
SBIG Software and Catalog
CD-ROM. It has an extensive tutorial section
and detailed information about the software and its capabilities.
