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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.
For critical focus, an exposure of about 1 second 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 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. In
order to avoid saturation, move to a dimmer star if the peak brightness counts are 40,000 or
more. 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.
Once the best focus is found, the focusing operation can be greatly shortened the second
time by removing the CCD head, being careful not to touch the focus knob. Insert a high power
eyepiece and slide it back and forth to find the best visual focus, and then scribe the outside of
the eyepiece barrel. The next time the CCD 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.
3.5.
Finding and Centering the Object
Once best focus is achieved, we suggest using "Dim" 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
CCD head and use FOCUS mode with an exposure time of about ten seconds, if it is dim.
Center the object using the telescope hand controls.
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.
3.6.
Taking an Image
Take a CCD image of the object by selecting the Grab command and setting the exposure time.
Start out with the Image size set to full and Auto Display and Auto contrast enabled. 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.
Section 3 - At the Telescope with a CCD Camera
Page 37
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